fix: enable FileReader wpt and align to spec (#10063)

This adds some algorithms from the whatwg mimesniff, whatwg infra, and
whatwg encoding specs that FileReader needs to use internally.

1 files changed, 28 insertions, 0 deletions

diff --git a/op_crates/web/08_text_encoding.js b/op_crates/web/08_text_encoding.js
index 73cb38311..1fda1a816 100644
--- a/op_crates/web/08_text_encoding.js
+++ b/op_crates/web/08_text_encoding.js
@@ -4545,10 +4545,38 @@
     fromByteArray,
   };
 
+  /**
+   * @param {Uint8Array} bytes 
+   */
+  function decode(bytes, encoding) {
+    const BOMEncoding = BOMSniff(bytes);
+    let start = 0;
+    if (BOMEncoding !== null) {
+      encoding = BOMEncoding;
+      if (BOMEncoding === "UTF-8") start = 3;
+      else start = 2;
+    }
+    return new TextDecoder(encoding).decode(bytes.slice(start));
+  }
+
+  /**
+   * @param {Uint8Array} bytes 
+   */
+  function BOMSniff(bytes) {
+    const BOM = bytes.subarray(0, 3);
+    if (BOM[0] === 0xEF && BOM[1] === 0xBB && BOM[2] === 0xBF) {
+      return "UTF-8";
+    }
+    if (BOM[0] === 0xFE && BOM[1] === 0xFF) return "UTF-16BE";
+    if (BOM[0] === 0xFF && BOM[1] === 0xFE) return "UTF-16LE";
+    return null;
+  }
+
   window.TextEncoder = TextEncoder;
   window.TextDecoder = TextDecoder;
   window.atob = atob;
   window.btoa = btoa;
   window.__bootstrap = window.__bootstrap || {};
+  window.__bootstrap.encoding = { decode };
   window.__bootstrap.base64 = base64;
 })(this);