com.google.debugging.sourcemap.Base64VLQ Maven / Gradle / Ivy
/*
* 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.
* @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.
*/
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);
}
}