prescheme-demo/lib/ps-string.scm

;;; ps-string: string utilities for Pre-Scheme

(define (string-copy! target offset source start end)
  (do ((tgt offset (+ tgt 1))
       (src start (+ src 1)))
      ((= src end))
    (string-set! target tgt (string-ref source src)))
  (unspecific))

(define (string-append a b)
  (let* ((len-a (string-length a))
         (len-b (string-length b))
         (target (make-string (+ len-a len-b))))
    (string-copy! target 0 a 0 len-a)
    (string-copy! target len-a b 0 len-b)
    target))

(define (string-repeat source n)
  (let* ((len (string-length source))
         (total (* len n))
         (target (make-string total)))
    (do ((ix 0 (+ ix len)))
        ((= ix total))
      (string-copy! target ix source 0 len))
    target))