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