const MI_TO_KM = 1.609344; // exact, international mile

function clock(totalMinutes) {
  let totalSeconds = Math.round(totalMinutes * 60);
  const hours = Math.floor(totalSeconds / 3600);
  totalSeconds -= hours * 3600;
  const minutes = Math.floor(totalSeconds / 60);
  const seconds = totalSeconds - minutes * 60;
  const pad = (n) => String(n).padStart(2, "0");
  return `${hours}:${pad(minutes)}:${pad(seconds)}`;
}

module.exports = {
  slug: "running-pace-and-finish-time",
  title: "Running pace and finish time",
  deck_category: "health",
  description: "Computes finish time from distance and pace, and converts distance and pace between kilometers and miles using the exact international mile.",
  inputs: [
    { name: "distance", type: "number", unit: "km or mi", required: true, description: "Race or run distance", min: 0 },
    { name: "distance_unit", type: "string", required: true, description: "Unit of the distance value", enum: ["km", "mi"] },
    { name: "pace_minutes_per_unit", type: "number", unit: "minutes per distance_unit", required: true, description: "Pace expressed in minutes per the given distance_unit", min: 0 }
  ],
  outputs: [
    { name: "finish_time_minutes", unit: "minutes", description: "Total finish time in decimal minutes" },
    { name: "finish_time_clock", unit: "h:mm:ss", description: "Total finish time formatted as hours:minutes:seconds" },
    { name: "equivalent_distance", unit: "the other unit", description: "Distance converted to the other unit" },
    { name: "equivalent_pace", unit: "minutes per the other unit", description: "Pace converted to the other unit" }
  ],
  steps: [
    "finish_time_minutes = distance * pace_minutes_per_unit, rounded to 2 decimals.",
    "finish_time_clock converts finish_time_minutes to hours:minutes:seconds.",
    "If distance_unit is km: equivalent_distance (mi) = distance / 1.609344; equivalent_pace (min/mi) = pace_minutes_per_unit * 1.609344.",
    "If distance_unit is mi: equivalent_distance (km) = distance * 1.609344; equivalent_pace (min/km) = pace_minutes_per_unit / 1.609344.",
    "Round equivalent_distance and equivalent_pace to 2 decimals."
  ],
  constants: [
    { name: "mi_to_km", value: 1.609344, unit: "km per mi", source: 0 }
  ],
  sources: [
    { kind: "edition_page", value: "International Yard and Pound Agreement, 1959", title: "Exact statute mile to kilometer conversion factor", publisher: "NIST Handbook 44, Appendix C", retrieved_utc: "2026-09-30T19:40:00Z", note: "Defines 1 international mile = 1.609344 km exactly." }
  ],
  disclaimer: "General formula only. Not medical advice. Dosing decisions belong to a licensed clinician and the product's label.",
  questionText(inputs) {
    return `At a pace of ${inputs.pace_minutes_per_unit} minutes per ${inputs.distance_unit}, what is the finish time for ${inputs.distance} ${inputs.distance_unit}?`;
  },
  run(inputs) {
    const { distance, distance_unit, pace_minutes_per_unit } = inputs;
    if (typeof distance !== "number" || distance <= 0) throw new Error("distance must be a positive number");
    if (typeof pace_minutes_per_unit !== "number" || pace_minutes_per_unit <= 0) throw new Error("pace_minutes_per_unit must be a positive number");
    if (distance_unit !== "km" && distance_unit !== "mi") throw new Error("distance_unit must be km or mi");
    const finishRaw = distance * pace_minutes_per_unit;
    const finishTime = Math.round(finishRaw * 100) / 100;
    let equivDistanceRaw, equivPaceRaw;
    if (distance_unit === "km") {
      equivDistanceRaw = distance / MI_TO_KM;
      equivPaceRaw = pace_minutes_per_unit * MI_TO_KM;
    } else {
      equivDistanceRaw = distance * MI_TO_KM;
      equivPaceRaw = pace_minutes_per_unit / MI_TO_KM;
    }
    const equivDistance = Math.round(equivDistanceRaw * 100) / 100;
    const equivPace = Math.round(equivPaceRaw * 100) / 100;
    const otherUnit = distance_unit === "km" ? "mi" : "km";
    const finishClock = clock(finishRaw);
    const answer = `Finish time ${finishClock} (${finishTime.toFixed(2)} min); equivalent ${equivDistance.toFixed(2)} ${otherUnit} at ${equivPace.toFixed(2)} min/${otherUnit}`;
    return {
      answer,
      values: { finish_time_minutes: finishTime, finish_time_clock: finishClock, equivalent_distance: equivDistance, equivalent_pace: equivPace }
    };
  },
  examples: [
    {
      name: "worked example",
      inputs: { distance: 10, distance_unit: "km", pace_minutes_per_unit: 5 },
      expected: { answer: "Finish time 0:50:00 (50.00 min); equivalent 6.21 mi at 8.05 min/mi" },
      check: "10*5=50 min = 0:50:00. 10/1.609344=6.21371 rounds to 6.21. 5*1.609344=8.04672 rounds to 8.05."
    },
    {
      name: "test vector: miles input",
      inputs: { distance: 13.1, distance_unit: "mi", pace_minutes_per_unit: 9 },
      expected: { answer: "Finish time 1:57:54 (117.90 min); equivalent 21.08 km at 5.59 min/km" },
      check: "13.1*9=117.9 min = 1 hr 57.9 min = 1:57:54 (0.9 min = 54 sec). 13.1*1.609344=21.08241 rounds to 21.08. 9/1.609344=5.5924 rounds to 5.59."
    }
  ]
};