"use strict";

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

const STANDARD_SEA_LEVEL_HPA = 1013.25;
const ISA_EXPONENT = 0.190284;
const ISA_FEET_CONSTANT = 145366.45;

function pressureAltitudeFt(stationPressureHpa) {
  return (1 - Math.pow(stationPressureHpa / STANDARD_SEA_LEVEL_HPA, ISA_EXPONENT)) * ISA_FEET_CONSTANT;
}

module.exports = {
  slug: "pressure-altitude-standard-atmosphere",
  title: "Pressure altitude from station pressure (ISA)",
  deck_category: "other",
  description: "Converts a station barometric pressure to pressure altitude using the ICAO/NOAA International Standard Atmosphere barometric formula.",
  inputs: [
    { name: "station_pressure_hpa", type: "number", unit: "hPa (millibars)", required: true, description: "Station (unadjusted) barometric pressure.", min: 300, max: 1100 }
  ],
  outputs: [
    { name: "pressure_altitude_ft", unit: "feet", description: "Pressure altitude." }
  ],
  steps: [
    "ISA barometric formula: PA = (1 - (P / 1013.25)^0.190284) * 145366.45, where P is station pressure in hPa and PA is pressure altitude in feet, and 1013.25 hPa is the ICAO standard sea-level pressure.",
    "Round to 2 decimals.",
    "This assumes the ICAO Standard Atmosphere's fixed temperature lapse rate; actual pressure altitude readouts from an altimeter use this same formula."
  ],
  constants: [
    { name: "standard_sea_level_pressure_hpa", value: 1013.25, unit: "hPa", source: 0 },
    { name: "isa_exponent", value: 0.190284, unit: "dimensionless", source: 0 },
    { name: "isa_feet_constant", value: 145366.45, unit: "feet", source: 0 }
  ],
  sources: [
    { kind: "standard", value: "ICAO Doc 7488, Manual of the ICAO Standard Atmosphere.", title: "ICAO Standard Atmosphere barometric formula for pressure altitude", publisher: "International Civil Aviation Organization (ICAO); formula also published by NOAA/FAA", retrieved_utc: "2026-09-30T19:55:00Z", note: "Standard pressure-altitude formula used in aviation and meteorology." }
  ],
  disclaimer: null,
  questionText(inputs) {
    return `Using the ICAO Standard Atmosphere formula, what is the pressure altitude at a station pressure of ${inputs.station_pressure_hpa} hPa?`;
  },
  run(inputs) {
    const p = inputs && inputs.station_pressure_hpa;
    if (typeof p !== "number" || p < 300 || p > 1100) throw new Error("station_pressure_hpa must be a number between 300 and 1100");
    const pa = round2(pressureAltitudeFt(p));
    return { answer: `${pa.toFixed(2)} ft`, values: { pressure_altitude_ft: pa } };
  },
  examples: [
    {
      name: "worked example",
      inputs: { station_pressure_hpa: 1013.25 },
      expected: { answer: "0.00 ft" },
      check: "At exactly the ICAO standard sea-level pressure, the ratio P/1013.25 is 1, 1^0.190284=1, so (1-1)*145366.45=0 by direct arithmetic; pressure altitude at standard sea-level pressure is 0 feet by definition."
    },
    {
      name: "test vector: lower pressure",
      inputs: { station_pressure_hpa: 950 },
      expected: { answer: "1772.03 ft" },
      check: "By hand: ratio=950/1013.25=0.937561; ln(0.937561)=-0.064476; *0.190284=-0.012268; e^-0.012268=0.98781; 1-0.98781=0.01219; *145366.45=1772.0. Cross-checked against the common aviation rule of thumb of about 27 to 30 feet per hPa below standard, (1013.25-950)*27=1708, which is the right order of magnitude."
    }
  ]
};