const GAS_MARK_TABLE = [
  { mark: "1/4", c: 110, f: 225 },
  { mark: "1/2", c: 120, f: 250 },
  { mark: "1", c: 140, f: 275 },
  { mark: "2", c: 150, f: 300 },
  { mark: "3", c: 170, f: 325 },
  { mark: "4", c: 180, f: 350 },
  { mark: "5", c: 190, f: 375 },
  { mark: "6", c: 200, f: 400 },
  { mark: "7", c: 220, f: 425 },
  { mark: "8", c: 230, f: 450 },
  { mark: "9", c: 240, f: 475 }
];

function nearestGasMark(celsius) {
  let best = GAS_MARK_TABLE[0];
  let bestDiff = Math.abs(GAS_MARK_TABLE[0].c - celsius);
  for (const row of GAS_MARK_TABLE) {
    const diff = Math.abs(row.c - celsius);
    if (diff < bestDiff) {
      best = row;
      bestDiff = diff;
    }
  }
  return best.mark;
}

module.exports = {
  slug: "oven-temperature-conversion",
  title: "Oven temperature conversion",
  deck_category: "cooking",
  description: "Converts an oven temperature between Fahrenheit and Celsius, finds the nearest UK gas mark, and gives the standard fan-oven adjustment.",
  inputs: [
    { name: "temperature", type: "number", required: true, description: "Oven temperature value" },
    { name: "unit", type: "string", required: true, description: "Unit of the given temperature", enum: ["F", "C"] }
  ],
  outputs: [
    { name: "celsius", unit: "C", description: "Temperature in Celsius" },
    { name: "fahrenheit", unit: "F", description: "Temperature in Fahrenheit" },
    { name: "gas_mark", unit: null, description: "Nearest UK gas mark from the cited table" },
    { name: "fan_oven_celsius", unit: "C", description: "Suggested fan (convection) oven temperature, conventional minus 20 C" }
  ],
  steps: [
    "If given in Fahrenheit: celsius = (fahrenheit - 32) * 5 / 9. If given in Celsius: fahrenheit = celsius * 9 / 5 + 32.",
    "Round celsius and fahrenheit to the nearest whole degree.",
    "gas_mark is the entry in the cited gas mark table whose Celsius value is nearest to the rounded celsius value.",
    "Fan (convection) ovens run hotter than conventional (static) ovens; the standard adjustment is to reduce the conventional Celsius temperature by about 20 C (or the Fahrenheit temperature by about 25 F) for a fan oven.",
    "fan_oven_celsius = celsius - 20, rounded to the nearest whole degree."
  ],
  constants: [
    { name: "gas_mark_1_4_c", value: 110, unit: "C", source: 0 },
    { name: "gas_mark_1_2_c", value: 120, unit: "C", source: 0 },
    { name: "gas_mark_1_c", value: 140, unit: "C", source: 0 },
    { name: "gas_mark_2_c", value: 150, unit: "C", source: 0 },
    { name: "gas_mark_3_c", value: 170, unit: "C", source: 0 },
    { name: "gas_mark_4_c", value: 180, unit: "C", source: 0 },
    { name: "gas_mark_5_c", value: 190, unit: "C", source: 0 },
    { name: "gas_mark_6_c", value: 200, unit: "C", source: 0 },
    { name: "gas_mark_7_c", value: 220, unit: "C", source: 0 },
    { name: "gas_mark_8_c", value: 230, unit: "C", source: 0 },
    { name: "gas_mark_9_c", value: 240, unit: "C", source: 0 },
    { name: "fan_oven_reduction_c", value: 20, unit: "C", source: 1 }
  ],
  sources: [
    { kind: "edition_page", value: "Rombauer IS, Becker MR, Becker E. The Joy of Cooking, 2006 edition, oven temperature and gas mark conversion table, p. 3.", title: "Standard Fahrenheit/Celsius/gas mark oven temperature table", publisher: "Scribner", retrieved_utc: "2026-09-30T19:55:00Z", note: "Source of the gas mark to Celsius/Fahrenheit correspondence used here." },
    { kind: "url", value: "https://www.which.co.uk/reviews/built-in-ovens/article/oven-temperature-conversion-table-aA5Ol9b157On", title: "Oven Temperature Conversion Table", publisher: "Which? (Which? Ltd, UK Consumers' Association)", retrieved_utc: "2026-09-30T22:29:13Z", note: "Confirmed table matches this card exactly: gas mark 4 = 350F/180C/160C fan, gas mark 6 = 400F/200C/180C fan, gas mark 7 = 425F/220C/200C fan; the page also states fan ovens run about 20C cooler than conventional ovens." }
  ],
  disclaimer: null,
  questionText(inputs) {
    return `What is ${inputs.temperature} degrees ${inputs.unit} in the other temperature scale, the nearest gas mark, and the fan-oven adjusted temperature?`;
  },
  run(inputs) {
    const { temperature, unit } = inputs;
    if (typeof temperature !== "number") throw new Error("temperature must be a number");
    let celsiusRaw, fahrenheitRaw;
    if (unit === "F") {
      fahrenheitRaw = temperature;
      celsiusRaw = (temperature - 32) * 5 / 9;
    } else if (unit === "C") {
      celsiusRaw = temperature;
      fahrenheitRaw = (temperature * 9) / 5 + 32;
    } else {
      throw new Error("unit must be F or C");
    }
    const celsius = Math.round(celsiusRaw);
    const fahrenheit = Math.round(fahrenheitRaw);
    const gasMark = nearestGasMark(celsius);
    const fanOvenCelsius = Math.round(celsius - 20);
    const answer = `${celsius} C / ${fahrenheit} F, gas mark ${gasMark}, fan oven ${fanOvenCelsius} C`;
    return { answer, values: { celsius, fahrenheit, gas_mark: gasMark, fan_oven_celsius: fanOvenCelsius } };
  },
  examples: [
    {
      name: "worked example",
      inputs: { temperature: 180, unit: "C" },
      expected: { answer: "180 C / 356 F, gas mark 4, fan oven 160 C" },
      check: "180*9/5+32 = 324+32 = 356. Nearest gas mark table entry to 180 C is gas mark 4 (180 C) exactly. Fan oven = 180-20 = 160."
    },
    {
      name: "test vector: Fahrenheit input",
      inputs: { temperature: 425, unit: "F" },
      expected: { answer: "218 C / 425 F, gas mark 7, fan oven 198 C" },
      check: "(425-32)*5/9 = 393*5/9 = 218.333, rounds to 218. Nearest gas mark table entry to 218 C is gas mark 7 (220 C), diff 2, closer than gas mark 6 (200 C, diff 18). Fan oven = 218-20 = 198."
    }
  ]
};