property.floor-area Unreviewed

export function floorArea(rooms: readonly RoomDimensions[], lengthUnit: FloorAreaUnit, decimals: number): FloorArea

export type FloorAreaUnit = "m" | "ft";

/** One rectangular room or area. */
export interface RoomDimensions {
  readonly name: string;
  /** plain decimal text, at most 3 decimal places, greater than 0 and below 1000 */
  readonly length: string;
  /** the same */
  readonly width: string;
}

/** Room areas and the total, exact in the measured unit and rounded in both. */
export interface FloorArea {
  /** each room's length × width, exact, in square lengthUnit, in input order */
  readonly roomAreas: readonly string[];
  /** the sum of roomAreas, exact, in square lengthUnit */
  readonly total: string;
  /** the total in m², rounded to decimals */
  readonly squareMetres: string;
  /** the total in sq ft, rounded to decimals */
  readonly squareFeet: string;
}

import { floorArea } from "#fune/property.floor-area@^1";

import { convertUnits } from "./units_convert.ts";  ← from units.convert ^1.0.0 · built alongside by fune
import { type FloorArea, type FloorAreaUnit, type RoomDimensions } from "./property_floor_area_types.ts";

const DIMENSION = /^[0-9]{1,3}(\.[0-9]{1,3})?$/;
const MAX_ROOMS = 1000;

/** A dimension as an integer number of thousandths. */
function thousandths(room: string, field: string, text: string): number {
  if (typeof text !== "string" || !DIMENSION.test(text)) {
    throw new RangeError(`room "${room}": ${field} must be a decimal below 1000 with at most 3 decimal places, received "${text}"`);
  }
  const [whole, fraction = ""] = text.split(".");
  const value = Number(whole) * 1000 + Number(fraction.padEnd(3, "0"));
  if (value === 0) {
    throw new RangeError(`room "${room}": ${field} must be greater than zero`);
  }
  return value;
}

/** An integer number of millionths as the shortest exact decimal. */
function decimal(millionths: number): string {
  const whole = Math.floor(millionths / 1_000_000);
  const fraction = String(millionths % 1_000_000).padStart(6, "0").replace(/0+$/, "");
  return fraction === "" ? String(whole) : `${whole}.${fraction}`;
}

/**
 * Room areas and their total, exact, from length × width of each room, and
 * the total in square metres and square feet rounded once at the end.
 */
export function floorArea(rooms: readonly RoomDimensions[], lengthUnit: FloorAreaUnit, decimals: number): FloorArea {
  if (lengthUnit !== "m" && lengthUnit !== "ft") {
    throw new RangeError(`unknown length unit "${lengthUnit}": use m or ft`);
  }
  if (rooms.length === 0) {
    throw new RangeError("rooms must not be empty");
  }
  if (rooms.length > MAX_ROOMS) {
    throw new RangeError(`at most ${MAX_ROOMS} rooms, received ${rooms.length}`);
  }
  // Each area is below 1e12 millionths and the total below 1e15: exact numbers.
  const areas = rooms.map((r) => thousandths(r.name, "length", r.length) * thousandths(r.name, "width", r.width));
  const total = decimal(areas.reduce((a, b) => a + b, 0));
  const from = lengthUnit === "m" ? "m2" : "ft2";
  return {
    roomAreas: areas.map(decimal),
    total,
    squareMetres: convertUnits(total, from, "m2", decimals),
    squareFeet: convertUnits(total, from, "ft2", decimals),
  };
}

Install

fune build

With that line in your source, in a TypeScript project (language typescript in fune.project), fune build resolves it and its 1 dependency, pins them in fune.lock, downloads only the TypeScript package of each, and builds the code above into your project’s .fune/build, one readable file per capability with a header linking back here. Or pin a range in fune.project and build in one step:

fune add property.floor-area

The manifest, vectors and README with only the TypeScript implementation. Install it without the registry with fune add ./property.floor-area-1.0.2-typescript.fune, or fetch it from a terminal with fune pull property.floor-area@1.0.2:typescript.

The whole function, every language, is one file too: property.floor-area-1.0.2.fune, 21,244 bytes, sha256 f59699a3b1318c87c90a61fe248c8ef838984c17b27452ee4d538dcd807727ed. It installs into a project of any language.

// fune: before property.floor-area

// fune: after property.floor-area

// fune: replace units.convert in property.floor-area

// fune: step property.floor-area after <n|label>

More from the author

## Why decimal text

Dimensions are read as exact decimals, never floats, so 4.1 × 3.3 is 13.53, not 13.529999999999999. Each room's area and the total are exact (up to six decimal places, from dimensions with up to three), and the only rounding is the final one to `decimals` places in each unit, done by `units.convert` with the exact definition 1 ft = 0.3048 m (1 sq ft = 0.09290304 m²). The total is rounded once, not the rooms: rounding each room first and adding can be off in the last place.

## Edge cases and limits

- Rooms are rectangles. An L-shaped room is two entries; a bay is its own entry; deduct nothing, since the function never subtracts. - Dimensions are greater than 0 and below 1000 of the unit, with at most 3 decimal places (a millimetre in metres), and there may be at most 1000 rooms. An empty list is an error, as is a room of zero size. - Feet and inches must be converted to decimal feet first (10 ft 6 in is "10.5"). - This is the area of the rooms measured, not a RICS gross internal area: walls, stairs and circulation space count only if they are measured in.

1.0.1 fixes Python accepting a trailing newline in a room's length or width; adds tests.

## Before you rely on this

**Not professional advice.** This capability calculates property figures from published rules. It is a software component for developers, not legal or financial advice. Rules change and every rate here has an effective date. Check that the dates cover your case. Verify results against the official sources listed above, and have a conveyancer or tax adviser review how you use it, before anyone relies on the output. Provided "as is" under its licence, without warranty.

**Unreviewed.** This capability's implementations agree in every language and pass its published test vectors, which were worked out from the official sources cited. But no qualified conveyancer or tax adviser has yet checked those vectors, or confirmed that the capability covers the cases it claims. Treat it as a draft. Do not use it for real people, money or decisions without your own expert review. Once a qualified reviewer signs off, this notice is replaced with their name, qualification and the date. Each new version needs fresh sign-off.

1.0.2 marks it unreviewed. The code and the tests are unchanged.

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