3 files changed, 36 insertions, 1 deletions

diff --git a/src/gleam/bit_array.gleam b/src/gleam/bit_array.gleam
index 909d9f1..8c3ce48 100644
--- a/src/gleam/bit_array.gleam
+++ b/src/gleam/bit_array.gleam
@@ -204,7 +204,18 @@ fn do_inspect(input: BitArray, accumulator: String) -> String {
 /// // -> Eq
 /// ```
 ///
+/// Only supported on Erlang target for now.
+///
 pub fn compare(first: BitArray, second: BitArray) -> order.Order {
+  do_compare(first, second)
+}
+
+@target(javascript)
+@external(javascript, "../gleam_stdlib.mjs", "bit_array_compare")
+fn do_compare(first: BitArray, second: BitArray) -> order.Order
+
+@target(erlang)
+fn do_compare(first: BitArray, second: BitArray) -> order.Order {
   case first, second {
     <<first_byte, first_rest:bits>>, <<second_byte, second_rest:bits>> ->
       int.compare(first_byte, second_byte)
@@ -217,11 +228,11 @@ pub fn compare(first: BitArray, second: BitArray) -> order.Order {
     <<>>, _ -> order.Lt
     // This happens when there's unusually sized elements.
     // Handle these special cases via custom erlang function.
-    // This cannot be reached in JS right now.
     first, second ->
       int.compare(bit_array_to_int(first), bit_array_to_int(second))
   }
 }
 
+@target(erlang)
 @external(erlang, "gleam_stdlib", "bit_array_to_int")
 fn bit_array_to_int(first: BitArray) -> Int
diff --git a/src/gleam_stdlib.mjs b/src/gleam_stdlib.mjs
index a89c829..9fe6caa 100644
--- a/src/gleam_stdlib.mjs
+++ b/src/gleam_stdlib.mjs
@@ -16,6 +16,7 @@ import {
 } from "./gleam/regex.mjs";
 import { DecodeError } from "./gleam/dynamic.mjs";
 import { Some, None } from "./gleam/option.mjs";
+import { Eq, Gt, Lt } from "./gleam/order.mjs";
 import Dict from "./dict.mjs";
 
 const Nil = undefined;
@@ -916,3 +917,25 @@ export function base16_decode(string) {
 export function bit_array_inspect(bits, acc) {
   return `${acc}${[...bits.buffer].join(", ")}`;
 }
+
+export function bit_array_compare(first, second) {
+  for (let i = 0; i < first.length; i++) {
+    if (i >= second.length) {
+      return new Gt();  // first has more items
+    }
+    const f = first.buffer[i];
+    const s = second.buffer[i];
+    if (f > s) {
+      return new Gt();
+    }
+    if (f < s) {
+      return new Lt()
+    }
+  }
+  // This means that either first did not have any items
+  // or all items in first were equal to second.
+  if (first.length === second.length) {
+    return new Eq();
+  }
+  return new Lt();  // second has more items
+}
diff --git a/test/gleam/bit_array_test.gleam b/test/gleam/bit_array_test.gleam
index 38dcb2e..5af8cee 100644
--- a/test/gleam/bit_array_test.gleam
+++ b/test/gleam/bit_array_test.gleam
@@ -333,6 +333,7 @@ pub fn inspect_partial_bytes_test() {
   |> should.equal("<<182, 1:size(1)>>")
 }
 
+@target(erlang)
 pub fn compare_test() {
   bit_array.compare(<<1, 2, 3>>, <<1, 2, 3>>)
   |> should.equal(order.Eq)