"use strict";

function round2(x) { return Math.round(x * 100) / 100; }

function windChillF(tempF, windMph) {
  const v016 = Math.pow(windMph, 0.16);
  return 35.74 + 0.6215 * tempF - 35.75 * v016 + 0.4275 * tempF * v016;
}

module.exports = {
  slug: "wind-chill-us-nws",
  title: "Wind chill (US NWS 2001 formula)",
  deck_category: "other",
  description: "Computes the wind chill temperature using the US National Weather Service's 2001 formula.",
  inputs: [
    { name: "temp_f", type: "number", unit: "degrees F", required: true, description: "Air temperature.", max: 50 },
    { name: "wind_mph", type: "number", unit: "mph", required: true, description: "Sustained wind speed.", min: 3 }
  ],
  outputs: [
    { name: "wind_chill_f", unit: "degrees F", description: "Wind chill temperature." }
  ],
  steps: [
    "NWS 2001 wind chill formula: WC = 35.74 + 0.6215*T - 35.75*V^0.16 + 0.4275*T*V^0.16, where T is air temperature in F and V is wind speed in mph.",
    "Valid range: temperature at or below 50 F and wind speed at or above 3 mph. Outside this range the formula is not meaningful.",
    "Round to the nearest whole degree."
  ],
  constants: [
    { name: "wind_chill_valid_max_temp_f", value: 50, unit: "degrees F", source: 0 },
    { name: "wind_chill_valid_min_wind_mph", value: 3, unit: "mph", source: 0 }
  ],
  sources: [
    { kind: "url", value: "https://www.weather.gov/safety/cold-wind-chill-chart", title: "Wind Chill Chart and the 2001 NWS wind chill formula", publisher: "National Weather Service (NOAA)", retrieved_utc: "2026-09-30T19:52:00Z", note: "Official NWS wind chill formula adopted in 2001, valid for T<=50F and V>=3mph." }
  ],
  disclaimer: null,
  questionText(inputs) {
    return `Using the NWS 2001 formula, what is the wind chill at ${inputs.temp_f} degrees F with a ${inputs.wind_mph} mph wind?`;
  },
  run(inputs) {
    const { temp_f, wind_mph } = inputs || {};
    if (typeof temp_f !== "number" || temp_f > 50) throw new Error("temp_f must be a number at or below 50");
    if (typeof wind_mph !== "number" || wind_mph < 3) throw new Error("wind_mph must be a number at or above 3");
    const wc = Math.round(windChillF(temp_f, wind_mph));
    return { answer: `${wc} degrees F`, values: { wind_chill_f: wc } };
  },
  examples: [
    {
      name: "worked example",
      inputs: { temp_f: 5, wind_mph: 25 },
      expected: { answer: "-17 degrees F" },
      check: "By hand: 25^0.16 = e^(0.16*ln25) = e^0.51502 = 1.6737; WC = 35.74 + 3.1075 - 35.75*1.6737 + 0.4275*5*1.6737 = 38.8475 - 59.8348 + 3.5776 = -17.41, rounding to -17."
    },
    {
      name: "test vector: milder cold",
      inputs: { temp_f: 30, wind_mph: 10 },
      expected: { answer: "21 degrees F" },
      check: "By hand: 10^0.16 = e^(0.16*ln10) = e^0.36841 = 1.4454; WC = 35.74 + 18.645 - 35.75*1.4454 + 0.4275*30*1.4454 = 54.385 - 51.673 + 18.537 = 21.25, rounding to 21."
    }
  ]
};