String suffix and prefix functions.

Changes

@@ -111,6 +111,10 @@ Working version
    and more generally all other data that is not a well-formed OCaml value
    (Xavier Leroy, review by Damien Doligez and Gabriel Scherer)
 
+- #9533: Added String.starts_with and String.ends_with.
+  (Bernhard Schommer, review by Daniel Bünzli, Gabriel Scherer and
+  Alain Frisch)
+
 ### Other libraries:
 
 * #9206, #9419: update documentation of the threads library;

stdlib/filename.ml

@@ -100,9 +100,7 @@ module Unix : SYSDEPS = struct
     && (String.length n < 2 || String.sub n 0 2 <> "./")
     && (String.length n < 3 || String.sub n 0 3 <> "../")
   let check_suffix name suff =
-    String.length name >= String.length suff &&
-    String.sub name (String.length name - String.length suff)
-                    (String.length suff) = suff
+    String.ends_with ~suffix:suff name
 
   let chop_suffix_opt ~suffix filename =
     let len_s = String.length suffix and len_f = String.length filename in

stdlib/string.ml

@@ -231,3 +231,22 @@ let to_seq s = bos s |> B.to_seq
 let to_seqi s = bos s |> B.to_seqi
 
 let of_seq g = B.of_seq g |> bts
+
+let starts_with ~prefix s =
+  let len_s = length s
+  and len_pre = length prefix in
+  let rec aux i =
+    if i = len_pre then true
+    else if unsafe_get s i <> unsafe_get prefix i then false
+    else aux (i + 1)
+  in len_s >= len_pre && aux 0
+
+let ends_with ~suffix s =
+  let len_s = length s
+  and len_suf = length suffix in
+  let diff = len_s - len_suf in
+  let rec aux i =
+    if i = len_suf then true
+    else if unsafe_get s (diff + i) <> unsafe_get suffix i then false
+    else aux (i + 1)
+  in diff >= 0 && aux 0

stdlib/string.mli

@@ -316,6 +316,14 @@ val equal: t -> t -> bool
 (** The equal function for strings.
     @since 4.03.0 *)
 
+val starts_with : prefix:t -> t -> bool
+(** [String.starts_with prefix s] tests if [s] starts with [prefix]
+    @since 4.12.0 *)
+
+val ends_with : suffix:t -> t -> bool
+(** [String.ends_with suffix s] tests if [s] ends with [suffix]
+    @since 4.12.0 *)
+
 val split_on_char: char -> string -> string list
 (** [String.split_on_char sep s] returns the list of all (possibly empty)
     substrings of [s] that are delimited by the [sep] character.

stdlib/stringLabels.mli

@@ -282,6 +282,14 @@ val equal: t -> t -> bool
 (** The equal function for strings.
     @since 4.05.0 *)
 
+val starts_with : prefix:t -> t -> bool
+(** [String.starts_with prefix s] tests if [s] starts with [prefix]
+    @since 4.12.0 *)
+
+val ends_with : suffix:t -> t -> bool
+(** [String.ends_with suffix s] tests if [s] ends with [suffix]
+    @since 4.12.0 *)
+
 val split_on_char: sep:char -> string -> string list
 (** [String.split_on_char sep s] returns the list of all (possibly empty)
     substrings of [s] that are delimited by the [sep] character.

testsuite/tests/lib-string/test_string.ml

@@ -51,5 +51,17 @@ let ()  =
     while !sz >= 0 do push big l; sz += Sys.max_string_length done;
     while !sz <= 0 do push big l; sz += Sys.max_string_length done;
     try ignore (String.concat "" !l); assert false
-    with Invalid_argument _ -> ()
+    with Invalid_argument _ -> ();
+    assert(String.starts_with ~prefix:"foob" "foobarbaz");
+    assert(String.starts_with ~prefix:"" "foobarbaz");
+    assert(String.starts_with ~prefix:"" "");
+    assert(not (String.starts_with ~prefix:"foobar" "bar"));
+    assert(not (String.starts_with ~prefix:"foo" ""));
+    assert(not (String.starts_with ~prefix:"fool" "foobar"));
+    assert(String.ends_with ~suffix:"baz" "foobarbaz");
+    assert(String.ends_with ~suffix:"" "foobarbaz");
+    assert(String.ends_with ~suffix:"" "");
+    assert(not (String.ends_with ~suffix:"foobar" "bar"));
+    assert(not (String.ends_with ~suffix:"foo" ""));
+    assert(not (String.ends_with ~suffix:"obaz" "foobar"));
   end