Longest common substring

Longest common substring
You are encouraged to solve this task according to the task description, using any language you may know.

Write a function that returns the longest common substring of two strings.

Use it within a program that demonstrates sample output from the function, which will consist of the longest common substring between "thisisatest" and "testing123testing".

Note that substrings are consecutive characters within a string.   This distinguishes them from subsequences, which is any sequence of characters within a string, even if there are extraneous characters in between them.

Hence, the longest common subsequence between "thisisatest" and "testing123testing" is "tsitest", whereas the longest common substring is just "test".

References

11l

Translation of: Python
```F longest_common_substring(s1, s2)
V ir = 0
V jr = -1
L(i1) 0 .< s1.len
V? i2 = s2.find(s1[i1])
L i2 != N
V (j1, j2) = (i1, i2)
L j1 < s1.len & j2 < s2.len & s2[j2] == s1[j1]
I j1 - i1 >= jr - ir
(ir, jr) = (i1, j1)
j1++
j2++
i2 = s2.find(s1[i1], i2 + 1)
R s1[ir..jr]

print(longest_common_substring(‘thisisatest’, ‘testing123testing’))```
Output:
```test
```

Action!

```BYTE Func Equals(CHAR ARRAY a,b)
BYTE i

IF a(0)#b(0) THEN
RETURN (0)
FI

FOR i=1 TO a(0)
DO
IF a(i)#b(i) THEN
RETURN (0)
FI
OD
RETURN (1)

PROC Lcs(CHAR ARRAY a,b,res)
CHAR ARRAY t(100)
BYTE i,j,len

IF a(0)<b(0) THEN
len=a(0)
ELSE
len=b(0)
FI

WHILE len>0
DO
FOR i=1 to a(0)-len+1
DO
SCopyS(res,a,i,i+len-1)
FOR j=1 to b(0)-len+1
DO
SCopyS(t,b,j,j+len-1)
IF Equals(res,t) THEN
RETURN
FI
OD
OD
len==-1
OD
res(0)=0
RETURN

PROC Test(CHAR ARRAY a,b)
CHAR ARRAY res(100)

Lcs(a,b,res)
PrintF("lcs(""%S"",""%S"")=""%S""%E",a,b,res)
RETURN

PROC Main()
Test("thisisatest","testing123testing")
RETURN```
Output:
```lcs("thisisatest","testing123testing")="test"
```

```with Ada.Text_Io;

procedure Longest_Common_Substring is

function Common (Left, Right: String) return String is
Com : array (Left'Range, Right'Range) of Natural := (others => (others => 0));
Longest : Natural := 0;
Last    : Natural := 0;
begin
for L in Left'Range loop
for R in Right'Range loop
if Left (L) = Right (R) then
if L > Left'First and R > Right'First then
Com (L, R) := Com (L - 1, R - 1) + 1;
else
Com (L, R) := 1;
end if;
if Com (L, R) > Longest then
Longest := Com (L, R);
Last    := L;
end if;
end if;
end loop;
end loop;
return Left (Last - Longest + 1 .. Last);
end Common;

begin
end Longest_Common_Substring;
```
Output:
```test
```

Aime

```void
test_string(text &g, v, l)
{
integer n;

n = prefix(v, l);
if (~g < n) {
g = cut(l, 0, n);
}
}

longest(text u, v)
{
record r;
text g, l, s;

while (~u) {
r[u] = 0;
u = delete(u, 0);
}
while (~v) {
if (rsk_lower(r, v, l)) {
test_string(g, v, l);
}
if (rsk_upper(r, v, l)) {
test_string(g, v, l);
}
v = delete(v, 0);
}

g;
}```
`o_(longest("thisisatest", "testing123testing"), "\n");`

ALGOL 68

```BEGIN
# returns the longest common substring of s and t #
PROC longest common substring = ( STRING s, t )STRING:
BEGIN
STRING s1       = s[ @ 1 ]; # normalise bounds to 1 : ... #
STRING s2       = t[ @ 1 ];
STRING result  := "";
INT result len := 0;
FOR i TO UPB s1 DO
FOR j TO UPB s2 DO
IF s1[ i ] = s2[ j ] THEN
INT k := 1;
WHILE INT ik = i + k;
INT jk = j + k;
IF ik > UPB s1 OR jk > UPB s2
THEN FALSE
ELSE s1[ ik ] = s2[ jk ]
FI
DO
k +:= 1
OD;
IF k > result len THEN
# found a longer substring #
result len := k;
result     := s1[ i : ( i + k ) - 1 ]
FI
FI
OD
OD;
result
END # longest common substring # ;

print( ( longest common substring( "thisisatest", "testing123testing" ), newline ) )
END```
Output:
```test
```

APL

```lcs←{
sb←∪⊃,/{⌽¨,\⌽⍵}¨,\⍵
match←(sb(∨/⍷)¨⊂⍺)/sb
⊃((⌈/=⊢)≢¨match)/match
}
```
Output:
```      'testing123testing' lcs 'thisisatest'
test
```

AppleScript

Iterative

This allows for the possibility of co-longest substrings, returning one instance of each. If either input string is empty, it's taken as meaning there are no common substrings.

```on LCS(a, b)
-- Identify the shorter string. The longest common substring won't be longer than it!
set lengthA to a's length
set lengthB to b's length
if (lengthA < lengthB) then
set {shorterString, shorterLength, longerString} to {a, lengthA, b}
else
set {shorterString, shorterLength, longerString} to {b, lengthB, a}
end if

set longestMatches to {}
set longestMatchLength to 0
-- Find the longest matching substring starting at each character in the shorter string.
repeat with i from 1 to shorterLength
repeat with j from shorterLength to i by -1
set thisSubstring to text i thru j of shorterString
if (longerString contains thisSubstring) then
-- Match found. If it's longer than the previously found match, or a new string of the same length, remember it.
set matchLength to j - i + 1
if (matchLength > longestMatchLength) then
set longestMatches to {thisSubstring}
set longestMatchLength to matchLength
else if ((matchLength = longestMatchLength) and (thisSubstring is not in longestMatches)) then
set end of longestMatches to thisSubstring
end if
-- Don't bother with the match's own substrings.
exit repeat
end if
end repeat
end repeat

return longestMatches
end LCS
```
```LCS("thisisatest", "testing123testing")
```
Output:
```{"test"}
```

Or:

```LCS("thisisthebesttest", "besting123testing")
```
Output:
```{"best", "test"}
```

Functional

Using library functions wherever possible, for better productivity, (and for more granular Rosetta comparison):

```------------------ LONGEST COMMON SUBSTRING ----------------

-- longestCommon :: Eq a => [a] -> [a] -> [a]
on longestCommon(a, b)
-- The longest common substring of two given strings.
script substrings
on |λ|(s)
map(my concat, concatMap(my tails, rest of inits(s)))
end |λ|
end script

set {xs, ys} to map(substrings, {a, b})
maximumBy(comparing(my |length|), intersect(xs, ys))
end longestCommon

-------------------------- TEST ---------------------------
on run
longestCommon("testing123testing", "thisisatest")
end run

-------------------- GENERIC FUNCTIONS --------------------

-- comparing :: (a -> b) -> (a -> a -> Ordering)
on comparing(f)
script
on |λ|(a, b)
tell mReturn(f)
set fa to |λ|(a)
set fb to |λ|(b)
if fa < fb then
-1
else if fa > fb then
1
else
0
end if
end tell
end |λ|
end script
end comparing

-- concat :: [String] -> String
on concat(xs)
script go
on |λ|(a, x)
a & x
end |λ|
end script
foldl(go, "", xs)
end concat

-- concatMap :: (a -> [b]) -> [a] -> [b]
on concatMap(f, xs)
set lng to length of xs
set acc to {}
tell mReturn(f)
repeat with i from 1 to lng
set acc to acc & (|λ|(item i of xs, i, xs))
end repeat
end tell
return acc
end concatMap

-- foldl :: (a -> b -> a) -> a -> [b] -> a
on foldl(f, startValue, xs)
tell mReturn(f)
set v to startValue
set lng to length of xs
repeat with i from 1 to lng
set v to |λ|(v, item i of xs, i, xs)
end repeat
return v
end tell
end foldl

-- inits :: String -> [String]
on inits(xs)
script charInit
on |λ|(_, i, xs)
text 1 thru i of xs
end |λ|
end script

{""} & map(charInit, xs)
end inits

-- intersect :: (Eq a) => [a] -> [a] -> [a]
on intersect(xs, ys)
if length of xs < length of ys then
set {shorter, longer} to {xs, ys}
else
set {longer, shorter} to {xs, ys}
end if
if shorter ≠ {} then
set lst to {}
repeat with x in shorter
if longer contains x then set end of lst to contents of x
end repeat
lst
else
{}
end if
end intersect

-- length :: [a] -> Int
on |length|(xs)
set c to class of xs
if list is c or string is c then
length of xs
else
(2 ^ 29 - 1) -- (maxInt - simple proxy for non-finite)
end if
end |length|

-- maximumBy :: (a -> a -> Ordering) -> [a] -> a
on maximumBy(f, xs)
set cmp to mReturn(f)
script max
on |λ|(a, b)
if a is missing value or cmp's |λ|(a, b) < 0 then
b
else
a
end if
end |λ|
end script

foldl(max, missing value, xs)
end maximumBy

-- mReturn :: First-class m => (a -> b) -> m (a -> b)
on mReturn(f)
-- 2nd class handler function lifted into 1st class script wrapper.
if script is class of f then
f
else
script
property |λ| : f
end script
end if
end mReturn

-- map :: (a -> b) -> [a] -> [b]
on map(f, xs)
-- The list obtained by applying f
-- to each element of xs.
tell mReturn(f)
set lng to length of xs
set lst to {}
repeat with i from 1 to lng
set end of lst to |λ|(item i of xs, i, xs)
end repeat
return lst
end tell
end map

-- tails :: String -> [String]
on tails(xs)
set es to characters of xs
script residue
on |λ|(_, i)
items i thru -1 of es
end |λ|
end script
map(residue, es) & {""}
end tails
```
Output:
`"test"`

Applesoft BASIC

Translation of: BASIC256
``` 0 A\$ = "thisisatest":B\$ = "testing123testing": GOSUB 100"LONGEST COMMON SUBSTRING": PRINT R\$;: END
100  LET R\$ = ""
110  LET A =  LEN (A\$)
120  LET B =  LEN (B\$)
130  IF A = 0 OR B = 0 THEN  RETURN
140  FOR B = B TO 1 STEP  - 1
150      FOR J = B TO 1 STEP  - 1
160          FOR K = 1 TO A
170              IF  MID\$ (A\$,K,J) <  >  LEFT\$ (B\$,J) THEN  NEXT K
180          LET R\$ =  LEFT\$ (B\$,J)
190          IF A > K THEN  RETURN
200      NEXT J
210      LET B\$ =  MID\$ (B\$,2)
220  NEXT B
230  LET R\$ = ""
240  RETURN```
Output:
```test
```

Arturo

```lcs: function [a,b][
lengths: map 0..size a => [map 0..size b => 0]
greatestLength: 0
result: ""
loop.with:'i a 'x [
loop.with:'j b 'y [
if x=y [
if? or? i=0 j=0 ->
lengths\[i]\[j]: 0
else ->
lengths\[i]\[j]: 1 + lengths\[i-1]\[j-1]

if greatestLength < lengths\[i]\[j] [
greatestLength: lengths\[i]\[j]
result: slice a (i-greatestLength)+1 i
]
]
]
]
return result
]

print lcs "thisisatest", "testing123testing"
```
Output:
`test`

AutoHotkey

Using Text Comparison

```LCS(a, b){
x := i := 1
while StrLen(x)
Loop % StrLen(a)
IfInString, b, % x := SubStr(a, i:=StrLen(x)=1 ? i+1 : i, n:=StrLen(a)+1-A_Index)
res := StrLen(res) > StrLen(x) ? res : x
return res
}
```
Examples:
```MsgBox % LCS("thisisatest", "testing123testing")
```
Outputs:
`test`

Using RegEx

```LCS(a, b){
while pos := RegExMatch(a "`n" b, "(.+)(?=.*\R.*\1)", m, pos?pos+StrLen(m):1)
res := StrLen(res) > StrLen(m1) ? res : m1
return res
}
```
Examples:
```MsgBox % LCS("thisisatest", "testing123testing")
```
Outputs:
`test`

BaCon

```FUNCTION Common_Sub\$(haystack\$, needle\$)

WHILE LEN(needle\$)
FOR x = LEN(needle\$) DOWNTO 1
IF INSTR(haystack\$, LEFT\$(needle\$, x)) THEN RETURN LEFT\$(needle\$, x)
NEXT
needle\$ = MID\$(needle\$, 2)
WEND
EXIT

ENDFUNC

PRINT Common_Sub\$("thisisatest", "testing123testing")```
Output:
`test`

BASIC

QBasic

Works with: QBasic version 1.1
Works with: QuickBasic version 4.5
```CALL LCS("thisisatest", "testing123testing")
END

SUB LCS (a\$, b\$)
IF LEN(a\$) = 0 OR LEN(b\$) = 0 THEN PRINT "": EXIT SUB
WHILE LEN(b\$)
FOR j = LEN(b\$) TO 1 STEP -1
IF INSTR(a\$, LEFT\$(b\$, j)) THEN PRINT LEFT\$(b\$, j): EXIT SUB
NEXT j
b\$ = MID\$(b\$, 2)
WEND
END SUB
```
Output:
`Same as FreeBASIC entry.`

Run BASIC

Works with: Just BASIC
Works with: Liberty BASIC
```call LCS "thisisatest", "testing123testing"
end

sub LCS a\$, b\$
if len(a\$) = 0 or len(b\$) = 0 then print "": exit sub
while len(b\$)
for j = len(b\$) to 1 step -1
if instr(a\$, left\$(b\$, j)) then print left\$(b\$, j): exit sub
next j
b\$ = mid\$(b\$, 2)
wend
end sub```
Output:
`Same as FreeBASIC entry.`

True BASIC

```SUB lcs (a\$,b\$)
IF LEN(a\$) = 0 OR LEN(b\$) = 0 THEN
PRINT ""
EXIT SUB
END IF
DO WHILE LEN(b\$)<>0
FOR j = LEN(b\$) TO 1 STEP -1
IF POS(a\$,(b\$)[1:j])<>0 THEN
PRINT (b\$)[1:j]
EXIT SUB
END IF
NEXT j
LET b\$ = (b\$)[2:maxnum]
LOOP
END SUB

CALL lcs ("thisisatest", "testing123testing")
END
```
Output:
`Same as FreeBASIC entry.`

BASIC256

Translation of: FreeBASIC
```function LCS(a, b)
if length(a) = 0 or length(b) = 0 then return ""
while length(b)
for j = length(b) to 1 step -1
if instr(a, left(b, j)) then return left(b, j)
next j
b = mid\$(b, 2)
end while
end function

print LCS("thisisatest", "testing123testing")
end```

Bracmat

```  ( lcs
=   X a b x L
.   !arg:(?a.?b)
& 0:?L
& :?X
& ( @( !a
:   ?
( ?x
& @(!x:? [>!L)
& @(!b:? !x ?)
& @(!x:? [?L:?X)
)
(?&~)
)
| !X
)
)
& out\$(lcs\$(thisisatest.testing123testing))```

Output

`test`

C

Translation of: Modula-2
```#include <stdio.h>

void lcs(const char * const sa, const char * const sb, char ** const beg, char ** const end) {
size_t apos, bpos;
ptrdiff_t len;

*beg = 0;
*end = 0;
len = 0;

for (apos = 0; sa[apos] != 0; ++apos) {
for (bpos = 0; sb[bpos] != 0; ++bpos) {
if (sa[apos] == sb[bpos]) {
len = 1;
while (sa[apos + len] != 0 && sb[bpos + len] != 0 && sa[apos + len] == sb[bpos + len]) {
len++;
}
}

if (len > *end - *beg) {
*beg = sa + apos;
*end = *beg + len;
len = 0;
}
}
}
}

int main() {
char *s1 = "thisisatest";
char *s2 = "testing123testing";
char *beg, *end, *it;

lcs(s1, s2, &beg, &end);

for (it = beg; it != end; it++) {
putchar(*it);
}
printf("\n");

return 0;
}
```
Output:
`test`

C#

Using dynamic programming

```using System;

namespace LongestCommonSubstring
{
class Program
{
static void Main(string[] args)
{
Console.WriteLine(lcs("thisisatest", "testing123testing"));
}

public static string lcs(string a, string b)
{
var lengths = new int[a.Length, b.Length];
int greatestLength = 0;
string output = "";
for (int i = 0; i < a.Length; i++)
{
for (int j = 0; j < b.Length; j++)
{
if (a[i] == b[j])
{
lengths[i, j] = i == 0 || j == 0 ? 1 : lengths[i - 1, j - 1] + 1;
if (lengths[i, j] > greatestLength)
{
greatestLength = lengths[i, j];
output = a.Substring(i - greatestLength + 1, greatestLength);
}
}
else
{
lengths[i, j] = 0;
}
}
}
return output;
}
}
}
```
Output:
```test
```

Searching for smaller substrings of a in b

Translation of: REXX
```//C# program tests the LCSUBSTR (Longest Common Substring) subroutine.
using System;
namespace LongestCommonSubstring
{
class Program
{
static void Main(string[] args)
{
string a = args.Length >= 1 ? args[0] : "";                                             /*get two arguments (strings).   */
string b = args.Length == 2 ? args[1] : "";
if (a == "") a = "thisisatest";                                                         /*use this string for a default. */
if (b == "") b = "testing123testing";                                                   /* "    "     "    "  "    "     */
Console.WriteLine("string A = {0}", a);                                                 /*echo string  A  to screen.     */
Console.WriteLine("string B = {0}", b);                                                 /*echo string  B  to screen.     */
Console.WriteLine("LCsubstr = {0}", LCsubstr(a, b));                                    /*tell Longest Common Substring. */
}                                                                                           /*stick a fork in it, we're done.*/

/*─────────────────────────────────LCSUBSTR subroutine─────────────────────────────────*/
public static string LCsubstr(string x, string y)                                           /*Longest Common Substring.      */
{
string output = "";
int lenx = x.Length;                                                                    /*shortcut for using the X length*/
for (int j = 0; j < lenx; j++)                                                          /*step through start points in X.*/
{
for (int k = lenx - j; k > -1; k--)                                                 /*step through string lengths.   */
{
string common = x.Substring(j, k);                                              /*extract a common substring.    */
if (y.IndexOf(common) > -1 && common.Length > output.Length) output = common;   /*longest?*/
}                                                                                   /*k*/
}                                                                                       /*j*/
return output;                                                                          /*\$  is "" if no common string.  */
}
}
}
```

output when using the default inputs:

```   string A = thisisatest
string B = testing123testing
LCsubstr = test
```

Searching for smaller substrings of a in b (simplified)

Translation of: zkl
```//C# program tests the LCS (Longest Common Substring) subroutine.
using System;
namespace LongestCommonSubstring
{
class Program
{
static void Main(string[] args)
{
string a = args.Length >= 1 ? args[0] : "";                                             /*get two arguments (strings).   */
string b = args.Length == 2 ? args[1] : "";
if (a == "") a = "thisisatest";                                                         /*use this string for a default. */
if (b == "") b = "testing123testing";                                                   /* "    "     "    "  "    "     */
Console.WriteLine("string A = {0}", a);                                                 /*echo string  A  to screen.     */
Console.WriteLine("string B = {0}", b);                                                 /*echo string  B  to screen.     */
Console.WriteLine("LCS = {0}", lcs(a, b));                                              /*tell Longest Common Substring. */
}                                                                                           /*stick a fork in it, we're done.*/

/*─────────────────────────────────LCS subroutine─────────────────────────────────*/
private static string lcs(string a, string b)
{
if(b.Length<a.Length){ string t=a; a=b; b=t; }
for (int n = a.Length; n > 0; n--)
{
for (int m = a.Length-n; m <= a.Length-n; m++)
{
string s=a.Substring(m,n);
if(b.Contains(s)) return(s);
}
}
return "";
}
}
```

output when using the default inputs:

```   string A = thisisatest
string B = testing123testing
LCS = test
```

C++

Works with: C++14
```#include <string>
#include <algorithm>
#include <iostream>
#include <set>
#include <vector>

auto collectSubStrings( const std::string& s, int maxSubLength )
{
int l = s.length();
auto res = std::set<std::string>();
for ( int start = 0; start < l; start++ )
{
int m = std::min( maxSubLength, l - start + 1 );
for ( int length = 1; length < m; length++ )
{
res.insert( s.substr( start, length ) );
}
}
return res;
}

std::string lcs( const std::string& s0, const std::string& s1 )
{
// collect substring set
auto maxSubLength = std::min( s0.length(), s1.length() );
auto set0 = collectSubStrings( s0, maxSubLength );
auto set1 = collectSubStrings( s1, maxSubLength );

// get commons into a vector
auto common = std::vector<std::string>();
std::set_intersection( set0.begin(), set0.end(), set1.begin(), set1.end(),
std::back_inserter( common ) );

// get the longest one
std::nth_element( common.begin(), common.begin(), common.end(),
[]( const std::string& s1, const std::string& s2 ) {
return s1.length() > s2.length();
} );
return *common.begin();
}

int main( int argc, char* argv[] )
{
auto s1 = std::string( "thisisatest" );
auto s2 = std::string( "testing123testing" );
std::cout << "The longest common substring of " << s1 << " and " << s2
<< " is:\n";
std::cout << "\"" << lcs( s1, s2 ) << "\" !\n";
return 0;
}
```
Output:
```The longest common substring of thisisatest and testing123testing is:
"test" !
```

Common Lisp

```(defun longest-common-substring (a b)
"Return the longest substring common to a and b"
;; Found at https://en.wikibooks.org/wiki/Algorithm_Implementation/Strings/Longest_common_substring#Common_Lisp
(let ((L (make-array (list (length a) (length b)) :initial-element 0))
(z 0)
(result '()) )
(dotimes (i (length a))
(dotimes (j (length b))
(when (char= (char a i) (char b j))
(setf (aref L i j)
(if (or (zerop i) (zerop j))
1
(1+ (aref L (1- i) (1- j))) ))
(when (> (aref L i j) z)
(setf z (aref L i j)
result '() ))
(when (= (aref L i j) z)
(pushnew (subseq a (1+ (- i z)) (1+ i))
result :test #'equal )))))
result ))
```
Output:
```(longest-common-substring "thisisatest" "testing123testing") => ("test")
```

Cowgol

```include "cowgol.coh";
include "strings.coh";

sub Contains(s1: [uint8], s2: [uint8]): (r: uint8) is
r := 0;
while [s1] != 0 loop
var a := s1;
var b := s2;
while [b] != 0 and [a] == [b] loop
a := @next a;
b := @next b;
end loop;
if [b] == 0 then
r := 1;
return;
end if;
s1 := @next s1;
end loop;
end sub;

sub LCS(s1: [uint8], s2: [uint8], outbuf: [uint8]) is
if StrLen(s1) < StrLen(s2) then
var temp := s1;
s1 := s2;
s2 := temp;
end if;

var maxlen := StrLen(s2);
var length := maxlen;
while length > 0 loop
var start: intptr := 0;
while start + length <= maxlen loop
MemCopy(s2 + start, length, outbuf);
[outbuf + length + 1] := 0;
if Contains(s1, outbuf) != 0 then
return;
end if;
start := start + 1;
end loop;
length := length - 1;
end loop;
[outbuf] := 0;
end sub;

var lcs: uint8[64];
LCS("thisisatest", "testing123testing", &lcs[0]);
print(&lcs[0]);
print_nl();```
Output:
`test`

D

Translation of: C#
```import std.stdio;

string lcs(string a, string b) {
int[][] lengths;
lengths.length = a.length;
for (int i=0; i<a.length; i++) {
lengths[i].length = b.length;
}

int greatestLength;
string output;
for (int i=0; i<a.length; i++) {
for (int j=0; j<b.length; j++) {
if (a[i]==b[j]) {
lengths[i][j] = i==0 || j==0 ? 1 : lengths[i-1][j-1] + 1;
if (lengths[i][j] > greatestLength) {
greatestLength = lengths[i][j];
int start = i-greatestLength+1;
output = a[start..start+greatestLength];
}
} else {
lengths[i][j] = 0;
}
}
}
return output;
}

void main() {
writeln(lcs("testing123testing", "thisisatest"));
}
```
Output:
`test`

Delphi

Translation of: C#
```program Longest_Common_Substring;

{\$APPTYPE CONSOLE}

{\$R *.res}

uses
System.SysUtils;

function lcs(x, y: string): string;
var
n, m, Alength: Integer;
t, common: string;
j: Integer;
k: Integer;
begin

Result := '';
Alength := x.Length;

for j := 0 to Alength - 1 do
for k := Alength - j downto 0 do
begin
common := x.Substring(j, k);
if (y.IndexOf(common) > -1) and (common.Length > Result.Length) then
Result := common;
end;
end;

var
a, b: string;

begin
a := 'thisisatest';
b := 'testing123testing';
if ParamCount = 2 then
begin
if not ParamStr(1).IsEmpty then
a := ParamStr(1);
if not ParamStr(2).IsEmpty then
b := ParamStr(2);
end;

Writeln('string A = ', a);
Writeln('string B = ', b);
Writeln('LCsubstr = ', lcs(a, b));
end.
```
Output:
```string A = thisisatest123
string B = testing123testing
LCsubstr = test
```

Dyalect

Translation of: Swift
```func lComSubStr(w1, w2) {
var (len, end) = (0, 0)
var mat  = Array.Empty(w1.Length() + 1, () => Array.Empty(w2.Length() + 1, 0))
var (i, j) = (0, 0)

for sLett in w1 {
for tLett in w2 {
if tLett == sLett {
let curLen = mat[i][j] + 1
mat[i + 1][j + 1] = curLen
if curLen > len {
len = curLen
end = i
}
}
j += 1
}
j = 0
i += 1
}
String(values: w1).Substring((end + 1) - len, len)
}

func comSubStr(w1, w2) {
return String(lComSubStr(w1.Iterate().ToArray(), w2.Iterate().ToArray()))
}

comSubStr("thisisatest", "testing123testing") // "test"```

EasyLang

```func\$ lcs a\$ b\$ .
if a\$ = "" or b\$ = ""
return ""
.
while b\$ <> ""
for j = len b\$ downto 1
l\$ = substr b\$ 1 j
for k = 1 to len a\$ - j + 1
if substr a\$ k j = l\$
if len l\$ > len max\$
max\$ = l\$
.
break 2
.
.
.
b\$ = substr b\$ 2 -1
.
return max\$
.
print lcs "thisisatest" "testing123testing"
print lcs "thisisatest" "stesting123testing"
print lcs "thisisatestxestinoo" "xxtesting123testing"```

Elixir

Works with: Elixir version 1.3
```defmodule LCS do
def longest_common_substring(a,b) do
alist = to_charlist(a) |> Enum.with_index
blist = to_charlist(b) |> Enum.with_index
lengths = for i <- 0..length(alist)-1, j <- 0..length(blist), into: %{}, do: {{i,j},0}
Enum.reduce(alist, {lengths,0,""}, fn {x,i},acc ->
Enum.reduce(blist, acc, fn {y,j},{map,gleng,lcs} ->
if x==y do
len = if i==0 or j==0, do: 1, else: map[{i-1,j-1}]+1
map = %{map | {i,j} => len}
if len > gleng, do: {map, len, String.slice(a, i - len + 1, len)},
else: {map, gleng, lcs}
else
{map, gleng, lcs}
end
end)
end)
|> elem(2)
end
end

IO.puts LCS.longest_common_substring("thisisatest", "testing123testing")
```
Output:
```test
```

Factor

Works with: Factor version 0.99 2020-07-03
```USING: io sequences.extras ;

"thisisatest" "testing123testing" longest-subseq print
```
Output:
```test
```

Fortran

```program main
implicit none
call  compare('testing123testingthing', 'thisis',                 'thi')
call  compare('testing',                'sting',                  'sting')
call  compare('thisisatest_stinger',    'testing123testingthing', 'sting')
call  compare('thisisatest_stinger',    'thisis',                 'thisis')
call  compare('thisisatest',            'testing123testing',      'test')
call  compare('thisisatest',            'thisisatest',            'thisisatest')
contains
character(len=:),allocatable :: a2, match
character(len=*),parameter :: g='(*(g0))'
integer :: i
a2=a ! should really make a2 the shortest and b the longest
match=''
do i=1,len(a2)-1
call compare_sub(a2,b,match)
if(len(a2).lt.len(match))exit
a2=a2(:len(a2)-1)
enddo
& ' longest match found: "',match,'"; expected "',answer,'"', &
& ' comparing "',a,'" and "',b,'"'
end subroutine
subroutine compare_sub(a,b,match)
character(len=*),intent(in) :: a, b
character(len=:),allocatable :: match
integer :: left, foundat, len_a
len_a=len(a)
do left=1,len_a
foundat=index(b,a(left:))
if(foundat.ne.0.and.len(match).lt.len_a-left+1)then
if(len(a(left:)).gt.len(match))then
match=a(left:)
exit
endif
endif
enddo
end subroutine compare_sub
end program main
```
Output:
```(PASSED) longest match found: "thi"; expected "thi" comparing "testing123testingthing" and "thisis"
(PASSED) longest match found: "sting"; expected "sting" comparing "testing" and "sting"
(PASSED) longest match found: "sting"; expected "sting" comparing "thisisatest_stinger" and "testing123testingthing"
(PASSED) longest match found: "thisis"; expected "thisis" comparing "thisisatest_stinger" and "thisis"
(PASSED) longest match found: "test"; expected "test" comparing "thisisatest" and "testing123testing"
(PASSED) longest match found: "thisisatest"; expected "thisisatest" comparing "thisisatest" and "thisisatest"
```

FreeBASIC

```Function LCS(a As String, b As String) As String
If Len(a) = 0 Or Len(b) = 0 Then Return ""
While Len(b)
For j As Integer = Len(b) To 1 Step -1
If Instr(a, Left(b, j)) Then Return Left(b, j)
Next j
b = Mid(b, 2)
Wend
End Function

Print LCS("thisisatest", "testing123testing")
Sleep```

Go

Translation of: C#
```package main

import "fmt"

func lcs(a, b string) (output string) {
lengths := make([]int, len(a)*len(b))
greatestLength := 0
for i, x := range a {
for j, y := range b {
if x == y {
if i == 0 || j == 0 {
lengths[i*len(b)+j] = 1
} else {
lengths[i*len(b)+j] = lengths[(i-1)*len(b)+j-1] + 1
}
if lengths[i*len(b)+j] > greatestLength {
greatestLength = lengths[i*len(b)+j]
output = a[i-greatestLength+1 : i+1]
}
}
}
}
return
}

func main() {
fmt.Println(lcs("thisisatest", "testing123testing"))
}
```
Output:
```test
```

```import Data.Ord (comparing)
import Data.List (maximumBy, intersect)

subStrings :: [a] -> [[a]]
subStrings s =
let intChars = length s
in [ take n \$ drop i s
| i <- [0 .. intChars - 1]
, n <- [1 .. intChars - i] ]

longestCommon :: Eq a => [a] -> [a] -> [a]
longestCommon a b =
maximumBy (comparing length) (subStrings a `intersect` subStrings b)

main :: IO ()
main = putStrLn \$ longestCommon "testing123testing" "thisisatest"
```
Output:
`test`

Or, fusing subStrings as tail . inits <=< tails

```import Control.Monad ((<=<))
import Data.List (inits, intersect, maximumBy, tails)
import Data.Ord (comparing)

----------------- LONGEST COMMON SUBSTRING ---------------

longestCommon :: Eq a => [a] -> [a] -> [a]
longestCommon a b =
let pair [x, y] = (x, y)
in maximumBy (comparing length) \$
(uncurry intersect . pair) \$
[tail . inits <=< tails] <*> [a, b]

--------------------------- TEST -------------------------
main :: IO ()
main =
putStrLn \$
longestCommon "testing123testing" "thisisatest"
```
Output:
`test`

J

This algorithm starts by comparing each character in the one string to each character in the other, and then iterates on this result until it finds the length of the longest common substring. So if Lx is the length of one argument string, Ly is the length of the other argument string, and Lr is the length of the result string, this algorithm uses space on the order of Lx*Ly and time on the order of Lx*Ly*Lr.

In other words: this can be suitable for small problems, but you might want something better if you're comparing gigabyte length strings with high commonality.

```lcstr=:4 :0
C=. ({.~ 1+\$) x=/y
M=. >./ (* * * >. * + (_1&|.)@:|:^:2)^:_  C
N=. >./ M
y {~ (M i. N)-i.-N
)
```

Intermedate results:

```   C shows which characters are in common between the two strings.
M marks the length of the longest common substring ending at each position in the right argument
N is the length of the longest common substring
```

Example use:

```   'thisisatest' lcs 'testing123testing'
test
```

Java

```public class LongestCommonSubstring {

public static void main(String[] args) {
System.out.println(lcs("testing123testing", "thisisatest"));
System.out.println(lcs("test", "thisisatest"));
System.out.println(lcs("testing", "sting"));
System.out.println(lcs("testing", "thisisasting"));
}

static String lcs(String a, String b) {
if (a.length() > b.length())
return lcs(b, a);

String res = "";
for (int ai = 0; ai < a.length(); ai++) {
for (int len = a.length() - ai; len > 0; len--) {

for (int bi = 0; bi <= b.length() - len; bi++) {

if (a.regionMatches(ai, b, bi, len) && len > res.length()) {
res = a.substring(ai, ai + len);
}
}
}
}
return res;
}
}
```
`test`
`test`
`sting`
`sting`

JavaScript

```(() => {
'use strict';

// longestCommon :: String -> String -> String
const longestCommon = (s1, s2) => maximumBy(
comparing(length),
intersect(...apList(
[s => map(
concat,
concatMap(tails, compose(tail, inits)(s))
)],
[s1, s2]
))
);

// main :: IO ()
const main = () =>
console.log(
longestCommon(
"testing123testing",
"thisisatest"
)
);

// GENERIC FUNCTIONS ----------------------------

// Each member of a list of functions applied to each
// of a list of arguments, deriving a list of new values.

// apList (<*>) :: [(a -> b)] -> [a] -> [b]
const apList = (fs, xs) => //
fs.reduce((a, f) => a.concat(
xs.reduce((a, x) => a.concat([f(x)]), [])
), []);

// comparing :: (a -> b) -> (a -> a -> Ordering)
const comparing = f =>
(x, y) => {
const
a = f(x),
b = f(y);
return a < b ? -1 : (a > b ? 1 : 0);
};

// compose (<<<) :: (b -> c) -> (a -> b) -> a -> c
const compose = (f, g) => x => f(g(x));

// concat :: [[a]] -> [a]
// concat :: [String] -> String
const concat = xs =>
0 < xs.length ? (() => {
const unit = 'string' !== typeof xs[0] ? (
[]
) : '';
return unit.concat.apply(unit, xs);
})() : [];

// concatMap :: (a -> [b]) -> [a] -> [b]
const concatMap = (f, xs) =>
xs.reduce((a, x) => a.concat(f(x)), []);

// inits([1, 2, 3]) -> [[], [1], [1, 2], [1, 2, 3]
// inits('abc') -> ["", "a", "ab", "abc"]

// inits :: [a] -> [[a]]
// inits :: String -> [String]
const inits = xs => [
[]
]
.concat(('string' === typeof xs ? xs.split('') : xs)
.map((_, i, lst) => lst.slice(0, i + 1)));

// intersect :: (Eq a) => [a] -> [a] -> [a]
const intersect = (xs, ys) =>
xs.filter(x => -1 !== ys.indexOf(x));

// Returns Infinity over objects without finite length.
// This enables zip and zipWith to choose the shorter
// argument when one is non-finite, like cycle, repeat etc

// length :: [a] -> Int
const length = xs =>
(Array.isArray(xs) || 'string' === typeof xs) ? (
xs.length
) : Infinity;

// map :: (a -> b) -> [a] -> [b]
const map = (f, xs) => xs.map(f);

// maximumBy :: (a -> a -> Ordering) -> [a] -> a
const maximumBy = (f, xs) =>
0 < xs.length ? (
xs.slice(1)
.reduce((a, x) => 0 < f(x, a) ? x : a, xs[0])
) : undefined;

// tail :: [a] -> [a]
const tail = xs => 0 < xs.length ? xs.slice(1) : [];

// tails :: [a] -> [[a]]
const tails = xs => {
const
es = ('string' === typeof xs) ? (
xs.split('')
) : xs;
return es.map((_, i) => es.slice(i))
.concat([
[]
]);
};

// MAIN ---
return main();
})();
```
Output:
`test`

jq

Translation of: C#, Go, Ruby
Works with: jq version 1.4

Utility functions:

```# Create an m x n matrix
def matrix(m; n; init):
if m == 0 then []
elif m == 1 then [range(0;n) | init]
elif m > 0 then
matrix(1;n;init) as \$row
| [range(0;m) | \$row ]
else error("matrix\(m);_;_) invalid")
end;

def set(i;j; value):
setpath([i,j]; value);```

Longest Common Substring:

```def lcs(a; b):
matrix(a|length; b|length; 0) as \$lengths
# state: [ \$lengths, greatestLength, answer ]
| [\$lengths, 0]
| reduce range(0; a|length) as \$i
(.;
reduce range(0; b|length) as \$j
(.;
if a[\$i:\$i+1] == b[\$j:\$j+1] then
(if \$i == 0 or \$j == 0 then 1
else .[0][\$i-1][\$j-1] + 1
end) as \$x
| .[0] |= set(\$i; \$j; \$x)
| if \$x > .[1] then
.[1] = \$x
| .[2] = a[1+\$i - \$x : 1+\$i] # output
else .
end
else .
end )) | .[2];```

Example:

`lcs("thisisatest"; "testing123testing")`
Output:
```\$ jq -n -f Longest_common_substring.jq
"test"
```

Julia

Works with: Julia version 1.5
```function lcs(s1::AbstractString, s2::AbstractString)::String
l, r, sub_len = 1, 0, 0
for i in eachindex(s1)
for j in i:length(s1)
contains(s2, SubString(s1, i, j)) || break
if sub_len ≤ j - i
l, r = i, j
sub_len = j - i
end
end
end
return s1[l:r]
end

@show lcs("thisisatest", "testing123testing")
```

Kotlin

Translation of: Java
```// version 1.1.2

fun lcs(a: String, b: String): String {
if (a.length > b.length) return lcs(b, a)
var res = ""
for (ai in 0 until a.length) {
for (len in a.length - ai downTo 1) {
for (bi in 0 until b.length - len) {
if (a.regionMatches(ai, b, bi,len) && len > res.length) {
res = a.substring(ai, ai + len)
}
}
}
}
return res
}

fun main(args: Array<String>) = println(lcs("testing123testing", "thisisatest"))
```
Output:
```test
```

Lambdatalk

1) A pure lambdatalk version

```{def lcs
{def lcs.rec
{lambda {:a :b :w}
{if {or {< {W.length :a} 2} {< {W.length :b} 2} }
then {W.rest :w}
else {if {W.equal? {W.first :a} {W.first :b}}
then {lcs.rec {W.rest :a} {W.rest :b} :w{W.first :a}}
else {let { {:x {lcs.rec :a {W.rest :b} :w}}
{:y {lcs.rec {W.rest :a} :b :w}}
} {if {> {W.length :x} {W.length :y}}
then :x
else :y} }}}}}
{lambda {:a :b}
{lcs.rec :a# :b# #}}}
-> lcs

{lcs testing123testing thisisatest}
-> tsitest   // 23000ms
```

2) The pure lambdatalk version is very, very slow, 23000ms.

A much more easier and faster way is to build an interface with the javascript code entry,
Translation of: Javascript
, used as it is.
```{jslcs testing123testing thisisatest}
-> tsitest    // 130ms

{script
// the lcs function code is in the javascript entry

LAMBDATALK.DICT["jslcs"] = function() {
var args = arguments[0].split(" ");
return lcs( args[0], args[1] )
};
}
```

langur

Translation of: Julia
```val .lcs = f(.s1, .s2) {
var .l, .r, .sublen = 1, 0, 0
for .i of .s1 {
for .j in .i .. len(.s1) {
if not matching(s2s(.s1, .i .. .j), .s2): break
if .sublen <= .j - .i {
.l, .r = .i, .j
.sublen = .j - .i
}
}
}
if .r == 0: return ""
s2s .s1, .l .. .r
}

writeln .lcs("thisisatest", "testing123testing")```
Output:
```test
```

Lobster

Translation of: Go
```import std
def lcs(a, b) -> string:
var out = ""
let lengths = map(a.length * b.length): 0
var greatestLength = 0
for(a) x, i:
for(b) y, j:
if x == y:
if i == 0 or j == 0:
lengths[i * b.length + j] = 1
else:
lengths[i * b.length + j] = lengths[(i-1) * b.length + j - 1] + 1
if lengths[i * b.length + j] > greatestLength:
greatestLength = lengths[i * b.length + j]
out = a.substring(i - greatestLength + 1, greatestLength)
return out```
Translation of: C#
```import std
def lcs2(a, b) -> string:
var out = ""
let lengths = map(b.length): map(a.length): 0
var greatestLength = 0
for(a) x, i:
for(b) y, j:
if x == y:
if i == 0 or j == 0:
lengths[j][i] = 1
else:
lengths[j][i] = lengths[j-1][i-1] + 1
if lengths[j][i] > greatestLength:
greatestLength = lengths[j][i]
out = a.substring(i - greatestLength + 1, greatestLength)
return out```

Maple

`StringTools:-LongestCommonSubString()` returns the longest common substring of two strings. `StringTools:-CommonSubSequence()` returns the longest common subsequence() of two strings.

`StringTools:-LongestCommonSubString("thisisatest","testing123testing");`

Mathematica/Wolfram Language

The function `LongestCommonSubsequence` returns the longest common substring, and `LongestCommonSequence` returns the longest common subsequence.

```Print[LongestCommonSubsequence["thisisatest", "testing123testing"]];
```
Output:
`test`

Modula-2

```MODULE LCS;
FROM FormatString IMPORT FormatString;

PROCEDURE WriteSubstring(s : ARRAY OF CHAR; b,e : CARDINAL);
VAR i : CARDINAL;
BEGIN
IF b=e THEN RETURN END;
IF e>HIGH(s) THEN e := HIGH(s) END;

FOR i:=b TO e DO
Write(s[i])
END
END WriteSubstring;

TYPE
Pair = RECORD
a,b : CARDINAL;
END;

PROCEDURE lcs(sa,sb : ARRAY OF CHAR) : Pair;
VAR
output : Pair;
a,b,len : CARDINAL;
BEGIN
output := Pair{0,0};

FOR a:=0 TO HIGH(sa) DO
FOR b:=0 TO HIGH(sb) DO
IF (sa[a]#0C) AND (sb[b]#0C) AND (sa[a]=sb[b]) THEN
len := 1;
WHILE (a+len<HIGH(sa)) AND (b+len<HIGH(sb)) DO
IF sa[a+len] = sb[b+len] THEN
INC(len)
ELSE
BREAK
END
END;
DEC(len);

IF len>output.b-output.a THEN
output := Pair{a,a+len}
END
END
END
END;

RETURN output
END lcs;

VAR res : Pair;
BEGIN
res := lcs("testing123testing", "thisisatest");
WriteSubstring("testing123testing", res.a, res.b);
WriteLn;

END LCS.
```

Nim

Translation of: Go
```# Longest common substring.

import sequtils

func lcs(a, b: string): string =
var lengths = newSeqWith(a.len, newSeq[int](b.len))
var greatestLength = 0
for i, x in a:
for j, y in b:
if x == y:
lengths[i][j] = if i == 0 or j == 0: 1 else: lengths[i - 1][j - 1] + 1
if lengths[i][j] > greatestLength:
greatestLength = lengths[i][j]
result = a[(i - greatestLength + 1)..i]

echo lcs("thisisatest", "testing123testing")
```
Output:
`test`

Pascal

using FreePascal

Translation of: Delphi
Works with: Free Pascal version 3.2.2
```PROGRAM LongestCommonSubString.pas;

{\$IFDEF FPC}
{\$mode objfpc}{\$H+}{\$J-}{\$m+}{\$R+}{\$T+}
{\$ELSE}
{\$APPTYPE CONSOLE}
{\$ENDIF}

(*)

Free Pascal Compiler version 3.2.2 [2022/08/01] for x86_64

The free and readable alternative at C/C++ speeds
compiles natively to almost any platform, including raspberry PI *
Can run independently from DELPHI / Lazarus

Version without `USES    SysUtils, Variants ;` and without `SubStr`, we do not need it here...

(*)

FUNCTION IFF ( Cond: boolean;  A, B: string ) : string ;

BEGIN		IF ( Cond ) THEN IFF := A ELSE IFF := B ;		END ;

FUNCTION lcss( S1, S2: string ) : string ;

VAR

j :	Integer =	0 ;

k :	Integer =	0 ;

S :	string  =  '' ;

BEGIN

lcss	:= ''	  ;

FOR j := 0 TO length ( S1 ) DO 	BEGIN

FOR k := length ( S1 ) - j DOWNTO 1 DO	BEGIN

S := Copy(S1, (j + 1), (k + j + 1)) ;

IF	( pos ( S, S2 )  > 0  )	AND
( length ( S ) > length ( lcss ) ) THEN	 BEGIN

lcss := S ;

BREAK ;

END ;

END ;

END ;

END ;	(*)	FUNCTION lcss	(*)

VAR

S1: string =  'thisisatest'       		;

S2: string =  'testing123isatesting'	;

BEGIN

IF ParamCount = 2 THEN  BEGIN

S1 := IFF( ( ParamStr ( 1 ) > '' ), ParamStr ( 1 ) , S1 );

S2 := IFF( ( ParamStr ( 2 ) > '' ), ParamStr ( 2 ) , S1 );

END;

Writeln ( 'string A = ', S1 )  ;

Writeln ( 'string B = ', S2 )  ;

WriteLn (  Lcss ( S1, S2 )  )  ;

END. (*) Of PROGRAM LongestCommonSubString.pas (*)

(*)
```
```JPD 2021/06/18
Output:

string A = thisisatest

string B = testing123testing

test
```
(*)

Perl

```#!/usr/bin/perl
use strict ;
use warnings ;

sub longestCommonSubstr {
my \$first = shift ;
my \$second = shift ;
my %firstsubs = findSubstrings ( \$first );
my %secondsubs = findSubstrings ( \$second ) ;
my @commonsubs ;
foreach my \$subst ( keys %firstsubs ) {
if ( exists \$secondsubs{ \$subst } ) {
push ( @commonsubs , \$subst ) ;
}
}
my @sorted = sort { length \$b <=> length \$a } @commonsubs ;
return \$sorted[0] ;
}

sub findSubstrings {
my \$string = shift ;
my %substrings ;
my \$l = length \$string ;
for ( my \$start = 0 ; \$start < \$l ; \$start++ ) {
for ( my \$howmany = 1 ; \$howmany < \$l - \$start + 1 ; \$howmany++) {
\$substrings{substr( \$string , \$start , \$howmany) } = 1 ;
}
}
return %substrings ;
}

my \$longest = longestCommonSubstr( "thisisatest" ,"testing123testing" ) ;
print "The longest common substring of <thisisatest> and <testing123testing> is \$longest !\n" ;
```
Output:
`The longest common substring of <thisisatest> and <testing123testing> is test !`

Alternate letting regex do the work

```#!/usr/bin/perl

use strict; # https://rosettacode.org/wiki/Longest_Common_Substring
use warnings;

my \$one = 'thisisatest';
my \$two = 'testing123testing';

my @best;
"\$one\n\$two" =~ /(.+).*\n.*\1(?{ \$best[length \$1]{\$1}++})(*FAIL)/;
print "\$_\n" for sort keys %{ \$best[-1] };
```
Output:

test

Phix

```function lcs(string a, b)
integer longest = 0
string best = ""
for i=1 to length(a) do
integer ch = a[i]
for j=1 to length(b) do
if ch=b[j] then
integer n=1
while i+n<=length(a)
and j+n<=length(b)
and a[i+n]=b[j+n] do
n += 1
end while
if n>longest then
longest = n
best = a[i..i+n-1]
end if
end if
end for
end for
return best
end function
?lcs("thisisatest", "testing123testing")
?lcs("testing123testing","thisisatest")
```
Output:
```"test"
"test"
```

PicoLisp

```(de longestCommonSubstring (Str1 Str2)
(setq Str1 (chop Str1)  Str2 (chop Str2))
(let Res NIL
(map
'((Lst1)
(map
'((Lst2)
(let Len 0
(find
'((A B) (nand (= A B) (inc 'Len)))
Lst1
Lst2 )
(when (> Len (length Res))
(setq Res (head Len Lst1)) ) ) )
Str2 ) )
Str1 )
(pack Res) ) )```

Test:

```: (longestCommonSubstring "thisisatest" "testing123testing")
-> "test"```

PowerShell

```function lcs([String]\$a, [String]\$b)
{
if ([String]::IsNullOrEmpty(\$a) -or [String]::IsNullOrEmpty(\$b))
{
return ""
}
\$startIndex, \$size = -1, -1
for (\$k = 0; \$k -lt \$a.Length; ++\$k)
{
for (\$i, \$j, \$d = \$k, 0, 0; (\$i -lt \$a.Length) -and (\$j -lt \$b.Length); ++\$i, ++\$j)
{
if (\$a.Chars(\$i) -eq \$b.Chars(\$j))
{
\$d += 1
if (\$size -lt \$d)
{
\$startIndex = \$i - \$d + 1
\$size = \$d
}
}
else
{
\$d = 0
}
}
}
for (\$k = 1; \$k -lt \$b.Length; ++\$k)
{
for (\$i, \$j, \$d = 0, \$k, 0; (\$i -lt \$a.Length) -and (\$j -lt \$b.Length); ++\$i, ++\$j)
{
if (\$a.Chars(\$i) -eq \$b.Chars(\$j))
{
\$d += 1
if (\$size -lt \$d)
{
\$startIndex = \$i - \$d + 1
\$size = \$d
}
}
else
{
\$d = 0
}
}
}
if (\$size -lt 0)
{
return ""
}
return \$a.Substring(\$startIndex, \$size)
}

function Print-Lcs([String]\$a, [String]\$b)
{
return "lcs \$a \$b = \$(lcs \$a \$b)"
}
Print-Lcs 'thisisatest' 'testing123testing'
Print-Lcs 'testing' 'sting'
Print-Lcs 'thisisatest_stinger' 'testing123testingthing'
Print-Lcs 'thisisatest_stinger' 'thisis'
Print-Lcs 'testing123testingthing' 'thisis'
Print-Lcs 'thisisatest' 'thisisatest'
```
Output:
```lcs thisisatest testing123testing = test
lcs testing sting = sting
lcs thisisatest_stinger testing123testingthing = sting
lcs thisisatest_stinger thisis = thisis
lcs testing123testingthing thisis = thi
lcs thisisatest thisisatest = thisisatest
```

Prolog

```common_sublist(A, B, M) :-
append(_, Ma, A),
append(M, _, Ma),
append(_, Mb, B),
append(M, _, Mb).

longest_list([], L, _, L).
longest_list([L|Ls], LongestList, LongestLength, Result) :-
length(L, Len),
Len >= LongestLength -> longest_list(Ls, L, Len, Result)
; longest_list(Ls, LongestList, LongestLength, Result).

longest_substring(A, B, Result) :-
string_chars(A, AChars),
string_chars(B, BChars),
findall(SubString, (
dif(SubString, []), common_sublist(AChars, BChars, SubString)
), AllSubstrings),
longest_list(AllSubstrings, [], 0, LongestSubString),
string_chars(Result, LongestSubString).
```
Output:
```?- longest_substring("thisisatest", "testing123testing", Longest).
Longest = "test".
```

PureBasic

```Procedure.s lcs(a\$,b\$)
If Len(a\$)>Len(b\$) : Swap a\$,b\$ : EndIf
l=Len(a\$)
For c=1 To l
For i=1 To 1+l-c
If FindString(b\$,Mid(a\$,i,c))
res\$=Mid(a\$,i,c)
EndIf
Next
Next
ProcedureReturn res\$
EndProcedure

t1\$="testing123testing"
t2\$="thisisatest"
Debug lcs(t1\$,t2\$)```
Output:
`test`

Python

Python: Idiomatic

Python: Using Indexes

```s1 = "thisisatest"
s2 = "testing123testing"
len1, len2 = len(s1), len(s2)
ir, jr = 0, -1
for i1 in range(len1):
i2 = s2.find(s1[i1])
while i2 >= 0:
j1, j2 = i1, i2
while j1 < len1 and j2 < len2 and s2[j2] == s1[j1]:
if j1-i1 >= jr-ir:
ir, jr = i1, j1
j1 += 1; j2 += 1
i2 = s2.find(s1[i1], i2+1)
print (s1[ir:jr+1])
```
Output:
`"test"`

Python: Set of substrings

From my explanatory blog post.

```def _set_of_substrings(s:str) -> set:
"_set_of_substrings('ABBA') == {'A', 'AB', 'ABB', 'ABBA', 'B', 'BA', 'BB', 'BBA'}"
len_s = len(s)
return {s[m: n] for m in range(len_s) for n in range(m+1, len_s +1)}

def _set_of_common_substrings(s:str, common: set) -> set:
"Substrings of s that are also in common"
len_s = len(s)
return {this for m in range(len_s) for n in range(m+1, len_s +1)
if (this := s[m:n]) in common}

def lcs_ss(*strings):
"longest of the common substrings of all substrings of each string"
strings_iter  = (s for s in strings)
common = _set_of_substrings(next(strings_iter)) # First string substrings
for s in strings_iter:
if not common:
break
common = _set_of_common_substrings(s, common) # Accumulate the common

return max(common, key= lambda x: len(x)) if common else ''

s0 = "thisisatest_stinger"
s1 = "testing123testingthing"
s2 = "thisis"

ans = lcs_ss(s0, s1)
print(f"\n{repr(s0)}, {repr(s1)} ->> {repr(ans)}")
ans = lcs_ss(s0, s2)
print(f"\n{repr(s0)}, {repr(s2)} ->> {repr(ans)}")
ans = lcs_ss(s1, s2)
print(f"\n{repr(s1)}, {repr(s2)} ->> {repr(ans)}")
ans = lcs_ss(s0, s1, s2)
print(f"\n{repr(s0)}, {repr(s1)}, {repr(s2)} ->> {repr(ans)}")
```
Output:
```'thisisatest_stinger', 'testing123testingthing' ->> 'sting'

'thisisatest_stinger', 'thisis' ->> 'thisis'

'testing123testingthing', 'thisis' ->> 'thi'

'thisisatest_stinger', 'testing123testingthing', 'thisis' ->> 'thi'```

Functional

Translation of: JavaScript

Expressed as a composition of pure generic functions:

```'''Longest common substring'''

from itertools import accumulate, chain
from functools import reduce

# longestCommon :: String -> String -> String
def longestCommon(s1):
'''The longest common substring of
two given strings.
'''
def go(s2):
return max(intersect(
*map(lambda s: map(
concat,
concatMap(tails)(
compose(tail, list, inits)(s)
)
), [s1, s2])
), key=len)
return go

# ------------------------- TEST -------------------------
def main():
'''Test'''
print(
longestCommon(
"testing123testing"
)(
"thisisatest"
)
)

# ----------------------- GENERIC ------------------------

# compose :: ((a -> a), ...) -> (a -> a)
def compose(*fs):
'''Composition, from right to left,
of a series of functions.
'''
def go(f, g):
def fg(x):
return f(g(x))
return fg
return reduce(go, fs, lambda x: x)

# concat :: [String] -> String
def concat(xs):
'''The concatenation of all the elements
in a list or iterable.
'''
return ''.join(xs)

# concatMap :: (a -> [b]) -> [a] -> [b]
def concatMap(f):
'''A concatenated list over which a function has been
mapped.
The list monad can be derived by using a function f
which wraps its output in a list, (using an empty
list to represent computational failure).
'''
def go(xs):
return chain.from_iterable(map(f, xs))
return go

# inits :: [a] -> [[a]]
def inits(xs):
'''all initial segments of xs, shortest first.'''
return accumulate(chain([[]], xs), lambda a, x: a + [x])

# intersect :: [a] -> [a] -> [a]
def intersect(xs, ys):
'''The ordered intersection of xs and ys.
intersect([1,2,2,3,4])([6,4,4,2]) -> [2,2,4]
'''
s = set(ys)
return (x for x in xs if x in s)

# scanl :: (b -> a -> b) -> b -> [a] -> [b]
def scanl(f):
'''scanl is like reduce, but defines a succession of
intermediate values, building from the left.
'''
def go(a):
def g(xs):
return accumulate(chain([a], xs), f)
return g
return go

# tail :: [a] -> [a]
# tail :: Gen [a] -> [a]
def tail(xs):
'''The elements following the head of a
(non-empty) list.
'''
return xs[1:]

# tails :: [a] -> [[a]]
def tails(xs):
'''All final segments of xs,
longest first.
'''
return [
xs[i:] for i in
range(0, 1 + len(xs))
]

# MAIN ---
main()
```
`test`

Quackery

```  [ 0 temp put
0 temp put
tuck dup size times
[ 2dup swap
0 temp put
0 swap witheach
[ unrot
over size
over = iff
[ drop
conclude ]
done
rot dip
[ 2dup peek ]
= tuck * +
dup temp take
max temp put ]
2drop
temp take
dup temp share > iff
[ temp release
i^ temp replace
temp put ]
else drop
2drop
temp take dip
[ temp take split nip ]
split drop ]              is lcs ( \$ \$ --> \$ )

\$ "thisisatest" \$ "testing123testing" lcs echo\$ cr```
Output:
`test`

Racket

A chance to show off how to use `HashTable` types in typed/racket

```#lang typed/racket
(: lcs (String String -> String))
(define (lcs a b)
(: all-substrings# (String -> (HashTable String Boolean)))
(define (all-substrings# str)
(define l (string-length str))
(for*/hash : (HashTable String Boolean)
(values (substring str s e) #t)))

(define a# (all-substrings# a))

(define b# (all-substrings# b))

(define-values (s l)
(for/fold : (Values String Nonnegative-Integer)
((s "") (l : Nonnegative-Integer 0))
((a_ (in-hash-keys a#))
#:when (and (> (string-length a_) l) (hash-ref b# a_ #f)))
(values a_ (string-length a_))))

s)

(module+ test
("thisisatest" . lcs . "testing123testing"))
```
Output:
`"test"`

Raku

(formerly Perl 6)

```sub createSubstrings( Str \$word --> Array ) {
my \$length = \$word.chars ;
my @substrings ;
for (0..\$length - 1) -> \$start {
for (1..\$length - \$start) -> \$howmany {
@substrings.push( \$word.substr( \$start , \$howmany ) ) ;
}
}
return @substrings ;
}

sub findLongestCommon( Str \$first , Str \$second --> Str ) {
my @substringsFirst  = createSubstrings( \$first ) ;
my @substringsSecond = createSubstrings( \$second ) ;
my \$firstset  = set( @substringsFirst ) ;
my \$secondset = set( @substringsSecond ) ;
my \$common = \$firstset (&) \$secondset ;
return \$common.keys.sort({\$^b.chars <=> \$^a.chars})[0] ; # or: sort(-*.chars)[0]
}

sub MAIN( Str \$first , Str \$second ) {
say "The longest common substring of \$first and \$second is " ~
"{findLongestCommon( \$first , \$second ) } !" ;
}
```
Output:
`The longest common substring of thisisatest and testing123testing is test !`

Functional

```sub substrings (\$s)       { (flat (0..\$_ X 1..\$_).grep:{\$_ ≥ [+] @_}).map: { \$s.substr(\$^a, \$^b) } given \$s.chars }
sub infix:<LCS>(\$s1, \$s2) { ([∩] (\$s1, \$s2)».&substrings).keys.sort(*.chars).tail }

my \$first  = 'thisisatest';
my \$second = 'testing123testing';
say "The longest common substring between '\$first' and '\$second' is '{\$first LCS \$second}'.";
```
Output:
`The longest common substring between 'thisisatest' and 'testing123testing' is 'test'.`

REXX

```/*REXX program determines the   LCSUBSTR   (Longest Common Substring)  via a function.  */
parse arg a b .                                  /*obtain optional arguments from the CL*/
if a==''  then a= "thisisatest"                  /*Not specified?  Then use the default.*/
if b==''  then b= "testing123testing"            /* "      "         "   "     "    "   */
say '   string A ='            a                 /*echo string A to the terminal screen.*/
say '   string B ='               b              /*  "     "   B  "  "      "       "   */
say '   LCsubstr ='   LCsubstr(a, b)             /*display the Longest Common Substring.*/
exit 0                                           /*stick a fork in it,  we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
LCsubstr: procedure;  parse arg x,y,,\$;     #= 0 /*LCsubstr:  Longest Common Substring. */
L= length(x);     w= length(y)         /*placeholders for string length of X,Y*/
if w<L  then do;  parse arg y,x;  L= w /*switch X & Y   if Y is shorter than X*/
end
do j=1  for L  while j<=L-#         /*step through start points in string X*/
do k=L-j+1   to #   by -1        /*step through string lengths.         */
_= substr(x, j, k)               /*extract a possible common substring. */
if pos(_, y)\==0  then  if k>#  then do;     \$= _;     #= k;      end
end   /*k*/                      /* [↑]  determine if string _ is longer*/
end      /*j*/                      /*#:  the current length of  \$  string.*/
return \$                               /*\$:  (null if there isn't common str.)*/
```
output   when using the default inputs:
```   string A = thisisatest
string B = testing123testing
LCsubstr = test
```

Ring

```# Project : Longest Common Substring

str1 = "testing123testing"
str2 = "tsitest"
see longest(str1, str2)

func  longest(str1, str2)
subarr = []
for n=1 to len(str1)
for m=1 to len(str1)
sub = substr(str1, n, m)
if substr(str2, sub) > 0
ok
next
next

temp = 0
for n=1 to len(subarr)
if len(subarr[n]) > temp
temp = len(subarr[n])
subend = subarr[n]
ok
next
see subend + nl```

Output:

```test
```

Ruby

Translation of: C#
```def longest_common_substring(a,b)
lengths = Array.new(a.length){Array.new(b.length, 0)}
greatestLength = 0
output = ""
a.each_char.with_index do |x,i|
b.each_char.with_index do |y,j|
next if x != y
lengths[i][j] = (i.zero? || j.zero?) ? 1 : lengths[i-1][j-1] + 1
if lengths[i][j] > greatestLength
greatestLength = lengths[i][j]
output = a[i - greatestLength + 1, greatestLength]
end
end
end
output
end

p longest_common_substring("thisisatest", "testing123testing")
```
Output:
```"test"
```

Rust

```fn longest_common_substring(s1: &str, s2: &str) -> String {
let s1_chars: Vec<char> = s1.chars().collect();
let s2_chars: Vec<char> = s2.chars().collect();
let mut lcs = "".to_string();

for i in 0..s1_chars.len() {
for j in 0..s2_chars.len() {
if s1_chars[i] == s2_chars[j] {
let mut tmp_lcs = s2_chars[j].to_string();
let mut tmp_i = i + 1;
let mut tmp_j = j + 1;

while tmp_i < s1_chars.len() && tmp_j < s2_chars.len() && s1_chars[tmp_i] == s2_chars[tmp_j] {
tmp_lcs = format!("{}{}", tmp_lcs, s1_chars[tmp_i]);
tmp_i += 1;
tmp_j += 1;
}

if tmp_lcs.len() > lcs.len() {
lcs = tmp_lcs;
}
}
}
}

lcs
}

fn main() {
let s1 = "thisisatest";
let s2 = "testing123testing";
let lcs = longest_common_substring(s1, s2);
println!("{}", lcs);
}
```
Output:
```"test"
```

Scala

The two algorithms below are Scala optimized versions of the Dynamic Programming algorithm pseudocode solution found on the "Longest Common Substring" Wikipedia page.

For a more in-depth look at the Scala solution space for this problem, please see this StackOverflow answer.

longestCommonSubstringsOptimizedPureFP

This algorithm sticks to "Pure" FP (Functional Programming) in that it uses tail recursion while avoiding any use of mutable collections or vars within the function's implementation.

Explore and experiment withing the online Scala playgrounds that run in your favorite browser: ScalaFiddle (ES a.k.a. JavaScript, non JVM) or Scastie (remote JVM).

```def longestCommonSubstringsOptimizedPureFP(left: String, right: String): Option[Set[String]] =
if (left.nonEmpty && right.nonEmpty) {
val (shorter, longer) =
if (left.length < right.length) (left, right)
else (right, left)

@scala.annotation.tailrec
def recursive(
indexLonger: Int = 0,
indexShorter: Int = 0,
currentLongestLength: Int = 0,
lengthsPrior: List[Int] = List.fill(shorter.length)(0),
lengths: List[Int] = Nil,
accumulator: List[Int] = Nil
): (Int, List[Int]) =
if (indexLonger < longer.length) {
val length =
if (longer(indexLonger) != shorter(indexShorter)) 0
val newCurrentLongestLength =
if (length > currentLongestLength) length
else currentLongestLength
val newAccumulator =
if ((length < currentLongestLength) || (length == 0)) accumulator
else {
val entry = indexShorter - length + 1
if (length > currentLongestLength) List(entry)
else entry :: accumulator
}
if (indexShorter < shorter.length - 1)
recursive(
indexLonger,
indexShorter + 1,
newCurrentLongestLength,
lengthsPrior.tail,
length :: lengths,
newAccumulator
)
else
recursive(
indexLonger + 1,
0,
newCurrentLongestLength,
0 :: lengths.reverse,
Nil,
newAccumulator
)
}
else (currentLongestLength, accumulator)

val (length, indexShorters) = recursive()
if (indexShorters.nonEmpty)
Some(
indexShorters
.map {
indexShorter =>
shorter.substring(indexShorter, indexShorter + length)
}
.toSet
)
else None
}
else None

println(longestCommonSubstringsOptimizedPureFP("thisisatest", "testing123testing"))
```
Output:
```"Some(Set(test))"
```

longestCommonSubstringsOptimizedReferentiallyTransparentFP

While this algorithm remains consistent with the FP concept of referential transparency, it does use both a mutable collection and a var within the function's implementation which provide an almost three times performance improvement over the above longestCommonSubstringsOptimizedPureFP implementation.

Explore this visual diff to see the changes between the longestCommonSubstringsOptimizedPureFP (above) and longestCommonSubstringsOptimizedReferentiallyTransparentFP (below) implementations

Explore and experiment withing the online Scala playgrounds that run in your favorite browser: ScalaFiddle (ES a.k.a. JavaScript, non JVM) or Scastie (remote JVM).

```def longestCommonSubstringsOptimizedReferentiallyTransparentFP(left: String, right: String): Option[Set[String]] =
if (left.nonEmpty && right.nonEmpty) {
val (shorter, longer) =
if (left.length < right.length) (left, right)
else (right, left)
val lengths: Array[Int] = new Array(shorter.length) //mutable

@scala.annotation.tailrec
def recursive(
indexLonger: Int = 0,
indexShorter: Int = 0,
currentLongestLength: Int = 0,
lastIterationLength: Int = 0,
accumulator: List[Int] = Nil
): (Int, List[Int]) =
if (indexLonger < longer.length) {
val length =
if (longer(indexLonger) != shorter(indexShorter)) 0
else
if (indexShorter == 0) 1
else lastIterationLength + 1
val newLastIterationLength = lengths(indexShorter)
lengths(indexShorter) = length //mutation
val newCurrentLongestLength =
if (length > currentLongestLength) length
else currentLongestLength
val newAccumulator =
if ((length < currentLongestLength) || (length == 0)) accumulator
else {
val entry = indexShorter - length + 1
if (length > currentLongestLength) List(entry)
else entry :: accumulator
}
if (indexShorter < shorter.length - 1)
recursive(
indexLonger,
indexShorter + 1,
newCurrentLongestLength,
newLastIterationLength,
newAccumulator
)
else
recursive(
indexLonger + 1,
0,
newCurrentLongestLength,
newLastIterationLength,
newAccumulator
)
}
else (currentLongestLength, accumulator)

val (length, indexShorters) = recursive()
if (indexShorters.nonEmpty)
Some(
indexShorters
.map {
indexShorter =>
shorter.substring(indexShorter, indexShorter + length)
}
.toSet
)
else None
}
else None

println(longestCommonSubstringsOptimizedReferentiallyTransparentFP("thisisatest", "testing123testing"))
```
Output:
```"Some(Set(test))"
```

SETL

```program longest_common_substring;
print(lcs("thisisatest", "testing123testing"));

proc lcs(s1, s2);
if #s1 < #s2 then [s1, s2] := [s2, s1]; end if;

loop for l in [#s2, #s2-1..1] do
loop for s in [1..#s2-l+1] do
if (substr := s2(s..s+l)) in s1 then
return substr;
end if;
end loop;
end loop;

return "";
end proc;
end program;```
Output:
`test`

Sidef

Translation of: Raku
```func createSubstrings(String word) -> Array {
gather {
combinations(word.len+1, 2, {|i,j|
take(word.substr(i, j-i))
})
}
}

func findLongestCommon(String first, String second) -> String {
createSubstrings(first) & createSubstrings(second) -> max_by { .len }
}

say findLongestCommon("thisisatest", "testing123testing")
```
Output:
`test`

Swift

```func lComSubStr<
S0: Sliceable, S1: Sliceable, T: Equatable where
S0.Generator.Element == T, S1.Generator.Element == T,
S0.Index.Distance == Int, S1.Index.Distance == Int
>(w1: S0, _ w2: S1) -> S0.SubSlice {

var (len, end) = (0, 0)

let empty = Array(Repeat(count: w2.count + 1, repeatedValue: 0))
var mat: [[Int]] = Array(Repeat(count: w1.count + 1, repeatedValue: empty))

for (i, sLett) in w1.enumerate() {
for (j, tLett) in w2.enumerate() where tLett == sLett {
let curLen = mat[i][j] + 1
mat[i + 1][j + 1] = curLen
if curLen > len {
len = curLen
end = i
}
}
}
}

func lComSubStr(w1: String, _ w2: String) -> String {
return String(lComSubStr(w1.characters, w2.characters))
}
```

Output:

```lComSubStr("thisisatest", "testing123testing") // "test"
```

VBA

```Function Longest_common_substring(string1 As String, string2 As String) As String
Dim i As Integer, j As Integer, temp As String, result As String
For i = 1 To Len(string1)
For j = 1 To Len(string1)
temp = Mid(string1, i, j)
If InStr(string2, temp) Then
If Len(temp) > Len(result) Then result = temp
End If
Next
Next
Longest_common_substring = result
End Function

Sub MainLCS()
Debug.Print Longest_common_substring("thisisatest", "testing123testing")
End Sub```
Output:

Invoke the script calling "MainLCS".

```test
```

VBScript

```Function lcs(string1,string2)
For i = 1 To Len(string1)
tlcs = tlcs & Mid(string1,i,1)
If InStr(string2,tlcs) Then
If Len(tlcs) > Len(lcs) Then
lcs = tlcs
End If
Else
tlcs = ""
End If
Next
End Function

WScript.Echo lcs(WScript.Arguments(0),WScript.Arguments(1))```
Output:

Invoke the script from a command prompt.

```C:\>cscript.exe /nologo lcs.vbs "thisisatest" "testing123testing"
test
```

V (Vlang)

Translation of: C#
```fn main()
{
println(lcs("thisisatest", "testing123testing"))
}

fn lcs(a string, b string) string {
mut lengths := map[int]int{}
mut output :=''
mut greatest_length := 0

for i, x in a {
for j, y in b {
if x == y {
if i == 0 || j == 0 {lengths[i * b.len + j] = 1} else {lengths[i * b.len + j] = lengths[(i-1) * b.len + j-1] + 1}
if lengths[i * b.len + j] > greatest_length {
greatest_length = lengths[i * b.len + j]
output += x.ascii_str()
}
}
}
}
return output
}```
Output:
```test
```

Wren

Translation of: Go
```var lcs = Fn.new { |a, b|
var la = a.count
var lb = b.count
var lengths = List.filled(la * lb, 0)
var greatestLength = 0
var output = ""
var i = 0
for (x in a) {
var j = 0
for (y in b) {
if (x == y) {
lengths[i*lb + j] = (i == 0 || j == 0) ? 1 : lengths[(i-1)*lb+j-1] + 1
}
if (lengths[i*lb+j] > greatestLength) {
greatestLength = lengths[i*lb+j]
output = a[i-greatestLength+1..i]
}
j = j + 1
}
i = i + 1
}
return output
}

System.print(lcs.call("thisisatest", "testing123testing"))```
Output:
```test
```

XPL0

Translation of: C
```string 0;

proc LCS(SA, SB, Beg, End);
char SA, SB;
int  Beg, End;
int  APos, BPos, Len;
[Beg(0):= 0;  End(0):= 0;  Len:= 0;
APos:= 0;
while SA(APos) # 0 do
[BPos:= 0;
while SB(BPos) # 0 do
[if SA(APos) = SB(BPos) then
[Len:= 1;
while SA(APos+Len) # 0 and SB(BPos+Len) # 0 and
SA(APos+Len) = SB(BPos+Len) do Len:= Len+1;
];
if Len > End(0) - Beg(0) then
[Beg(0):= SA + APos;
End(0):= Beg(0) + Len;
Len:= 0;
];
BPos:= BPos+1;
];
APos:= APos+1;
];
];

char S1, S2, It;
int  Beg, End;
[S1:= "thisisatest";
S2:= "testing123testing";
LCS(S1, S2, @Beg, @End);
for It:= Beg to End-1 do
ChOut(0, It(0));
CrLf(0);
]```
Output:
```test
```

Yabasic

Translation of: FreeBASIC
```sub LCS\$(a\$, b\$)
if len(a\$) = 0 or len(b\$) = 0 then return "" : endif
while len(b\$)
for j = len(b\$) to 1 step -1
if instr(a\$, left\$(b\$, j)) then return left\$(b\$, j) : endif
next j
b\$ = mid\$(b\$, 2)
wend
end sub

print LCS\$("thisisatest", "testing123testing")
end```

zkl

```fcn lcd(a,b){
if(b.len()<a.len()){ t:=a; a=b; b=t; }
foreach n,m in ([a.len()..1,-1],a.len()-n+1){
s:=a[m,n];
if(b.holds(s)) return(s);
}
""
}```
`lcd("testing123testing","thisisatest").println();`
Output:
`test`