Reinventing the Wheel

Sure, there's existing code. Somebody Else's Code. It works fine, maybe not as fast as you'd like, or the interface isn't quite right. That's how it often is with me and SRFI-13. Olin Shivers is a skilled Schemer, back when that wasn't cool (OK, it's still not cool), but some of his APIs enshrined in early SRFIs drive me a little nuts, and the implementation is slow because it's so generalized.

So after a few false starts and failed tests, I now have these pretty things: (updated 2020-11-10, enforced hstart, hend boundaries)

;; Returns index of `needle` in `haystack`, or #f if not found.
;; `cmp`: Comparator. Default `char=?`, `char-ci=?` is the most useful alternate comparator.
;; `hstart`: Starting index, default 0.
;; `hend`: Ending index, default (- haystack-length needle-length)
(define string-find (case-lambda
    [(haystack needle)  (string-find haystack needle char=? 0 #f)]
    [(haystack needle cmp)  (string-find haystack needle cmp 0 #f)]
    [(haystack needle cmp hstart)  (string-find haystack needle cmp hstart #f) ]
    [(haystack needle cmp hstart hend)
        (let* [ (hlen (string-length haystack))  (nlen (string-length needle)) ]
            (set! hstart (max 0 (min hstart (sub1 hlen))))
            (unless hend (set! hend (fx- hlen nlen)))
            (set! hend (max 0 (min hend hlen)) )
            (if (or (fxzero? hlen) (fxzero? nlen))
                #f
                (let loop [ (hi hstart)  (ni 0) ]
                    ;; assume (< ni nlen)
                    ;(errprintln "hi=" hi ", ni=" ni ", hsub=" (substr haystack hi hlen) ", bsub=" (substr needle ni nlen))
                    (cond
                        [(cmp (string-ref haystack (fx+ hi ni)) (string-ref needle ni))  (set! ni (fx+ ni 1))
                            ;; end of needle?
                            (if (fx>=? ni nlen)  hi  (loop hi ni) )
                        ]
                        [else  (set! hi (fx+ hi 1))
                            ;; end of haystack?
                            (if (fx>? hi hend)  #f  (loop hi 0) )
                        ]
        ))))
    ]
))

;; Test whether 'haystack' starts with 'needle'.
(define (string-has-prefix? haystack needle)
    (let [ (i (string-find haystack needle char=? 0 0)) ]
        (and i (fxzero? i))
))

;; Test whether 'haystack' ends with 'needle'.
(define (string-has-suffix? haystack needle)
    (let* [ (hlen (string-length haystack))  (nlen (string-length needle))
            (i (string-find haystack needle char=? (fx- hlen nlen)))
        ]
        (and i (fx=? i (fx- hlen nlen)))
))

Written for Chez Scheme, caveat implementor. BSD license, do what thou wilt. If you find a bug, send me a failing test case.

I don't normally bother with fx (fixnum) operations, but in tight loops it makes a difference over generic numeric tower +, etc.