add string-startswith? and string-endswith?

pkgs/racket-doc/scribblings/reference/strings.scrbl

@@ -397,6 +397,13 @@ one between @racket[list] and @racket[list*].
 ]}
 
 
+@defproc[(string-endswith? [str string?]
+                           [suffix string?])
+         boolean?]{
+
+Returns @racket[#t] if the second argument is a suffix of the first.}
+
+
 @defproc[(string-join [strs (listof string?)] [sep string? " "]
                       [#:before-first before-first string? ""]
                       [#:before-last  before-last  string? sep]
@@ -459,6 +466,13 @@ replaced if @racket[all?] is @racket[#f].
 ]}
 
 
+@defproc[(string-startswith? [str string?]
+                             [prefix string?])
+         boolean?]{
+
+Returns @racket[#t] if the second argument is a prefix of the first.}
+
+
 @defproc[(string-split [str string?]
                        [sep (or/c string? regexp?) #px"\\s+"]
                        [#:trim? trim? any/c #t]

pkgs/racket-test-core/tests/racket/string.rktl

@@ -490,4 +490,23 @@
   (test "_1_ !!!" string-replace "_1_ _2_" str "!!!") ;verify that the new str is used
 )
 
+;; ---------- string-starts/endswith ----------
+(let ()
+  (test #t string-startswith? "racket" "")
+  (test #t string-startswith? "racket" "r")
+  (test #t string-startswith? "racket" "rack")
+  (test #t string-startswith? "racket" "racket")
+  (test #t string-endswith?   "racket" "")
+  (test #t string-endswith?   "racket" "t")
+  (test #t string-endswith?   "racket" "cket")
+  (test #t string-endswith?   "racket" "racket")
+  (test #f string-startswith? ""       "racket")
+  (test #f string-startswith? "racket" "R")
+  (test #f string-startswith? "racket" "rak")
+  (test #f string-startswith? "racket" "racket2")
+  (test #f string-endswith?   ""       "racket")
+  (test #f string-endswith?   "racket" "T")
+  (test #f string-endswith?   "racket" "r")
+  (test #f string-endswith?   "racket" "kat"))
+
 (report-errs)

racket/collects/racket/string.rkt

@@ -6,7 +6,9 @@
          string-normalize-spaces
          string-split
          string-replace
-         non-empty-string?)
+         non-empty-string?
+         string-startswith?
+         string-endswith?)
 
 (define string-append*
   (case-lambda [(strs) (apply string-append strs)] ; optimize common cases
@@ -138,3 +140,19 @@
 
 (define (non-empty-string? x)
   (and (string? x) (not (zero? (string-length x)))))
+
+(define (string-startswith? str prefix)
+  (and
+    (<= (string-length prefix) (string-length str))
+    (for/and ([c1 (in-string str)]
+              [c2 (in-string prefix)])
+      (char=? c1 c2))))
+
+(define (string-endswith? str suffix)
+  ;; Skip all but the last `suffix` characters of `str`
+  (define offset (- (string-length str) (string-length suffix)))
+  (and
+    (not (negative? offset))
+    (for/and ([c1 (in-string str offset)]
+              [c2 (in-string suffix)])
+      (char=? c1 c2))))