import {
  DurationStats,
  IssueReadModel,
  IssueTimelineEntry,
  ProjectAnalyticsReadModel,
  ReleaseBurnupReadModel,
  ReleaseReadModel,
  SprintBurndownReadModel,
  SprintReadModel,
  StatusCategory,
  UserRefReadModel,
} from '@task-stream/api/tracker/domain';

/*
 * Statistics are computed from each ticket's timeline (status and release
 * steps). Pure functions: no Nest, no database, easy to test.
 */

const DAY = 86_400_000;
const STARTED = [StatusCategory.IN_PROGRESS, StatusCategory.QA];

interface Step {
  at: number;
  status: string;
  category: StatusCategory;
  assigneeId: string | null;
}

/** "2026-10-06" for a timestamp (UTC). */
export const dayOf = (time: number) =>
  new Date(time).toISOString().slice(0, 10);

/** Monday of the ISO week of a timestamp (UTC), as "YYYY-MM-DD". */
export function weekOf(time: number): string {
  const date = new Date(time);
  const shift = (date.getUTCDay() + 6) % 7;
  return dayOf(
    Date.UTC(
      date.getUTCFullYear(),
      date.getUTCMonth(),
      date.getUTCDate() - shift,
    ),
  );
}

/** End of each UTC day between two instants (inclusive). */
function days(from: number, to: number): number[] {
  const out: number[] = [];
  const first = Date.UTC(
    new Date(from).getUTCFullYear(),
    new Date(from).getUTCMonth(),
    new Date(from).getUTCDate(),
  );
  for (let day = first; day <= to; day += DAY) out.push(day + DAY - 1);
  return out;
}

function weeks(from: number, to: number): string[] {
  const out: string[] = [];
  for (let day = Date.parse(weekOf(from)); day <= to; day += 7 * DAY)
    out.push(dayOf(day));
  return out;
}

/** Status steps per ticket; tickets with no timeline get one from their creation. */
function stepsByIssue(
  issues: IssueReadModel[],
  timeline: IssueTimelineEntry[],
): Map<string, Step[]> {
  const steps = new Map<string, Step[]>();
  for (const entry of timeline) {
    if (entry.kind !== 'status' || !entry.status || !entry.category) continue;
    const list = steps.get(entry.issueId) ?? [];
    list.push({
      at: Date.parse(entry.at),
      status: entry.status,
      category: entry.category,
      assigneeId: entry.assigneeId ?? null,
    });
    steps.set(entry.issueId, list);
  }
  for (const issue of issues) {
    if (!steps.has(issue.id)) {
      steps.set(issue.id, [
        {
          at: Date.parse(issue.createdAt),
          status: issue.status,
          category: issue.statusCategory,
          assigneeId: issue.assignee?.id ?? null,
        },
      ]);
    }
    steps.get(issue.id)?.sort((a, b) => a.at - b.at);
  }
  return steps;
}

/** Last step at or before `time`. */
function stepAt(steps: Step[], time: number): Step | null {
  let current: Step | null = null;
  for (const step of steps) {
    if (step.at > time) break;
    current = step;
  }
  return current;
}

function durations(values: number[]): DurationStats {
  if (values.length === 0)
    return { count: 0, averageDays: null, p85Days: null };
  const sorted = [...values].sort((a, b) => a - b);
  const average = values.reduce((sum, v) => sum + v, 0) / values.length;
  const p85 = sorted[Math.max(0, Math.ceil(0.85 * sorted.length) - 1)];
  const round = (ms: number) => Math.round((ms / DAY) * 10) / 10;
  return {
    count: values.length,
    averageDays: round(average),
    p85Days: round(p85),
  };
}

export interface AnalyticsInput {
  issues: IssueReadModel[];
  timeline: IssueTimelineEntry[];
  /** Status ids in the project's order (series follow it). */
  statusOrder: string[];
  from: Date;
  to: Date;
}

export function computeProjectAnalytics(
  input: AnalyticsInput,
): ProjectAnalyticsReadModel {
  const from = input.from.getTime();
  const to = input.to.getTime();
  const inRange = (time: number) => time >= from && time <= to;
  const steps = stepsByIssue(input.issues, input.timeline);
  const byId = new Map(input.issues.map((issue) => [issue.id, issue]));
  const people = new Map<string, UserRefReadModel>();
  for (const issue of input.issues) {
    if (issue.assignee) people.set(issue.assignee.id, issue.assignee);
    people.set(issue.reporter.id, issue.reporter);
  }

  // Created vs resolved, per week.
  const weekKeys = weeks(from, to);
  const created = new Map(weekKeys.map((week) => [week, 0]));
  const resolved = new Map(weekKeys.map((week) => [week, 0]));
  for (const issue of input.issues) {
    const at = Date.parse(issue.createdAt);
    if (inRange(at))
      created.set(weekOf(at), (created.get(weekOf(at)) ?? 0) + 1);
  }

  // Walk every transition once: resolutions, cycle/lead time, throughput, QA.
  const cycle: number[] = [];
  const lead: number[] = [];
  const throughput = new Map<
    string,
    { week: string; user: string | null; count: number }
  >();
  let passed = 0;
  let failed = 0;
  for (const [issueId, list] of steps) {
    const issue = byId.get(issueId);
    for (let i = 1; i < list.length; i++) {
      const previous = list[i - 1];
      const step = list[i];
      if (!inRange(step.at)) continue;
      if (
        previous.category === StatusCategory.QA &&
        step.category !== StatusCategory.QA
      ) {
        if (step.category === StatusCategory.DONE) passed++;
        else if (step.category !== StatusCategory.CANCELLED) failed++;
      }
      if (
        step.category !== StatusCategory.DONE ||
        previous.category === StatusCategory.DONE
      )
        continue;
      const week = weekOf(step.at);
      resolved.set(week, (resolved.get(week) ?? 0) + 1);
      if (issue) lead.push(step.at - Date.parse(issue.createdAt));
      const start = list.slice(0, i).find((s) => STARTED.includes(s.category));
      if (start) cycle.push(step.at - start.at);
      const key = `${week}|${step.assigneeId ?? ''}`;
      const row = throughput.get(key) ?? {
        week,
        user: step.assigneeId,
        count: 0,
      };
      row.count++;
      throughput.set(key, row);
    }
  }

  // Cumulative flow: tickets per status at the end of each day.
  const dayEnds = days(from, to);
  const counts = new Map<string, number[]>();
  for (const list of steps.values()) {
    dayEnds.forEach((end, index) => {
      const step = stepAt(list, end);
      if (!step) return;
      const series =
        counts.get(step.status) ?? new Array(dayEnds.length).fill(0);
      series[index]++;
      counts.set(step.status, series);
    });
  }
  const order = (status: string) => {
    const index = input.statusOrder.indexOf(status);
    return index === -1 ? Number.MAX_SAFE_INTEGER : index;
  };

  return {
    from: input.from.toISOString(),
    to: input.to.toISOString(),
    createdVsResolved: weekKeys.map((week) => ({
      week,
      created: created.get(week) ?? 0,
      resolved: resolved.get(week) ?? 0,
    })),
    cumulativeFlow: {
      dates: dayEnds.map(dayOf),
      series: [...counts.entries()]
        .sort(([a], [b]) => order(a) - order(b))
        .map(([status, values]) => ({ status, counts: values })),
    },
    cycleTime: durations(cycle),
    leadTime: durations(lead),
    throughput: [...throughput.values()]
      .sort((a, b) => a.week.localeCompare(b.week) || b.count - a.count)
      .map((row) => ({
        week: row.week,
        user: row.user
          ? (people.get(row.user) ?? { id: row.user, name: '?' })
          : null,
        count: row.count,
      })),
    qa: {
      passed,
      failed,
      failureRate:
        passed + failed
          ? Math.round((failed / (passed + failed)) * 1000) / 1000
          : null,
    },
  };
}

export interface BurnupInput {
  release: ReleaseReadModel;
  issues: IssueReadModel[];
  timeline: IssueTimelineEntry[];
  now: Date;
}

/**
 * Per day, from the release's creation until it ships (or today): tickets in
 * it (scope, cancelled ones aside) and how many of those are done.
 */
export function computeReleaseBurnup(
  input: BurnupInput,
): ReleaseBurnupReadModel {
  const { release } = input;
  const steps = stepsByIssue(input.issues, input.timeline);
  const membership = new Map<
    string,
    { at: number; releaseId: string | null }[]
  >();
  for (const entry of input.timeline) {
    if (entry.kind !== 'release') continue;
    const list = membership.get(entry.issueId) ?? [];
    list.push({ at: Date.parse(entry.at), releaseId: entry.releaseId ?? null });
    membership.set(entry.issueId, list);
  }
  for (const issue of input.issues) {
    if (!membership.has(issue.id) && issue.release) {
      membership.set(issue.id, [
        { at: Date.parse(issue.createdAt), releaseId: issue.release.id },
      ]);
    }
    membership.get(issue.id)?.sort((a, b) => a.at - b.at);
  }

  const end = release.releasedAt
    ? Date.parse(release.releasedAt)
    : input.now.getTime();
  // At most the last 180 days, so a long-lived release stays readable.
  const start = Math.max(Date.parse(release.createdAt), end - 179 * DAY);
  const dayEnds = days(start, end);
  const scope: number[] = [];
  const done: number[] = [];
  for (const time of dayEnds) {
    let inScope = 0;
    let finished = 0;
    for (const [issueId, list] of membership) {
      const last = list.filter((m) => m.at <= time).at(-1);
      if (last?.releaseId !== release.id) continue;
      const step = stepAt(steps.get(issueId) ?? [], time);
      if (!step || step.category === StatusCategory.CANCELLED) continue;
      inScope++;
      if (step.category === StatusCategory.DONE) finished++;
    }
    scope.push(inScope);
    done.push(finished);
  }

  return {
    releaseId: release.id,
    targetDate: release.targetDate,
    dates: dayEnds.map(dayOf),
    scope,
    done,
  };
}

export interface SprintBurndownInput {
  sprint: SprintReadModel;
  issues: IssueReadModel[];
  timeline: IssueTimelineEntry[];
  now: Date;
}

/**
 * Per day of the sprint (start to end date, UTC): tickets in it, how many
 * are still open and their points, and the ideal line. Points are today's
 * estimates (their history is not kept). Days after today, or after the
 * sprint closed, are null.
 */
export function computeSprintBurndown(
  input: SprintBurndownInput,
): SprintBurndownReadModel {
  const { sprint } = input;
  const steps = stepsByIssue(input.issues, input.timeline);
  const points = new Map(input.issues.map((i) => [i.id, i.storyPoints ?? 0]));
  const membership = new Map<
    string,
    { at: number; sprintId: string | null }[]
  >();
  for (const entry of input.timeline) {
    if (entry.kind !== 'sprint') continue;
    const list = membership.get(entry.issueId) ?? [];
    list.push({ at: Date.parse(entry.at), sprintId: entry.sprintId ?? null });
    membership.set(entry.issueId, list);
  }
  for (const list of membership.values()) list.sort((a, b) => a.at - b.at);

  const start = Date.parse(`${sprint.startDate}T00:00:00Z`);
  const end = Date.parse(`${sprint.endDate}T00:00:00Z`);
  const last = Math.min(
    sprint.closedAt ? Date.parse(sprint.closedAt) : input.now.getTime(),
    end + DAY - 1,
  );
  const dayEnds = days(start, end + DAY - 1);
  const scope: (number | null)[] = [];
  const remaining: (number | null)[] = [];
  const remainingPoints: (number | null)[] = [];
  for (const time of dayEnds) {
    // The day under way counts up to now; the ones to come are unknown.
    const at = Math.min(time, last);
    if (time - DAY >= last) {
      scope.push(null);
      remaining.push(null);
      remainingPoints.push(null);
      continue;
    }
    let inScope = 0;
    let open = 0;
    let openPoints = 0;
    for (const [issueId, list] of membership) {
      const current = list.filter((m) => m.at <= at).at(-1);
      if (current?.sprintId !== sprint.id) continue;
      const step = stepAt(steps.get(issueId) ?? [], at);
      if (!step || step.category === StatusCategory.CANCELLED) continue;
      inScope++;
      if (step.category !== StatusCategory.DONE) {
        open++;
        openPoints += points.get(issueId) ?? 0;
      }
    }
    scope.push(inScope);
    remaining.push(open);
    remainingPoints.push(openPoints);
  }
  const first = remaining[0] ?? 0;
  const span = Math.max(dayEnds.length - 1, 1);
  return {
    sprintId: sprint.id,
    dates: dayEnds.map(dayOf),
    scope,
    remaining,
    remainingPoints,
    ideal: dayEnds.map(
      (_, i) => Math.round((first * (span - i) * 100) / span) / 100,
    ),
  };
}