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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.debugging.sourcemap;

import java.io.IOException;

/**
 * We encode our variable length numbers as base64 encoded strings with the least significant digit
 * coming first. Each base64 digit encodes a 5-bit value (0-31) and a continuation bit. Signed
 * values can be represented by using the least significant bit of the value as the sign bit.
 */
public final class Base64VLQ {
  // Utility class.
  private Base64VLQ() {}

  // A Base64 VLQ digit can represent 5 bits, so it is base-32.
  private static final int VLQ_BASE_SHIFT = 5;
  private static final int VLQ_BASE = 1 << VLQ_BASE_SHIFT;

  // A mask of bits for a VLQ digit (11111), 31 decimal.
  private static final int VLQ_BASE_MASK = VLQ_BASE-1;

  // The continuation bit is the 6th bit.
  private static final int VLQ_CONTINUATION_BIT = VLQ_BASE;

  /**
   * Converts from a two-complement value to a value where the sign bit is
   * is placed in the least significant bit.  For example, as decimals:
   *   1 becomes 2 (10 binary), -1 becomes 3 (11 binary)
   *   2 becomes 4 (100 binary), -2 becomes 5 (101 binary)
   */
  private static int toVLQSigned(int value) {
    if (value < 0) {
      return ((-value) << 1) + 1;
    } else {
      return (value << 1) + 0;
    }
  }

  /**
   * Converts to a two-complement value from a value where the sign bit is
   * is placed in the least significant bit.  For example, as decimals:
   *   2 (10 binary) becomes 1, 3 (11 binary) becomes -1
   *   4 (100 binary) becomes 2, 5 (101 binary) becomes -2
   */
  private static int fromVLQSigned(int value) {
    boolean negate = (value & 1) == 1;
    value = value >>> 1;
    if (!negate) {
      return value;
    }
    // We need to OR 0x80000000 here to ensure the 32nd bit (the sign bit) is
    // always set for negative numbers. If `value` were 1, (meaning `negate` is
    // true and all other bits were zeros), `value` would now be 0. -0 is just
    // 0, and doesn't flip the 32nd bit as intended. All positive numbers will
    // successfully flip the 32nd bit without issue, so it's a noop for them.
    return -value | 0x80000000;
  }

  /** Writes a VLQ encoded value to the provide appendable. */
  public static void encode(Appendable out, int value) throws IOException {
    value = toVLQSigned(value);
    do {
      int digit = value & VLQ_BASE_MASK;
      value >>>= VLQ_BASE_SHIFT;
      if (value > 0) {
        digit |= VLQ_CONTINUATION_BIT;
      }
      out.append(Base64.toBase64(digit));
    } while (value > 0);
  }

  /**
   * A simple interface for advancing through a sequence of characters, that communicates that
   * advance back to the source.
   */
  public interface CharIterator {
    public boolean hasNext();

    public char next();
  }

  /**
   * Decodes the next VLQValue from the provided CharIterator.
   */
  public static int decode(CharIterator in) {
    int result = 0;
    boolean continuation;
    int shift = 0;
    do {
      char c = in.next();
      int digit = Base64.fromBase64(c);
      continuation = (digit & VLQ_CONTINUATION_BIT) != 0;
      digit &= VLQ_BASE_MASK;
      result = result + (digit << shift);
      shift = shift + VLQ_BASE_SHIFT;
    } while (continuation);

    return fromVLQSigned(result);
  }
}