Optimized utf8 decoder

lib/utils/strings.js

@@ -10,6 +10,13 @@ var STR_APPLY_OK = true;
 try { String.fromCharCode.apply(null, [0]); } catch(__) { STR_APPLY_OK = false; }
 
 
+// Table with utf8 lengths
+var utf8len = new utils.Buf8(256);
+for (var i=0; i<256; i++) {
+  utf8len[i] = (i >= 252 ? 6 : i >= 248 ? 5 : i >= 240 ? 4 : i >= 224 ? 3 : i >= 192 ? 2 : 1);
+}
+
+
 // convert string to array (typed, when possible)
 // src: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Base64_encoding_and_decoding
 exports.string2buf = function (str) {
@@ -68,34 +75,6 @@ exports.string2buf = function (str) {
 };
 
 
-// convert array to string
-// src: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Base64_encoding_and_decoding
-exports.buf2string = function (buf, max) {
-  var str = '';
-
-  for (var part, len = max || buf.length, i = 0; i < len; i++) {
-    part = buf[i];
-    str += String.fromCharCode(
-      part > 251 && part < 254 && i + 5 < len ? /* six bytes */
-        /* (part - 252 << 32) is not possible in ECMAScript! So...: */
-        (part - 252) * 1073741824 + (buf[++i] - 128 << 24) + (buf[++i] - 128 << 18) + (buf[++i] - 128 << 12) + (buf[++i] - 128 << 6) + buf[++i] - 128
-      : part > 247 && part < 252 && i + 4 < len ? /* five bytes */
-        (part - 248 << 24) + (buf[++i] - 128 << 18) + (buf[++i] - 128 << 12) + (buf[++i] - 128 << 6) + buf[++i] - 128
-      : part > 239 && part < 248 && i + 3 < len ? /* four bytes */
-        (part - 240 << 18) + (buf[++i] - 128 << 12) + (buf[++i] - 128 << 6) + buf[++i] - 128
-      : part > 223 && part < 240 && i + 2 < len ? /* three bytes */
-        (part - 224 << 12) + (buf[++i] - 128 << 6) + buf[++i] - 128
-      : part > 191 && part < 224 && i + 1 < len ? /* two bytes */
-        (part - 192 << 6) + buf[++i] - 128
-      : /* part < 127 ? */ /* one byte */
-        part
-    );
-  }
-
-  return str;
-};
-
-
 // Convert byte array to binary string
 exports.buf2binstring = function(buf) {
   // use fallback for big arrays to avoid stack overflow
@@ -121,7 +100,81 @@ exports.binstring2buf = function(str) {
 };
 
 
+// convert array to string
+// src: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Base64_encoding_and_decoding
+exports.buf2string = function (buf, max) {
+  /*jshint nonstandard:true*/
+  // That's not as fast as via String.fromCharCode.appy
+  /*return decodeURIComponent(escape(exports.buf2binstring(
+    (buf.length === max) ?
+      buf
+    :
+      buf.subarray ? buf.subarray(0, max) : buf.slice(0, max)
+  )));*/
+
+  var str = '', i, out, part, char_len;
+  var len = max || buf.length;
+  var out_len = 0;
+  var utf16buf;
+
+  // Calculate output length
+  for (i=0; i < len;) {
+    i += utf8len[buf[i]];
+    out_len++;
+  }
+
+  utf16buf = new utils.Buf32(out_len);
+
+  for (out=0, i=0; i<len; i++) {
+    part = buf[i];
+    char_len = utf8len[part];
+
+    // edge case - broken sequence
+    if (i + char_len > len) {
+      utf16buf[out++] = 0x7f;
+      break;
+    }
+    switch (char_len) {
+      case 1:
+        utf16buf[out++] = part;
+        break;
+      case 2:
+        utf16buf[out++] = ((part & 0x1f) << 6) | (buf[++i] & 0x7f);
+        break;
+      case 3:
+        utf16buf[out++] = ((part & 0x0f) << 12) | ((buf[++i] & 0x3f) << 6) | (buf[++i] & 0x3f);
+        break;
+      case 4:
+        utf16buf[out++] = ((part & 0x07) << 18) | ((buf[++i] & 0x3f) << 12) | ((buf[++i] & 0x3f) << 6) + (buf[++i] & 0x3f);
+        break;
+      // 5 & 6 bytes uticodes not supported in UTF16 (JS), so fill with dummy symbol
+      case 5:
+        i += 4;
+        utf16buf[out++] = 0x7f;
+        //utf16buf[out++] = (part - 248 << 24) + (buf[++i] - 128 << 18) + (buf[++i] - 128 << 12) + (buf[++i] - 128 << 6) + buf[++i] - 128;
+        break;
+      case 6:
+        i += 5;
+        utf16buf[out++] = 0x7f;
+                          // (part - 252 << 32) is not possible in ECMAScript! So...:
+        //utf16buf[out++] = (part - 252) * 1073741824 + (buf[++i] - 128 << 24) + (buf[++i] - 128 << 18) + (buf[++i] - 128 << 12) + (buf[++i] - 128 << 6) + buf[++i] - 128;
+        break;
+    }
+  }
+
+  if (STR_APPLY_OK) {
+    return String.fromCharCode.apply(null, utf16buf);
+  }
+
+  // Fallback, when String.fromCharCode.apply not available
+  for (i=0, len=utf16buf.length; i<len; i++) {
+    str += String.fromCharCode(utf16buf[i]);
+  }
+  return str;
+};
+
+
 // calculate tail size of utf8 char by current byte value
 exports.utf8tail = function(code) {
-  return code >= 252 ? 6 : code >= 248 ? 5 : code >= 240 ? 4 : code >= 224 ? 3 : code >= 192 ? 2 : 1;
+  return utf8len[code];
 };