Data commit

Task/Generate-Chess960-starting-position/Scheme/generate-chess960-starting-position.ss

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+(import (scheme base) (scheme write)
+        (srfi 1)    ; list library
+        (srfi 27))  ; random numbers
+
+(random-source-randomize! default-random-source)
+
+;; Random integer in [start, end)
+(define (random-between start end)
+  (let ((len (- end start 1)))
+    (if (< len 2)
+      start
+      (+ start (random-integer len)))))
+
+;; Random item in list
+(define (random-pick lst)
+  (if (= 1 (length lst))
+    (car lst)
+    (list-ref lst (random-integer (length lst)))))
+
+;; Construct a random piece placement for Chess960
+(define (random-piece-positions)
+  (define (free-indices positions) ; return list of empty slot indices
+    (let loop ((i 0)
+               (free '()))
+      (if (= 8 i)
+        free
+        (loop (+ 1 i)
+              (if (string=? "." (vector-ref positions i))
+                (cons i free)
+                free)))))
+  ;
+  (define (place-king+rooks positions)
+    (let ((king-posn (random-between 1 8)))
+      (vector-set! positions king-posn "K")
+      ; left-rook is between left-edge and king
+      (vector-set! positions (random-between 0 king-posn) "R")
+      ; right-rook is between right-edge and king
+      (vector-set! positions (random-between (+ 1 king-posn) 8) "R")))
+  ;
+  (define (place-bishops positions)
+    (let-values (((evens odds) (partition even? (free-indices positions))))
+                (vector-set! positions (random-pick evens) "B")
+                (vector-set! positions (random-pick odds) "B")))
+  ;
+  (let ((positions (make-vector 8 ".")))
+    (place-king+rooks positions)
+    (place-bishops positions)
+    ;; place the queen in a random remaining slot
+    (vector-set! positions (random-pick (free-indices positions)) "Q")
+    ;; place the two knights in the remaining slots
+    (for-each (lambda (idx) (vector-set! positions idx "N"))
+              (free-indices positions))
+
+    positions))
+
+(display "First rank: ") (display (random-piece-positions)) (newline)