/**
 * PeriodPicker - pure logic helpers.
 *
 * No React, no DOM. All date math is local-date based (no timezone shifting).
 * Use ISO `YYYY-MM-DD` strings as the wire format.
 */

import type {
  PeriodConstraints,
  PeriodPreset,
  PeriodPresetKind,
  PeriodUnit,
  PeriodValue,
} from './PeriodPickerTypes';

// ---------------------------------------------------------------------------
// Date primitives
// ---------------------------------------------------------------------------

const _pad = (n: number): string => String(n).padStart(2, '0');

export function toIsoDate(d: Date): string {
  return `${d.getFullYear()}-${_pad(d.getMonth() + 1)}-${_pad(d.getDate())}`;
}

export function fromIsoDate(s: string | null | undefined): Date | null {
  if (!s) return null;
  const parts = s.split('-').map(Number);
  if (parts.length !== 3 || parts.some(Number.isNaN)) return null;
  return new Date(parts[0], parts[1] - 1, parts[2]);
}

export function daysInRange(fromIso: string, toIso: string): number {
  const from = fromIsoDate(fromIso);
  const to = fromIsoDate(toIso);
  if (!from || !to) return 0;
  const ms = to.getTime() - from.getTime();
  return Math.max(1, Math.round(ms / (1000 * 60 * 60 * 24)) + 1);
}

export function todayDate(): Date {
  const d = new Date();
  d.setHours(0, 0, 0, 0);
  return d;
}

function _addDays(d: Date, n: number): Date {
  const r = new Date(d);
  r.setDate(r.getDate() + n);
  return r;
}
function _addMonths(d: Date, n: number): Date {
  const r = new Date(d);
  r.setMonth(r.getMonth() + n);
  return r;
}
function _addYears(d: Date, n: number): Date {
  const r = new Date(d);
  r.setFullYear(r.getFullYear() + n);
  return r;
}
function _startOfMonth(d: Date): Date { return new Date(d.getFullYear(), d.getMonth(), 1); }
function _endOfMonth(d: Date): Date { return new Date(d.getFullYear(), d.getMonth() + 1, 0); }
function _startOfYear(d: Date): Date { return new Date(d.getFullYear(), 0, 1); }
function _endOfYear(d: Date): Date { return new Date(d.getFullYear(), 11, 31); }
function _startOfQuarter(d: Date): Date {
  return new Date(d.getFullYear(), Math.floor(d.getMonth() / 3) * 3, 1);
}
function _endOfQuarter(d: Date): Date {
  const s = _startOfQuarter(d);
  return new Date(s.getFullYear(), s.getMonth() + 3, 0);
}

function _shiftBy(d: Date, amount: number, unit: PeriodUnit): Date {
  switch (unit) {
    case 'day':   return _addDays(d, amount);
    case 'week':  return _addDays(d, amount * 7);
    case 'month': return _addMonths(d, amount);
    case 'year':  return _addYears(d, amount);
  }
}

// ---------------------------------------------------------------------------
// Preset resolver
// ---------------------------------------------------------------------------

// Sentinel bounds used when the user picked ``Alle`` (no date filter). We keep
// the values *inside* ``PeriodValue`` so downstream code that reads
// ``fromDate``/``toDate`` doesn't break; callers that want to forward "no
// filter" to the backend should check ``preset.kind === 'allTime'`` and drop
// the dates explicitly before building the request.
export const ALL_TIME_FROM = '1970-01-01';
export const ALL_TIME_TO = '2999-12-31';

export function resolvePeriod(preset: PeriodPreset, prevValue?: PeriodValue | null): { fromDate: string; toDate: string } {
  const today = todayDate();
  switch (preset.kind) {
    case 'allTime':
      return { fromDate: ALL_TIME_FROM, toDate: ALL_TIME_TO };
    case 'ytd':
      return { fromDate: toIsoDate(_startOfYear(today)), toDate: toIsoDate(today) };
    case 'lastYear': {
      const ly = _addYears(today, -1);
      return { fromDate: toIsoDate(_startOfYear(ly)), toDate: toIsoDate(_endOfYear(ly)) };
    }
    case 'nextYear': {
      const ny = _addYears(today, 1);
      return { fromDate: toIsoDate(_startOfYear(ny)), toDate: toIsoDate(_endOfYear(ny)) };
    }
    case 'last12Months':
      return { fromDate: toIsoDate(_addMonths(today, -12)), toDate: toIsoDate(today) };
    case 'next12Months':
      return { fromDate: toIsoDate(today), toDate: toIsoDate(_addMonths(today, 12)) };
    case 'thisMonth':
      return { fromDate: toIsoDate(_startOfMonth(today)), toDate: toIsoDate(_endOfMonth(today)) };
    case 'lastMonth': {
      const lm = _addMonths(today, -1);
      return { fromDate: toIsoDate(_startOfMonth(lm)), toDate: toIsoDate(_endOfMonth(lm)) };
    }
    case 'thisQuarter':
      return { fromDate: toIsoDate(_startOfQuarter(today)), toDate: toIsoDate(_endOfQuarter(today)) };
    case 'lastQuarter': {
      const lq = _addMonths(_startOfQuarter(today), -3);
      return { fromDate: toIsoDate(_startOfQuarter(lq)), toDate: toIsoDate(_endOfQuarter(lq)) };
    }
    case 'lastN':
      return { fromDate: toIsoDate(_shiftBy(today, -preset.amount, preset.unit)), toDate: toIsoDate(today) };
    case 'nextN':
      return { fromDate: toIsoDate(today), toDate: toIsoDate(_shiftBy(today, preset.amount, preset.unit)) };
    case 'custom':
      // Custom holds whatever was last picked; rely on the previous value if available,
      // otherwise default to a single-day range at today to give the calendar an anchor.
      return {
        fromDate: prevValue?.fromDate || toIsoDate(today),
        toDate: prevValue?.toDate || toIsoDate(today),
      };
  }
}

// ---------------------------------------------------------------------------
// Constraints
// ---------------------------------------------------------------------------

export function isDateDisabled(d: Date, cfg: PeriodConstraints): boolean {
  const min = fromIsoDate(cfg.minDate);
  const max = fromIsoDate(cfg.maxDate);
  if (min && d < min) return true;
  if (max && d > max) return true;
  if (cfg.direction === 'past' && d > todayDate()) return true;
  if (cfg.direction === 'future' && d < todayDate()) return true;
  return false;
}

const _FUTURE_PRESETS: PeriodPresetKind[] = ['nextYear', 'next12Months', 'nextN'];
const _PAST_PRESETS: PeriodPresetKind[] = ['lastYear', 'last12Months', 'lastN', 'lastMonth', 'lastQuarter'];

export function isPresetDisabled(kind: PeriodPresetKind, cfg: PeriodConstraints): boolean {
  if (cfg.enabledPresets && !cfg.enabledPresets.includes(kind)) return true;
  if (cfg.direction === 'past'   && _FUTURE_PRESETS.includes(kind)) return true;
  if (cfg.direction === 'future' && _PAST_PRESETS.includes(kind))   return true;
  return false;
}

export function isValueAllowed(value: PeriodValue | null, cfg: PeriodConstraints): boolean {
  if (!value) return false;
  if (isPresetDisabled(value.preset.kind, cfg)) return false;
  if (value.preset.kind === 'custom') {
    const f = fromIsoDate(value.fromDate);
    const tt = fromIsoDate(value.toDate);
    if (!f || !tt) return false;
    if (isDateDisabled(f, cfg)) return false;
    if (isDateDisabled(tt, cfg)) return false;
  }
  return true;
}

// Clamp an ISO date to the direction/min/max window defined by ``cfg``. Used
// for ``<input type="date">`` ``min``/``max`` attributes so the browser
// refuses invalid years instead of us silently falling back to the default
// preset afterwards.
export function clampIsoDate(iso: string | undefined, cfg: PeriodConstraints, side: 'min' | 'max'): string | undefined {
  const today = toIsoDate(todayDate());
  let lo: string | undefined = cfg.minDate;
  let hi: string | undefined = cfg.maxDate;
  if (cfg.direction === 'past')   hi = hi && hi < today ? hi : today;
  if (cfg.direction === 'future') lo = lo && lo > today ? lo : today;
  if (side === 'min') return lo;
  return hi;
}

// ---------------------------------------------------------------------------
// Label formatting
// ---------------------------------------------------------------------------

/**
 * Returns the human label for a preset kind. Caller wraps with `t()` because
 * `t()` only accepts string literals (no variables).
 */
export function presetLiteralKey(kind: PeriodPresetKind): string {
  switch (kind) {
    case 'allTime':       return 'Alle';
    case 'ytd':           return 'Laufendes Jahr';
    case 'lastYear':      return 'Letztes Jahr';
    case 'nextYear':      return 'Nächstes Jahr';
    case 'last12Months':  return 'Letzte 12 Monate';
    case 'next12Months':  return 'Nächste 12 Monate';
    case 'thisMonth':     return 'Dieser Monat';
    case 'lastMonth':     return 'Letzter Monat';
    case 'thisQuarter':   return 'Dieses Quartal';
    case 'lastQuarter':   return 'Letztes Quartal';
    case 'lastN':         return 'Letzte N';
    case 'nextN':         return 'Nächste N';
    case 'custom':        return 'Benutzerdefiniert';
  }
}

export function formatIsoDateDe(iso: string): string {
  const d = fromIsoDate(iso);
  if (!d) return iso;
  return `${_pad(d.getDate())}.${_pad(d.getMonth() + 1)}.${d.getFullYear()}`;
}

// ---------------------------------------------------------------------------
// Calendar grid helper
// ---------------------------------------------------------------------------

export interface CalendarCell {
  date: Date;
  iso: string;
  inMonth: boolean;
  isToday: boolean;
}

/**
 * Returns 6x7 = 42 cells starting on Monday for the given month anchor.
 */
export function buildMonthCells(anchor: Date): CalendarCell[] {
  const start = _startOfMonth(anchor);
  const leading = (start.getDay() + 6) % 7;   // Monday-first
  const today = todayDate();
  const cells: CalendarCell[] = [];
  for (let i = 0; i < 42; i++) {
    const d = _addDays(start, i - leading);
    cells.push({
      date: d,
      iso: toIsoDate(d),
      inMonth: d.getMonth() === anchor.getMonth(),
      isToday: _isSameDay(d, today),
    });
  }
  return cells;
}

export function _isSameDay(a: Date, b: Date): boolean {
  return a.getFullYear() === b.getFullYear()
    && a.getMonth() === b.getMonth()
    && a.getDate() === b.getDate();
}

export function addMonthsToDate(d: Date, n: number): Date {
  return _addMonths(d, n);
}

export function startOfMonth(d: Date): Date {
  return _startOfMonth(d);
}