'use strict';
// mass-conversion.cjs -- METHOD DECK (X063), reversible mass unit conversion.
// Factors are the exact kilogram-equivalents from NIST SP 811 (2008 ed.), Appendix B.8 "Mass".

function roundSig(x, sig) {
  if (x === 0) return { value: 0, decimals: 0 };
  const neg = x < 0;
  const ax = Math.abs(x);
  let exp = Math.floor(Math.log10(ax));
  let decimals = Math.max(sig - exp - 1, 0);
  let factor = Math.pow(10, decimals);
  let value = Math.round(ax * factor) / factor;
  const newExp = value === 0 ? exp : Math.floor(Math.log10(value));
  if (newExp !== exp) {
    decimals = Math.max(sig - newExp - 1, 0);
    factor = Math.pow(10, decimals);
    value = Math.round(ax * factor) / factor;
  }
  return { value: neg ? -value : value, decimals };
}
function fmt(x, decimals) {
  return x.toLocaleString('en-US', { minimumFractionDigits: decimals, maximumFractionDigits: decimals });
}
function fmtSig(x, sig) {
  const r = roundSig(x, sig);
  return fmt(r.value, r.decimals);
}

// Kilograms per 1 unit. All exact by definition (NIST SP 811 Appendix B.8).
const TO_KG = {
  mg: 0.000001,
  g: 0.001,
  kg: 1,
  t: 1000,
  oz: 0.028349523125,
  lb: 0.45359237,
  st: 6.35029318,
};
const UNITS = Object.keys(TO_KG);

module.exports = {
  slug: 'mass-conversion',
  title: 'Mass conversion',
  deck_category: 'units',
  description: 'Converts a mass between metric and avoirdupois units using exact NIST SP 811 kilogram-equivalents.',
  inputs: [
    { name: 'value', type: 'number', unit: 'from_unit', required: true, description: 'The mass to convert' },
    { name: 'from_unit', type: 'string', required: true, description: 'Unit of the input value', enum: UNITS },
    { name: 'to_unit', type: 'string', required: true, description: 'Unit to convert into', enum: UNITS },
  ],
  outputs: [ { name: 'result', unit: 'to_unit', description: 'The converted mass' } ],
  steps: [
    'Look up the exact kilogram-equivalent of from_unit and of to_unit from the NIST SP 811 table.',
    'Convert the input to kilograms: kilograms = value * kg_per(from_unit).',
    'Convert kilograms to the target unit: result = kilograms / kg_per(to_unit).',
    'Round result to 6 significant figures.',
  ],
  constants: [
    { name: 'mg', value: 0.000001, unit: 'kg', source: 0 },
    { name: 'g', value: 0.001, unit: 'kg', source: 0 },
    { name: 'kg', value: 1, unit: 'kg', source: 0 },
    { name: 't', value: 1000, unit: 'kg', source: 0 },
    { name: 'oz', value: 0.028349523125, unit: 'kg', source: 0 },
    { name: 'lb', value: 0.45359237, unit: 'kg', source: 0 },
    { name: 'st', value: 6.35029318, unit: 'kg', source: 0 },
  ],
  sources: [
    { kind: 'standard', value: 'NIST Special Publication 811 (2008 edition), Appendix B.8, Table for mass (avoirdupois)', title: 'Guide for the Use of the International System of Units (SI)', publisher: 'National Institute of Standards and Technology', retrieved_utc: '2026-09-30T19:29:48Z', note: 'oz and lb are exact avoirdupois definitions; st (stone) = 14 lb exactly; t is the metric tonne (1000 kg exactly).' },
  ],
  disclaimer: null,
  questionText(inputs) {
    return `How many ${inputs.to_unit} are in ${inputs.value} ${inputs.from_unit}?`;
  },
  run(inputs) {
    const { value, from_unit, to_unit } = inputs || {};
    if (typeof value !== 'number' || !isFinite(value)) throw new Error('value must be a finite number');
    if (!TO_KG.hasOwnProperty(from_unit)) throw new Error('from_unit must be one of: ' + UNITS.join(', '));
    if (!TO_KG.hasOwnProperty(to_unit)) throw new Error('to_unit must be one of: ' + UNITS.join(', '));
    const kg = value * TO_KG[from_unit];
    const result = kg / TO_KG[to_unit];
    return { answer: `${fmtSig(result, 6)} ${to_unit}`, values: { result } };
  },
  examples: [
    { name: 'worked example', inputs: { value: 10, from_unit: 'lb', to_unit: 'kg' },
      expected: { answer: '4.53592 kg' },
      check: 'By hand: 10 lb * 0.45359237 kg/lb = 4.5359237 kg, rounded to 6 significant figures is 4.53592.' },
    { name: 'test vector: stone to pounds', inputs: { value: 1, from_unit: 'st', to_unit: 'lb' },
      expected: { answer: '14.0000 lb' },
      check: 'By definition 1 stone = 14 lb exactly: 6.35029318 kg / 0.45359237 kg/lb = 14 exactly.' },
    { name: 'test vector: kilograms to ounces', inputs: { value: 5, from_unit: 'kg', to_unit: 'oz' },
      expected: { answer: '176.370 oz' },
      check: 'By hand: 5 kg / 0.028349523125 kg/oz = 176.3698...; rounded to 6 significant figures is 176.370.' },
  ],
};