2 files changed, 61 insertions, 17 deletions
diff --git a/src/gleam/base.gleam b/src/gleam/base.gleam
index 0995f1b..6807767 100644
--- a/src/gleam/base.gleam
+++ b/src/gleam/base.gleam
@@ -20,18 +20,23 @@ if javascript {
     "../gleam_stdlib.js" "encode64"
 }
 
+/// Decodes a base 64 encoded string into a BitString.
+pub fn decode64(encoded: String) -> Result(BitString, Nil) {
+  let padded = case bit_string.byte_size(bit_string.from_string(encoded)) % 4 {
+    0 -> encoded
+    n -> string.append(encoded, string.repeat("=", 4 - n))
+  }
+  do_decode64(padded)
+}
+
 if erlang {
-  external fn erl_decode64(String) -> Result(BitString, Nil) =
+  external fn do_decode64(String) -> Result(BitString, Nil) =
     "gleam_stdlib" "base_decode64"
+}
 
-  /// Decodes a base 64 encoded string into a BitString.
-  pub fn decode64(encoded: String) -> Result(BitString, Nil) {
-    let padded = case bit_string.byte_size(bit_string.from_string(encoded)) % 4 {
-      0 -> encoded
-      n -> string.append(encoded, string.repeat("=", 4 - n))
-    }
-    erl_decode64(padded)
-  }
+if javascript {
+  external fn do_decode64(String) -> Result(BitString, Nil) =
+    "../gleam_stdlib.js" "decode64"
 }
 
 /// Encodes a BitString into a base 64 encoded string with URL and filename safe alphabet.
@@ -41,12 +46,10 @@ pub fn url_encode64(input: BitString, padding: Bool) -> String {
   |> string.replace("/", "_")
 }
 
-if erlang {
-  /// Decodes a base 64 encoded string with URL and filename safe alphabet into a BitString.
-  pub fn url_decode64(encoded: String) -> Result(BitString, Nil) {
-    encoded
-    |> string.replace("-", "+")
-    |> string.replace("_", "/")
-    |> decode64()
-  }
+/// Decodes a base 64 encoded string with URL and filename safe alphabet into a BitString.
+pub fn url_decode64(encoded: String) -> Result(BitString, Nil) {
+  encoded
+  |> string.replace("-", "+")
+  |> string.replace("_", "/")
+  |> decode64()
 }
diff --git a/src/gleam_stdlib.js b/src/gleam_stdlib.js
index d26a4b6..bb63cbb 100644
--- a/src/gleam_stdlib.js
+++ b/src/gleam_stdlib.js
@@ -426,3 +426,44 @@ function uint6ToB64(nUint6) {
     ? 47
     : 65;
 }
+
+// From https://developer.mozilla.org/en-US/docs/Glossary/Base64#Solution_2_%E2%80%93_rewrite_the_DOMs_atob()_and_btoa()_using_JavaScript's_TypedArrays_and_UTF-8
+function b64ToUint6(nChr) {
+  return nChr > 64 && nChr < 91
+    ? nChr - 65
+    : nChr > 96 && nChr < 123
+    ? nChr - 71
+    : nChr > 47 && nChr < 58
+    ? nChr + 4
+    : nChr === 43
+    ? 62
+    : nChr === 47
+    ? 63
+    : 0;
+}
+
+// From https://developer.mozilla.org/en-US/docs/Glossary/Base64#Solution_2_%E2%80%93_rewrite_the_DOMs_atob()_and_btoa()_using_JavaScript's_TypedArrays_and_UTF-8
+export function decode64(sBase64) {
+  if (sBase64.match(/[^A-Za-z0-9\+\/=]/g)) return new Error(Nil);
+  let sB64Enc = sBase64.replace(/=/g, "");
+  let nInLen = sB64Enc.length;
+  let nOutLen = (nInLen * 3 + 1) >> 2;
+  let taBytes = new Uint8Array(nOutLen);
+
+  for (
+    let nMod3, nMod4, nUint24 = 0, nOutIdx = 0, nInIdx = 0;
+    nInIdx < nInLen;
+    nInIdx++
+  ) {
+    nMod4 = nInIdx & 3;
+    nUint24 |= b64ToUint6(sB64Enc.charCodeAt(nInIdx)) << (6 * (3 - nMod4));
+    if (nMod4 === 3 || nInLen - nInIdx === 1) {
+      for (nMod3 = 0; nMod3 < 3 && nOutIdx < nOutLen; nMod3++, nOutIdx++) {
+        taBytes[nOutIdx] = (nUint24 >>> ((16 >>> nMod3) & 24)) & 255;
+      }
+      nUint24 = 0;
+    }
+  }
+
+  return new Ok(new BitString(taBytes));
+}