[Gauche] 途中で xmin の値が変更されてしまう。mx, my の演算に有理数を使うとこの問題が発生しない所まで確認。(0.9.2, 0.9.3.3)

buggy.scm

(define xmin 0)
(define xmax 0)
(define ymin 0)
(define ymax 0)

(define (f x0 y0 x1 y1 depth)
  (when (>= depth 0)
    (when (< x0 xmin) (set! xmin x0))
    (when (< x1 xmin) (set! xmin x1))
    (when (< xmax x0) (set! xmax x0))
    (when (< xmax x1) (set! xmax x1))

    (when (< y0 ymin) (set! ymin y0))
    (when (< y1 ymin) (set! ymin y1))
    (when (< ymax y0) (set! ymax y0))
    (when (< ymax y1) (set! ymax y1))

    (when (> xmin 0) (print "ERR?1 " xmin) (exit))

    (let* ([dx (- x1 x0)]
           [dy (- y1 y0)]
           [x0+ (- x0 dy)]
           [y0+ (+ y0 dx)]
           [x1+ (- x1 dy)]
           [y1+ (+ y1 dx)]
           [mx (+ x0+ (* .5 dx) (* -.5 dy))] ;; 浮動小数点演算だと駄目
           [my (+ y0+ (* .5 dy) (* .5 dx))])
;           [mx (+ x0+ (* 1/2 dx) (* -1/2 dy))] ;; 有理数だと問題ない
;           [my (+ y0+ (* 1/2 dy) (* 1/2 dx))])

      (when (> xmin 0) (print "ERR?3 " xmin) (exit))

      (f x0+ y0+ mx my (- depth 1))
      (f mx my x1+ y1+ (- depth 1))
      (when (> xmin 0) (print "ERR?5 " xmin) (exit))
      )))

;(disasm f)

(f 0 0 1 0 11)
(format #t "x:[~a ~a] y:[~a ~a]¥n" xmin xmax ymin ymax)