// https://github.com/python/cpython/blob/a74eea238f5baba15797e2e8b570d153bc8690a7/Modules/mathmodule.c#L1423
export class Adder {
  constructor() {
    this._partials = new Float64Array(32);
    this._n = 0;
  }
  add(x) {
    const p = this._partials;
    let i = 0;
    for (let j = 0; j < this._n && j < 32; j++) {
      const y = p[j],
        hi = x + y,
        lo = Math.abs(x) < Math.abs(y) ? x - (hi - y) : y - (hi - x);
      if (lo) p[i++] = lo;
      x = hi;
    }
    p[i] = x;
    this._n = i + 1;
    return this;
  }
  valueOf() {
    const p = this._partials;
    let n = this._n, x, y, lo, hi = 0;
    if (n > 0) {
      hi = p[--n];
      while (n > 0) {
        x = hi;
        y = p[--n];
        hi = x + y;
        lo = y - (hi - x);
        if (lo) break;
      }
      if (n > 0 && ((lo < 0 && p[n - 1] < 0) || (lo > 0 && p[n - 1] > 0))) {
        y = lo * 2;
        x = hi + y;
        if (y == x - hi) hi = x;
      }
    }
    return hi;
  }
}

export function fsum(values, valueof) {
  const adder = new Adder();
  if (valueof === undefined) {
    for (let value of values) {
      if (value = +value) {
        adder.add(value);
      }
    }
  } else {
    let index = -1;
    for (let value of values) {
      if (value = +valueof(value, ++index, values)) {
        adder.add(value);
      }
    }
  }
  return +adder;
}

export function fcumsum(values, valueof) {
  const adder = new Adder();
  let index = -1;
  return Float64Array.from(values, valueof === undefined
      ? v => adder.add(+v || 0)
      : v => adder.add(+valueof(v, ++index, values) || 0)
  );
}