/**
 * Little League World Series (LLWS) Bracket Simulator
 *
 * Monte Carlo simulation of the LLWS (20-team double-elimination format, 2025+).
 *
 * The tournament is two independent 10-team double-elimination brackets — United
 * States and International — each producing a side champion, then a World
 * Championship game and a Consolation game between the side runners-up. There is no
 * pool play. This mirrors the llws_20 bracket template so simulated placements line
 * up with the bracket admins actually score.
 *
 * Algorithm:
 *   1. Load all 20 participants for the sports season from DB
 *      (must be exactly 10 US + 10 International, identified by externalId)
 *   2. Load championship futures odds from participantExpectedValues.sourceOdds
 *      (entered via Admin → Futures Odds; American format)
 *   3. Convert odds to normalized championship probabilities (vig removed).
 *      These drive per-game win probability: p1 / (p1 + p2). Falls back to 50/50.
 *   4. Per simulation:
 *      a. Shuffle each side's 10 teams into the 10 bracket slots (8 opening-round
 *         teams + 2 byes). The draw is modelled as random — a specific known draw
 *         is not yet expressible in participant config.
 *      b. Simulate the 10-team double-elimination bracket for each side
 *         (see simulateSideBracket for the exact game-by-game structure)
 *      c. Consolation game: US side loser vs Intl side loser → 3rd / 4th
 *      d. World Championship: US champion vs Intl champion → 1st / 2nd
 *   5. Track placement counts across all simulations.
 *   6. Convert counts to probability distributions.
 *
 * Side assignment (externalId): "US" or "Intl". The legacy pool suffixes
 * ("US:A", "US:B", "Intl:A", "Intl:B") are still accepted and read as the side
 * alone, so seasons configured for the old pool-play format keep working — pools
 * no longer exist, so the suffix has no effect.
 *
 * Placement tiers → SimulationProbabilities mapping (matches llws_20's scoring):
 *   probFirst            = World Championship winner   (1 per sim)
 *   probSecond           = World Championship loser    (1 per sim)
 *   probThird            = Consolation winner          (1 per sim)
 *   probFourth           = Consolation loser           (1 per sim)
 *   probFifth/probSixth  = Elimination Final losers    (2 per sim — 1 per side)
 *   probSeventh/probEighth = Elimination Round 4 losers (2 per sim — 1 per side)
 *   Everyone else        → all 0 (12 teams out in Elimination Rounds 1–3)
 *
 * Admin setup:
 *   1. Create a Sport with simulatorType = "llws_bracket"
 *   2. Create a Sports Season and add exactly 20 participants (10 US, 10 International)
 *   3. Set externalId on each participant via Admin → Manage Participants to "US" or
 *      "Intl" (optional — names starting with "US " infer US, all others infer Intl)
 *   4. Enter championship futures odds via Admin → Futures Odds (sourceOdds)
 *   5. Run simulation via Admin → Simulate
 */

import { database } from "~/database/context";
import { eq } from "drizzle-orm";
import * as schema from "~/database/schema";
import { convertAmericanOddsToProbability } from "~/services/probability-engine";
import type { Simulator, SimulationResult } from "./types";
import { positiveConfigNumber } from "./config-access";

// ─── Simulation parameters ────────────────────────────────────────────────────

const NUM_SIMULATIONS = 50_000;
const US_TEAM_COUNT = 10;
const INTL_TEAM_COUNT = 10;

// ─── Types ────────────────────────────────────────────────────────────────────

type Side = "US" | "Intl";

interface Team {
  participantId: string;
  side: Side;
  /** Normalized championship win probability (0–1, vig removed). */
  oddsProb: number;
}

interface PlacementCounts {
  champion: number;
  finalist: number;
  thirdPlace: number;
  fourthPlace: number;
  /** Lost the Elimination Final — the 5th–6th tier (1 per side per sim). */
  elimFinalLoser: number;
  /** Lost Elimination Round 4 — the 7th–8th tier (1 per side per sim). */
  elimRound4Loser: number;
}

// ─── Helpers ─────────────────────────────────────────────────────────────────

function zeroCounts(): PlacementCounts {
  return {
    champion: 0, finalist: 0, thirdPlace: 0, fourthPlace: 0,
    elimFinalLoser: 0, elimRound4Loser: 0,
  };
}

function simGame(t1: Team, t2: Team): { winner: Team; loser: Team } {
  // If either team has no odds entered, treat the game as a coin flip.
  // The 50/50 fallback must cover the one-sided case (one team known, one not)
  // because oddsProb=0 would otherwise give the unknown team a 0% win rate.
  let p1Win: number;
  if (t1.oddsProb === 0 || t2.oddsProb === 0) {
    p1Win = 0.5;
  } else {
    p1Win = t1.oddsProb / (t1.oddsProb + t2.oddsProb);
  }
  return Math.random() < p1Win ? { winner: t1, loser: t2 } : { winner: t2, loser: t1 };
}

/**
 * Fisher-Yates shuffle (in-place).
 */
function shuffle<T>(arr: T[]): T[] {
  for (let i = arr.length - 1; i > 0; i--) {
    const j = Math.floor(Math.random() * (i + 1));
    [arr[i], arr[j]] = [arr[j], arr[i]];
  }
  return arr;
}

/**
 * Simulate one side's 10-team double-elimination bracket.
 *
 * `slots` holds the side's teams in bracket order, matching the llws_20 participant
 * layout: slots[0..7] are the four opening-round games (two teams each) and
 * slots[8], slots[9] are the two bye teams entering Winners Round 2.
 *
 * Structure (side-local, mirroring LLWS_ADVANCEMENT in models/playoff-match):
 *   Winners bracket
 *     OP1 s0 v s1   OP2 s2 v s3   OP3 s4 v s5   OP4 s6 v s7
 *     WR2-1 s8 v OP1w            WR2-2 s9 v OP2w
 *     WSF1 OP3w v WR2-1w         WSF2 WR2-2w v OP4w
 *     WF   WSF1w v WSF2w         → winner to the side championship
 *   Elimination bracket (a loss here is final)
 *     ER1-1 OP2l v OP3l          ER1-2 OP1l v OP4l
 *     ER2-1 WR2-1l v ER1-1w      ER2-2 WR2-2l v ER1-2w
 *     ER3-1 WSF1l v ER2-2w       ER3-2 WSF2l v ER2-1w   (cross-over)
 *     ER4   ER3-2w v ER3-1w      → loser is the 7th–8th tier
 *     EF    WFl v ER4w           → loser is the 5th–6th tier
 *   Side championship: WFw v EFw → loser drops to the consolation game
 *
 * Note the double-chance path: the Winners Final loser is NOT out, it drops to the
 * Elimination Final. There is no "if necessary" game, so the side championship is
 * decided in one game.
 *
 * Returns { sideChampion, sideLoser }; the two scoring elimination losers are
 * bumped into the counts directly.
 */
function simulateSideBracket(
  slots: Team[],
  bump: (id: string, key: keyof PlacementCounts) => void
): { sideChampion: Team; sideLoser: Team } {
  // ── Winners bracket ────────────────────────────────────────────────────────
  const op1 = simGame(slots[0], slots[1]);
  const op2 = simGame(slots[2], slots[3]);
  const op3 = simGame(slots[4], slots[5]);
  const op4 = simGame(slots[6], slots[7]);

  const wr21 = simGame(slots[8], op1.winner);
  const wr22 = simGame(slots[9], op2.winner);

  const wsf1 = simGame(op3.winner, wr21.winner);
  const wsf2 = simGame(wr22.winner, op4.winner);

  const wf = simGame(wsf1.winner, wsf2.winner);

  // ── Elimination bracket ────────────────────────────────────────────────────
  const er11 = simGame(op2.loser, op3.loser);
  const er12 = simGame(op1.loser, op4.loser);

  const er21 = simGame(wr21.loser, er11.winner);
  const er22 = simGame(wr22.loser, er12.winner);

  // Cross-over: each semifinal loser meets the winner from the opposite half.
  const er31 = simGame(wsf1.loser, er22.winner);
  const er32 = simGame(wsf2.loser, er21.winner);

  const er4 = simGame(er32.winner, er31.winner);
  bump(er4.loser.participantId, "elimRound4Loser"); // 7th–8th tier

  // The Winners Final loser gets its second chance here.
  const ef = simGame(wf.loser, er4.winner);
  bump(ef.loser.participantId, "elimFinalLoser"); // 5th–6th tier

  // ── Side championship ──────────────────────────────────────────────────────
  const sideChampionship = simGame(wf.winner, ef.winner);

  return { sideChampion: sideChampionship.winner, sideLoser: sideChampionship.loser };
}

// ─── Validation helpers ───────────────────────────────────────────────────────

/**
 * Parse a participant's externalId into a side.
 *
 * The legacy pool-play suffixes ("US:A", "Intl:B", …) are still accepted so seasons
 * configured before the format change keep loading; the pool part is ignored because
 * the tournament no longer has pools.
 */
function parseExternalId(raw: string | null): { side: Side } | null {
  if (!raw) return null;
  const side = raw.toUpperCase().split(":")[0];
  if (side === "US") return { side: "US" };
  if (side === "INTL") return { side: "Intl" };
  return null;
}

/**
 * Infer an externalId from a participant name when none is stored.
 * Teams whose name is exactly "US" or starts with "US " (case-insensitive)
 * are assigned to the US side; all others are assigned to Intl.
 * The inferred value never has a pool suffix, so pools will be randomized.
 */
function inferExternalIdFromName(name: string): string {
  const upper = name.trim().toUpperCase();
  return upper === "US" || upper.startsWith("US ") ? "US" : "Intl";
}

// ─── Simulator ────────────────────────────────────────────────────────────────

export class LLWSSimulator implements Simulator {
  constructor(private numSimulations = NUM_SIMULATIONS) {}

  async simulate(sportsSeasonId: string, config: Record<string, unknown> = {}): Promise<SimulationResult[]> {
    const numSimulations = Math.round(positiveConfigNumber(config, "iterations", this.numSimulations));
    const db = database();

    // 1. Load all participants.
    const participants = await db
      .select({ id: schema.seasonParticipants.id, name: schema.seasonParticipants.name, externalId: schema.seasonParticipants.externalId })
      .from(schema.seasonParticipants)
      .where(eq(schema.seasonParticipants.sportsSeasonId, sportsSeasonId));

    if (participants.length !== US_TEAM_COUNT + INTL_TEAM_COUNT) {
      throw new Error(
        `LLWS simulator requires exactly ${US_TEAM_COUNT + INTL_TEAM_COUNT} participants, ` +
        `found ${participants.length}.`
      );
    }

    // 2. Load championship futures odds.
    const evRows = await db
      .select({
        participantId: schema.seasonParticipantExpectedValues.participantId,
        sourceOdds: schema.seasonParticipantExpectedValues.sourceOdds,
      })
      .from(schema.seasonParticipantExpectedValues)
      .where(eq(schema.seasonParticipantExpectedValues.sportsSeasonId, sportsSeasonId));

    const rawOddsMap = new Map<string, number>();
    for (const row of evRows) {
      if (row.sourceOdds !== null) {
        rawOddsMap.set(row.participantId, convertAmericanOddsToProbability(row.sourceOdds));
      }
    }

    // 3. Normalize odds (remove vig) to get championship probability per team.
    const normalizedOddsMap = new Map<string, number>();
    if (rawOddsMap.size > 0) {
      const rawSum = [...rawOddsMap.values()].reduce((a, b) => a + b, 0);
      for (const [id, prob] of rawOddsMap) {
        normalizedOddsMap.set(id, rawSum > 0 ? prob / rawSum : 0);
      }
    }

    // 4. Parse externalId for each participant to determine which side they're on.
    const teams: Team[] = [];
    for (const p of participants) {
      const raw = p.externalId ?? inferExternalIdFromName(p.name);
      const parsed = parseExternalId(raw);
      if (!parsed) {
        throw new Error(
          `Participant ${p.id} has invalid externalId "${p.externalId}". ` +
          `Expected: "US" or "Intl".`
        );
      }
      teams.push({
        participantId: p.id,
        side: parsed.side,
        oddsProb: normalizedOddsMap.get(p.id) ?? 0,
      });
    }

    // Validate team counts per side.
    const usTeams = teams.filter((t) => t.side === "US");
    const intlTeams = teams.filter((t) => t.side === "Intl");

    if (usTeams.length !== US_TEAM_COUNT) {
      throw new Error(`Expected ${US_TEAM_COUNT} US teams, found ${usTeams.length}.`);
    }
    if (intlTeams.length !== INTL_TEAM_COUNT) {
      throw new Error(`Expected ${INTL_TEAM_COUNT} International teams, found ${intlTeams.length}.`);
    }

    // 5. Initialise placement count accumulators for all participants.
    const allIds = participants.map((p) => p.id);
    const counts = new Map<string, PlacementCounts>(allIds.map((id) => [id, zeroCounts()]));
    const bump = (id: string, key: keyof PlacementCounts) => {
      const entry = counts.get(id);
      if (entry) entry[key]++;
    };

    // 6. Run Monte Carlo simulations.
    for (let s = 0; s < numSimulations; s++) {
      // The draw is modelled as random: shuffle each side into the 10 bracket slots
      // (8 opening-round teams, then the 2 bye teams).
      const { sideChampion: usChamp, sideLoser: usLose } =
        simulateSideBracket(shuffle([...usTeams]), bump);
      const { sideChampion: intlChamp, sideLoser: intlLose } =
        simulateSideBracket(shuffle([...intlTeams]), bump);

      // Consolation game: 3rd / 4th place.
      const consolation = simGame(usLose, intlLose);
      bump(consolation.winner.participantId, "thirdPlace");
      bump(consolation.loser.participantId, "fourthPlace");

      // World Championship: 1st / 2nd place.
      const ws = simGame(usChamp, intlChamp);
      bump(ws.winner.participantId, "champion");
      bump(ws.loser.participantId, "finalist");
    }

    // 7. Convert counts to probability distributions.
    //    Each of the two 5–8 tiers takes exactly 2 teams per sim (one per side), and
    //    the teams within a tier are tied, so the tier probability is split across
    //    its two positions.
    const tierDivisor = 2 * numSimulations;

    const empty = zeroCounts();
    return allIds.map((id) => {
      const c = counts.get(id) ?? empty;
      const upperTier = c.elimFinalLoser / tierDivisor;   // 5th–6th
      const lowerTier = c.elimRound4Loser / tierDivisor;  // 7th–8th
      return {
        participantId: id,
        probabilities: {
          probFirst: c.champion / numSimulations,
          probSecond: c.finalist / numSimulations,
          probThird: c.thirdPlace / numSimulations,
          probFourth: c.fourthPlace / numSimulations,
          probFifth: upperTier,
          probSixth: upperTier,
          probSeventh: lowerTier,
          probEighth: lowerTier,
        },
        source: "llws_monte_carlo",
      };
    });
  }
}