Add split and split_once to bit_array module

CHANGELOG.md

@@ -12,6 +12,7 @@
 - `uri.query_to_string` now correctly handles `+` in query params.
 - The deprecated `result.then`, `result.unwrap_both`, `function.tap`,
   `int.digits`, and `int.undigits` functions have been removed.
+- The `bit_array` module gains the `split` and `split_once` functions.
 
 ## v0.67.1 - 2025-12-03
 

src/gleam/bit_array.gleam

@@ -57,6 +57,52 @@ pub fn slice(
   take length: Int,
 ) -> Result(BitArray, Nil)
 
+/// Splits a bit array into two parts at the location of the pattern.
+/// 
+/// The result will not include the pattern, and returns an error if the
+/// pattern is not found.
+/// 
+/// This function runs in linear time.
+///
+/// ## Examples
+/// 
+/// ```gleam
+/// split_once(from: <<1, 2, 3>>, on: <<2>>)
+/// // -> Ok(#(<<1>>, <<3>>))
+/// 
+/// split_once(from: <<0>>, on: <<1>>)
+/// // -> Error(Nil)
+/// ```
+@external(erlang, "gleam_stdlib", "bit_array_split_once")
+@external(javascript, "../gleam_stdlib.mjs", "bit_array_split_once")
+pub fn split_once(
+  from bits: BitArray,
+  on pattern: BitArray,
+) -> Result(#(BitArray, BitArray), Nil)
+
+/// Splits a bit array into parts at the locations of the pattern.
+///
+/// The result will not include the pattern, and returns the input
+/// as is if the pattern is not found.
+///
+/// This function runs in linear time.
+///
+/// ## Examples
+///
+/// ```gleam
+/// split(from: <<0, 1, 0, 2, 0, 3>>, on: <<0>>)
+/// // -> Ok([<<1>>, <<2>>, <<3>>])
+/// 
+/// split(from: <<0>>, on: <<1>>)
+/// // -> Ok([<<0>>])
+/// ```
+@external(erlang, "gleam_stdlib", "bit_array_split")
+@external(javascript, "../gleam_stdlib.mjs", "bit_array_split")
+pub fn split(
+  from bits: BitArray,
+  on pattern: BitArray,
+) -> Result(List(BitArray), Nil)
+
 /// Tests to see whether a bit array is valid UTF-8.
 ///
 pub fn is_utf8(bits: BitArray) -> Bool {

src/gleam_stdlib.erl

@@ -11,7 +11,8 @@
     int_from_base_string/2, utf_codepoint_list_to_string/1, contains_string/2,
     crop_string/2, base16_encode/1, base16_decode/1, string_replace/3, slice/3,
     bit_array_to_int_and_size/1, bit_array_pad_to_bytes/1, index/2, list/5,
-    dict/1, int/1, float/1, bit_array/1, is_null/1
+    dict/1, int/1, float/1, bit_array/1, is_null/1, bit_array_split/2, 
+    bit_array_split_once/2
 ]).
 
 %% Taken from OTP's uri_string module
@@ -154,6 +155,21 @@ bit_array_slice(Bin, Pos, Len) ->
     catch error:badarg -> {error, nil}
     end.
 
+bit_array_split_once(Bin, Sub) ->
+    try
+        case binary:split(Bin, [Sub]) of
+            [<<>>, <<>>] -> {error, nil};
+            [A, B] -> {ok, {A, B}};
+            _ -> {error, nil}
+        end
+    catch error:badarg -> {error, nil}
+    end.
+
+bit_array_split(Bin, Sub) ->
+    try {ok, binary:split(Bin, [Sub], [global, trim_all])}
+    catch error:badarg -> {error, nil}
+    end.
+
 base64_decode(S) ->
     try {ok, base64:decode(S)}
     catch error:_ -> {error, nil}

src/gleam_stdlib.mjs

@@ -451,6 +451,80 @@ export function bit_array_slice(bits, position, length) {
   return new Ok(bitArraySlice(bits, start * 8, end * 8));
 }
 
+export function bit_array_split_once(bits, pattern) {
+  try {
+    const patternEmpty = pattern.buffer.length < 1
+    const patternLongerThanBits = pattern.buffer.length >= bits.buffer.length
+    const incorrectArguments = !(bits instanceof BitArray) || !(pattern instanceof BitArray)
+    if (incorrectArguments || patternEmpty || patternLongerThanBits) {
+      return new Error(Nil);
+    }
+
+    const n = bits.buffer.length - pattern.buffer.length + 1;
+    find: for (let i = 0; i < n; i++) {
+      for (let j = 0; j < pattern.buffer.length; j++) {
+        if (bits.buffer[i + j] !== pattern.buffer[j]) {
+          continue find;
+        }
+      }
+      const before = bits.buffer.slice(0, i);
+      const after = bits.buffer.slice(i + pattern.buffer.length);
+      return new Ok([new BitArray(before), new BitArray(after)]);
+    }
+
+    return new Error(Nil);
+  } catch (e) {
+    return new Error(Nil);
+  }
+}
+
+export function bit_array_split(bits, pattern) {
+  try {
+    const patternEmpty = pattern.buffer.length < 1
+    const incorrectArguments = !(bits instanceof BitArray) || !(pattern instanceof BitArray)
+    if (incorrectArguments || patternEmpty) {
+      return new Error(Nil);
+    }
+
+    const bitsShorter = bits.buffer.length < pattern.buffer.length
+    if (bitsShorter) {
+      return new Ok(List.fromArray([bits]))
+    }
+
+    const results = [];
+    let lastIndex = 0;
+    const n = bits.buffer.length - pattern.buffer.length + 1;
+
+    find: for (let i = 0; i < n; i++) {
+      for (let j = 0; j < pattern.buffer.length; j++) {
+        if (bits.buffer[i + j] !== pattern.buffer[j]) {
+          continue find;
+        }
+      }
+
+      const bitsEqualsPattern = bits.buffer.length === pattern.buffer.length
+      if (bitsEqualsPattern) {
+        return new Ok(List.fromArray([]));
+      }
+
+      if (i > lastIndex) {
+        results.push(new BitArray(bits.buffer.slice(lastIndex, i)));
+      }
+
+      lastIndex = i + pattern.buffer.length;
+      i = lastIndex - 1;
+    }
+
+    if (lastIndex < bits.buffer.length) {
+      results.push(new BitArray(bits.buffer.slice(lastIndex)));
+    }
+
+    return new Ok(List.fromArray(results))
+  } catch (e) {
+    return new Error(Nil);
+  }
+}
+
 export function codepoint(int) {
   return new UtfCodepoint(int);
 }

test/gleam/bit_array_test.gleam

@@ -118,6 +118,92 @@ pub fn slice_test() {
     == Error(Nil)
 }
 
+pub fn split_once_test() {
+  <<"hello":utf8>>
+  |> bit_array.split_once(<<"l":utf8>>)
+  |> should.equal(Ok(#(<<"he":utf8>>, <<"lo":utf8>>)))
+
+  <<"hello":utf8>>
+  |> bit_array.split_once(<<"o":utf8>>)
+  |> should.equal(Ok(#(<<"hell":utf8>>, <<>>)))
+
+  <<"hello":utf8>>
+  |> bit_array.split_once(<<"h":utf8>>)
+  |> should.equal(Ok(#(<<>>, <<"ello":utf8>>)))
+
+  <<0, 1, 0, 2, 0, 3>>
+  |> bit_array.split_once(<<0, 2>>)
+  |> should.equal(Ok(#(<<0, 1>>, <<0, 3>>)))
+
+  <<0, 1, 2, 0, 3, 4, 5>>
+  |> bit_array.split_once(<<>>)
+  |> should.equal(Error(Nil))
+
+  <<>>
+  |> bit_array.split_once(<<1>>)
+  |> should.equal(Error(Nil))
+
+  <<1>>
+  |> bit_array.split_once(<<1>>)
+  |> should.equal(Error(Nil))
+
+  <<0>>
+  |> bit_array.split_once(<<1>>)
+  |> should.equal(Error(Nil))
+}
+
+// This test is target specific since it's using non byte-aligned BitArrays
+// and those are not supported on the JavaScript target.
+@target(erlang)
+pub fn split_once_erlang_only_test() {
+  <<0, 1, 2:7>>
+  |> bit_array.split_once(<<1>>)
+  |> should.equal(Error(Nil))
+}
+
+pub fn split_test() {
+  <<"hello":utf8>>
+  |> bit_array.split(<<"l":utf8>>)
+  |> should.equal(Ok([<<"he":utf8>>, <<"o":utf8>>]))
+
+  <<0, 1, 0, 2, 0, 3>>
+  |> bit_array.split(<<0>>)
+  |> should.equal(Ok([<<1>>, <<2>>, <<3>>]))
+
+  <<0, 1, 0, 2, 0, 3>>
+  |> bit_array.split(<<0, 2>>)
+  |> should.equal(Ok([<<0, 1>>, <<0, 3>>]))
+
+  <<1, 0>>
+  |> bit_array.split(<<0>>)
+  |> should.equal(Ok([<<1>>]))
+
+  <<1, 0>>
+  |> bit_array.split(<<1>>)
+  |> should.equal(Ok([<<0>>]))
+
+  <<1>>
+  |> bit_array.split(<<0>>)
+  |> should.equal(Ok([<<1>>]))
+
+  <<1, 2>>
+  |> bit_array.split(<<1, 2>>)
+  |> should.equal(Ok([]))
+
+  <<0, 1, 2, 0, 3, 4, 5>>
+  |> bit_array.split(<<>>)
+  |> should.equal(Error(Nil))
+}
+
+// This test is target specific since it's using non byte-aligned BitArrays
+// and those are not supported on the JavaScript target.
+@target(erlang)
+pub fn split_erlang_only_test() {
+  <<0, 1, 2:7>>
+  |> bit_array.split(<<1>>)
+  |> should.equal(Error(Nil))
+}
+
 pub fn to_string_test() {
   assert bit_array.to_string(<<>>) == Ok("")