import type { DaySummary, TimeEntry, WeekSummary } from "./types";

const DAY_MS = 24 * 60 * 60 * 1000;

function dayStartMs(value: number): number {
  const d = new Date(value);
  return new Date(d.getFullYear(), d.getMonth(), d.getDate()).getTime();
}

function dayKey(value: number): string {
  const d = new Date(value);
  const y = d.getFullYear();
  const m = String(d.getMonth() + 1).padStart(2, "0");
  const day = String(d.getDate()).padStart(2, "0");
  return `${y}-${m}-${day}`;
}

function isoWeekKey(value: number): string {
  const d = new Date(value);
  const date = new Date(Date.UTC(d.getFullYear(), d.getMonth(), d.getDate()));
  const dayNum = date.getUTCDay() || 7;
  date.setUTCDate(date.getUTCDate() + 4 - dayNum);
  const yearStart = new Date(Date.UTC(date.getUTCFullYear(), 0, 1));
  const weekNo = Math.ceil((((date.getTime() - yearStart.getTime()) / DAY_MS) + 1) / 7);
  return `${date.getUTCFullYear()}-W${String(weekNo).padStart(2, "0")}`;
}

export function splitEntryAcrossDays(entry: TimeEntry): DaySummary[] {
  const result: DaySummary[] = [];
  let cursor = entry.startedAtEpochMs;
  const end = entry.endedAtEpochMs;

  while (cursor < end) {
    const startOfDay = dayStartMs(cursor);
    const nextDay = startOfDay + DAY_MS;
    const segmentEnd = Math.min(end, nextDay);
    result.push({
      day: dayKey(cursor),
      durationMs: segmentEnd - cursor
    });
    cursor = segmentEnd;
  }

  return result;
}

export function computeDaySummaries(entries: TimeEntry[]): DaySummary[] {
  const map = new Map<string, number>();
  for (const entry of entries) {
    for (const segment of splitEntryAcrossDays(entry)) {
      map.set(segment.day, (map.get(segment.day) ?? 0) + segment.durationMs);
    }
  }

  return [...map.entries()]
    .map(([day, durationMs]) => ({ day, durationMs }))
    .sort((a, b) => a.day.localeCompare(b.day));
}

export function computeWeekSummaries(entries: TimeEntry[]): WeekSummary[] {
  const daySummaries = computeDaySummaries(entries);
  const map = new Map<string, number>();

  for (const day of daySummaries) {
    const week = isoWeekKey(new Date(`${day.day}T12:00:00`).getTime());
    map.set(week, (map.get(week) ?? 0) + day.durationMs);
  }

  return [...map.entries()]
    .map(([week, durationMs]) => ({ week, durationMs }))
    .sort((a, b) => a.week.localeCompare(b.week));
}