| // 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 - $a + 10; |
| } |
| |
| throw FormatException( |
| "Invalid hexadecimal code unit " |
| "U+${codeUnit.toRadixString(16).padLeft(4, '0')}.", |
| codeUnits, |
| index); |
| } |