/*
* Copyright 2011 The Closure Compiler Authors. All rights reserved.
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are
* met:
*
* * Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above
* copyright notice, this list of conditions and the following
* disclaimer in the documentation and/or other materials provided
* with the distribution.
* * Neither the name of Google Inc. nor the names of its
* contributors may be used to endorse or promote products derived
* from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
* "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
* LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
* A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
* OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
* SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
* LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
* OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
package com.github.sommeri.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.
*
* @author johnlenz@google.com (John Lenz)
*/
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;
return negate ? -value : value;
}
/**
* Writes a VLQ encoded value to the provide appendable.
* @throws IOException
*/
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.
*/
interface CharIterator {
boolean hasNext();
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);
}
}