Find Chess960 starting position identifier

From Rosetta Code
Task
Find Chess960 starting position identifier
You are encouraged to solve this task according to the task description, using any language you may know.

As described on the Chess960 page, Chess960 (a.k.a Fischer Random Chess, Chess9LX) is a variant of chess where the array of pieces behind the pawns is randomized at the start of the game to minimize the value of opening theory "book knowledge". That task is to generate legal starting positions, and some of the solutions accept a standard Starting Position Identifier number ("SP-ID"), and generate the corresponding position.

Task

This task is to go the other way: given a starting array of pieces (provided in any form that suits your implementation, whether string or list or array, of letters or Unicode chess symbols or enum values, etc.), derive its unique SP-ID. For example, given the starting array QNRBBNKR (or ♕♘♖♗♗♘♔♖ or ♛♞♜♝♝♞♚♜), which we assume is given as seen from White's side of the board from left to right, your (sub)program should return 105; given the starting lineup of standard chess, it should return 518.

You may assume the input is a valid Chess960 position; detecting invalid input (including illegal characters or starting arrays with the bishops on the same color square or the king not between the two rooks) is optional.

Algorithm

The derivation is the inverse of the algorithm given at Wikipedia, and goes like this (we'll use the standard chess setup as an example).

1. Ignoring the Queen and Bishops, find the positions of the Knights within the remaining five spaces (in the standard array they're in the second and fourth positions), and then find the index number of that combination. There's a table at the above Wikipedia article, but it's just the possible positions sorted left to right and numbered 0 to 9: 0=NN---, 1=N-N--, 2=N--N-, 3=N---N, 4=-NN--, etc; our pair is combination number 5. Call this number N. N=5

2. Still ignoring the Bishops, find the position of the Queen in the remaining 6 spaces; number them 0..5 from left to right and call the index of the Queen's position Q. In our example, Q=2.

3. Finally, find the positions of the two bishops within their respective sets of four like-colored squares. It's important to note here that the board in chess is placed such that the leftmost position on the home row is on a dark square and the rightmost a light. So if we number the squares of each color 0..3 from left to right, the dark bishop in the standard position is on square 1 (D=1), and the light bishop is on square 2 (L=2).

4. Then the position number is given by 4(4(6N + Q)+D)+L, which reduces to 96N + 16Q + 4D + L. In our example, that's 96×5 + 16×2 + 4×1 + 2 = 480 + 32 + 4 + 2 = 518.

Note that an earlier iteration of this page contained an incorrect description of the algorithm which would give the same SP-ID for both of the following two positions.

   RQNBBKRN = 601
   RNQBBKRN = 617

BASIC[edit]

Commodore BASIC[edit]

Works with: Commodore BASIC version 2.0

Unlike the solution for the reverse task, which uses DO/LOOP and so requires at least Commodore BASIC 3.5, this should work on any version.

100 REM DERIVE SP-ID FROM CHESS960 POS
110 READ A$: IF A$="" THEN END
120 PRINT A$":";
130 GOSUB 170
140 PRINT SP
150 GOTO 110
160 DATA QNRBBNKR, RNBQKBNR, RQNBBKRN, RNQBBKRN,
170 IF LEN(A$)=8 THEN 190
180 PRINT "ARRAY MUST BE 8 PIECES.": SP=-1: RETURN
190 K=0:Q=0:B=0:N=0:R=0
200 FOR I=0 TO 7
210 : K(I)=0:Q(I)=0:B(I)=0:N(I)=0:R(I)=0
220 NEXT I
230 FOR I=1 TO 8
240 : P$=MID$(A$,I,1)
250 : IF P$="Q" THEN Q(Q)=I: Q=Q+1:  GOTO 310
260 : IF P$="K" THEN K(K)=I: K=K+1:  GOTO 310
270 : IF P$="B" THEN B(B)=I: B=B+1:  GOTO 310
280 : IF P$="N" THEN N(N)=I: N=N+1:  GOTO 310
290 : IF P$="R" THEN R(R)=I: R=R+1:  GOTO 310
300 : PRINT "ILLEGAL PIECE '"P$"'.": SP=-1: RETURN
310 NEXT I
320 IF K<>1 THEN PRINT "THERE MUST BE EXACTLY ONE KING.":    SP=-1: RETURN
330 IF Q<>1 THEN PRINT "THERE MUST BE EXACTLY ONE QUEEN.":   SP=-1: RETURN
340 IF B<>2 THEN PRINT "THERE MUST BE EXACTLY TWO BISHOPS.": SP=-1: RETURN
350 IF N<>2 THEN PRINT "THERE MUST BE EXACTLY TWO KNIGHTS.": SP=-1: RETURN
360 IF R<>2 THEN PRINT "THERE MUST BE EXACTLY TWO ROOKS.":   SP=-1: RETURN
370 IF (K(0) > R(0)) AND (K(0) < R(1)) THEN 390
380 PRINT "KING MUST BE BETWEEN THE ROOKS.": SP=-1: RETURN
390 IF (B(0) AND 1) <> (B(1) AND 1) THEN 410
400 PRINT "BISHOPS MUST BE ON OPPOSITE COLORS.": SP=-1: RETURN
410 FOR I=0 TO 1
420 : N=N(I)
430 : IF N(I)>Q(I) THEN N=N-1
440 : FOR J=0 TO 1
450 :  IF N(I)>B(J) THEN N=N-1
460 : NEXT J
470 : N(I)=N
480 NEXT I
490 N0=1: N1=2
500 FOR N=0 TO 9
510 : IF N0=N(0) AND N1=N(1) THEN 550
520 : N1=N1+1
530 : IF N1>5 THEN N0=N0+1: N1=N0+1
540 NEXT N
550 Q=Q(0)-1
560 FOR I=0 TO 1
570 : IF Q(0)>B(I) THEN Q=Q-1
580 NEXT I
590 FOR I=0 TO 1
600 : B=B(I)-1
610 : IF B AND 1 THEN L=INT(B/2)
620 : IF (B AND 1)=0 THEN D=B/2
630 NEXT I
640 SP = 96*N+16*Q+4*D+L
650 RETURN
Output:
READY.
RUN
QNRBBNKR: 105
RNBQKBNR: 518
RQNBBKRN: 601
RNQBBKRN: 617

READY.

FreeBASIC[edit]

Sub SP_ID(PosicPiezas As String)
    Dim As String pieza
    Dim As Integer pQ(), pK(), pB(), pN(), pR(), i, j
    Dim As Integer Q, K, B, N, R, L, D
    
    For i = 1 To 8
        pieza = Mid(PosicPiezas, i, 1)
        Select Case pieza
        Case "Q"
            Redim Preserve pQ(Q) : pQ(Q) = i: Q += 1
        Case "K"
            Redim Preserve pK(K) : pK(K) = i: K += 1
        Case "B"
            Redim Preserve pB(B) : pB(B) = i: B += 1
        Case "N"
            Redim Preserve pN(N) : pN(N) = i: N += 1
        Case "R"
            Redim Preserve pR(R) : pR(R) = i: R += 1
        Case Else 
            Print "ILLEGAL PIECE '"; pieza; "'.": Exit Sub
        End Select
    Next i
    
    If K <> 1 Then Print "THERE MUST BE EXACTLY ONE KING."
    If Q <> 1 Then Print "THERE MUST BE EXACTLY ONE QUEEN."
    If B <> 2 Then Print "THERE MUST BE EXACTLY TWO BISHOPS."
    If N <> 2 Then Print "THERE MUST BE EXACTLY TWO KNIGHTS."
    If R <> 2 Then Print "THERE MUST BE EXACTLY TWO ROOKS."
    If Not (pK(0) > pR(0)) And (pK(0) < pR(1)) Then Print "KING MUST BE BETWEEN THE ROOKS."
    If Not (pB(0) And 1) <> (pB(1) And 1) Then Print "BISHOPS MUST BE ON OPPOSITE COLORS."

    For i = 0 To 1
        N = pN(i)
        If pN(i) > pQ(i) Then N -= 1
        For j = 0 To 1
            If pN(i) > pB(j) Then N -= 1
        Next j
        pN(i) = N
    Next i

    Dim As Integer N0 = 1, N1 = 2
    For N = 0 To 9
        If N0 = pN(0) And N1 = pN(1) Then Exit For
        N1 += 1
        If N1 > 5 Then N0 += 1: N1 = N0 + 1
    Next N

    Q = pQ(0) - 1
    For i = 0 To 1
        If pQ(0) > pB(i) Then Q -= 1
    Next i
    For i = 0 To 1
        B = pB(i) - 1
        If B And 1 Then L = Int(B / 2)
        If (B And 1) = 0 Then D = B / 2
    Next i
    
    Print PosicPiezas; " has SP_ID of"; 96 * N + 16 * Q + 4 * D + L
End Sub

SP_ID("QNRBBNKR")
Print
SP_ID("RNBQKBNR")
Print
SP_ID("RQNBBKRN")
Print
SP_ID("RNQBBKRN")
Sleep
Output:
QNRBBNKR has SP_ID of 105

RNBQKBNR has SP_ID of 518

RQNBBKRN has SP_ID of 601

RNQBBKRN has SP_ID of 617

QBasic[edit]

Works with: QBasic version 1.1
Translation of: Commodore BASIC
Cls
Print "Enter start array as seen by white."
120 Print
Print "Starting array";
Input Ar$
Print
If Len(Ar$) = 0 Then End
If Len(Ar$) = 8 Then 170
Print "Array must be 8 pieces.": GoTo 120

170 For I = 1 To 8
    P$ = Mid$(Ar$, I, 1)
    If P$ = "Q" Or P$ = "q" Then Q(Q) = I: Q = Q + 1: GoTo 250
    If P$ = "K" Or P$ = "k" Then K(K) = I: K = K + 1: GoTo 250
    If P$ = "B" Or P$ = "b" Then B(B) = I: B = B + 1: GoTo 250
    If P$ = "N" Or P$ = "n" Then N(N) = I: N = N + 1: GoTo 250
    If P$ = "R" Or P$ = "r" Then R(R) = I: R = R + 1: GoTo 250
    Print "Illegal piece '"; P$; "'.": GoTo 120
250 Next I

If K <> 1 Then Print "There must be exactly one King.": GoTo 120
If Q <> 1 Then Print "There must be exactly one Queen.": GoTo 120
If B <> 2 Then Print "There must be exactly two Bishops.": GoTo 120
If N <> 2 Then Print "There must be exactly two Knights.": GoTo 120
If R <> 2 Then Print "There must be exactly two Rooks.": GoTo 120
If (K(0) > R(0)) And (K(0) < R(1)) Then 330
Print "King must be between the Rooks.": GoTo 120

330 If (B(0) And 1) <> (B(1) And 1) Then 350
Print "Bishops must be on opposite colors.": GoTo 120

350 For I = 0 To 1
    N = N(I)
    If N(I) > Q(I) Then N = N - 1
    For J = 0 To 1
        If N(I) > B(J) Then N = N - 1
    Next J
    N(I) = N
Next I
N0 = 1: N1 = 2

For N = 0 To 9
    If N0 = N(0) And N1 = N(1) Then 490
    N1 = N1 + 1
    If N1 > 5 Then N0 = N0 + 1: N1 = N0 + 1
Next N
490 Q = Q(0) - 1

For I = 0 To 1
    If Q(0) > B(I) Then Q = Q - 1
Next I

For I = 0 To 1
    B = B(I) - 1
    If B And 1 Then L = Int(B / 2)
    If (B And 1) = 0 Then D = B / 2
Next I
Print "SP-ID ="; 96 * N + 16 * Q + 4 * D + L
End
Output:
Enter start array as seen by White.

Starting array? qnrbbnkr

SP-ID = 105

Starting array? RNBQKBNR

SP-ID = 518

Starting array? RQNBBKRN

SP-ID = 601

Starting array? RNQBBKRN

SP-ID = 617

Common Lisp[edit]

; make sure string is a valid Chess960 starting array
(defun valid-array-p (start-array)
   (and (string-equal (sort (copy-seq start-array) #'string-lessp) "BBKNNQRR") ; right pieces
        (not (equal (mod (position #\B start-array) 2)                         ; bishops on opposite colors
                    (mod (position #\B start-array :from-end t) 2)))
        (< (position #\R start-array) (position #\K start-array))              ; king between two rooks
        (< (position #\K start-array) (position #\R start-array :from-end t))))

; find Start Position IDentifier for a Chess960 setup
(defun sp-id (start-array)
   (if (not (valid-array-p start-array))
      -1
     (let* ((bishopless   (remove #\B start-array))
            (queenless    (remove #\Q bishopless))
            (n5n-pattern  (substitute-if-not #\- (lambda (ch) (eql ch #\N)) queenless))
            (n5n-table   '("NN---" "N-N--" "N--N-" "N---N" "-NN--" "-N-N-" "-N--N" "--NN-" "--N-N" "---NN"))
            (knights      (position n5n-pattern n5n-table :test #'string-equal))
            (queen        (position #\Q bishopless))
            (left-bishop  (position #\B start-array))
            (right-bishop (position #\B start-array :from-end t)))

       ; map each bishop to its color complex and position within those four squares
       (destructuring-bind (dark-bishop light-bishop)
         (mapcar (lambda (p) (floor p 2))
           (cond ((zerop (mod left-bishop 2)) (list left-bishop  right-bishop))
                  (t                          (list right-bishop left-bishop))))

           (+ (* 96 knights) (* 16 queen) (* 4 dark-bishop) light-bishop)))))

(loop for ary in '("RNBQKBNR" "QNRBBNKR" "RQNBBKRN" "RNQBBKRN") doing
  (format t "~a: ~a~%" ary (sp-id ary)))
Output:
RNBQKBNR: 518
QNRBBNKR: 105
RQNBBKRN: 601
RNQBBKRN: 617

Factor[edit]

Works with: Factor version 0.99 2021-06-02
USING: assocs assocs.extras combinators formatting kernel
literals math math.combinatorics sequences sequences.extras sets
strings ;

IN: scratchpad

! ====== optional error-checking ======

: check-length ( str -- )
    length 8 = [ "Must have 8 pieces." throw ] unless ;

: check-one ( str -- )
    "KQ" counts [ nip 1 = not ] assoc-find nip
    [ 1string "Must have one %s." sprintf throw ] [ drop ] if ;

: check-two ( str -- )
    "BNR" counts [ nip 2 = not ] assoc-find nip
    [ 1string "Must have two %s." sprintf throw ] [ drop ] if ;

: check-king ( str -- )
    "QBN" without "RKR" =
    [ "King must be between rooks." throw ] unless ;

: check-bishops ( str -- )
    CHAR: B swap indices sum odd?
    [ "Bishops must be on opposite colors." throw ] unless ;

: check-sp ( str -- )
    {
        [ check-length ]
        [ check-one ]
        [ check-two ]
        [ check-king ]
        [ check-bishops ]
    } cleave ;

! ====== end optional error-checking ======


CONSTANT: convert $[ "RNBQK" "♖♘♗♕♔" zip ]

CONSTANT: table $[ "NN---" all-unique-permutations ]

: knightify ( str -- newstr )
    [ dup CHAR: N = [ drop CHAR: - ] unless ] map ;

: n ( str -- n ) "QB" without knightify table index ;

: q ( str -- q ) "B" without CHAR: Q swap index ;

: d ( str -- d ) CHAR: B swap <evens> index ;

: l ( str -- l ) CHAR: B swap <odds> index ;

: sp-id ( str -- n )
    dup check-sp
    { [ n 96 * ] [ q 16 * + ] [ d 4 * + ] [ l + ] } cleave ;

: sp-id. ( str -- )
    dup [ convert substitute ] [ sp-id ] bi
    "%s / %s: %d\n" printf ;

"QNRBBNKR" sp-id.
"RNBQKBNR" sp-id.
"RQNBBKRN" sp-id.
"RNQBBKRN" sp-id.
Output:
QNRBBNKR / ♕♘♖♗♗♘♔♖: 105
RNBQKBNR / ♖♘♗♕♔♗♘♖: 518
RQNBBKRN / ♖♕♘♗♗♔♖♘: 601
RNQBBKRN / ♖♘♕♗♗♔♖♘: 617

Go[edit]

Translation of: Wren
package main

import (
    "fmt"
    "log"
    "strings"
)

var glyphs = []rune("♜♞♝♛♚♖♘♗♕♔")
var names = map[rune]string{'R': "rook", 'N': "knight", 'B': "bishop", 'Q': "queen", 'K': "king"}
var g2lMap = map[rune]string{
    '♜': "R", '♞': "N", '♝': "B", '♛': "Q", '♚': "K",
    '♖': "R", '♘': "N", '♗': "B", '♕': "Q", '♔': "K",
}

var ntable = map[string]int{"01": 0, "02": 1, "03": 2, "04": 3, "12": 4, "13": 5, "14": 6, "23": 7, "24": 8, "34": 9}

func g2l(pieces string) string {
    lets := ""
    for _, p := range pieces {
        lets += g2lMap[p]
    }
    return lets
}

func spid(pieces string) int {
    pieces = g2l(pieces) // convert glyphs to letters

    /* check for errors */
    if len(pieces) != 8 {
        log.Fatal("There must be exactly 8 pieces.")
    }
    for _, one := range "KQ" {
        count := 0
        for _, p := range pieces {
            if p == one {
                count++
            }
        }
        if count != 1 {
            log.Fatalf("There must be one %s.", names[one])
        }
    }
    for _, two := range "RNB" {
        count := 0
        for _, p := range pieces {
            if p == two {
                count++
            }
        }
        if count != 2 {
            log.Fatalf("There must be two %s.", names[two])
        }
    }
    r1 := strings.Index(pieces, "R")
    r2 := strings.Index(pieces[r1+1:], "R") + r1 + 1
    k := strings.Index(pieces, "K")
    if k < r1 || k > r2 {
        log.Fatal("The king must be between the rooks.")
    }
    b1 := strings.Index(pieces, "B")
    b2 := strings.Index(pieces[b1+1:], "B") + b1 + 1
    if (b2-b1)%2 == 0 {
        log.Fatal("The bishops must be on opposite color squares.")
    }

    /* compute SP_ID */
    piecesN := strings.ReplaceAll(pieces, "Q", "")
    piecesN = strings.ReplaceAll(piecesN, "B", "")
    n1 := strings.Index(piecesN, "N")
    n2 := strings.Index(piecesN[n1+1:], "N") + n1 + 1
    np := fmt.Sprintf("%d%d", n1, n2)
    N := ntable[np]

    piecesQ := strings.ReplaceAll(pieces, "B", "")
    Q := strings.Index(piecesQ, "Q")

    D := strings.Index("0246", fmt.Sprintf("%d", b1))
    L := strings.Index("1357", fmt.Sprintf("%d", b2))
    if D == -1 {
        D = strings.Index("0246", fmt.Sprintf("%d", b2))
        L = strings.Index("1357", fmt.Sprintf("%d", b1))
    }

    return 96*N + 16*Q + 4*D + L
}

func main() {
    for _, pieces := range []string{"♕♘♖♗♗♘♔♖", "♖♘♗♕♔♗♘♖", "♖♕♘♗♗♔♖♘", "♖♘♕♗♗♔♖♘"} {
        fmt.Printf("%s or %s has SP-ID of %d\n", pieces, g2l(pieces), spid(pieces))
    }
}
Output:
♕♘♖♗♗♘♔♖ or QNRBBNKR has SP-ID of 105
♖♘♗♕♔♗♘♖ or RNBQKBNR has SP-ID of 518
♖♕♘♗♗♔♖♘ or RQNBBKRN has SP-ID of 601
♖♘♕♗♗♔♖♘ or RNQBBKRN has SP-ID of 617

J[edit]

Implementation:
REF=: {{
  'N Q B0 B1'=. 0 6 4 4 #: y
  s=. 'B' (0 1+2*B0,B1)} 8#' '
  s=. 'Q' (Q{I.' '=s)} s
  s=. 'N' ((N{(#~ 2=+/"1)#:i.-32){&I.' '=s)} s
  'RKR' (I.' '=s)} s
}}"0 i.960

c960=: {{ r=. REF i. rplc&((u:9812+i.12);&>12$'KQRBNP') 7 u:deb y assert. r<#REF }}

Examples:

   c960'♕♘♖♗♗♘♔♖'
105
   c960'♛♞♜♝♝♞♚♜'
105
   c960'RNBQKBNR'
518
   c960'RQNBBKRN'
601
   c960'RNQBBKRN'
617

Julia[edit]

const whitepieces = "♖♘♗♕♔♗♘♖♙"
const whitechars = "rnbqkp"
const blackpieces = "♜♞♝♛♚♝♞♜♟"
const blackchars = "RNBQKP"
const piece2ascii = Dict(zip("♖♘♗♕♔♗♘♖♙♜♞♝♛♚♝♞♜♟", "rnbqkbnrpRNBQKBNRP"))

""" Derive a chess960 position's SP-ID from its string representation. """
function chess960spid(position::String = "♖♘♗♕♔♗♘♖", errorchecking = true)
    if errorchecking
        @assert length(position) == 8 "Need exactly 8 pieces"
        @assert all(p -> p in whitepieces || p in blackpieces, position) "Invalid piece character"
        @assert all(p -> p in whitepieces, position) || all(p -> p in blackpieces, position) "Side of pieces is mixed"
        @assert all(p -> !(p in "♙♟"), position) "No pawns allowed"
    end
    a = uppercase(String([piece2ascii[c] for c in position]))

    if errorchecking
        @assert all(p -> count(x -> x == p, a) == 1, "KQ") "Need exactly one of each K and Q"
        @assert all(p -> count(x -> x == p, a) == 2, "RNB") "Need exactly 2 of each R, N, B"
        @assert findfirst(p -> p == 'R', a) < findfirst(p -> p == 'K', a) < findlast(p -> p == 'R', a) "King must be between rooks"
        @assert isodd(findfirst(p -> p == 'B', a) + findlast(p -> p == 'B', a)) "Bishops must be on different colors"
    end

    knighttable = [12, 13, 14, 15, 23, 24, 25, 34, 35, 45]
    noQB = replace(a, r"[QB]" => "")
    knightpos1, knightpos2 = findfirst(c -> c =='N', noQB), findlast(c -> c =='N', noQB)
    N = findfirst(s -> s == 10 * knightpos1 + knightpos2, knighttable) - 1
    Q = findfirst(c -> c == 'Q', replace(a, "B" => "")) - 1
    bishoppositions = [findfirst(c -> c =='B', a), findlast(c -> c =='B', a)]
    if isodd(bishoppositions[2])
        bishoppositions = reverse(bishoppositions) # dark color bishop first
    end
    D, L = bishoppositions[1] ÷ 2, bishoppositions[2] ÷ 2 - 1

    return 96N + 16Q + 4D + L
end

for position in ["♕♘♖♗♗♘♔♖", "♖♘♗♕♔♗♘♖", "♖♕♘♗♗♔♖♘", "♖♘♕♗♗♔♖♘"]
    println(collect(position), " => ", chess960spid(position))
end
Output:
['♕', '♘', '♖', '♗', '♗', '♘', '♔', '♖'] => 105
['♖', '♘', '♗', '♕', '♔', '♗', '♘', '♖'] => 518
['♖', '♕', '♘', '♗', '♗', '♔', '♖', '♘'] => 601
['♖', '♘', '♕', '♗', '♗', '♔', '♖', '♘'] => 617

Nim[edit]

Translation of: Wren
import sequtils, strformat, strutils, sugar, tables, unicode

type Piece {.pure.} = enum Rook = "R", Knight = "N", Bishop = "B", Queen = "Q", King = "K"

const
  GlypthToPieces = {"♜": Rook, "♞": Knight, "♝": Bishop, "♛": Queen, "♚": King,
                    "♖": Rook, "♘": Knight, "♗": Bishop, "♕": Queen, "♔": King}.toTable
  Names = [Rook: "rook", Knight: "knight", Bishop: "bishop", Queen: "queen", King: "king"]
  NTable = {[0, 1]: 0, [0, 2]: 1, [0, 3]: 2, [0, 4]: 3, [1, 2]: 4,
            [1, 3]: 5, [1, 4]: 6, [2, 3]: 7, [2, 4]: 8, [3, 4]: 9}.toTable

func toPieces(glyphs: string): seq[Piece] =
  collect(newSeq, for glyph in glyphs.runes: GlypthToPieces[glyph.toUTF8])

func isEven(n: int): bool = (n and 1) == 0

func positions(pieces: seq[Piece]; piece: Piece): array[2, int] =
  var idx = 0
  for i, p in pieces:
    if p == piece:
      result[idx] = i
      inc idx

func spid(glyphs: string): int =

  let pieces = glyphs.toPieces()

  # Check for errors.
  if pieces.len != 8:
    raise newException(ValueError, "there must be exactly 8 pieces.")
  for piece in [King, Queen]:
    if pieces.count(piece) != 1:
      raise newException(ValueError, &"there must be one {Names[piece]}.")
  for piece in [Rook, Knight, Bishop]:
    if pieces.count(piece) != 2:
      raise newException(ValueError, &"there must be two {Names[piece]}s.")
  let r = pieces.positions(Rook)
  let k = pieces.find(King)
  if k < r[0] or k > r[1]:
    raise newException(ValueError, "the king must be between the rooks.")
  var b = pieces.positions(Bishop)
  if isEven(b[1] - b[0]):
    raise newException(ValueError, "the bishops must be on opposite color squares.")

  # Compute SP_ID.
  let piecesN = pieces.filterIt(it notin [Queen, Bishop])
  let n = NTable[piecesN.positions(Knight)]

  let piecesQ = pieces.filterIt(it != Bishop)
  let q = piecesQ.find(Queen)

  if b[1].isEven: swap b[0], b[1]
  let d = [0, 2, 4, 6].find(b[0])
  let l = [1, 3, 5, 7].find(b[1])

  result = 96 * n + 16 * q + 4 * d + l


for glyphs in ["♕♘♖♗♗♘♔♖", "♖♘♗♕♔♗♘♖", "♖♕♘♗♗♔♖♘", "♖♘♕♗♗♔♖♘"]:
  echo &"{glyphs} or {glyphs.toPieces().join()} has SP-ID of {glyphs.spid()}"
Output:
♕♘♖♗♗♘♔♖ or QNRBBNKR has SP-ID of 105
♖♘♗♕♔♗♘♖ or RNBQKBNR has SP-ID of 518
♖♕♘♗♗♔♖♘ or RQNBBKRN has SP-ID of 601
♖♘♕♗♗♔♖♘ or RNQBBKRN has SP-ID of 617

Perl[edit]

Translation of: Raku
use v5.36;
use List::AllUtils 'indexes';

sub sp_id ($setup) {
    8 == length $setup                          or return 'Illegal position: should have exactly eight pieces';
    1 == @{[ $setup =~ /$_/g ]}                 or return "Illegal position: should have exactly one $_"        for <K Q>;
    2 == @{[ $setup =~ /$_/g ]}                 or return "Illegal position: should have exactly two $_\'s"     for <B N R>;
    $setup =~ m/R .* K .* R/x                   or return 'Illegal position: King not between rooks.';
    index($setup,'B')%2 != rindex($setup,'B')%2 or return 'Illegal position: Bishops not on opposite colors.';

    my @knights = indexes { 'N' eq $_ } split '', $setup =~ s/[QB]//gr;
    my $knight  = indexes { join('', @knights) eq $_ } <01 02 03 04 12 13 14 23 24 34>; # combinations(5,2)
    my @bishops = indexes { 'B' eq $_ } split '', $setup;
    my $dark    = int ((grep { $_ % 2 == 0 } @bishops)[0]) / 2;
    my $light   = int ((grep { $_ % 2 == 1 } @bishops)[0]) / 2;
    my $queen   = index(($setup =~ s/B//gr), 'Q');

    int 4*(4*(6*$knight + $queen)+$dark)+$light;
}

say "$_ " . sp_id($_) for <QNRBBNKR RNBQKBNR RQNBBKRN RNQBBKRN QNBRBNKR>;
Output:
QNRBBNKR 105
RNBQKBNR 518
RQNBBKRN 601
RNQBBKRN 617
QNBRBNKR Illegal position: Bishops not on opposite colors.

Phix[edit]

with javascript_semantics
function spid(string s)
    if sort(s)!="BBKNNQRR" then return -1 end if
    if filter(s,"in","RK")!="RKR" then return -1 end if
    sequence b = find_all('B',s)
    if even(sum(b)) then return -1 end if
    integer {n1,n2} = find_all('N',filter(s,"out","QB")),
            N = {-2,1,3,4}[n1]+n2,
            Q = find('Q',filter(s,"!=",'B'))-1,
            D = filter(b,odd)[1]-1, -- (nb not /2)
            L = filter(b,even)[1]/2-1
    return 96*N + 16*Q + 2*D + L
end function

procedure test(string s)
    printf(1,"%s : %d\n",{s,spid(s)})
end procedure

test("QNRBBNKR")
test("RNBQKBNR")
test("RQNBBKRN")
test("RNQBBKRN")
Output:
QNRBBNKR : 105
RNBQKBNR : 518
RQNBBKRN : 601
RNQBBKRN : 617

To support all those crazy unicode characters just change the start of spid() to:

function spid(string u)
    sequence u32 = utf8_to_utf32(u),
             c32 = utf8_to_utf32("♜♞♝♛♚♖♘♗♕♔"),
             s32 = substitute_all(u32,c32,"RNBQKRNBQK")
    string s = utf32_to_utf8(s32)

--... and add:
test("♕♘♖♗♗♘♔♖")
test("♖♘♗♕♔♗♘♖")
test("♜♛♞♝♝♚♜♞")
test("♜♞♛♝♝♚♜♞")
Output:

Note that output on a windows terminal is as expected far from pretty, this is from pwa/p2js

QNRBBNKR : 105
RNBQKBNR : 518
RQNBBKRN : 601
RNQBBKRN : 617
♕♘♖♗♗♘♔♖ : 105
♖♘♗♕♔♗♘♖ : 518
♜♛♞♝♝♚♜♞ : 601
♜♞♛♝♝♚♜♞ : 617

Python[edit]

Works with: Python version 3.10.5 2022-06-28
# optional, but task function depends on it as written
def validate_position(candidate: str):
    assert (
        len(candidate) == 8
    ), f"candidate position has invalide len = {len(candidate)}"

    valid_pieces = {"R": 2, "N": 2, "B": 2, "Q": 1, "K": 1}
    assert {
        piece for piece in candidate
    } == valid_pieces.keys(), f"candidate position contains invalid pieces"
    for piece_type in valid_pieces.keys():
        assert (
            candidate.count(piece_type) == valid_pieces[piece_type]
        ), f"piece type '{piece_type}' has invalid count"

    bishops_pos = [index for index, 
                   value in enumerate(candidate) if value == "B"]
    assert (
        bishops_pos[0] % 2 != bishops_pos[1] % 2
    ), f"candidate position has both bishops in the same color"

    assert [piece for piece in candidate if piece in "RK"] == [
        "R",
        "K",
        "R",
    ], "candidate position has K outside of RR"


def calc_position(start_pos: str):
    try:
        validate_position(start_pos)
    except AssertionError:
        raise AssertionError
    # step 1
    subset_step1 = [piece for piece in start_pos if piece not in "QB"]
    nights_positions = [
        index for index, value in enumerate(subset_step1) if value == "N"
    ]
    nights_table = {
        (0, 1): 0,
        (0, 2): 1,
        (0, 3): 2,
        (0, 4): 3,
        (1, 2): 4,
        (1, 3): 5,
        (1, 4): 6,
        (2, 3): 7,
        (2, 4): 8,
        (3, 4): 9,
    }
    N = nights_table.get(tuple(nights_positions))

    # step 2
    subset_step2 = [piece for piece in start_pos if piece != "B"]
    Q = subset_step2.index("Q")

    # step 3
    dark_squares = [
        piece for index, piece in enumerate(start_pos) if index in range(0, 9, 2)
    ]
    light_squares = [
        piece for index, piece in enumerate(start_pos) if index in range(1, 9, 2)
    ]
    D = dark_squares.index("B")
    L = light_squares.index("B")

    return 4 * (4 * (6*N + Q) + D) + L

if __name__ == '__main__':
    for example in ["QNRBBNKR", "RNBQKBNR", "RQNBBKRN", "RNQBBKRN"]:
        print(f'Position: {example}; Chess960 PID= {calc_position(example)}')
Output:
Position: QNRBBNKR; Chess960 PID= 105
Position: RNBQKBNR; Chess960 PID= 518
Position: RQNBBKRN; Chess960 PID= 601
Position: RNQBBKRN; Chess960 PID= 617

Raku[edit]

sub c960-spid($array) {
    # standardize on letters for easier processing
    my $ascii = $array.trans('♜♞♝♛♚♖♘♗♕♔' => 'RNBQK');

    # error-checking
    my %Names = <Q Queen K King R Rook N Knight B Bishop>;
    return 'Illegal position: should have exactly eight pieces' unless 8 == $ascii.chars;
    return 'Illegal position: Bishops not on opposite colors.'  unless 1 == sum $ascii.indices('B').map(* % 2);
    return 'Illegal position: King not between rooks.'          unless $ascii ~~ /'R' .* 'K' .* 'R'/;
    for <K 1 Q 1 B 2 N 2 R 2> -> $piece, $count {
        return "Illegal position: should have exactly $count %Names{$piece}\(s\)\n" unless $count == $ascii.indices($piece)
    }

    # Work backwards through the placement rules.
    # King and rooks are forced during placement, so ignore them. 

    # 1. Figure out which knight combination was used:
    my @knights = $ascii.subst(/<[QB]>/, '', :g).indices('N');
    my $knight = combinations(5,2).kv.grep( -> $i, @c { @c eq @knights } ).flat.first;

    # 2. Then which queen position:
    my $queen = $ascii.subst('B', '', :g).index('Q'); 

    # 3. Finally the two bishops:
    my @bishops = $ascii.indices('B');
    my ($dark,$light) = (@bishops.first %% 2 ?? @bishops !! @bishops.reverse) Xdiv 2;

    $ascii.trans('RNBQK' => '♖♘♗♕♔') ~ ' ' ~ 4 × (4 × (6 × $knight + $queen) + $dark) + $light;
}

say .&c960-spid for <♖♘♗♕♔♗♘♖ ♛♞♜♝♝♞♚♜ RQNBBKRN RNQBBKRN QNBRBNKR>;
Output:
♖♘♗♕♔♗♘♖ 518
♕♘♖♗♗♘♔♖ 105
♖♕♘♗♗♔♖♘ 601
♖♘♕♗♗♔♖♘ 617
Illegal position: Bishops not on opposite colors.

Ruby[edit]

CHESS_PIECES = %w<♖♘♗♕♔ ♜♞♝♛♚>
def chess960_to_spid(pos)
  start_str = pos.tr(CHESS_PIECES.join, "RNBQKRNBQK")
  #1 knights score
  s = start_str.delete("QB")
  n = [0,1,2,3,4].combination(2).to_a.index( [s.index("N"), s.rindex("N")] )
  #2 queen score
  q = start_str.delete("B").index("Q")
  #3 bishops
  bs = start_str.index("B"), start_str.rindex("B")
  d = bs.detect(&:even?).div(2)
  l = bs.detect(&:odd? ).div(2)

  96*n + 16*q + 4*d + l
end

%w<QNRBBNKR RNBQKBNR RQNBBKRN RNQBBKRN>.each_with_index do |array, i|
  pieces = array.tr("RNBQK", CHESS_PIECES[i%2])
  puts "#{pieces} (#{array}):  #{chess960_to_spid array}"
end
Output:
♕♘♖♗♗♘♔♖ (QNRBBNKR):  105
♜♞♝♛♚♝♞♜ (RNBQKBNR):  518
♖♕♘♗♗♔♖♘ (RQNBBKRN):  601
♜♞♛♝♝♚♜♞ (RNQBBKRN):  617

Wren[edit]

Library: Wren-trait
import "/trait" for Indexed

var glyphs  = "♜♞♝♛♚♖♘♗♕♔".toList
var letters = "RNBQKRNBQK"
var names = { "R": "rook", "N": "knight", "B": "bishop", "Q": "queen", "K": "king" }
var g2lMap = {}
for (se in Indexed.new(glyphs)) g2lMap[glyphs[se.index]] = letters[se.index]

var g2l = Fn.new { |pieces| pieces.reduce("") { |acc, p| acc + g2lMap[p] } }

var ntable = { "01":0, "02":1, "03":2, "04":3, "12":4, "13":5, "14":6, "23":7, "24":8, "34":9 }

var spid = Fn.new { |pieces|
    pieces = g2l.call(pieces) // convert glyphs to letters

    /* check for errors */
    if (pieces.count != 8) Fiber.abort("There must be exactly 8 pieces.")
    for (one in "KQ") {
        if (pieces.count { |p| p == one } != 1 ) Fiber.abort("There must be one %(names[one]).")
    }
    for (two in "RNB") {
        if (pieces.count { |p| p == two } != 2 ) Fiber.abort("There must be two %(names[two])s.")
    }
    var r1 = pieces.indexOf("R")
    var r2 = pieces.indexOf("R", r1 + 1)
    var k  = pieces.indexOf("K")
    if (k < r1 || k > r2) Fiber.abort("The king must be between the rooks.")
    var b1 = pieces.indexOf("B")
    var b2 = pieces.indexOf("B", b1 + 1)
    if ((b2 - b1) % 2 == 0) Fiber.abort("The bishops must be on opposite color squares.")

    /* compute SP_ID */
    var piecesN = pieces.replace("Q", "").replace("B", "")
    var n1 = piecesN.indexOf("N")
    var n2 = piecesN.indexOf("N", n1 + 1)
    var np = "%(n1)%(n2)"
    var N = ntable[np]

    var piecesQ = pieces.replace("B", "")
    var Q = piecesQ.indexOf("Q")

    var D = "0246".indexOf(b1.toString)
    var L = "1357".indexOf(b2.toString)
    if (D == -1) {
        D = "0246".indexOf(b2.toString)
        L = "1357".indexOf(b1.toString)
    }

    return 96*N + 16*Q + 4*D + L
}

for (pieces in ["♕♘♖♗♗♘♔♖", "♖♘♗♕♔♗♘♖", "♜♛♞♝♝♚♜♞", "♜♞♛♝♝♚♜♞"]) {
    System.print("%(pieces) or %(g2l.call(pieces)) has SP-ID of %(spid.call(pieces))")
}
Output:
♕♘♖♗♗♘♔♖ or QNRBBNKR has SP-ID of 105
♖♘♗♕♔♗♘♖ or RNBQKBNR has SP-ID of 518
♜♛♞♝♝♚♜♞ or RQNBBKRN has SP-ID of 601
♜♞♛♝♝♚♜♞ or RNQBBKRN has SP-ID of 617