IFC length unit and placement matrices

export function unitMm(u) {
  if (!u) return 1000;
  if (u.Name && val(u.Name) === 'METRE' && !u.ConversionFactor) {
    const pre = val(u.Prefix);
    return pre ? (PREFIX[pre] ?? 1000) : 1000;
  }
  if (u.ConversionFactor) {
    const f = u.ConversionFactor;   // IfcMeasureWithUnit: value × its unit (usually SI metre)
    // web-ifc: {ValueComponent, UnitComponent}; cc-ifc: {name:'IFCMEASUREWITHUNIT', value:[factor, unit]}. The unit is a
    // {ref} when it was not inlined; it is the SI metre in every file we write, so a ref reads as metres.
    const arr = Array.isArray(f.value) ? f.value : null;
    const v = Number(arr ? val(arr[0]) : val(f.ValueComponent));
    const uc = arr ? arr[1] : f.UnitComponent;
    const base = uc && !uc.ref && typeof uc === 'object' && (uc.Name || uc.ConversionFactor) ? unitMm(uc) : 1000;
    if (Number.isFinite(v) && v > 0) return v * base;
    const n = String(val(u.Name) || '').toUpperCase();
    return n.includes('FOOT') ? 304.8 : n.includes('INCH') ? 25.4 : 1000;
  }
  return 1000;
}

/** mm per length unit of the file (project units; SI metre with prefix, or a conversion-based unit). */
export function lengthUnitMm(api, mid) {
  const proj = ids(api, mid, 'IFCPROJECT')[0];
  let assign = null;
  if (proj) {
    const p = api.GetLine(mid, proj, false);
    if (p.UnitsInContext) assign = api.GetLine(mid, p.UnitsInContext.value, true);
  }
  if (!assign) { const a = ids(api, mid, 'IFCUNITASSIGNMENT')[0]; if (a) assign = api.GetLine(mid, a, true); }
  for (const u of assign?.Units || []) {
    if (!u || val(u.UnitType) !== 'LENGTHUNIT') continue;
    return unitMm(u);
  }
  return 1000;   // IFC's default length unit is the metre
}

export function mul4(a, b) {
  const o = new Array(16);
  for (let c = 0; c < 4; c++) for (let r = 0; r < 4; r++) {
    let s = 0;
    for (let k = 0; k < 4; k++) s += a[k * 4 + r] * b[c * 4 + k];
    o[c * 4 + r] = s;
  }
  return o;
}

export function apply4(m, x, y, z) {
  return [m[0] * x + m[4] * y + m[8] * z + m[12], m[1] * x + m[5] * y + m[9] * z + m[13], m[2] * x + m[6] * y + m[10] * z + m[14]];
}

export function inv4(m) {
  const a = m, o = new Array(16);
  o[0] = a[5] * a[10] * a[15] - a[5] * a[11] * a[14] - a[9] * a[6] * a[15] + a[9] * a[7] * a[14] + a[13] * a[6] * a[11] - a[13] * a[7] * a[10];
  o[4] = -a[4] * a[10] * a[15] + a[4] * a[11] * a[14] + a[8] * a[6] * a[15] - a[8] * a[7] * a[14] - a[12] * a[6] * a[11] + a[12] * a[7] * a[10];
  o[8] = a[4] * a[9] * a[15] - a[4] * a[11] * a[13] - a[8] * a[5] * a[15] + a[8] * a[7] * a[13] + a[12] * a[5] * a[11] - a[12] * a[7] * a[9];
  o[12] = -a[4] * a[9] * a[14] + a[4] * a[10] * a[13] + a[8] * a[5] * a[14] - a[8] * a[6] * a[13] - a[12] * a[5] * a[10] + a[12] * a[6] * a[9];
  o[1] = -a[1] * a[10] * a[15] + a[1] * a[11] * a[14] + a[9] * a[2] * a[15] - a[9] * a[3] * a[14] - a[13] * a[2] * a[11] + a[13] * a[3] * a[10];
  o[5] = a[0] * a[10] * a[15] - a[0] * a[11] * a[14] - a[8] * a[2] * a[15] + a[8] * a[3] * a[14] + a[12] * a[2] * a[11] - a[12] * a[3] * a[10];
  o[9] = -a[0] * a[9] * a[15] + a[0] * a[11] * a[13] + a[8] * a[1] * a[15] - a[8] * a[3] * a[13] - a[12] * a[1] * a[11] + a[12] * a[3] * a[9];
  o[13] = a[0] * a[9] * a[14] - a[0] * a[10] * a[13] - a[8] * a[1] * a[14] + a[8] * a[2] * a[13] + a[12] * a[1] * a[10] - a[12] * a[2] * a[9];
  o[2] = a[1] * a[6] * a[15] - a[1] * a[7] * a[14] - a[5] * a[2] * a[15] + a[5] * a[3] * a[14] + a[13] * a[2] * a[7] - a[13] * a[3] * a[6];
  o[6] = -a[0] * a[6] * a[15] + a[0] * a[7] * a[14] + a[4] * a[2] * a[15] - a[4] * a[3] * a[14] - a[12] * a[2] * a[7] + a[12] * a[3] * a[6];
  o[10] = a[0] * a[5] * a[15] - a[0] * a[7] * a[13] - a[4] * a[1] * a[15] + a[4] * a[3] * a[13] + a[12] * a[1] * a[7] - a[12] * a[3] * a[5];
  o[14] = -a[0] * a[5] * a[14] + a[0] * a[6] * a[13] + a[4] * a[1] * a[14] - a[4] * a[2] * a[13] - a[12] * a[1] * a[6] + a[12] * a[2] * a[5];
  o[3] = -a[1] * a[6] * a[11] + a[1] * a[7] * a[10] + a[5] * a[2] * a[11] - a[5] * a[3] * a[10] - a[9] * a[2] * a[7] + a[9] * a[3] * a[6];
  o[7] = a[0] * a[6] * a[11] - a[0] * a[7] * a[10] - a[4] * a[2] * a[11] + a[4] * a[3] * a[10] + a[8] * a[2] * a[7] - a[8] * a[3] * a[6];
  o[11] = -a[0] * a[5] * a[11] + a[0] * a[7] * a[9] + a[4] * a[1] * a[11] - a[4] * a[3] * a[9] - a[8] * a[1] * a[7] + a[8] * a[3] * a[5];
  o[15] = a[0] * a[5] * a[10] - a[0] * a[6] * a[9] - a[4] * a[1] * a[10] + a[4] * a[2] * a[9] + a[8] * a[1] * a[6] - a[8] * a[2] * a[5];
  const det = a[0] * o[0] + a[1] * o[4] + a[2] * o[8] + a[3] * o[12];
  if (!det) return null;
  for (let i = 0; i < 16; i++) o[i] /= det;
  return o;
}

/**
 * Federation (0.4 B16): where model k goes in the scene. Each model is loaded moved to the origin (web-ifc
 * COORDINATE_TO_ORIGIN, its coordination matrix C); undoing C and applying the first model's (refC) puts it in the first
 * model's coordinates, then `offset` [x, y, z] (metres, IFC axes: Z up) moves it — web-ifc's viewer space is Y up, so
 * IFC (x, y, z) → viewer (x, z, −y). Column-major 4 × 4.
 */
export function placementMatrix(refC, C, offset = [0, 0, 0]) {
  const [dx, dy, dz] = offset;
  const T = [1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1, 0, dx, dz, -dy, 1];
  return mul4(T, mul4(refC || C, inv4(C)));
}