"use strict";

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

module.exports = {
  slug: "rectangle-room-paint-coverage",
  title: "Paint needed for a rectangular room",
  deck_category: "geometry",
  description: "Computes the liters of paint needed to cover a rectangular room's walls, given openings, number of coats, and coverage per liter.",
  inputs: [
    { name: "length", type: "number", unit: "meters", required: true, description: "Room length.", min: 0 },
    { name: "width", type: "number", unit: "meters", required: true, description: "Room width.", min: 0 },
    { name: "height", type: "number", unit: "meters", required: true, description: "Wall height.", min: 0 },
    { name: "openings_area", type: "number", unit: "square meters", required: true, description: "Total area of doors and windows to subtract.", min: 0 },
    { name: "coats", type: "integer", unit: "coats", required: true, description: "Number of coats of paint to apply.", min: 1 },
    { name: "coverage_per_liter", type: "number", unit: "square meters per liter", required: true, description: "Manufacturer-stated coverage of the paint, one coat.", min: 0 }
  ],
  outputs: [
    { name: "wall_area", unit: "square meters", description: "Net paintable wall area, one coat." },
    { name: "paint_needed_liters", unit: "liters", description: "Total paint needed for all coats." }
  ],
  steps: [
    "Wall perimeter = 2 * (length + width).",
    "Gross wall area = perimeter * height.",
    "Net wall area = gross wall area - openings_area.",
    "Paint needed (liters) = net wall area * coats / coverage_per_liter.",
    "Round both outputs to 2 decimals."
  ],
  constants: [],
  sources: [
    { kind: "url", value: "https://www.thisoldhouse.com/painting/21097077/how-much-paint-do-i-need", title: "Paint Calculator: How Much Paint Do I Need?", publisher: "This Old House", retrieved_utc: "2026-09-30T22:29:13Z", note: "Confirms the standard method: wall area (perimeter times height, minus openings) times number of coats, divided by the paint's stated coverage rate per gallon/liter." }
  ],
  disclaimer: null,
  questionText(inputs) {
    return `How many liters of paint are needed for a ${inputs.length}m by ${inputs.width}m room with ${inputs.height}m walls, ${inputs.openings_area} square meters of openings, ${inputs.coats} coat(s), and paint that covers ${inputs.coverage_per_liter} square meters per liter?`;
  },
  run(inputs) {
    const { length, width, height, openings_area, coats, coverage_per_liter } = inputs || {};
    for (const [name, v] of Object.entries({ length, width, height, openings_area, coverage_per_liter })) {
      if (typeof v !== "number" || v < 0) throw new Error(`${name} must be a non-negative number`);
    }
    if (typeof coats !== "number" || !Number.isInteger(coats) || coats < 1) throw new Error("coats must be a positive integer");
    if (coverage_per_liter <= 0) throw new Error("coverage_per_liter must be positive");
    const perimeter = 2 * (length + width);
    const grossArea = perimeter * height;
    const wallArea = grossArea - openings_area;
    if (wallArea < 0) throw new Error("openings_area exceeds the gross wall area");
    const paintNeeded = (wallArea * coats) / coverage_per_liter;
    const wallAreaR = round2(wallArea);
    const paintNeededR = round2(paintNeeded);
    return { answer: `${paintNeededR.toFixed(2)} liters`, values: { wall_area: wallAreaR, paint_needed_liters: paintNeededR } };
  },
  examples: [
    {
      name: "worked example",
      inputs: { length: 4, width: 3, height: 2.5, openings_area: 3, coats: 2, coverage_per_liter: 10 },
      expected: { answer: "6.40 liters" },
      check: "By hand: perimeter=2*(4+3)=14 m; gross wall area=14*2.5=35 sq m; net area=35-3=32 sq m; paint=32*2/10=6.4 liters."
    },
    {
      name: "test vector: one coat",
      inputs: { length: 5, width: 5, height: 3, openings_area: 4, coats: 1, coverage_per_liter: 8 },
      expected: { answer: "7.00 liters" },
      check: "By hand: perimeter=2*(5+5)=20 m; gross wall area=20*3=60 sq m; net area=60-4=56 sq m; paint=56*1/8=7.00 liters."
    }
  ]
};