145 lines
3.5 KiB
JavaScript
145 lines
3.5 KiB
JavaScript
// Copyright 2012 The Obvious Corporation.
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/*
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* bits: Bitwise buffer utilities. The utilities here treat a buffer
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* as a little-endian bigint, so the lowest-order bit is bit #0 of
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* `buffer[0]`, and the highest-order bit is bit #7 of
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* `buffer[buffer.length - 1]`.
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*/
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/*
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* Modules used
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*/
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"use strict";
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/*
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* Exported bindings
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*/
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/**
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* Extracts the given number of bits from the buffer at the indicated
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* index, returning a simple number as the result. If bits are requested
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* that aren't covered by the buffer, the `defaultBit` is used as their
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* value.
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*
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* The `bitLength` must be no more than 32. The `defaultBit` if not
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* specified is taken to be `0`.
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*/
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export function extract(buffer, bitIndex, bitLength, defaultBit) {
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if (bitLength < 0 || bitLength > 32) {
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throw new Error("Bad value for bitLength.");
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}
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if (defaultBit === undefined) {
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defaultBit = 0;
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} else if (defaultBit !== 0 && defaultBit !== 1) {
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throw new Error("Bad value for defaultBit.");
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}
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var defaultByte = defaultBit * 0xff;
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var result = 0; // All starts are inclusive. The {endByte, endBit} pair is exclusive, but
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// if endBit !== 0, then endByte is inclusive.
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var lastBit = bitIndex + bitLength;
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var startByte = Math.floor(bitIndex / 8);
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var startBit = bitIndex % 8;
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var endByte = Math.floor(lastBit / 8);
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var endBit = lastBit % 8;
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if (endBit !== 0) {
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// `(1 << endBit) - 1` is the mask of all bits up to but not including
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// the endBit.
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result = get(endByte) & (1 << endBit) - 1;
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}
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while (endByte > startByte) {
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endByte--;
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result = result << 8 | get(endByte);
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}
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result >>>= startBit;
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return result;
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function get(index) {
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var result = buffer[index];
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return result === undefined ? defaultByte : result;
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}
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}
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/**
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* Injects the given bits into the given buffer at the given index. Any
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* bits in the value beyond the length to set are ignored.
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*/
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export function inject(buffer, bitIndex, bitLength, value) {
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if (bitLength < 0 || bitLength > 32) {
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throw new Error("Bad value for bitLength.");
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}
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var lastByte = Math.floor((bitIndex + bitLength - 1) / 8);
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if (bitIndex < 0 || lastByte >= buffer.length) {
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throw new Error("Index out of range.");
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} // Just keeping it simple, until / unless profiling shows that this
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// is a problem.
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var atByte = Math.floor(bitIndex / 8);
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var atBit = bitIndex % 8;
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while (bitLength > 0) {
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if (value & 1) {
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buffer[atByte] |= 1 << atBit;
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} else {
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buffer[atByte] &= ~(1 << atBit);
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}
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value >>= 1;
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bitLength--;
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atBit = (atBit + 1) % 8;
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if (atBit === 0) {
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atByte++;
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}
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}
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}
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/**
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* Gets the sign bit of the given buffer.
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*/
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export function getSign(buffer) {
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return buffer[buffer.length - 1] >>> 7;
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}
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/**
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* Gets the zero-based bit number of the highest-order bit with the
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* given value in the given buffer.
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*
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* If the buffer consists entirely of the other bit value, then this returns
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* `-1`.
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*/
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export function highOrder(bit, buffer) {
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var length = buffer.length;
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var fullyWrongByte = (bit ^ 1) * 0xff; // the other-bit extended to a full byte
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while (length > 0 && buffer[length - 1] === fullyWrongByte) {
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length--;
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}
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if (length === 0) {
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// Degenerate case. The buffer consists entirely of ~bit.
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return -1;
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}
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var byteToCheck = buffer[length - 1];
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var result = length * 8 - 1;
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for (var i = 7; i > 0; i--) {
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if ((byteToCheck >> i & 1) === bit) {
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break;
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}
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result--;
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}
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return result;
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} |