Add a percent-encoding converter.

lib/src/utils.dart

+// Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+library convert.utils;
+
+import 'package:charcode/ascii.dart';
+
+/// Returns the digit (0 through 15) corresponding to the hexadecimal code unit
+/// at index [i] in [codeUnits].
+///
+/// If the given code unit isn't valid hexadecimal, throws a [FormatException].
+int digitForCodeUnit(List<int> codeUnits, int index) {
+  // If the code unit is a numeral, get its value. XOR works because 0 in ASCII
+  // is `0b110000` and the other numerals come after it in ascending order and
+  // take up at most four bits.
+  //
+  // We check for digits first because it ensures there's only a single branch
+  // for 10 out of 16 of the expected cases. We don't count the `digit >= 0`
+  // check because branch prediction will always work on it for valid data.
+  var codeUnit = codeUnits[index];
+  var digit = $0 ^ codeUnit;
+  if (digit <= 9) {
+    if (digit >= 0) return digit;
+  } else {
+    // If the code unit is an uppercase letter, convert it to lowercase. This
+    // works because uppercase letters in ASCII are exactly `0b100000 = 0x20`
+    // less than lowercase letters, so if we ensure that that bit is 1 we ensure
+    // that the letter is lowercase.
+    var letter = 0x20 | codeUnit;
+    if ($a <= letter && letter <= $f) return letter - $a + 10;
+  }
+
+  throw new FormatException(
+      "Invalid hexadecimal code unit "
+          "U+${codeUnit.toRadixString(16).padLeft(4, '0')}.",
+      codeUnits, index);
+}
+