lib/internal/webidl.js

'use strict';

const {
  MathAbs,
  MathMax,
  MathMin,
  MathPow,
  MathSign,
  MathTrunc,
  NumberIsNaN,
  NumberMAX_SAFE_INTEGER,
  NumberMIN_SAFE_INTEGER,
} = primordials;

const {
  codes: {
    ERR_INVALID_ARG_VALUE,
  },
} = require('internal/errors');
const { kEmptyObject } = require('internal/util');

// https://webidl.spec.whatwg.org/#abstract-opdef-integerpart
const integerPart = MathTrunc;

/* eslint-disable node-core/non-ascii-character */
// Round x to the nearest integer, choosing the even integer if it lies halfway
// between two, and choosing +0 rather than -0.
// This is different from Math.round, which rounds to the next integer in the
// direction of +∞ when the fraction portion is exactly 0.5.
/* eslint-enable node-core/non-ascii-character */
function evenRound(x) {
  // Convert -0 to +0.
  const i = integerPart(x) + 0;
  const reminder = MathAbs(x % 1);
  const sign = MathSign(i);
  if (reminder === 0.5) {
    return i % 2 === 0 ? i : i + sign;
  }
  const r = reminder < 0.5 ? i : i + sign;
  // Convert -0 to +0.
  if (r === 0) {
    return 0;
  }
  return r;
}

function pow2(exponent) {
  // << operates on 32 bit signed integers.
  if (exponent < 31) {
    return 1 << exponent;
  }
  if (exponent === 31) {
    return 0x8000_0000;
  }
  if (exponent === 32) {
    return 0x1_0000_0000;
  }
  return MathPow(2, exponent);
}

// https://tc39.es/ecma262/#eqn-modulo
// The notation “x modulo y” computes a value k of the same sign as y.
function modulo(x, y) {
  const r = x % y;
  // Convert -0 to +0.
  if (r === 0) {
    return 0;
  }
  return r;
}

// https://webidl.spec.whatwg.org/#abstract-opdef-converttoint
function convertToInt(name, value, bitLength, options = kEmptyObject) {
  const { signed = false, enforceRange = false, clamp = false } = options;

  let upperBound;
  let lowerBound;
  // 1. If bitLength is 64, then:
  if (bitLength === 64) {
    // 1.1. Let upperBound be 2^53 − 1.
    upperBound = NumberMAX_SAFE_INTEGER;
    // 1.2. If signedness is "unsigned", then let lowerBound be 0.
    // 1.3. Otherwise let lowerBound be −2^53 + 1.
    lowerBound = !signed ? 0 : NumberMIN_SAFE_INTEGER;
  } else if (!signed) {
    // 2. Otherwise, if signedness is "unsigned", then:
    // 2.1. Let lowerBound be 0.
    // 2.2. Let upperBound be 2^bitLength − 1.
    lowerBound = 0;
    upperBound = pow2(bitLength) - 1;
  } else {
    // 3. Otherwise:
    // 3.1. Let lowerBound be -2^(bitLength − 1).
    // 3.2. Let upperBound be 2^(bitLength − 1) − 1.
    lowerBound = -pow2(bitLength - 1);
    upperBound = pow2(bitLength - 1) - 1;
  }

  // 4. Let x be ? ToNumber(V).
  let x = +value;
  // 5. If x is −0, then set x to +0.
  if (x === 0) {
    x = 0;
  }

  // 6. If the conversion is to an IDL type associated with the [EnforceRange]
  // extended attribute, then:
  if (enforceRange) {
    // 6.1. If x is NaN, +∞, or −∞, then throw a TypeError.
    if (NumberIsNaN(x) || x === Infinity || x === -Infinity) {
      throw new ERR_INVALID_ARG_VALUE(name, x);
    }
    // 6.2. Set x to IntegerPart(x).
    x = integerPart(x);

    // 6.3. If x < lowerBound or x > upperBound, then throw a TypeError.
    if (x < lowerBound || x > upperBound) {
      throw new ERR_INVALID_ARG_VALUE(name, x);
    }

    // 6.4. Return x.
    return x;
  }

  // 7. If x is not NaN and the conversion is to an IDL type associated with
  // the [Clamp] extended attribute, then:
  if (clamp && !NumberIsNaN(x)) {
    // 7.1. Set x to min(max(x, lowerBound), upperBound).
    x = MathMin(MathMax(x, lowerBound), upperBound);

    // 7.2. Round x to the nearest integer, choosing the even integer if it
    // lies halfway between two, and choosing +0 rather than −0.
    x = evenRound(x);

    // 7.3. Return x.
    return x;
  }

  // 8. If x is NaN, +0, +∞, or −∞, then return +0.
  if (NumberIsNaN(x) || x === 0 || x === Infinity || x === -Infinity) {
    return 0;
  }

  // 9. Set x to IntegerPart(x).
  x = integerPart(x);

  // 10. Set x to x modulo 2^bitLength.
  x = modulo(x, pow2(bitLength));

  // 11. If signedness is "signed" and x ≥ 2^(bitLength − 1), then return x −
  // 2^bitLength.
  if (signed && x >= pow2(bitLength - 1)) {
    return x - pow2(bitLength);
  }

  // 12. Otherwise, return x.
  return x;
}

module.exports = {
  convertToInt,
  evenRound,
};