Variable-length quantity

From Rosetta Code
Task
Variable-length quantity
You are encouraged to solve this task according to the task description, using any language you may know.

Implement some operations on variable-length quantities, at least including conversions from a normal number in the language to the binary representation of the variable-length quantity for that number, and vice versa. Any variants are acceptable.

Task : With above operations,

  • convert these two numbers 0x200000 (2097152 in decimal) and 0x1fffff (2097151 in decimal) into sequences of octets (an eight-bit byte);
  • display these sequences of octets;
  • convert these sequences of octets back to numbers, and check that they are equal to original numbers.

Ada[edit]

with Ada.Containers.Vectors;
with Ada.Text_IO;
with Ada.Unchecked_Conversion;
 
procedure VLQ is
 
package Nat_IO is new Ada.Text_IO.Integer_IO (Natural);
 
type Byte is mod 2**8;
 
package Byte_IO is new Ada.Text_IO.Modular_IO (Byte);
 
type Int7 is mod 2**7;
 
package Int7_IO is new Ada.Text_IO.Modular_IO (Int7);
 
type VLQ_Octet is record
Value : Int7 := 0;
Next  : Boolean := True;
end record;
pragma Pack (VLQ_Octet);
for VLQ_Octet'Size use 8;
 
function VLQ_To_Byte is new Ada.Unchecked_Conversion (VLQ_Octet, Byte);
function Byte_To_VLQ is new Ada.Unchecked_Conversion (Byte, VLQ_Octet);
 
package VLQ_Vectors is new Ada.Containers.Vectors (Natural, VLQ_Octet);
 
procedure Hex_Print (Position : in VLQ_Vectors.Cursor) is
Value : Byte := VLQ_To_Byte (VLQ_Vectors.Element (Position));
begin
Ada.Text_IO.Put (':');
Byte_IO.Put (Item => Value, Width => 6, Base => 16);
end Hex_Print;
 
procedure Print (X : VLQ_Vectors.Vector) is
begin
X.Iterate (Hex_Print'Access);
Ada.Text_IO.New_Line;
end Print;
 
function To_VLQ (From : Natural) return VLQ_Vectors.Vector is
Result : VLQ_Vectors.Vector;
Current : Natural := From;
Element : VLQ_Octet;
begin
loop
Element.Value := Int7 (Current mod 2**7);
Result.Prepend (Element);
Current := Current / 2**7;
exit when Current = 0;
end loop;
Element := Result.Last_Element;
Element.Next := False;
VLQ_Vectors.Replace_Element (Result, Result.Last, Element);
return Result;
end To_VLQ;
 
function To_Int (From : VLQ_Vectors.Vector) return Natural is
use type VLQ_Vectors.Cursor;
Result : Natural := 0;
Iterator : VLQ_Vectors.Cursor := From.First;
begin
while Iterator /= VLQ_Vectors.No_Element loop
Result := Result * 2**7;
Result := Result + Natural(VLQ_Vectors.Element (Iterator).Value);
VLQ_Vectors.Next (Iterator);
end loop;
return Result;
end To_Int;
 
Test : VLQ_Vectors.Vector;
begin
Test := To_VLQ (16#7f#);
Nat_IO.Put (To_Int (Test), 10, 16); Ada.Text_IO.Put (" = ");
Print (Test);
Test := To_VLQ (16#4000#);
Nat_IO.Put (To_Int (Test), 10, 16); Ada.Text_IO.Put (" = ");
Print (Test);
Test := To_VLQ (16#0#);
Nat_IO.Put (To_Int (Test), 10, 16); Ada.Text_IO.Put (" = ");
Print (Test);
Test := To_VLQ (16#3FFFFE#);
Nat_IO.Put (To_Int (Test), 10, 16); Ada.Text_IO.Put (" = ");
Print (Test);
Test := To_VLQ (16#1FFFFF#);
Nat_IO.Put (To_Int (Test), 10, 16); Ada.Text_IO.Put (" = ");
Print (Test);
Test := To_VLQ (16#200000#);
Nat_IO.Put (To_Int (Test), 10, 16); Ada.Text_IO.Put (" = ");
Print (Test);
end VLQ;

Output:

    16#7F# = :16#7F#
  16#4000# = :16#81#:16#80#: 16#0#
     16#0# = : 16#0#
16#3FFFFE# = :16#81#:16#FF#:16#FF#:16#7E#
16#1FFFFF# = :16#FF#:16#FF#:16#7F#
16#200000# = :16#81#:16#80#:16#80#: 16#0#

Bracmat[edit]

Bracmat has no native octet array type. Luckily, the only octet that possibly can be zero in a VLQ is the last octet. Therefore a solitary VLQ can be expressed as a Bracmat string, which, just as a C string, is null terminated. If the last byte of the VLQ string has the high bit set, we know that the last octet contained 0-bits only. A problem is of course that VLQ's probably are meant to be concatenizable. With null bytes missing, this is no option for the VLQ's generated by this solution.

(  ( VLQ
= b07 b8 vlq
. 0:?b8
& :?vlq
& whl
' ( !arg:>0
& mod$(!arg.128):?b07
& (chr$(!b8+!b07)|) !vlq:?vlq
& 128:?b8
& div$(!arg.128):?arg
)
& str$!vlq
)
& ( NUM
= c num d
. 0:?num:?d
& whl
' ( @(!arg:%@?c ?arg)
& asc$!c:?c:~<128
& 128*(!c+-128+!num):?num
& 1+!d:?d
)
& (!c:<128&!c+!num:?num|)
& !num
)
& ( printVLQ
= c h
.  :?h
& whl
' ( @(!arg:%@?c ?arg)
& d2x$(asc$!c):?x
&  !h (@(!x:? [1)&0|) !x
 : ?h
)
& ( asc$!c:~<128&!h 00:?h
|
)
& out$("VLQ  :" str$!h)
)
& ( test
= vlq num
. out$("input:" !arg)
& VLQ$(x2d$!arg):?vlq
& printVLQ$!vlq
& NUM$!vlq:?num
& out$("back :" d2x$!num \n)
)
& test$200000
& test$1fffff
& test$00
& test$7f
& test$80
& test$81
& test$82
& test$894E410E0A
);

Output:

input: 200000
VLQ  : 81808000
back : 200000

input: 1fffff
VLQ  : FFFF7F
back : 1FFFFF

input: 00
VLQ  :
back : 0

input: 7f
VLQ  : 7F
back : 7F

input: 80
VLQ  : 8100
back : 80

input: 81
VLQ  : 8101
back : 81

input: 82
VLQ  : 8102
back : 82

input: 894E410E0A
VLQ  : 9194F2849C0A
back : 894E410E0A

C[edit]

#include <stdio.h>
#include <stdint.h>
 
void to_seq(uint64_t x, uint8_t *out)
{
int i, j;
for (i = 9; i > 0; i--) {
if (x & 127ULL << i * 7) break;
}
for (j = 0; j <= i; j++)
out[j] = ((x >> ((i - j) * 7)) & 127) | 128;
 
out[i] ^= 128;
}
 
uint64_t from_seq(uint8_t *in)
{
uint64_t r = 0;
 
do {
r = (r << 7) | (uint64_t)(*in & 127);
} while (*in++ & 128);
 
return r;
}
 
int main()
{
uint8_t s[10];
uint64_t x[] = { 0x7f, 0x4000, 0, 0x3ffffe, 0x1fffff, 0x200000, 0x3311a1234df31413ULL};
 
int i, j;
for (j = 0; j < sizeof(x)/8; j++) {
to_seq(x[j], s);
printf("seq from %llx: [ ", x[j]);
 
i = 0;
do { printf("%02x ", s[i]); } while ((s[i++] & 128));
printf("] back: %llx\n", from_seq(s));
}
 
return 0;
}
output
seq from 7f: [ 7f ] back: 7f
seq from 4000: [ 81 80 00 ] back: 4000
seq from 0: [ 00 ] back: 0
seq from 3ffffe: [ 81 ff ff 7e ] back: 3ffffe
seq from 1fffff: [ ff ff 7f ] back: 1fffff
seq from 200000: [ 81 80 80 00 ] back: 200000
seq from 3311a1234df31413: [ b3 88 e8 a4 b4 ef cc a8 13 ] back: 3311a1234df31413

C#[edit]

For methods involving a BinaryReader or BinaryWriter please refer to this page.

namespace Vlq
{
using System;
using System.Collections.Generic;
using System.Linq;
 
public static class VarLenQuantity
{
public static ulong ToVlq(ulong integer)
{
var array = new byte[8];
var buffer = ToVlqCollection(integer)
.SkipWhile(b => b == 0)
.Reverse()
.ToArray();
Array.Copy(buffer, array, buffer.Length);
return BitConverter.ToUInt64(array, 0);
}
 
public static ulong FromVlq(ulong integer)
{
var collection = BitConverter.GetBytes(integer).Reverse();
return FromVlqCollection(collection);
}
 
public static IEnumerable<byte> ToVlqCollection(ulong integer)
{
if (integer > Math.Pow(2, 56))
throw new OverflowException("Integer exceeds max value.");
 
var index = 7;
var significantBitReached = false;
var mask = 0x7fUL << (index * 7);
while (index >= 0)
{
var buffer = (mask & integer);
if (buffer > 0 || significantBitReached)
{
significantBitReached = true;
buffer >>= index * 7;
if (index > 0)
buffer |= 0x80;
yield return (byte)buffer;
}
mask >>= 7;
index--;
}
}
 
 
public static ulong FromVlqCollection(IEnumerable<byte> vlq)
{
ulong integer = 0;
var significantBitReached = false;
 
using (var enumerator = vlq.GetEnumerator())
{
int index = 0;
while (enumerator.MoveNext())
{
var buffer = enumerator.Current;
if (buffer > 0 || significantBitReached)
{
significantBitReached = true;
integer <<= 7;
integer |= (buffer & 0x7fUL);
}
 
if (++index == 8 || (significantBitReached && (buffer & 0x80) != 0x80))
break;
}
}
return integer;
}
 
public static void Main()
{
var integers = new ulong[] { 0x7fUL << 7 * 7, 0x80, 0x2000, 0x3FFF, 0x4000, 0x200000, 0x1fffff };
 
foreach (var original in integers)
{
Console.WriteLine("Original: 0x{0:X}", original);
 
//collection
var seq = ToVlqCollection(original);
Console.WriteLine("Sequence: 0x{0}", seq.Select(b => b.ToString("X2")).Aggregate(string.Concat));
 
var decoded = FromVlqCollection(seq);
Console.WriteLine("Decoded: 0x{0:X}", decoded);
 
//ints
var encoded = ToVlq(original);
Console.WriteLine("Encoded: 0x{0:X}", encoded);
 
decoded = FromVlq(encoded);
Console.WriteLine("Decoded: 0x{0:X}", decoded);
 
Console.WriteLine();
}
Console.WriteLine("Press any key to continue...");
Console.ReadKey();
}
}
}
output
Original: 0xFE000000000000
Sequence: 0xFF80808080808000
Decoded: 0xFE000000000000
Encoded: 0xFF80808080808000
Decoded: 0xFE000000000000
 
Original: 0x80
Sequence: 0x8100
Decoded: 0x80
Encoded: 0x8100
Decoded: 0x80
 
Original: 0x2000
Sequence: 0xC000
Decoded: 0x2000
Encoded: 0xC000
Decoded: 0x2000
 
Original: 0x3FFF
Sequence: 0xFF7F
Decoded: 0x3FFF
Encoded: 0xFF7F
Decoded: 0x3FFF
 
Original: 0x4000
Sequence: 0x818000
Decoded: 0x4000
Encoded: 0x818000
Decoded: 0x4000
 
Original: 0x200000
Sequence: 0x81808000
Decoded: 0x200000
Encoded: 0x81808000
Decoded: 0x200000
 
Original: 0x1FFFFF
Sequence: 0xFFFF7F
Decoded: 0x1FFFFF
Encoded: 0xFFFF7F
Decoded: 0x1FFFFF
 
Press any key to continue...

D[edit]

This implements a Variable-length Quantity struct for an ulong integer.

import std.stdio, std.string, std.file, std.algorithm;
 
/// Variable length quantity (unsigned long, max 63-bit).
struct VLQ {
ulong value;
 
// This allows VLQ to work like an ulong.
alias value this;
 
uint extract(in ubyte[] v) pure
in {
assert(v.length > 0);
} body {
immutable limit = min(v.length - 1, 8);
ulong t = 0;
size_t idx = 0;
while ((idx < limit) && ((v[idx] & 0x80) > 0))
t = (t << 7) | (0x7f & v[idx++]);
if (idx > limit)
throw new Exception(
"Too large for ulong or invalid format.");
else
value = (t << 7) | v[idx];
return idx + 1;
}
 
VLQ from(in ubyte[] v) pure {
extract(v);
return this;
}
 
@property ubyte[] toVLQ() const pure {
ubyte[] v = [0x7f & value];
for (ulong k = value >>> 7; k > 0; k >>>= 7)
v ~= (k & 0x7f) | 0x80;
if (v.length > 9)
throw new Exception("Too large value.");
v.reverse();
return v;
}
 
static ulong[] split(in ubyte[] b) pure {
ulong[] res;
VLQ v;
for (size_t i = 0; i < b.length; ) {
i += v.extract(b[i .. $]);
res ~= v.value;
}
return res;
}
 
string toString() const pure /*nothrow*/ {
return format("(%(%02X:%))", this.toVLQ);
}
}
 
 
void main() { // VLQ demo code.
VLQ a = VLQ(0x7f),
b = VLQ(0x4000),
c;
writefln("a:%8x = %s\nb:%8x = %s\nc:%8x = %s",
a.value, a, b.value, b, c.value, c);
 
// Some operations.
c = (a + 1) * b;
a = c - 1;
b = VLQ().from(a.toVLQ);
a <<= 1;
 
// Convert ulong to octet sequence.
writefln("\na:%8x = %s\nb:%8x = %s\nc:%8x = %s",
a.value, a, b.value, b, c.value, c);
 
// Write them to a binary file.
std.file.write("vlqtest.bin", a.toVLQ ~ b.toVLQ ~ c.toVLQ);
 
// Read them back.
const buf = cast(ubyte[])std.file.read("vlqtest.bin");
writefln("\nFile length: %d bytes.", buf.length);
 
// Convert octet sequence to ulongs.
foreach (immutable i, immutable v; VLQ.split(buf))
writefln("%d:%8x = %s", i + 1, v, VLQ(v));
}
Output:
a:      7f = (7F)
b:    4000 = (81:80:00)
c:       0 = (00)

a:  3ffffe = (81:FF:FF:7E)
b:  1fffff = (FF:FF:7F)
c:  200000 = (81:80:80:00)

File length: 11 bytes.
1:  3ffffe = (81:FF:FF:7E)
2:  1fffff = (FF:FF:7F)
3:  200000 = (81:80:80:00)

Erlang[edit]

This is built in.

7> binary:encode_unsigned(2097152).
<<32,0,0>>
8> binary:decode_unsigned(<<32,0,0>>).
2097152
13> binary:encode_unsigned(16#1fffff).
<<31,255,255>>
14> binary:decode_unsigned(<<31,255,255>>).
2097151

Euphoria[edit]

function vlq_encode(integer n)
sequence s
s = {}
while n > 0 do
s = prepend(s, #80 * (length(s) > 0) + and_bits(n, #7F))
n = floor(n / #80)
end while
if length(s) = 0 then
s = {0}
end if
return s
end function
 
function vlq_decode(sequence s)
integer n
n = 0
for i = 1 to length(s) do
n *= #80
n += and_bits(s[i], #7F)
if not and_bits(s[i], #80) then
exit
end if
end for
return n
end function
 
function svlg(sequence s)
sequence out
out = ""
for i = 1 to length(s) do
out &= sprintf("#%02x:", {s[i]})
end for
return out[1..$-1]
end function
 
constant testNumbers = { #200000, #1FFFFF, 1, 127, 128 }
sequence s
for i = 1 to length(testNumbers) do
s = vlq_encode(testNumbers[i])
printf(1, "#%02x -> %s -> #%02x\n", {testNumbers[i], svlg(s), vlq_decode(s)})
end for

Output:

#200000 -> #81:#80:#80:#00 -> #200000
#1FFFFF -> #FF:#FF:#7F -> #1FFFFF
#01 -> #01 -> #01
#7F -> #7F -> #7F
#80 -> #81:#00 -> #80

Go[edit]

Go has an implementation of variable length quantities in the standard library.

package main
 
import (
"fmt"
"encoding/binary"
)
 
func main() {
buf := make([]byte, binary.MaxVarintLen64)
for _, x := range []int64{0x200000, 0x1fffff} {
v := buf[:binary.PutVarint(buf, x)]
fmt.Printf("%d encodes into %d bytes: %x\n", x, len(v), v)
x, _ = binary.Varint(v)
fmt.Println(x, "decoded")
}
}

Output required by task:

2097152 encodes into 4 bytes: 80808002
2097152 decoded
2097151 encodes into 4 bytes: feffff01
2097151 decoded

More output showing negative numbers, the roll over from one byte to two, and larger numbers of different lengths:

0 encodes into 1 bytes: 00
0 decoded
1 encodes into 1 bytes: 02
1 decoded
2 encodes into 1 bytes: 04
2 decoded
-1 encodes into 1 bytes: 01
-1 decoded
-2 encodes into 1 bytes: 03
-2 decoded
63 encodes into 1 bytes: 7e
63 decoded
64 encodes into 2 bytes: 8001
64 decoded
589723405834 encodes into 6 bytes: 94b888e4a922
589723405834 decoded
3679899543542109203 encodes into 9 bytes: a6d098dfe9c8d09166
3679899543542109203 decoded

Groovy[edit]

Solution:

final RADIX = 7
final MASK = 2**RADIX - 1
 
def octetify = { n ->
def octets = []
for (def i = n; i != 0; i >>>= RADIX) {
octets << ((byte)((i & MASK) + (octets.empty ? 0 : MASK + 1)))
}
octets.reverse()
}
 
def deoctetify = { octets ->
octets.inject(0) { long n, octet ->
(n << RADIX) + ((int)(octet) & MASK)
}
}

Test (samples borrowed from Java example):

def testNumbers = [ 0x200000, 0x1fffff, 1, 127, 128, 589723405834L ]
 
testNumbers.each { a ->
def octets = octetify(a)
octets.each { printf "0x%02x ", it }; println ()
def a1 = deoctetify(octets)
assert a1 == a
}

Output:

0x81 0x80 0x80 0x00 
0xff 0xff 0x7f 
0x01 
0x7f 
0x81 0x00 
0x91 0x94 0xf2 0x84 0x9c 0x0a

Haskell[edit]

import Numeric (readOct, showOct)
import Data.List (intercalate)
 
to :: Int -> String
to = flip showOct ""
 
from :: String -> Int
from = fst . head . readOct
 
main :: IO ()
main =
mapM_
(putStrLn .
intercalate " <-> " . (pure (:) <*> to <*> (return . show . from . to)))
[2097152, 2097151]

Homemade Version:

import Data.List (intercalate)
 
base :: Int
base = 8
 
to :: Int -> [Int]
to 0 = []
to i = to (div i base) ++ [mod i base]
 
from :: [Int] -> Int
from = foldl1 ((+) . (base *))
 
main :: IO ()
main =
mapM_
(putStrLn .
intercalate " <-> " .
(((:) . concatMap show . to) <*> (return . show . from . to)))
[2097152, 2097151]
Output:
10000000 <-> 2097152
7777777 <-> 2097151

Icon and Unicon[edit]

procedure main()
every i := 2097152 | 2097151 | 1 | 127 | 128 | 589723405834 | 165 | 256 do
write(image(i)," = ",string2hex(v := uint2vlq(i))," = ",vlq2uint(v))
end
 
procedure vlq2uint(s) #: decode a variable length quantity
if *s > 0 then {
i := 0
s ? while h := ord(move(1)) do {
if (pos(0) & h > 128) | (not pos(0) & h < 128) then fail
i := 128 * i + h % 128
}
return i
}
end
 
procedure uint2vlq(i,c) #: encode a whole number as a variable length quantity
if "integer" == type(-1 < i) then
return if i = 0 then
char((/c := 0)) | ""
else
uint2vlq(i/128,1) || char((i % 128) + ((/c := 0) | 128) )
end
 
procedure string2hex(s) #: convert a string to hex
h := ""
every i := ord(!s) do
h ||:= "0123456789abcdef"[i/16+1] || "0123456789abcdef"[i%16+1]
return h
end
Output:
2097152 = 81808000 = 2097152
2097151 = ffff7f = 2097151
1 = 01 = 1
127 = 7f = 127
128 = 8100 = 128
589723405834 = 9194f2849c0a = 589723405834
165 = 8125 = 165
256 = 8200 = 256

J[edit]

N=: 128x
v2i=: (N&| N&#./.~ [: +/\ _1 |. N&>)@i.~&a.
i2v=: a. {~ [:;}.@(N+//.@,:N&#.inv)&.>
ifv=: v2i :. i2v
vfi=: i2v :. v2i

ifv is an invertible function which gets an (unsigned, arbitrary precision) integer sequence from a variable-length quantity sequence. vfi is an invertable function which gets a variable-length quantity sequence from an unsigned integer sequence. av displays character code numbers corresponding to the characters in its argument.

Example use:

   require'convert' 
numbers=: 16b7f 16b4000 0 16b3ffffe 16b1fffff 200000
av vlq=: vfi numbers
127 129 128 0 0 129 255 255 126 255 255 127 140 154 64
av (vfi 1 2 3 4 5 6) +&.ifv vlq
129 0 129 128 2 3 130 128 128 2 129 128 128 4 140 154 70

Java[edit]

public class VLQCode
{
public static byte[] encode(long n)
{
int numRelevantBits = 64 - Long.numberOfLeadingZeros(n);
int numBytes = (numRelevantBits + 6) / 7;
if (numBytes == 0)
numBytes = 1;
byte[] output = new byte[numBytes];
for (int i = numBytes - 1; i >= 0; i--)
{
int curByte = (int)(n & 0x7F);
if (i != (numBytes - 1))
curByte |= 0x80;
output[i] = (byte)curByte;
n >>>= 7;
}
return output;
}
 
public static long decode(byte[] b)
{
long n = 0;
for (int i = 0; i < b.length; i++)
{
int curByte = b[i] & 0xFF;
n = (n << 7) | (curByte & 0x7F);
if ((curByte & 0x80) == 0)
break;
}
return n;
}
 
public static String byteArrayToString(byte[] b)
{
StringBuilder sb = new StringBuilder();
for (int i = 0; i < b.length; i++)
{
if (i > 0)
sb.append(", ");
String s = Integer.toHexString(b[i] & 0xFF);
if (s.length() < 2)
s = "0" + s;
sb.append(s);
}
return sb.toString();
}
 
public static void main(String[] args)
{
long[] testNumbers = { 2097152, 2097151, 1, 127, 128, 589723405834L };
for (long n : testNumbers)
{
byte[] encoded = encode(n);
long decoded = decode(encoded);
System.out.println("Original input=" + n + ", encoded = [" + byteArrayToString(encoded) + "], decoded=" + decoded + ", " + ((n == decoded) ? "OK" : "FAIL"));
}
}
}
 

Output:

Original input=2097152, encoded = [81, 80, 80, 00], decoded=2097152, OK
Original input=2097151, encoded = [ff, ff, 7f], decoded=2097151, OK
Original input=1, encoded = [01], decoded=1, OK
Original input=127, encoded = [7f], decoded=127, OK
Original input=128, encoded = [81, 00], decoded=128, OK
Original input=589723405834, encoded = [91, 94, f2, 84, 9c, 0a], decoded=589723405834, OK

Julia[edit]

 
type VLQ
q::Array{Uint8,1}
end
 
function VLQ{T<:Integer}(n::T)
q = uint8(digits(n, 128))
for i in 2:length(q)
q[i] |= 0x80
end
VLQ(reverse(q))
end
 
function Base.uint(vlq::VLQ)
q = reverse(vlq.q)
n = shift!(q)
p = one(Uint64)
for i in q
p *= 0x80
n += p*(i&0x7f)
end
return n
end
 
test = [0x00200000, 0x001fffff, 0x00000000, 0x0000007f,
0x00000080, 0x00002000, 0x00003fff, 0x00004000,
0x08000000, 0x0fffffff]
 
for i in test
q = VLQ(i)
j = uint(q)
print(@sprintf " 0x%x => " i)
print(@sprintf "[%s]" join(["0x"*hex(r, 2) for r in q.q], ", "))
println(@sprintf " => 0x%x" j)
end
 
Output:
    0x200000 => [0x81, 0x80, 0x80, 0x00] => 0x200000
    0x1fffff => [0xff, 0xff, 0x7f] => 0x1fffff
    0x0 => [0x00] => 0x0
    0x7f => [0x7f] => 0x7f
    0x80 => [0x81, 0x00] => 0x80
    0x2000 => [0xc0, 0x00] => 0x2000
    0x3fff => [0xff, 0x7f] => 0x3fff
    0x4000 => [0x81, 0x80, 0x00] => 0x4000
    0x8000000 => [0xc0, 0x80, 0x80, 0x00] => 0x8000000
    0xfffffff => [0xff, 0xff, 0xff, 0x7f] => 0xfffffff

Kotlin[edit]

// version 1.0.6
 
fun Int.toOctets(): ByteArray {
var s = Integer.toBinaryString(this)
val r = s.length % 7
var z = s.length / 7
if (r > 0) {
z++
s = s.padStart(z * 7, '0')
}
s = Array(z) { "1" + s.slice(it * 7 until (it + 1) * 7) }.joinToString("")
s = s.take(s.length - 8) + "0" + s.takeLast(7)
return ByteArray(z) { Integer.parseInt(s.slice(it * 8 until (it + 1) * 8), 2).toByte() }
}
 
fun ByteArray.fromOctets(): Int {
var s = ""
for (b in this) s += Integer.toBinaryString(b.toInt()).padStart(7, '0').takeLast(7)
return Integer.parseInt(s, 2)
}
 
fun main(args: Array<String>) {
val tests = intArrayOf(0x7f, 0x3fff, 0x200000, 0x1fffff)
for (test in tests) {
val ba = test.toOctets()
print("${"0x%x".format(test).padEnd(8)} -> ")
var s = ""
ba.forEach { s += "0x%02x ".format(it) }
println("${s.padEnd(20)} <- ${"0x%x".format(ba.fromOctets())}")
}
}
Output:
0x7f     -> 0x7f                 <- 0x7f
0x3fff   -> 0xff 0x7f            <- 0x3fff
0x200000 -> 0x81 0x80 0x80 0x00  <- 0x200000
0x1fffff -> 0xff 0xff 0x7f       <- 0x1fffff

LiveCode[edit]

This task was completed a different (and better) way a long time ago in UDI's PMD/MakeSMF Lib for LiveCode (back when it was MetaCard). Here is my own (and probably slower) version. -- Paul McClernan

 
on DecToVLQ
Ask "Enter base 10 value:" -- input dialog box
if it is not empty then
if it is a number then
put it into theString
if isWholeNumString(theString) is false then -- I think there is built in equivalent for this but I rolled my own!
answer "Only Whole Decimal Numbers Are Allowed!"
exit DecToVLQ
end if
if theString>4294967295 then
answer "This function fails with whole numbers over 4294967295!"&cr\
& "4294967295 is the maximum allowed value for 32bits (4 bytes)"
exit DecToVLQ
end if
if theString>268435455 then
answer "This function is not accurate with whole numbers over 268435455!"&cr\
& "268435455 is the maximum allowed value for 28bit (7bits per byte) MIDI delta-time VLQ"
end if
put "Original Whole Number="& theString & cr & \
"Original Whole Number in Hex="& baseConvert(theString,10,16) & cr & \ --- LC's built in baseConvert function
"Variable Length Quantity in Hex=" & wholeNumToVLQ(theString) into fld "Output"
else
answer "Only Whole Decimal Numbers Are Allowed!"
end if
end if
end DecToVLQ
 
function wholeNumToVLQ theWholeNum
-- baseConvert(number,originalBase,destinationBase) -- there is also bitwise operations in LC but I took the long road
if theWholeNum < 127 then -- if it fits into a single 7bit byte value and theres no need to process it
put baseConvert(theWholeNum,10,16) into VQLinHex
if the number of chars in VQLinHex=1 then put "0" before VQLinHex
return VQLinHex
exit wholeNumToVLQ
end if
put baseConvert(theWholeNum,10,2) into theBits
put number of chars in theBits into x
put 0 into bitCounter
put empty into the7bitBytes
repeat
if char x of theBits is not empty then
put char x theBits before the7bitBytes
delete char x of theBits
if theBits is empty then exit repeat
put number of chars in theBits into x
add 1 to bitCounter
if bitCounter=7 then
put "," before the7bitBytes
put 0 into bitCounter
next repeat
end if
else
exit repeat
end if
end repeat
get the number of chars in item 1 of the7bitBytes
if it<7 then
put 7 - it into x
repeat x
put "0" before item 1 of the7bitBytes
end repeat
end if
put the number of items in the7bitBytes into y
repeat with x = 1 to y
if x is not y then
put "1" before item x of the7bitBytes
else
put "0" before item x of the7bitBytes
end if
put baseConvert(item x of the7bitBytes,2,16) into item x of the7bitBytes
if the number of chars in item x of the7bitBytes<2 then put "0" before item x of the7bitBytes
put item x of the7bitBytes after VQLinHex
end repeat
return VQLinHex
end wholeNumToVLQ
 
function isWholeNumString theString
put the number of chars in theString into y
repeat with x = 1 to y
if char x of theString is not in "0123456789" then
return false
exit isWholeNumString
end if
end repeat
return true
end isWholeNumString
 
Output:
Original Whole Number=2097152
Original Whole Number in Hex=200000
Variable Length Quantity in Hex=81808000

Convert back:

 
function VLQtoWholeNum theHexVLQ
-- The number must be an integer between zero and 4,294,967,295
put baseConvert(theHexVLQ,16,2) into theBits
put 0 into bitCounter
put empty into the8bitBytes
repeat
if char 1 of theBits is not empty then
put char 1 theBits after the8bitBytes
delete char 1 of theBits
if theBits is empty then exit repeat
add 1 to bitCounter
if bitCounter=8 then
put "," after the8bitBytes
put 0 into bitCounter
next repeat
end if
else
exit repeat
end if
end repeat
put the number of items in the8bitBytes into y
repeat with x = 1 to y
put char 1 of item x of the8bitBytes into lengthCntrlBit
delete char 1 of item x of the8bitBytes
if the number of chars in item x of the8bitBytes < 7 then
repeat 7 - (the number of chars in item x of the8bitBytes)
put "0" before item x of the8bitBytes
end repeat
end if
put item x of the8bitBytes after WholeNumInBinary
switch lengthCntrlBit
case "1"
next repeat
break
case "0"
exit repeat
break
end switch
end repeat
return baseConvert(WholeNumInBinary,2,10)
end VLQtoWholeNum
 
function isHexString theString
---again there is probably an easier way to do this:
if char 1 to 2 of theString is "0x" then delete char 1 to 2 of theString
put the number of chars in theString into y
repeat with x = 1 to y
if char x of theString is not in "abcdefABCDEF0123456789" then
return false
end if
end repeat
end isHexString
 
on VLQHexToWholeNum
Ask "Enter Variable Length Quantity Hex Value:" -- input dialog
if it is not empty then
if char 1 to 2 of it is "0x" then delete char 1 to 2 of it
put it into hexString
if isHexString(hexString) is false then
answer "Only Valid Hex Digits Are Allowed!"
exit VLQHexToWholeNum
else
put "Original Variable Length Quantity in Hex="& hexString & cr & \
"Whole Number=" & VLQtoWholeNum(hexString) into fld "Output"
end if
end if
end VLQHexToWholeNum
 
Output:
Original Variable Length Quantity in Hex=FFFF7F
Whole Number=2097151

Mathematica[edit]

toOctets[n_Integer] := 
StringJoin @@@
Partition[
PadLeft[Characters@IntegerString[n, 16],
2 Ceiling[Plus @@ DigitCount[n, 16]/2], {"0"}], 2]
 
fromOctets[octets_List] := FromDigits[StringJoin @@ octets, 16]
 
Grid[{#, toOctets@#, fromOctets[toOctets@#]} & /@ {16^^3ffffe,
16^^1fffff, 16^^200000}]
Output:

4194302 {3f,ff,fe} 4194302 2097151 {1f,ff,ff} 2097151 2097152 {20,00,00} 2097152

Nim[edit]

import unsigned, strutils
 
proc toSeq(x: uint64): seq[uint8] =
var x = x
result = @[]
var f = 0
for i in countdown(9, 1):
if (x and (127'u64 shl uint((i * 7)))) > 0'u64:
f = i
break
for j in 0..f:
result.add(uint8((x shr uint64((f - j) * 7)) and 127) or 128)
 
result[f] = result[f] xor 128'u8
 
proc fromSeq(xs): uint64 =
result = 0
for x in xs:
result = (result shl 7) or (x and 127)
 
for x in [0x7f'u64, 0x4000'u64, 0'u64, 0x3ffffe'u64, 0x1fffff'u64,
0x200000'u64, 0x3311a1234df31413'u64]:
let c = toSeq(x)
echo "seq from $#: $# back: $#".format(x, c, fromSeq(c))

Output:

seq from 127: @[127] back: 127
seq from 16384: @[129, 128, 0] back: 16384
seq from 0: @[0] back: 0
seq from 4194302: @[129, 255, 255, 126] back: 4194302
seq from 2097151: @[255, 255, 127] back: 2097151
seq from 2097152: @[129, 128, 128, 0] back: 2097152
seq from 3679899543542109203: @[179, 136, 232, 164, 180, 239, 204, 168, 19] back: 3679899543542109203

OCaml[edit]

let to_vlq n =
let a, b = n lsr 7, n land 0x7F in
let rec aux n acc =
let x = (n land 0x7F) lor 0x80
and xs = n lsr 7 in
if xs > 0
then aux xs (x::acc)
else x::acc
in
aux a [b]
 
let to_num = List.fold_left (fun n x -> n lsl 7 + (x land 0x7F)) 0
 
let v_rep n =
Printf.printf "%d ->" n;
let seq = to_vlq n in
List.iter (Printf.printf " 0x%02X") seq;
let num = to_num seq in
Printf.printf "-> %d\n%!" num;
assert (n = num)
 
let _ =
v_rep 0x200000;
v_rep 0x1FFFFF
 

Outputs:

$ ocaml variable_length.ml
2097152 -> 0x81 0x80 0x80 0x00 -> 2097152
2097151 -> 0xFF 0xFF 0x7F -> 2097151

PARI/GP[edit]

hex(s)=my(a=10,b=11,c=12,d=13,e=14,f=15);subst(Pol(eval(Vec(s))),'x,16);
n1=hex("200000");n2=hex("1fffff");
v1=digits(n1,256)
v2=digits(n2,256)
subst(Pol(v1),'x,256)==n1
subst(Pol(v2),'x,256)==n2
Output:
%1 = [32, 0, 0]
%2 = [31, 255, 255]
%3 = 1
%4 = 1

Perl[edit]

The vlg_encode sub returns an array of octets in most -> least significant order. Simply reverse the array to reverse the order.

 
use warnings;
use strict;
 
for my $testcase (
0, 0xa, 123, 254, 255, 256,
257, 65534, 65535, 65536, 65537, 0x1fffff,
0x200000
)
{
my @vlq = vlq_encode($testcase);
printf "%8s %12s %8s\n", $testcase, ( join ':', @vlq ), vlq_decode(@vlq);
}
 
sub vlq_encode {
my @vlq;
my $binary = sprintf "%s%b", 0 x 7, shift;
$binary =~ s/(.{7})$//;
@vlq = ( unpack 'H2', ( pack 'B8', '0' . $1 ) );
while ( 0 + $binary ) {
$binary =~ s/(.{7})$//;
unshift @vlq, ( unpack 'H2', pack 'B8', '1' . $1 );
}
return @vlq;
}
 
sub vlq_decode {
my $num;
$num .= sprintf "%07b", hex(shift @_) & 0x7f while @_;
return oct '0b' . $num;
}
 

Output:

       0           00        0
      10           0a       10
     123           7b      123
     254        81:7e      254
     255        81:7f      255
     256        82:00      256
     257        82:01      257
   65534     83:ff:7e    65534
   65535     83:ff:7f    65535
   65536     84:80:00    65536
   65537     84:80:01    65537
 2097151     ff:ff:7f  2097151
 2097152  81:80:80:00  2097152

Perl 6[edit]

vlq_encode() returns a string of characters whose ordinals are the encoded octets. vlq_decode() takes a string and returns a decimal number.

sub vlq_encode ($number is copy) {
my $string = '';
my $t = 0x7F +& $number;
$number +>= 7;
$string = $t.chr ~ $string;
while ($number) {
$t = 0x7F +& $number;
$string = (0x80 +| $t).chr ~ $string;
$number +>= 7;
}
return $string;
}
 
sub vlq_decode ($string is copy) {
my $number = '0b';
for $string.ords -> $oct {
$number ~= ($oct +& 0x7F).fmt("%07b");
}
return :2($number);
}
 
#test encoding and decoding
for (
0, 0xa, 123, 254, 255, 256,
257, 65534, 65535, 65536, 65537, 0x1fffff,
0x200000
) -> $testcase {
my $encoded = vlq_encode($testcase);
printf "%8s %12s %8s\n", $testcase,
( join ':', $encoded.ords>>.fmt("%02X") ),
vlq_decode($encoded);
}

Output:

       0           00        0
      10           0A       10
     123           7B      123
     254        81:7E      254
     255        81:7F      255
     256        82:00      256
     257        82:01      257
   65534     83:FF:7E    65534
   65535     83:FF:7F    65535
   65536     84:80:00    65536
   65537     84:80:01    65537
 2097151     FF:FF:7F  2097151
 2097152  81:80:80:00  2097152

Phix[edit]

Copy of Euphoria, modified to pack several numbers into a single stream. Also added an explicit check that (as per wp) only unsigned numbers are attempted.

function vlq_encode(sequence s)
sequence res = {}
integer n, msb
for i=length(s) to 1 by -1 do
n = s[i]
if n<0 then crash("unsigned integers only!") end if
msb = 0
while 1 do
res = prepend(res,msb+and_bits(n,#7F))
n = floor(n/#80)
if n=0 then exit end if
msb = #80
end while
end for
return res
end function
 
function vlq_decode(sequence s)
sequence res = {}
integer n = 0, byte
for i=1 to length(s) do
byte = s[i]
n = n*#80+and_bits(byte,#7F)
if not and_bits(byte,#80) then
res = append(res,n)
n = 0
end if
end for
return res
end function
 
function svlg(sequence s)
string res = ""
for i=1 to length(s) do
res &= sprintf("#%02x:",{s[i]})
end for
return res[1..$-1]
end function
 
constant testNumbers = { #200000, #1FFFFF, 1, 127, 128 }
sequence s = vlq_encode(testNumbers)
sequence decoded = vlq_decode(s)
printf(1,"%s -> %s -> %s\n",{svlg(testNumbers),svlg(s),svlg(decoded)})
if decoded!=testNumbers then crash("something wrong") end if
Output:
#200000:#1FFFFF:#01:#7F:#80 -> #81:#80:#80:#00:#FF:#FF:#7F:#01:#7F:#81:#00 -> #200000:#1FFFFF:#01:#7F:#80

PicoLisp[edit]

(de numToVlq (Num)
(let Res (cons (& Num 127))
(while (gt0 (setq Num (>> 7 Num)))
(push 'Res (| 128 (& Num 127))) )
Res ) )
 
(de vlqToNum (Vlq)
(let Res 0
(for N Vlq
(setq Res (| (>> -7 Res) (& N 127))) ) ) )
 
(for Num (0 15 16 127 128 255 2097151 2097152)
(let Vlq (numToVlq Num)
(tab (12 12 12) Num (glue ":" (mapcar hex Vlq)) (vlqToNum Vlq)) ) )

Output:

           0           0           0
          15           F          15
          16          10          16
         127          7F         127
         128        81:0         128
         255       81:7F         255
     2097151    FF:FF:7F     2097151
     2097152  81:80:80:0     2097152

PL/I[edit]

 
test: procedure options(main);
declare s character (20) varying;
declare c character (1);
declare v fixed binary (31);
declare (i, k) fixed binary;
 
get edit (s) (L);
s = trim (s);
v = 0;
do i = 1 to length(s);
c = substr(s, i, 1);
k = index('0123456789abcdef', c);
if k > 0 then v = v*16 + k - 1;
end;
put skip data (s, v);
 
/* Convert back to hex */
declare hex character(16) initial ('0123456789abcdef');
declare hs character (20) initial ('');
declare d fixed binary;
 
do i = length(hs) to 1 by -1 until (v = 0);
d = mod(v, 16) + 1;
substr(hs, i, 1) = substr(hex, d, 1);
v = v/16;
end;
put skip list (hs);
end test;
 

OUTPUT:

S='200000'              V=       2097152;
              200000 
S='1fffff'              V=       2097151;
              1fffff 

Python[edit]

The vlq format is computed in a form for printing. This could easily be changed to a series of 8 bit ASCII chars whose integer value corresponds to the vlq for saving or transmission.

When transmitting the Vlq, octets are sent from the rightmost of the Vlq first.

def tobits(n, _group=8, _sep='_', _pad=False):
'Express n as binary bits with separator'
bits = '{0:b}'.format(n)[::-1]
if _pad:
bits = '{0:0{1}b}'.format(n,
((_group+len(bits)-1)//_group)*_group)[::-1]
answer = _sep.join(bits[i:i+_group]
for i in range(0, len(bits), _group))[::-1]
answer = '0'*(len(_sep)-1) + answer
else:
answer = _sep.join(bits[i:i+_group]
for i in range(0, len(bits), _group))[::-1]
return answer
 
def tovlq(n):
return tobits(n, _group=7, _sep='1_', _pad=True)
 
def toint(vlq):
return int(''.join(vlq.split('_1')), 2)
 
def vlqsend(vlq):
for i, byte in enumerate(vlq.split('_')[::-1]):
print('Sent byte {0:3}: {1:#04x}'.format(i, int(byte,2)))


Sample Output The underscore separates groups of eight bits (octets), for readability

>>> for n in (254, 255, 256, 257, -2+(1<<16), -1+(1<<16), 1<<16, 1+(1<<16), 0x200000, 0x1fffff ):
print('int: %7i bin: %26s vlq: %35s vlq->int: %7i' % (n, tobits(n,_pad=True), tovlq(n), toint(tovlq(n))))
 
 
int: 254 bin: 11111110 vlq: 00000001_11111110 vlq->int: 254
int: 255 bin: 11111111 vlq: 00000001_11111111 vlq->int: 255
int: 256 bin: 00000001_00000000 vlq: 00000010_10000000 vlq->int: 256
int: 257 bin: 00000001_00000001 vlq: 00000010_10000001 vlq->int: 257
int: 65534 bin: 11111111_11111110 vlq: 00000011_11111111_11111110 vlq->int: 65534
int: 65535 bin: 11111111_11111111 vlq: 00000011_11111111_11111111 vlq->int: 65535
int: 65536 bin: 00000001_00000000_00000000 vlq: 00000100_10000000_10000000 vlq->int: 65536
int: 65537 bin: 00000001_00000000_00000001 vlq: 00000100_10000000_10000001 vlq->int: 65537
int: 2097152 bin: 00100000_00000000_00000000 vlq: 00000001_10000000_10000000_10000000 vlq->int: 2097152
int: 2097151 bin: 00011111_11111111_11111111 vlq: 01111111_11111111_11111111 vlq->int: 2097151
>>> vlqsend(tovlq(0x200000))
Sent byte 0: 0x80
Sent byte 1: 0x80
Sent byte 2: 0x80
Sent byte 3: 0x01
>>> vlqsend(tovlq(0x1fffff))
Sent byte 0: 0xff
Sent byte 1: 0xff
Sent byte 2: 0x7f
>>>


Racket[edit]

 
#lang racket
 
(define (try n)
(printf "Original number: ~s (0x~x)\n" n n)
(define 4octets (integer->integer-bytes n 4 #f))
(printf "Octets: ~a (byte-string: ~s)\n"
(string-join (map (λ(o) (~r o #:base 16))
(bytes->list 4octets))
":")
4octets)
(define m (integer-bytes->integer 4octets #f))
(printf "Back to a number: ~s (~a)\n"
m (if (= m n) "OK" "BAD")))
 
(for-each try '(#x200000 #x1fffff))
 

Output:

Original number: 2097152 (0x200000)
Octets: 0:0:20:0 (byte-string: #"\0\0 \0")
Back to a number: 2097152 (OK)
Original number: 2097151 (0x1fffff)
Octets: ff:ff:1f:0 (byte-string: #"\377\377\37\0")
Back to a number: 2097151 (OK)

REXX[edit]

/*REXX program displays  (and also tests/verifies)  some numbers as  octets.            */
nums = x2d(200000) x2d(1fffff) 2097172 2097151
#=words(nums)
say ' number hex octet original'
say '══════════ ══════════ ══════════ ══════════'
ok=1
do j=1 for #; @.j= word(nums,j)
onum.j=octet(@.j)
orig.j= x2d( space(onum.j, 0) )
w=10
say center(@.j, w) center(d2x(@.j), w) center(onum.j, w) center(orig.j, w)
if @.j\==orig.j then ok=0
end /*j*/
say
if ok then say 'All ' # " numbers are OK." /*all of the numbers are good. */
else say "Some numbers are not OK." /*some of the numbers are ¬good. */
exit /*stick a fork in it, we're all done. */
/*──────────────────────────────────────────────────────────────────────────────────────*/
octet: procedure; parse arg z,$ /*obtain Z from the passed arguments.*/
x=d2x(z) /*convert Z to a hexadecimal octet. */
do j=length(x) by -2 to 1 /*process the "little" end first. */
$= substr(x, j-1, 2, 0) $ /*pad odd hexadecimal characters with */
end /*j*/ /* ··· a zero on the left. */
return strip($)
output   when using the default input:
  number       hex       octet    original
══════════ ══════════ ══════════ ══════════
 2097152     200000    20 00 00   2097152
 2097151     1FFFFF    1F FF FF   2097151
 2097172     200014    20 00 14   2097172
 2097151     1FFFFF    1F FF FF   2097151

All  4  numbers are OK.

Ruby[edit]

Array#pack can encode the BER-compressed integer, which is identical to the variable-length quantity from the MIDI specification. String#unpack can decode it.

[0x200000, 0x1fffff].each do |i|
# Encode i => BER
ber = [i].pack("w")
hex = ber.unpack("C*").collect {|c| "%02x" % c}.join(":")
printf "%s => %s\n", i, hex
 
# Decode BER => j
j = ber.unpack("w").first
i == j or fail "BER not preserve integer"
end
2097152 => 81:80:80:00
2097151 => ff:ff:7f

Scala[edit]

object VlqCode {
def encode(x:Long)={
val result=scala.collection.mutable.Stack[Byte]()
result push (x&0x7f).toByte
var l = x >>> 7
while(l>0){
result push ((l&0x7f)|0x80).toByte
l >>>= 7
}
result.toArray
}
 
def decode(a:Array[Byte])=a.foldLeft(0L)((r, b) => r<<7|b&0x7f)
 
def toString(a:Array[Byte])=a map("%02x".format(_)) mkString("[", ", ", "]")
 
def test(x:Long)={
val enc=encode(x)
println("0x%x => %s => 0x%x".format(x, toString(enc), decode(enc)))
}
 
def main(args: Array[String]): Unit = {
val xs=Seq(0, 0x7f, 0x80, 0x2000, 0x3fff, 0x4000, 0x1FFFFF, 0x200000, 0x8000000,
0xFFFFFFF, 0xFFFFFFFFL, 0x842FFFFFFFFL, 0x0FFFFFFFFFFFFFFFL)
xs foreach test
}
}

Output:

0x0 => [00] => 0x0
0x7f => [7f] => 0x7f
0x80 => [81, 00] => 0x80
0x2000 => [c0, 00] => 0x2000
0x3fff => [ff, 7f] => 0x3fff
0x4000 => [81, 80, 00] => 0x4000
0x1fffff => [ff, ff, 7f] => 0x1fffff
0x200000 => [81, 80, 80, 00] => 0x200000
0x8000000 => [c0, 80, 80, 00] => 0x8000000
0xfffffff => [ff, ff, ff, 7f] => 0xfffffff
0xffffffff => [8f, ff, ff, ff, 7f] => 0xffffffff
0x842ffffffff => [82, 88, af, ff, ff, ff, 7f] => 0x842ffffffff
0xfffffffffffffff => [8f, ff, ff, ff, ff, ff, ff, ff, 7f] => 0xfffffffffffffff

Seed7[edit]

The example below uses bigInteger numbers, since variable-length quantities are able to represent integer numbers of unlimited size.

$ include "seed7_05.s7i";
include "bigint.s7i";
 
const func string: toSequence (in var bigInteger: number) is func
result
var string: sequence is "";
begin
sequence := str(chr(ord(number mod 128_)));
number >>:= 7;
while number <> 0_ do
sequence := str(chr(ord(number mod 128_) + 128)) & sequence;
number >>:= 7;
end while;
end func;
 
const func bigInteger: fromSequence (in string: sequence) is func
result
var bigInteger: number is 0_;
local
var integer: index is 1;
begin
while ord(sequence[index]) >= 128 do
number <<:= 7;
number +:= bigInteger conv (ord(sequence[index]) - 128);
incr(index);
end while;
number <<:= 7;
number +:= bigInteger conv ord(sequence[index]);
end func;
 
const proc: main is func
local
const array bigInteger: testValues is [] (
0_, 10_, 123_, 254_, 255_, 256_, 257_, 65534_, 65535_, 65536_, 65537_, 2097151_, 2097152_);
var string: sequence is "";
var bigInteger: testValue is 0_;
var char: element is ' ';
begin
for testValue range testValues do
sequence := toSequence(testValue);
write("sequence from " <& testValue <& ": [ ");
for element range sequence do
write(ord(element) radix 16 lpad0 2 <& " ");
end for;
writeln("] back: " <& fromSequence(sequence));
end for;
end func;

Output:

sequence from 0: [ 00 ] back: 0
sequence from 10: [ 0a ] back: 10
sequence from 123: [ 7b ] back: 123
sequence from 254: [ 81 7e ] back: 254
sequence from 255: [ 81 7f ] back: 255
sequence from 256: [ 82 00 ] back: 256
sequence from 257: [ 82 01 ] back: 257
sequence from 65534: [ 83 ff 7e ] back: 65534
sequence from 65535: [ 83 ff 7f ] back: 65535
sequence from 65536: [ 84 80 00 ] back: 65536
sequence from 65537: [ 84 80 01 ] back: 65537
sequence from 2097151: [ ff ff 7f ] back: 2097151
sequence from 2097152: [ 81 80 80 00 ] back: 2097152

Sidef[edit]

Translation of: Perl 6
func vlq_encode(num) {
var t = (0x7F & num)
var str = t.chr
while (num >>= 7) {
t = (0x7F & num)
str += chr(0x80 | t)
}
str.reverse
}
 
func vlq_decode(str) {
var num = ''
str.each_byte { |b|
num += ('%07b' % (b & 0x7F))
}
Num(num, 2)
}
 
var tests = [0, 0xa, 123, 254, 255, 256,
257, 65534, 65535, 65536, 65537, 0x1fffff,
0x200000]
 
tests.each { |t|
var vlq = vlq_encode(t)
printf("%8s %12s %8s\n", t,
vlq.bytes.join(':', { "%02X" % _ }), vlq_decode(vlq))
}
Output:
       0           00        0
      10           0A       10
     123           7B      123
     254        81:7E      254
     255        81:7F      255
     256        82:00      256
     257        82:01      257
   65534     83:FF:7E    65534
   65535     83:FF:7F    65535
   65536     84:80:00    65536
   65537     84:80:01    65537
 2097151     FF:FF:7F  2097151
 2097152  81:80:80:00  2097152

Tcl[edit]

package require Tcl 8.5
 
proc vlqEncode number {
if {$number < 0} {error "negative not supported"}
while 1 {
lappend digits [expr {$number & 0x7f}]
if {[set number [expr {$number >> 7}]] == 0} break
}
set out [format %c [lindex $digits 0]]
foreach digit [lrange $digits 1 end] {
set out [format %c%s [expr {0x80+$digit}] $out]
}
return $out
}
proc vlqDecode chars {
set n 0
foreach c [split $chars ""] {
scan $c %c c
set n [expr {($n<<7) | ($c&0x7f)}]
if {!($c&0x80)} break
}
return $n
}

Demo code:

proc numtohex {num} {
binary scan [string trimleft [binary format W $num] \0] H* hexEncoded
regsub -all "..(?=.)" $hexEncoded "&:"
}
proc strtohex {string} {
binary scan $string H* hexEncoded
regsub -all "..(?=.)" $hexEncoded "&:"
}
foreach testcase {
123
254 255 256 257
65534 65535 65536 65537
2097152 2097151
12345678901234566789
} {
set encoded [vlqEncode $testcase]
binary scan $encoded H* hexEncoded
regsub -all {..(?=.)} $hexEncoded &: hexEncoded
set decoded [vlqDecode $encoded]
puts "$testcase ([numtohex $testcase]) ==>\
[strtohex $encoded] ([string length $encoded] bytes) ==>\
$decoded"

}

Output:

123 (7b) ==> 7b (1 bytes) ==> 123
254 (fe) ==> 81:7e (2 bytes) ==> 254
255 (ff) ==> 81:7f (2 bytes) ==> 255
256 (01:00) ==> 82:00 (2 bytes) ==> 256
257 (01:01) ==> 82:01 (2 bytes) ==> 257
65534 (ff:fe) ==> 83:ff:7e (3 bytes) ==> 65534
65535 (ff:ff) ==> 83:ff:7f (3 bytes) ==> 65535
65536 (01:00:00) ==> 84:80:00 (3 bytes) ==> 65536
65537 (01:00:01) ==> 84:80:01 (3 bytes) ==> 65537
2097152 (20:00:00) ==> 81:80:80:00 (4 bytes) ==> 2097152
2097151 (1f:ff:ff) ==> ff:ff:7f (3 bytes) ==> 2097151
12345678901234566789 (ab:54:a9:8c:eb:1f:06:85) ==> 81:ab:aa:aa:b1:ce:d8:fc:8d:05 (10 bytes) ==> 12345678901234566789

TXR[edit]

TXR's carray type, closely associated with the Foreign Function Interface, has functions for converting between integers and foreign arrays. The arrays can use any element type. The integer is stored in big endian order, and "right justified" within the buffer, so that its least significant byte is aligned with the least significant byte of the last element of the array.

Two representations are supported: unsigned and signed. The unsigned representation takes only non-negative integers. It is a straightforward pure binary enumeration. The signed representation uses twos complement. The most significant byte of the array representation is in the range 80-FF if the value is negative, otherwise in the range 0 to 7F. This means that in some cases, a zero byte has to be added.

Interactive session:

1> (carray-num #x200000)
#<carray 3 #<ffi-type uchar>>
2> (carray-get *1)
#(32 0 0)
3> (carray-num #x1FFFFF)
#<carray 3 #<ffi-type uchar>>
4> (carray-get *3)
#(31 255 255)
5> (num-carray *1)
2097152
6> (num-carray *3)
2097151

Conversion to a carray not based on the default uchar:

1> (carray-num #x123456789 (ffi uint32))
#<carray 2 #<ffi-type uint32>>
2> (carray-get *1)
#(16777216 2305246499)

This number requires two 32-bit units to store. Because uint32 is in the native endian, opposite to the big endian storage of the integer, the words come out byte swapped. The be-uint32 type could be used to change this.

zkl[edit]

Translation of: C
fcn to_seq(x){ //--> list of ints
z:=(x.log2()/7);
(0).pump(z+1,List,'wrap(j){
x.shiftRight((z-j)*7).bitAnd(0x7f).bitOr((j!=z) and 0x80 or 0)
});
}
 
fcn from_seq(in){ in.reduce(fcn(p,n){ p.shiftLeft(7).bitOr(n.bitAnd(0x7f)) },0) }
ns:=T(0x7f, 0x4000, 0, 0x3ffffe, 0x1fffff, 0x200000, 0x3311a1234df31413);
ms:=ns.apply(to_seq);
ns.zipWith(fcn{"%8,x --> %s --> %,x".fmt(vm.arglist.xplode()).println()},
ms.apply("apply","%,x".fmt),
ms.apply(from_seq));
Output:
      7f --> L("7f") --> 7f
   40|00 --> L("81","80","0") --> 40|00
       0 --> L("0") --> 0
3f|ff|fe --> L("81","ff","ff","7e") --> 3f|ff|fe
1f|ff|ff --> L("ff","ff","7f") --> 1f|ff|ff
20|00|00 --> L("81","80","80","0") --> 20|00|00
33|11|a1|23|4d|f3|14|13 --> L("b3","88","e8","a4","b4","ef","cc","a8","13")
   --> 33|11|a1|23|4d|f3|14|13

Note: the strings in the output are numbers formatted to hex (ie to_seq returns a list of ints). A "|" is used between bytes for ease of reading.