41 lines
1.4 KiB
Dart
41 lines
1.4 KiB
Dart
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// Copyright (c) 2019, the Dart project authors. Please see the AUTHORS file
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// for details. All rights reserved. Use of this source code is governed by a
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// BSD-style license that can be found in the LICENSE file.
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import 'dart:math' as math;
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import 'package:string_scanner/string_scanner.dart';
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void main(List<String> args) {
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print(parseNumber(args.single));
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}
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num parseNumber(String source) {
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// Scan a number ("1", "1.5", "-3").
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final scanner = StringScanner(source);
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// [Scanner.scan] tries to consume a [Pattern] and returns whether or not it
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// succeeded. It will move the scan pointer past the end of the pattern.
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final negative = scanner.scan('-');
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// [Scanner.expect] consumes a [Pattern] and throws a [FormatError] if it
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// fails. Like [Scanner.scan], it will move the scan pointer forward.
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scanner.expect(RegExp(r'\d+'));
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// [Scanner.lastMatch] holds the [MatchData] for the most recent call to
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// [Scanner.scan], [Scanner.expect], or [Scanner.matches].
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var number = num.parse(scanner.lastMatch![0]!);
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if (scanner.scan('.')) {
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scanner.expect(RegExp(r'\d+'));
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final decimal = scanner.lastMatch![0]!;
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number += int.parse(decimal) / math.pow(10, decimal.length);
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}
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// [Scanner.expectDone] will throw a [FormatError] if there's any input that
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// hasn't yet been consumed.
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scanner.expectDone();
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return (negative ? -1 : 1) * number;
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}
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