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Closure Compiler is a JavaScript optimizing compiler. It parses your JavaScript, analyzes it, removes dead code and rewrites and minimizes what's left. It also checks syntax, variable references, and types, and warns about common JavaScript pitfalls. It is used in many of Google's JavaScript apps, including Gmail, Google Web Search, Google Maps, and Google Docs.

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/*
 * Copyright 2011 The Closure Compiler Authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.javascript.jscomp.parsing.parser;

import com.google.common.base.Preconditions;
import com.google.javascript.jscomp.parsing.parser.util.SourcePosition;
import com.google.javascript.jscomp.parsing.parser.util.SourceRange;
import java.util.Objects;

/** Utility for finding line and column offsets within a source file. */
final class LineNumberScanner {

  private final SourceFile sourceFile;
  private final String contents;
  private final int sourceLength;
  private int lastLine = -1;
  private int lastLineStart = -1;
  private int nextLineStart = 0;

  LineNumberScanner(SourceFile sourceFile) {
    this.sourceFile = sourceFile;
    this.contents = sourceFile.contents;
    this.sourceLength = contents.length();
  }

  /**
   * Returns the source position of character offset {@code offset}. This class expects this method
   * to be called with increasing values of {@code offset}, more or less. {@link #rewindTo} must be
   * called before backing up to a previous line.
   */
  SourcePosition getSourcePosition(int offset) {
    Preconditions.checkArgument(
        offset >= lastLineStart,
        "Must call rewindTo before calling getSourcePosition for an earlier line (%s < %s)",
        offset,
        lastLineStart);
    while (offset >= nextLineStart) {
      advanceLine();
    }
    return new SourcePosition(sourceFile, offset, lastLine, offset - lastLineStart);
  }

  SourceRange getSourceRange(int startOffset, int endOffset) {
    return new SourceRange(getSourcePosition(startOffset), getSourcePosition(endOffset));
  }

  /**
   * Call this method to rewind the scanner to an earlier position in the source file. This is
   * necessary if backing up to a previous line.
   */
  void rewindTo(SourcePosition position) {
    Preconditions.checkArgument(Objects.equals(position.source, sourceFile));
    if (position.offset < lastLineStart) {
      lastLine = position.line - 1;
      nextLineStart = position.offset - position.column;
      advanceLine();
    }
  }

  private void advanceLine() {
    lastLine++;

    lastLineStart = nextLineStart;
    for (int index = lastLineStart; index < sourceLength; index++) {
      char ch = contents.charAt(index);
      if (isLineTerminator(ch)) {
        if (ch == '\r' && index + 1 < sourceLength && contents.charAt(index + 1) == '\n') {
          index++;
        }
        nextLineStart = index + 1;
        return;
      }
    }
    nextLineStart = Integer.MAX_VALUE;
  }

  private static boolean isLineTerminator(char ch) {
    switch (ch) {
      case '\n': // Line Feed
      case '\r': // Carriage Return
      case '\u2028': // Line Separator
      case '\u2029': // Paragraph Separator
        return true;
      default:
        return false;
    }
  }
}