'use strict';
// pressure-conversion.cjs -- METHOD DECK (X063), reversible pressure unit conversion.
// Factors are exact pascal-equivalents from NIST SP 811 (2008 ed.), Appendix B.9 "Pressure".

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);
}

// psi: exact pound-force (0.45359237 kg * 9.80665 m/s2, both exact) per exact square inch (0.0254 m)^2.
const PSI_PA = (0.45359237 * 9.80665) / (0.0254 * 0.0254);
// Conventional mercury density 13595.1 kg/m3 * standard gravity 9.80665 m/s2 * column height.
const MMHG_PA = 13595.1 * 9.80665 * 0.001;
const INHG_PA = 13595.1 * 9.80665 * 0.0254;

// Pascals per 1 unit.
const TO_PA = {
  Pa: 1,
  kPa: 1000,
  bar: 100000,
  atm: 101325,
  psi: PSI_PA,
  mmHg: MMHG_PA,
  inHg: INHG_PA,
};
const UNITS = Object.keys(TO_PA);

module.exports = {
  slug: 'pressure-conversion',
  title: 'Pressure conversion',
  deck_category: 'units',
  description: 'Converts a pressure between Pa, kPa, bar, atm, psi, mmHg, and inHg using exact NIST SP 811 pascal-equivalents.',
  inputs: [
    { name: 'value', type: 'number', unit: 'from_unit', required: true, description: 'The pressure 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 pressure' } ],
  steps: [
    'Look up the exact pascal-equivalent of from_unit and of to_unit.',
    'Convert the input to pascals: pa = value * pa_per(from_unit).',
    'Convert pascals to the target unit: result = pa / pa_per(to_unit).',
    'Round result to 6 significant figures.',
  ],
  constants: [
    { name: 'Pa', value: 1, unit: 'Pa', source: 0 },
    { name: 'kPa', value: 1000, unit: 'Pa', source: 0 },
    { name: 'bar', value: 100000, unit: 'Pa', source: 0 },
    { name: 'atm', value: 101325, unit: 'Pa', source: 0 },
    { name: 'psi', value: PSI_PA, unit: 'Pa', source: 0 },
    { name: 'mmHg', value: MMHG_PA, unit: 'Pa', source: 0 },
    { name: 'inHg', value: INHG_PA, unit: 'Pa', source: 0 },
  ],
  sources: [
    { kind: 'standard', value: 'NIST Special Publication 811 (2008 edition), Appendix B.9, Table for pressure', 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: 'bar and standard atmosphere (atm) are exact by definition; psi is derived exactly from the pound-force and the square inch; mmHg and inHg use the conventional mercury density 13,595.1 kg/m3 at standard gravity 9.80665 m/s2.' },
  ],
  disclaimer: null,
  questionText(inputs) {
    return `What is ${inputs.value} ${inputs.from_unit} in ${inputs.to_unit}?`;
  },
  run(inputs) {
    const { value, from_unit, to_unit } = inputs || {};
    if (typeof value !== 'number' || !isFinite(value)) throw new Error('value must be a finite number');
    if (!TO_PA.hasOwnProperty(from_unit)) throw new Error('from_unit must be one of: ' + UNITS.join(', '));
    if (!TO_PA.hasOwnProperty(to_unit)) throw new Error('to_unit must be one of: ' + UNITS.join(', '));
    const pa = value * TO_PA[from_unit];
    const result = pa / TO_PA[to_unit];
    return { answer: `${fmtSig(result, 6)} ${to_unit}`, values: { result } };
  },
  examples: [
    { name: 'worked example', inputs: { value: 1, from_unit: 'atm', to_unit: 'psi' },
      expected: { answer: '14.6959 psi' },
      check: 'By hand: 101325 Pa / 6894.757293... Pa/psi = 14.69595...; rounded to 6 significant figures is 14.6959.' },
    { name: 'test vector: bar to kPa', inputs: { value: 1, from_unit: 'bar', to_unit: 'kPa' },
      expected: { answer: '100.000 kPa' },
      check: 'By definition 1 bar = 100,000 Pa = 100 kPa exactly.' },
    { name: 'test vector: inHg to mmHg', inputs: { value: 1, from_unit: 'inHg', to_unit: 'mmHg' },
      expected: { answer: '25.4000 mmHg' },
      check: 'By hand: both use the same mercury column density and gravity, so the ratio is exactly the length ratio: 1 inch / 1 mm = 25.4.' },
  ],
};