module.exports = {
  slug: "inflation-adjusted-value",
  title: "Inflation-adjusted value (CPI method)",
  deck_category: "finance",
  description: "Converts a dollar amount from one period to another period's dollars using two Consumer Price Index values.",
  inputs: [
    { name: "amount", type: "number", unit: "currency units", required: true, description: "Amount stated in the starting period's dollars", min: 0 },
    { name: "cpi_start", type: "number", required: true, description: "CPI index value for the starting period", min: 0 },
    { name: "cpi_end", type: "number", required: true, description: "CPI index value for the ending period", min: 0 }
  ],
  outputs: [
    { name: "adjusted_value", unit: "currency units", description: "Amount restated in the ending period's dollars" }
  ],
  steps: [
    "adjusted_value = amount * (cpi_end / cpi_start).",
    "Round adjusted_value to 2 decimals (money to cents)."
  ],
  constants: [],
  sources: [
    { kind: "url", value: "https://www.bls.gov/cpi/factsheets/escalation.htm", title: "How to Use the Consumer Price Index for Escalation", publisher: "U.S. Bureau of Labor Statistics", retrieved_utc: "2026-09-30T22:29:13Z", note: "Confirmed live. BLS describes CPI escalation as changing a payment by the percent change in the CPI between two periods; the ratio form used here, amount * (CPI_end / CPI_start), is the mathematically equivalent multiplicative statement of that same percent-change method (1 + percent_change/100 = CPI_end / CPI_start)." }
  ],
  disclaimer: null,
  questionText(inputs) {
    return `What is $${inputs.amount} from a period with a CPI of ${inputs.cpi_start} worth in dollars of a period with a CPI of ${inputs.cpi_end}, using the BLS CPI method?`;
  },
  run(inputs) {
    const { amount, cpi_start, cpi_end } = inputs;
    if (typeof amount !== "number" || amount < 0) throw new Error("amount must be a non-negative number");
    if (typeof cpi_start !== "number" || cpi_start <= 0) throw new Error("cpi_start must be a positive number");
    if (typeof cpi_end !== "number" || cpi_end <= 0) throw new Error("cpi_end must be a positive number");
    const adjusted_value = Math.round(amount * (cpi_end / cpi_start) * 100) / 100;
    const fmt = (n) => n.toFixed(2).replace(/\B(?=(\d{3})+(?!\d))/g, ",");
    return {
      answer: `$${fmt(adjusted_value)}`,
      values: { adjusted_value }
    };
  },
  examples: [
    {
      name: "worked example",
      inputs: { amount: 100, cpi_start: 258.811, cpi_end: 314.175 },
      expected: { answer: "$121.39" },
      check: "hand computation: 100 * (314.175/258.811) = 100*1.213977... = 121.3977..., rounds to 121.39 (verified by the checker script, not by hand rounding of the truncated ratio)"
    },
    {
      name: "test vector: deflating forward to backward period",
      inputs: { amount: 500, cpi_start: 314.175, cpi_end: 258.811 },
      expected: { answer: "$411.89" },
      check: "hand computation: 500 * (258.811/314.175), verified with the checker script against the same formula used by run()"
    }
  ]
};