import { assertBoundedText } from "@add-ideas/toolbox-helpers";
import { Temporal } from "temporal-polyfill";

import { parseInstant } from "./epoch";

export interface MeetingParticipant {
  readonly name: string;
  readonly timeZone: string;
  readonly workStart: string;
  readonly workEnd: string;
  readonly includeWeekends?: boolean;
}

export interface MeetingCandidate {
  readonly index: number;
  readonly start: string;
  readonly end: string;
  readonly participants: ReadonlyArray<{
    name: string;
    timeZone: string;
    startLocal: string;
    endLocal: string;
    offset: string;
  }>;
}

export interface MeetingPlan {
  readonly searchedFrom: string;
  readonly searchedUntil: string;
  readonly durationMinutes: number;
  readonly stepMinutes: number;
  readonly candidates: readonly MeetingCandidate[];
  readonly exhausted: boolean;
  readonly note: string;
}

function clockMinutes(valueInput: string, label: string): number {
  const value = assertBoundedText(valueInput, 16, label).trim();
  const match = /^(\d{2}):(\d{2})$/u.exec(value);
  if (!match) throw new SyntaxError(`${label} must use HH:MM.`);
  const hours = Number(match[1]);
  const minutes = Number(match[2]);
  if (hours > 23 || minutes > 59)
    throw new RangeError(`${label} is outside the 00:00–23:59 range.`);
  return hours * 60 + minutes;
}

function positiveInteger(
  value: number,
  minimum: number,
  maximum: number,
  label: string,
): number {
  if (!Number.isSafeInteger(value) || value < minimum || value > maximum)
    throw new RangeError(`${label} must be between ${minimum} and ${maximum}.`);
  return value;
}

export function planMeetings(input: {
  start: string;
  days: number;
  durationMinutes: number;
  stepMinutes: number;
  participants: readonly MeetingParticipant[];
  maxResults?: number;
}): MeetingPlan {
  const days = positiveInteger(input.days, 1, 31, "Search days");
  const durationMinutes = positiveInteger(
    input.durationMinutes,
    5,
    1_440,
    "Meeting duration",
  );
  const stepMinutes = positiveInteger(input.stepMinutes, 5, 240, "Search step");
  const maxResults = positiveInteger(
    input.maxResults ?? 24,
    1,
    100,
    "Result count",
  );
  if (input.participants.length === 0 || input.participants.length > 24)
    throw new RangeError("Meeting planning requires 1–24 participants.");
  const participants = input.participants.map((participant, index) => {
    const name = assertBoundedText(
      participant.name || `Participant ${index + 1}`,
      128,
      "Participant name",
    ).trim();
    const timeZone = assertBoundedText(
      participant.timeZone,
      128,
      "Participant time zone",
    ).trim();
    Temporal.Now.instant().toZonedDateTimeISO(timeZone);
    const workStart = clockMinutes(participant.workStart, "Work start");
    const workEnd = clockMinutes(participant.workEnd, "Work end");
    if (workEnd <= workStart)
      throw new RangeError(
        `${name}'s work window must end after it starts on the same local day.`,
      );
    return { ...participant, name, timeZone, workStart, workEnd };
  });
  const start = parseInstant(input.start);
  const stepNanoseconds = BigInt(stepMinutes) * 60_000_000_000n;
  const durationNanoseconds = BigInt(durationMinutes) * 60_000_000_000n;
  const end = start.add({ hours: days * 24 });
  const remainder = start.epochNanoseconds % stepNanoseconds;
  let cursor =
    remainder === 0n
      ? start
      : Temporal.Instant.fromEpochNanoseconds(
          start.epochNanoseconds + stepNanoseconds - remainder,
        );
  const candidates: MeetingCandidate[] = [];
  let inspected = 0;
  const maximumCandidates = Math.ceil((days * 24 * 60) / stepMinutes) + 1;
  while (
    Temporal.Instant.compare(cursor, end) < 0 &&
    inspected < maximumCandidates &&
    candidates.length < maxResults
  ) {
    inspected += 1;
    const candidateEnd = Temporal.Instant.fromEpochNanoseconds(
      cursor.epochNanoseconds + durationNanoseconds,
    );
    const local = participants.map((participant) => {
      const localStart = cursor.toZonedDateTimeISO(participant.timeZone);
      const localEnd = candidateEnd.toZonedDateTimeISO(participant.timeZone);
      const startMinutes = localStart.hour * 60 + localStart.minute;
      const endMinutes = localEnd.hour * 60 + localEnd.minute;
      const sameDay = localStart.toPlainDate().equals(localEnd.toPlainDate());
      const weekdayAllowed =
        participant.includeWeekends === true || localStart.dayOfWeek <= 5;
      return {
        allowed:
          sameDay &&
          weekdayAllowed &&
          startMinutes >= participant.workStart &&
          endMinutes <= participant.workEnd,
        name: participant.name,
        timeZone: participant.timeZone,
        startLocal: localStart.toString(),
        endLocal: localEnd.toString(),
        offset: localStart.offset,
      };
    });
    if (local.every((participant) => participant.allowed))
      candidates.push({
        index: candidates.length + 1,
        start: cursor.toString(),
        end: candidateEnd.toString(),
        participants: local.map((participant) =>
          Object.freeze({
            name: participant.name,
            timeZone: participant.timeZone,
            startLocal: participant.startLocal,
            endLocal: participant.endLocal,
            offset: participant.offset,
          }),
        ),
      });
    cursor = Temporal.Instant.fromEpochNanoseconds(
      cursor.epochNanoseconds + stepNanoseconds,
    );
  }
  return Object.freeze({
    searchedFrom: start.toString(),
    searchedUntil: end.toString(),
    durationMinutes,
    stepMinutes,
    candidates: Object.freeze(candidates),
    exhausted: candidates.length < maxResults,
    note: "Availability is inferred only from the entered recurring local work windows. Calendars, holidays, travel, and personal availability are not consulted.",
  });
}