/**
 * Built-in functions: a plain table { NAME -> fn(values) }.
 *
 * By the time a function runs, the evaluator has already:
 *   - resolved every range argument into a flat list of cell values,
 *   - flattened scalar arguments into that same list,
 *   - checked for errors among the values (an error short-circuits and is
 *     returned instead of calling the function).
 *
 * So a function only ever sees `(number | string | null)[]`. Each function
 * decides for itself how to treat text and empty cells. Per spec:
 *   - empty cell  -> ignored by every aggregate, never counted by COUNT
 *   - text        -> ignored by the numeric aggregates
 */

import { FormulaError, isError, type CellValue } from './types';
import { dateToSerial, timeSerial, todaySerial, nowSerial, strictNum } from './dates';

export type FunctionFn = (values: CellValue[]) => CellValue;

/** Keep only real numbers (finite). */
function nums(values: CellValue[]): number[] {
  const out: number[] = [];
  for (const v of values) {
    if (typeof v === 'number' && Number.isFinite(v)) out.push(v);
  }
  return out;
}

export const FUNCTIONS: Record<string, FunctionFn> = {
  SUM(values) {
    return nums(values).reduce((a, b) => a + b, 0);
  },

  AVG(values) {
    const n = nums(values);
    if (n.length === 0) return new FormulaError('#DIV/0!', 'AVG of no numbers');
    return n.reduce((a, b) => a + b, 0) / n.length;
  },

  MIN(values) {
    const n = nums(values);
    return n.length ? Math.min(...n) : 0;
  },

  MAX(values) {
    const n = nums(values);
    return n.length ? Math.max(...n) : 0;
  },

  COUNT(values) {
    return nums(values).length;
  },

  // IF(cond; then; else?) -- evaluator.ts special-cases this name *before*
  // ever reaching FUNCTIONS, giving it real short-circuit evaluation (only
  // the taken branch is computed, so e.g. IF(B1=0; 0; A1/B1) never runs the
  // division when B1=0). This entry only exists so IF is a registered
  // built-in for hasFunction()'s sake (the store's custom-function name
  // check then correctly refuses to let a user shadow it) and as a
  // defensive, best-effort fallback if this were ever reached directly
  // (values are already-eagerly-computed here, so it can't short-circuit --
  // see evaluator.ts's evalIf() for the real implementation).
  IF(values) {
    if (values.length !== 2 && values.length !== 3) {
      return new FormulaError('#ERR', 'IF expects 2 or 3 arguments');
    }
    const [cond, thenValue, elseValue] = values;
    if (isError(cond)) return cond;
    const truthy = typeof cond === 'number' ? cond !== 0 : cond !== null && cond !== '';
    if (truthy) return thenValue;
    return values.length === 3 ? elseValue : 0;
  },

  // Dates/times are plain numbers here (see dates.ts) -- DATE/TIME build one,
  // TODAY/NOW read the current one. `null` (empty cell) arguments count as 0,
  // same as everywhere else in this engine; a non-numeric text argument is
  // `#VALUE!`.
  DATE(values) {
    if (values.length !== 3) {
      return new FormulaError('#ERR', 'DATE expects 3 arguments (Jahr; Monat; Tag)');
    }
    const y = strictNum(values[0]);
    if (isError(y)) return y;
    const mo = strictNum(values[1]);
    if (isError(mo)) return mo;
    const d = strictNum(values[2]);
    if (isError(d)) return d;
    return dateToSerial(y, mo, d);
  },

  TIME(values) {
    if (values.length !== 3) {
      return new FormulaError('#ERR', 'TIME expects 3 arguments (Stunde; Minute; Sekunde)');
    }
    const h = strictNum(values[0]);
    if (isError(h)) return h;
    const mi = strictNum(values[1]);
    if (isError(mi)) return mi;
    const s = strictNum(values[2]);
    if (isError(s)) return s;
    return timeSerial(h, mi, s);
  },

  // Not "volatile": this engine only recalculates on precedent change, so a
  // cell holding =TODAY()/=NOW() refreshes when THAT cell is next re-edited
  // or recalculated, not spontaneously when the clock ticks over (documented
  // limitation, see README.md's "Bekannte Grenzen").
  TODAY(values) {
    if (values.length !== 0) return new FormulaError('#ERR', 'TODAY erwartet keine Argumente');
    return todaySerial();
  },

  NOW(values) {
    if (values.length !== 0) return new FormulaError('#ERR', 'NOW erwartet keine Argumente');
    return nowSerial();
  },

  // XLOOKUP(Suchwert; Suchbereich; Rückgabebereich; [WennNichtGefunden];
  // [Vergleichsmodus]; [Suchmodus]) -- like IF, evaluator.ts special-cases
  // this name *before* ever reaching FUNCTIONS. Two reasons, not just one:
  //   1. WennNichtGefunden must only be evaluated when actually needed
  //      (short-circuit, same reasoning as IF's branches).
  //   2. By the time a function gets here, EVERY argument -- including
  //      Suchbereich and Rückgabebereich, both ranges -- has already been
  //      flattened into ONE combined array (see the module comment above).
  //      That destroys the boundary between the two ranges, so there is no
  //      way to even attempt a best-effort fallback here: unlike IF's
  //      2-3 scalar arguments, XLOOKUP's array arguments can't be recovered
  //      from a flat `values` list of unknown split point. This entry exists
  //      purely so XLOOKUP is a registered built-in (hasFunction() /
  //      name-reservation) -- see evaluator.ts's evalXLookup() for the real
  //      implementation. Reaching this in practice would be an engine bug.
  XLOOKUP() {
    return new FormulaError('#ERR', 'XLOOKUP must be evaluated by evaluator.ts, not called directly');
  },
};

// AVERAGE is an alias of AVG.
FUNCTIONS.AVERAGE = FUNCTIONS.AVG;

export function hasFunction(name: string): boolean {
  return Object.prototype.hasOwnProperty.call(FUNCTIONS, name.toUpperCase());
}

export function getFunction(name: string): FunctionFn | undefined {
  return FUNCTIONS[name.toUpperCase()];
}