'use strict';
// unit-price-comparison.cjs -- METHOD DECK (X063), price per unit for two package sizes and
// which is cheaper. Standard consumer-math unit pricing: unit price = total price / quantity.

function fmt(x, decimals) {
  return x.toLocaleString('en-US', { minimumFractionDigits: decimals, maximumFractionDigits: decimals });
}

module.exports = {
  slug: 'unit-price-comparison',
  title: 'Unit price comparison',
  deck_category: 'currency',
  description: 'Computes the price per unit for two package sizes and reports which package is the better value.',
  inputs: [
    { name: 'price_a', type: 'number', required: true, description: 'Total price of package A', min: 0 },
    { name: 'quantity_a', type: 'number', required: true, description: 'Quantity (in a common unit, e.g. oz or count) of package A', min: 0 },
    { name: 'price_b', type: 'number', required: true, description: 'Total price of package B', min: 0 },
    { name: 'quantity_b', type: 'number', required: true, description: 'Quantity (in the same unit as quantity_a) of package B', min: 0 },
  ],
  outputs: [
    { name: 'unit_price_a', description: 'price_a / quantity_a' },
    { name: 'unit_price_b', description: 'price_b / quantity_b' },
    { name: 'cheaper', description: '"A", "B", or "tie"' },
  ],
  steps: [
    'Compute the unit price of package A: unit_price_a = price_a / quantity_a.',
    'Compute the unit price of package B: unit_price_b = price_b / quantity_b.',
    'The package with the lower unit price is the better value; equal unit prices are a tie.',
    'Round both unit prices to 4 decimal places (the conventional precision for per-unit shelf pricing).',
  ],
  constants: [],
  sources: [
    { kind: 'url', value: 'https://portal.ct.gov/dcp/common-elements/publications/saving-money-with-unit-pricing', title: 'Saving Money with Unit Pricing', publisher: 'Connecticut Department of Consumer Protection', retrieved_utc: '2026-09-30T22:29:13Z', note: 'Confirmed live: unit price is the amount paid per unit of measure (per pound, ounce, quart, gallon, etc.), used to compare package sizes of different brands and sizes on an equal footing; the page works a worked example of two package prices reduced to a common per-pound unit price.' },
  ],
  disclaimer: null,
  questionText(inputs) {
    return `Package A costs ${inputs.price_a} for ${inputs.quantity_a} units and package B costs ${inputs.price_b} for ${inputs.quantity_b} units. Which is cheaper per unit?`;
  },
  run(inputs) {
    const { price_a, quantity_a, price_b, quantity_b } = inputs || {};
    for (const [k, v] of [['price_a', price_a], ['quantity_a', quantity_a], ['price_b', price_b], ['quantity_b', quantity_b]]) {
      if (typeof v !== 'number' || !isFinite(v) || v < 0) throw new Error(k + ' must be a nonnegative finite number');
    }
    if (quantity_a === 0) throw new Error('quantity_a must be greater than zero');
    if (quantity_b === 0) throw new Error('quantity_b must be greater than zero');
    const unitA = Math.round((price_a / quantity_a) * 10000) / 10000;
    const unitB = Math.round((price_b / quantity_b) * 10000) / 10000;
    let cheaper;
    if (unitA < unitB) cheaper = 'A';
    else if (unitB < unitA) cheaper = 'B';
    else cheaper = 'tie';
    const verdict = cheaper === 'tie' ? 'the two packages are the same price per unit' : `package ${cheaper} is cheaper per unit`;
    const answer = `Package A: ${fmt(unitA, 4)} per unit; Package B: ${fmt(unitB, 4)} per unit; ${verdict}.`;
    return { answer, values: { unit_price_a: unitA, unit_price_b: unitB, cheaper } };
  },
  examples: [
    { name: 'worked example', inputs: { price_a: 4.99, quantity_a: 16, price_b: 7.99, quantity_b: 32 },
      expected: { answer: 'Package A: 0.3119 per unit; Package B: 0.2497 per unit; package B is cheaper per unit.' },
      check: 'By hand: 4.99/16 = 0.311875 -> 0.3119; 7.99/32 = 0.2496875 -> 0.2497. 0.2497 < 0.3119, so B is cheaper.' },
    { name: 'test vector: A is cheaper', inputs: { price_a: 3.00, quantity_a: 12, price_b: 5.00, quantity_b: 18 },
      expected: { answer: 'Package A: 0.2500 per unit; Package B: 0.2778 per unit; package A is cheaper per unit.' },
      check: 'By hand: 3.00/12 = 0.25 exactly; 5.00/18 = 0.277778 -> 0.2778. 0.25 < 0.2778, so A is cheaper.' },
    { name: 'test vector: tie', inputs: { price_a: 10, quantity_a: 5, price_b: 20, quantity_b: 10 },
      expected: { answer: 'Package A: 2.0000 per unit; Package B: 2.0000 per unit; the two packages are the same price per unit.' },
      check: 'By hand: 10/5 = 2 exactly and 20/10 = 2 exactly, so both unit prices are equal: a tie.' },
  ],
};