Add a scanner and parser for platform selectors.

lib/src/backend/platform_selector/parser.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 unittest.backend.platform_selector.parser;
+
+import 'package:source_span/source_span.dart';
+
+import 'ast.dart';
+import 'scanner.dart';
+import 'token.dart';
+
+/// A class for parsing a platform selector.
+///
+/// Platform selectors use a stripped-down version of the Dart expression
+/// syntax that only contains variables, parentheses, and boolean operators.
+/// Variables may also contain dashes, contrary to Dart's syntax; this allows
+/// consistency with command-line arguments.
+class Parser {
+  /// The scanner that tokenizes the selector.
+  final Scanner _scanner;
+
+  Parser(String selector)
+      : _scanner = new Scanner(selector);
+
+  /// Parses the selector.
+  ///
+  /// This must only be called once per parser.
+  Node parse() {
+    var selector = _conditional();
+
+    if (_scanner.peek().type != TokenType.endOfFile) {
+      throw new SourceSpanFormatException(
+          "Expected end of input.", _scanner.peek().span);
+    }
+
+    return selector;
+  }
+
+  /// Parses a conditional:
+  ///
+  ///     conditionalExpression:
+  ///       logicalOrExpression ("?" conditionalExpression ":"
+  ///           conditionalExpression)?
+  Node _conditional() {
+    var condition = _or();
+    if (!_scanner.scan(TokenType.questionMark)) return condition;
+
+    var whenTrue = _conditional();
+    if (!_scanner.scan(TokenType.colon)) {
+      throw new SourceSpanFormatException(
+          'Expected ":".', _scanner.peek().span);
+    }
+
+    var whenFalse = _conditional();
+    return new ConditionalNode(condition, whenTrue, whenFalse);
+  }
+
+  /// Parses a logical or:
+  ///
+  ///     logicalOrExpression:
+  ///       logicalAndExpression ("||" logicalOrExpression)?
+  Node _or() {
+    var left = _and();
+    if (!_scanner.scan(TokenType.or)) return left;
+    return new OrNode(left, _or());
+  }
+
+  /// Parses a logical and:
+  ///
+  ///     logicalAndExpression:
+  ///       simpleExpression ("&&" logicalAndExpression)?
+  Node _and() {
+    var left = _simpleExpression();
+    if (!_scanner.scan(TokenType.and)) return left;
+    return new AndNode(left, _and());
+  }
+
+  /// Parses a simple expression:
+  ///
+  ///     simpleExpression:
+  ///       "!" simpleExpression |
+  ///           "(" conditionalExpression ")" |
+  ///           IDENTIFIER
+  Node _simpleExpression() {
+    var token = _scanner.next();
+    switch (token.type) {
+      case TokenType.not:
+        var child = _simpleExpression();
+        return new NotNode(child, token.span.expand(child.span));
+
+      case TokenType.leftParen:
+        var child = _conditional();
+        if (!_scanner.scan(TokenType.rightParen)) {
+          throw new SourceSpanFormatException(
+              'Expected ")".', _scanner.peek().span);
+        }
+        return child;
+
+      case TokenType.identifier:
+        return new VariableNode(token.name, token.span);
+
+      default:
+        throw new SourceSpanFormatException("Expected expression.", token.span);
+    }
+  }
+}