nes-emu/helpers/Byte.js

A NES emulator

"use strict";

Object.defineProperty(exports, "__esModule", {
  value: true
});
exports.default = void 0;
/*
 * A byte helper. Signed bytes use the "Two's complement" representation.
 *
 * Positive values are: {value}             => [0  , 127]
 * Negative values are: -(256 - {value})   => [128, 255]
 */
var _default = exports.default = {
  /** Converts a signed `byte` to a number (254 => -2). */
  toNumber(byte) {
    return byte << 24 >> 24;
  },
  /** Converts a `number` to a signed byte (-2 => 254). */
  toSignedByte(number) {
    return number & 0xff;
  },
  /** Returns whether `value` can be represented as a single byte or not. */
  hasOverflow(value) {
    return value >= 256;
  },
  /** Returns whether `value` is positive or not. */
  isPositive(byte) {
    return !(byte >> 7 & 1);
  },
  /** Returns whether `value` is negative or not. */
  isNegative(byte) {
    return !!(byte >> 7 & 1);
  },
  /** Converts a signed `byte` to negative. */
  negate(byte) {
    return 256 - byte;
  },
  /** Forces a `value` to fit in 8 bits (256 => 0). */
  force8Bit(value) {
    return value & 0xff;
  },
  /** Forces a `value` to fit in 16 bits (65536 => 0). */
  force16Bit(value) {
    return value & 0xffff;
  },
  /** Returns the bit located at `position` in `number`. */
  getBit(number, position) {
    return number >> position & 1;
  },
  /** Returns a sub-number of `size` bits inside a `byte`, starting at `startPosition`. */
  getBits(byte, startPosition, size) {
    return byte >> startPosition & 0xff >> 8 - size;
  },
  /** Inserts a `value` of `size` bits inside a `byte`, starting at `startPosition`. */
  setBits(byte, startPosition, size, value) {
    const mask = (1 << size) - 1 << startPosition;
    return byte & ~mask | value << startPosition & mask;
  },
  /** Returns the most significant byte of a `twoBytesNumber`. */
  highPartOf(twoBytesNumber) {
    return twoBytesNumber >> 8;
  },
  /** Returns the least significant byte of a `twoBytesNumber`. */
  lowPartOf(twoBytesNumber) {
    return twoBytesNumber & 0x00ff;
  },
  /** Returns a two bytes value from the `highByte` and `lowByte`. */
  to16Bit(highByte, lowByte) {
    return (highByte & 0xff) << 8 | lowByte & 0xff;
  }
};