import { parseLocalDate } from "@/lib/utils";
import { monthKeyOf } from "@/lib/format";
import type { Transaction } from "@/types/finance";
import { countsAsSpend } from "@/lib/transfers";

export interface CashflowPoint {
  key: string;
  label: string;
  income: number;
  expenses: number;
}

export function buildCashflow(transactions: Transaction[], months: number): CashflowPoint[] {
  const byMonth = new Map<string, { income: number; expenses: number }>();
  for (const t of transactions) {
    if (!countsAsSpend(t)) continue;
    const key = monthKeyOf(t.date);
    if (!key || !Number.isFinite(t.amount)) continue;
    if (!byMonth.has(key)) byMonth.set(key, { income: 0, expenses: 0 });
    const bucket = byMonth.get(key)!;
    if (t.type === "income") bucket.income += Math.abs(t.amount);
    else bucket.expenses += Math.abs(t.amount);
  }
  return [...byMonth.entries()]
    .sort(([a], [b]) => a.localeCompare(b))
    .slice(-months)
    .map(([key, v]) => {
      const d = parseLocalDate(`${key}-01`);
      return {
        key,
        // The year is included: a 6-month window crossing a year boundary used
        // to render two identical "Jan" ticks.
        label: d.toLocaleDateString("en-US", { month: "short", year: "2-digit" }),
        income: Math.round(v.income * 100) / 100,
        expenses: Math.round(v.expenses * 100) / 100,
      };
    });
}