4 files changed, 77 insertions, 46 deletions

diff --git a/src/gleam/bit_string.gleam b/src/gleam/bit_string.gleam
index 6605ce9..8ed94f8 100644
--- a/src/gleam/bit_string.gleam
+++ b/src/gleam/bit_string.gleam
@@ -45,19 +45,38 @@ pub fn append(to first: BitString, suffix second: BitString) -> BitString {
   concat([first, second])
 }
 
+/// Extracts a sub-section of a bit string.
+///
+/// The slice will start at given position and continue up to specified
+/// length.
+/// A negative length can be used to extract bytes at the end of a bit string.
+///
+/// This function runs in constant time.
+///
+pub fn slice(
+  from string: BitString,
+  at position: Int,
+  take length: Int,
+) -> Result(BitString, Nil) {
+  do_slice(string, position, length)
+}
+
 if erlang {
-  /// Extracts part of a bit string.
-  ///
-  /// BitString part will start at given position and continue up to specified
-  /// length.
-  /// A negative length can be used to extract bytes at the end of a bit string.
-  ///
-  pub external fn part(
+  pub external fn do_slice(
+    string: BitString,
+    position: Int,
+    length: Int,
+  ) -> Result(BitString, Nil) =
+    "gleam_stdlib" "bit_string_slice"
+}
+
+if javascript {
+  pub external fn do_slice(
     string: BitString,
     position: Int,
     length: Int,
   ) -> Result(BitString, Nil) =
-    "gleam_stdlib" "bit_string_part_"
+    "../gleam_stdlib.js" "bit_string_slice"
 }
 
 /// Tests to see whether a bit string is valid UTF-8.
diff --git a/src/gleam_stdlib.erl b/src/gleam_stdlib.erl
index c3aa317..13968d2 100644
--- a/src/gleam_stdlib.erl
+++ b/src/gleam_stdlib.erl
@@ -9,7 +9,7 @@
          string_ends_with/2, string_pad/4, decode_tuple2/1, decode_tuple3/1,
          decode_tuple4/1, decode_tuple5/1, decode_tuple6/1, decode_map/1,
          bit_string_int_to_u32/1, bit_string_int_from_u32/1, decode_result/1,
-         bit_string_part_/3, decode_bit_string/1, compile_regex/2,
+         bit_string_slice/3, decode_bit_string/1, compile_regex/2,
          regex_match/2, regex_split/2, regex_scan/2, base_decode64/1,
          wrap_list/1, bit_string_concat/1]).
 
@@ -174,7 +174,7 @@ string_pop_grapheme(String) ->
 bit_string_concat(BitStrings) ->
     iolist_to_binary(BitStrings).
 
-bit_string_part_(Bin, Pos, Len) ->
+bit_string_slice(Bin, Pos, Len) ->
     try {ok, binary:part(Bin, Pos, Len)}
     catch error:badarg -> {error, nil}
     end.
diff --git a/src/gleam_stdlib.js b/src/gleam_stdlib.js
index b63a4ea..51de04a 100644
--- a/src/gleam_stdlib.js
+++ b/src/gleam_stdlib.js
@@ -213,8 +213,11 @@ export function log(term) {
 }
 
 export function stringify(data) {
-  let replacer = (_key, value) =>
-    typeof value === "bigint" ? value.toString() + "n" : value;
+  let replacer = (_key, value) => {
+    if (typeof value === "bigint") return value.toString() + "n";
+    if (typeof value === "undefined") return null;
+    return value;
+  };
   try {
     return JSON.stringify(data, replacer);
   } catch (_error) {
@@ -263,3 +266,10 @@ export function truncate(float) {
 export function power(base, exponent) {
   return Math.pow(base, exponent);
 }
+
+export function bit_string_slice(bits, position, length) {
+  let start = Math.min(position, position + length);
+  let end = Math.max(position, position + length);
+  if (start < 0 || end > bits.byteLength) return gleam_error(Nil);
+  return gleam_ok(new Uint8Array(bits.buffer, start, Math.abs(length)));
+}
diff --git a/test/gleam/bit_string_test.gleam b/test/gleam/bit_string_test.gleam
index e86dd5d..a395672 100644
--- a/test/gleam/bit_string_test.gleam
+++ b/test/gleam/bit_string_test.gleam
@@ -33,40 +33,42 @@ pub fn concat_test() {
   |> should.equal(<<1, 2, 3, 4>>)
 }
 
-if erlang {
-  pub fn part_test() {
-    bit_string.from_string("hello")
-    |> bit_string.part(0, 5)
-    |> should.equal(Ok(bit_string.from_string("hello")))
-
-    bit_string.from_string("hello")
-    |> bit_string.part(0, 0)
-    |> should.equal(Ok(bit_string.from_string("")))
-
-    bit_string.from_string("hello")
-    |> bit_string.part(2, 2)
-    |> should.equal(Ok(bit_string.from_string("ll")))
-
-    bit_string.from_string("hello")
-    |> bit_string.part(5, -2)
-    |> should.equal(Ok(bit_string.from_string("lo")))
-
-    bit_string.from_string("")
-    |> bit_string.part(0, 0)
-    |> should.equal(Ok(bit_string.from_string("")))
-
-    bit_string.from_string("hello")
-    |> bit_string.part(6, 0)
-    |> should.equal(Error(Nil))
-
-    bit_string.from_string("hello")
-    |> bit_string.part(-1, 1)
-    |> should.equal(Error(Nil))
-
-    bit_string.from_string("hello")
-    |> bit_string.part(1, 6)
-    |> should.equal(Error(Nil))
-  }
+pub fn slice_test() {
+  <<"hello":utf8>>
+  |> bit_string.slice(0, 5)
+  |> should.equal(Ok(<<"hello":utf8>>))
+
+  <<"hello":utf8>>
+  |> bit_string.slice(0, 0)
+  |> should.equal(Ok(<<"":utf8>>))
+
+  <<"hello":utf8>>
+  |> bit_string.slice(2, 2)
+  |> should.equal(Ok(<<"ll":utf8>>))
+
+  <<"hello":utf8>>
+  |> bit_string.slice(5, -2)
+  |> should.equal(Ok(<<"lo":utf8>>))
+
+  <<"":utf8>>
+  |> bit_string.slice(0, 0)
+  |> should.equal(Ok(<<"":utf8>>))
+
+  <<"hello":utf8>>
+  |> bit_string.slice(6, 0)
+  |> should.equal(Error(Nil))
+
+  <<"hello":utf8>>
+  |> bit_string.slice(1, -2)
+  |> should.equal(Error(Nil))
+
+  bit_string.from_string("hello")
+  |> bit_string.slice(-1, 1)
+  |> should.equal(Error(Nil))
+
+  bit_string.from_string("hello")
+  |> bit_string.slice(1, 6)
+  |> should.equal(Error(Nil))
 }
 
 pub fn to_string_test() {