string.starts_with & string.ends_with

2 files changed, 33 insertions, 21 deletions

diff --git a/src/gleam/string.gleam b/src/gleam/string.gleam
index 2cad6f2..93c686f 100644
--- a/src/gleam/string.gleam
+++ b/src/gleam/string.gleam
@@ -176,26 +176,26 @@ pub fn contains(does haystack: String, contain needle: String) -> Bool {
   erl_contains(haystack, needle)
 }
 
-// TODO
-// TODO: Not sure about the name and labels here
-// See if the second string starts with the first one.
-//
-// ## Examples
-//    > starts_with(does: "theory", start_with: "ory")
-//    False
-//
-//
-// pub fn starts_with(does string: String, start_with prefix: String) -> String {}
-// TODO
-// TODO: Not sure about the name and labels here
-// See if the second string ends with the first one.
-//
-// ## Examples
-//    > ends_with(does: "theory", end_with: "ory")
-//    True
-//
-//
-// pub fn ends_with(does string: String, end_with suffix: String) -> String {}
+/// See if the first string starts with the second one.
+///
+/// ## Examples
+///    > starts_with("theory", "ory")
+///    False
+///
+///
+pub external fn starts_with(String, String) -> Bool =
+  "gleam_stdlib" "string_starts_with"
+
+/// See if the first string ends with the second one.
+///
+/// ## Examples
+///    > ends_with("theory", "ory")
+///    True
+///
+///
+pub external fn ends_with(String, String) -> Bool =
+  "gleam_stdlib" "string_ends_with"
+
 /// Create a list of strings by splitting a given string on a given substring.
 ///
 /// ## Examples
diff --git a/src/gleam_stdlib.erl b/src/gleam_stdlib.erl
index 49a7e2a..fbd84da 100644
--- a/src/gleam_stdlib.erl
+++ b/src/gleam_stdlib.erl
@@ -7,7 +7,7 @@
          iodata_append/2, iodata_prepend/2, identity/1, decode_int/1,
          decode_string/1, decode_bool/1, decode_float/1, decode_thunk/1, decode_atom/1,
          decode_list/1, decode_field/2, decode_element/2, parse_int/1, parse_float/1, compare_strings/2,
-         string_contains/2]).
+         string_contains/2, string_starts_with/2, string_ends_with/2]).
 
 should_equal(Actual, Expected) -> ?assertEqual(Expected, Actual).
 should_not_equal(Actual, Expected) -> ?assertNotEqual(Expected, Actual).
@@ -127,3 +127,15 @@ string_contains(Haystack, Needle) ->
     _ ->
       true
   end.
+
+string_starts_with(_, <<>>) -> true;
+string_starts_with(String, Prefix) when byte_size(Prefix) > byte_size(String) -> false;
+string_starts_with(String, Prefix) ->
+    PrefixSize = byte_size(Prefix),
+    Prefix == binary_part(String, 0, PrefixSize).
+
+string_ends_with(_, <<>>) -> true;
+string_ends_with(String, Suffix) when byte_size(Suffix) > byte_size(String) -> false;
+string_ends_with(String, Suffix) ->
+    SuffixSize = byte_size(Suffix),
+    Suffix == binary_part(String, byte_size(String) - SuffixSize, SuffixSize).