import type { Transaction } from "@/types/finance";

export interface CalendarSelection { from: string; to: string }
export interface ActivityDay { date: string; inflow: number; outflow: number; count: number }
export type FlowDirection = "outflow" | "inflow";

/** Read a calendar date without UTC/local conversion or JavaScript's date rollover. */
export function calendarDate(value: string | null | undefined): string | null {
  if (!value || !/^\d{4}-\d{2}-\d{2}(?:$|T)/.test(value)) return null;
  const key = value.slice(0, 10);
  const date = new Date(`${key}T00:00:00Z`);
  return Number.isFinite(date.getTime()) && date.toISOString().slice(0, 10) === key ? key : null;
}

export function dailyCashflow(transactions: Transaction[]): Map<string, ActivityDay> {
  const days = new Map<string, ActivityDay>();
  for (const t of transactions) {
    const date = calendarDate(t.date);
    if (!date || !Number.isFinite(t.amount)) continue;
    const day = days.get(date) ?? { date, inflow: 0, outflow: 0, count: 0 };
    // Accumulate integer cents; direction follows the type, even on imports
    // whose expense amount is positive. Transfers are cash movement here.
    day[t.type === "income" ? "inflow" : "outflow"] += Math.round(Math.abs(t.amount) * 100);
    day.count += 1;
    days.set(date, day);
  }
  for (const day of days.values()) { day.inflow /= 100; day.outflow /= 100; }
  return days;
}

export function orderedSelection(a: string, b: string): CalendarSelection {
  return a <= b ? { from: a, to: b } : { from: b, to: a };
}

export function inCalendarSelection(date: string, selection: CalendarSelection): boolean {
  return date >= selection.from && date <= selection.to;
}

/** Sunday-first, column-major weeks. Padding cells are not dates/buttons. */
export function calendarWeeks(year: number): (string | null)[][] {
  const first = new Date(Date.UTC(year, 0, 1));
  const last = new Date(Date.UTC(year, 11, 31));
  const count = Math.round((last.getTime() - first.getTime()) / 86_400_000) + 1;
  const size = Math.ceil((first.getUTCDay() + count) / 7) * 7;
  const cells = Array.from({ length: size }, (_, i) => {
    const date = new Date(Date.UTC(year, 0, 1 + i - first.getUTCDay()));
    return date.getUTCFullYear() === year ? date.toISOString().slice(0, 10) : null;
  });
  return Array.from({ length: size / 7 }, (_, i) => cells.slice(i * 7, i * 7 + 7));
}

/** Rank-based levels keep a large bill from washing out every everyday purchase. */
export function activityThresholds(days: ReadonlyMap<string, ActivityDay>, direction: FlowDirection): number[] {
  const values = [...days.values()].map(d => d[direction]).filter(n => n > 0).sort((a, b) => a - b);
  return values.length ? [0.25, 0.5, 0.75].map(p => values[Math.floor((values.length - 1) * p)]) : [];
}

export function activityLevel(amount: number, thresholds: number[]): number {
  if (amount <= 0 || !thresholds.length) return 0;
  return 1 + thresholds.filter(n => amount > n).length;
}