felt/internal/seating/balance.go

package seating

import (
	"context"
	"database/sql"
	"encoding/json"
	"fmt"
	"sort"

	"github.com/felt-app/felt/internal/audit"
	"github.com/felt-app/felt/internal/server/ws"
)

// ---------- Types ----------

// BalanceStatus describes whether tables are balanced and what moves are needed.
type BalanceStatus struct {
	IsBalanced    bool         `json:"is_balanced"`
	MaxDifference int          `json:"max_difference"`
	Tables        []TableCount `json:"tables"`
	NeedsMoves    int          `json:"needs_moves"`
}

// TableCount is a summary of one table's player count.
type TableCount struct {
	TableID     int    `json:"table_id"`
	TableName   string `json:"table_name"`
	PlayerCount int    `json:"player_count"`
}

// BalanceSuggestion is a proposed move to balance tables.
type BalanceSuggestion struct {
	ID            int     `json:"id"`
	FromTableID   int     `json:"from_table_id"`
	FromTableName string  `json:"from_table_name"`
	ToTableID     int     `json:"to_table_id"`
	ToTableName   string  `json:"to_table_name"`
	PlayerID      *string `json:"player_id,omitempty"`
	PlayerName    *string `json:"player_name,omitempty"`
	Status        string  `json:"status"` // pending, accepted, cancelled, expired
	CreatedAt     int64   `json:"created_at"`
}

// ---------- Service ----------

// BalanceEngine manages table balancing suggestions per TDA rules.
type BalanceEngine struct {
	db    *sql.DB
	audit *audit.Trail
	hub   *ws.Hub
}

// NewBalanceEngine creates a new BalanceEngine.
func NewBalanceEngine(db *sql.DB, auditTrail *audit.Trail, hub *ws.Hub) *BalanceEngine {
	return &BalanceEngine{
		db:    db,
		audit: auditTrail,
		hub:   hub,
	}
}

// ---------- Balance Check ----------

// CheckBalance checks whether tables in a tournament are balanced.
// TDA rule: tables are unbalanced if the difference between the largest
// and smallest table exceeds 1.
func (e *BalanceEngine) CheckBalance(ctx context.Context, tournamentID string) (*BalanceStatus, error) {
	counts, err := e.getTableCounts(ctx, tournamentID)
	if err != nil {
		return nil, err
	}

	if len(counts) < 2 {
		return &BalanceStatus{
			IsBalanced: true,
			Tables:     counts,
		}, nil
	}

	// Find min and max player counts
	minCount := counts[0].PlayerCount
	maxCount := counts[0].PlayerCount
	for _, tc := range counts[1:] {
		if tc.PlayerCount < minCount {
			minCount = tc.PlayerCount
		}
		if tc.PlayerCount > maxCount {
			maxCount = tc.PlayerCount
		}
	}

	diff := maxCount - minCount
	balanced := diff <= 1

	// Calculate how many moves are needed to balance
	needsMoves := 0
	if !balanced {
		// Target: all tables should have either floor(total/n) or ceil(total/n) players
		total := 0
		for _, tc := range counts {
			total += tc.PlayerCount
		}
		n := len(counts)
		base := total / n
		extra := total % n

		// Sort by player count descending to count surplus
		sorted := make([]TableCount, len(counts))
		copy(sorted, counts)
		sort.Slice(sorted, func(i, j int) bool {
			return sorted[i].PlayerCount > sorted[j].PlayerCount
		})

		for i, tc := range sorted {
			target := base
			if i < extra {
				target = base + 1
			}
			if tc.PlayerCount > target {
				needsMoves += tc.PlayerCount - target
			}
		}
	}

	return &BalanceStatus{
		IsBalanced:    balanced,
		MaxDifference: diff,
		Tables:        counts,
		NeedsMoves:    needsMoves,
	}, nil
}

// ---------- Suggestions ----------

// SuggestMoves generates balance move suggestions. Suggestions are proposals
// that must be accepted by the TD -- never auto-applied.
func (e *BalanceEngine) SuggestMoves(ctx context.Context, tournamentID string) ([]BalanceSuggestion, error) {
	status, err := e.CheckBalance(ctx, tournamentID)
	if err != nil {
		return nil, err
	}

	if status.IsBalanced {
		return nil, nil
	}

	// Build table name map for later use
	tableNames := make(map[int]string)
	for _, tc := range status.Tables {
		tableNames[tc.TableID] = tc.TableName
	}

	// Calculate targets: each table should have floor(total/n) or ceil(total/n)
	total := 0
	for _, tc := range status.Tables {
		total += tc.PlayerCount
	}
	n := len(status.Tables)
	base := total / n
	extra := total % n

	// Sort tables by player count descending
	sorted := make([]TableCount, len(status.Tables))
	copy(sorted, status.Tables)
	sort.Slice(sorted, func(i, j int) bool {
		return sorted[i].PlayerCount > sorted[j].PlayerCount
	})

	// Assign targets: top 'extra' tables get base+1, rest get base
	targets := make(map[int]int)
	for i, tc := range sorted {
		if i < extra {
			targets[tc.TableID] = base + 1
		} else {
			targets[tc.TableID] = base
		}
	}

	// Generate moves: from tables with surplus to tables with deficit
	type surplus struct {
		tableID int
		excess  int
	}
	type deficit struct {
		tableID int
		need    int
	}

	var surplusTables []surplus
	var deficitTables []deficit

	for _, tc := range sorted {
		target := targets[tc.TableID]
		if tc.PlayerCount > target {
			surplusTables = append(surplusTables, surplus{tc.TableID, tc.PlayerCount - target})
		} else if tc.PlayerCount < target {
			deficitTables = append(deficitTables, deficit{tc.TableID, target - tc.PlayerCount})
		}
	}

	var suggestions []BalanceSuggestion

	si, di := 0, 0
	for si < len(surplusTables) && di < len(deficitTables) {
		s := &surplusTables[si]
		d := &deficitTables[di]

		// Pick a player from the surplus table (prefer player farthest from button for fairness)
		playerID, playerName := e.pickPlayerToMove(ctx, tournamentID, s.tableID)

		// Insert suggestion into DB
		result, err := e.db.ExecContext(ctx,
			`INSERT INTO balance_suggestions
			 (tournament_id, from_table_id, to_table_id, player_id, status, reason)
			 VALUES (?, ?, ?, ?, 'pending', 'balance')`,
			tournamentID, s.tableID, d.tableID, playerID,
		)
		if err != nil {
			return nil, fmt.Errorf("insert suggestion: %w", err)
		}

		id, _ := result.LastInsertId()
		var createdAt int64
		_ = e.db.QueryRowContext(ctx,
			`SELECT created_at FROM balance_suggestions WHERE id = ?`, id,
		).Scan(&createdAt)

		sugg := BalanceSuggestion{
			ID:            int(id),
			FromTableID:   s.tableID,
			FromTableName: tableNames[s.tableID],
			ToTableID:     d.tableID,
			ToTableName:   tableNames[d.tableID],
			PlayerID:      playerID,
			PlayerName:    playerName,
			Status:        "pending",
			CreatedAt:     createdAt,
		}
		suggestions = append(suggestions, sugg)

		s.excess--
		d.need--
		if s.excess == 0 {
			si++
		}
		if d.need == 0 {
			di++
		}
	}

	e.recordAudit(ctx, tournamentID, audit.ActionSeatBalance, "balance", tournamentID, nil,
		map[string]interface{}{"suggestions": len(suggestions)})
	e.broadcast(tournamentID, "balance.suggestions", suggestions)

	return suggestions, nil
}

// AcceptSuggestion re-validates and executes a balance suggestion.
// If the suggestion is stale (state changed), it is cancelled and an error is returned.
func (e *BalanceEngine) AcceptSuggestion(ctx context.Context, tournamentID string, suggestionID, fromSeat, toSeat int) error {
	// Load suggestion
	var sugg BalanceSuggestion
	var playerID sql.NullString
	err := e.db.QueryRowContext(ctx,
		`SELECT id, from_table_id, to_table_id, player_id, status
		 FROM balance_suggestions
		 WHERE id = ? AND tournament_id = ?`,
		suggestionID, tournamentID,
	).Scan(&sugg.ID, &sugg.FromTableID, &sugg.ToTableID, &playerID, &sugg.Status)
	if err == sql.ErrNoRows {
		return fmt.Errorf("suggestion not found")
	}
	if err != nil {
		return fmt.Errorf("load suggestion: %w", err)
	}

	if sugg.Status != "pending" {
		return fmt.Errorf("suggestion is %s, not pending", sugg.Status)
	}

	// Re-validate: is the source table still larger than destination?
	var fromCount, toCount int
	err = e.db.QueryRowContext(ctx,
		`SELECT COUNT(*) FROM tournament_players
		 WHERE tournament_id = ? AND seat_table_id = ? AND status IN ('registered', 'active')`,
		tournamentID, sugg.FromTableID,
	).Scan(&fromCount)
	if err != nil {
		return fmt.Errorf("count from table: %w", err)
	}

	err = e.db.QueryRowContext(ctx,
		`SELECT COUNT(*) FROM tournament_players
		 WHERE tournament_id = ? AND seat_table_id = ? AND status IN ('registered', 'active')`,
		tournamentID, sugg.ToTableID,
	).Scan(&toCount)
	if err != nil {
		return fmt.Errorf("count to table: %w", err)
	}

	// Stale check: if source is not larger than destination by at least 2, suggestion is stale
	if fromCount-toCount < 2 {
		// Cancel the stale suggestion
		_, _ = e.db.ExecContext(ctx,
			`UPDATE balance_suggestions SET status = 'expired', resolved_at = unixepoch()
			 WHERE id = ?`, suggestionID)
		e.broadcast(tournamentID, "balance.suggestion_expired", map[string]interface{}{
			"suggestion_id": suggestionID, "reason": "stale",
		})
		return fmt.Errorf("suggestion is stale: table counts no longer justify this move (from=%d, to=%d)", fromCount, toCount)
	}

	// Determine which player to move
	pid := ""
	if playerID.Valid {
		pid = playerID.String
	}
	if pid == "" {
		return fmt.Errorf("no player specified in suggestion")
	}

	// Verify player is still at the source table
	var playerTable sql.NullInt64
	err = e.db.QueryRowContext(ctx,
		`SELECT seat_table_id FROM tournament_players
		 WHERE tournament_id = ? AND player_id = ? AND status IN ('registered', 'active')`,
		tournamentID, pid,
	).Scan(&playerTable)
	if err != nil {
		return fmt.Errorf("check player location: %w", err)
	}
	if !playerTable.Valid || int(playerTable.Int64) != sugg.FromTableID {
		_, _ = e.db.ExecContext(ctx,
			`UPDATE balance_suggestions SET status = 'expired', resolved_at = unixepoch()
			 WHERE id = ?`, suggestionID)
		return fmt.Errorf("suggestion is stale: player no longer at source table")
	}

	// Verify destination seat is empty
	var occCount int
	err = e.db.QueryRowContext(ctx,
		`SELECT COUNT(*) FROM tournament_players
		 WHERE tournament_id = ? AND seat_table_id = ? AND seat_position = ?
		   AND status IN ('registered', 'active')`,
		tournamentID, sugg.ToTableID, toSeat,
	).Scan(&occCount)
	if err != nil {
		return fmt.Errorf("check destination seat: %w", err)
	}
	if occCount > 0 {
		return fmt.Errorf("destination seat %d is already occupied", toSeat)
	}

	// Execute the move
	_, err = e.db.ExecContext(ctx,
		`UPDATE tournament_players SET seat_table_id = ?, seat_position = ?, updated_at = unixepoch()
		 WHERE tournament_id = ? AND player_id = ? AND status IN ('registered', 'active')`,
		sugg.ToTableID, toSeat, tournamentID, pid,
	)
	if err != nil {
		return fmt.Errorf("move player: %w", err)
	}

	// Mark suggestion as accepted
	_, err = e.db.ExecContext(ctx,
		`UPDATE balance_suggestions SET status = 'accepted', from_seat = ?, to_seat = ?, resolved_at = unixepoch()
		 WHERE id = ?`,
		fromSeat, toSeat, suggestionID,
	)
	if err != nil {
		return fmt.Errorf("mark accepted: %w", err)
	}

	e.recordAudit(ctx, tournamentID, audit.ActionSeatBalance, "player", pid,
		map[string]interface{}{"table_id": sugg.FromTableID, "seat": fromSeat},
		map[string]interface{}{"table_id": sugg.ToTableID, "seat": toSeat})
	e.broadcast(tournamentID, "balance.accepted", map[string]interface{}{
		"suggestion_id": suggestionID, "player_id": pid,
		"from_table_id": sugg.FromTableID, "to_table_id": sugg.ToTableID,
		"from_seat": fromSeat, "to_seat": toSeat,
	})

	return nil
}

// CancelSuggestion cancels a pending balance suggestion.
func (e *BalanceEngine) CancelSuggestion(ctx context.Context, tournamentID string, suggestionID int) error {
	result, err := e.db.ExecContext(ctx,
		`UPDATE balance_suggestions SET status = 'cancelled', resolved_at = unixepoch()
		 WHERE id = ? AND tournament_id = ? AND status = 'pending'`,
		suggestionID, tournamentID,
	)
	if err != nil {
		return fmt.Errorf("cancel suggestion: %w", err)
	}
	n, _ := result.RowsAffected()
	if n == 0 {
		return fmt.Errorf("suggestion not found or not pending")
	}

	e.recordAudit(ctx, tournamentID, audit.ActionSeatBalance, "suggestion", fmt.Sprintf("%d", suggestionID), nil,
		map[string]interface{}{"status": "cancelled"})
	e.broadcast(tournamentID, "balance.cancelled", map[string]interface{}{
		"suggestion_id": suggestionID,
	})
	return nil
}

// InvalidateStaleSuggestions checks all pending suggestions and cancels those
// that are no longer valid due to state changes. This implements the "live and
// adaptive" behavior from CONTEXT.md.
func (e *BalanceEngine) InvalidateStaleSuggestions(ctx context.Context, tournamentID string) error {
	rows, err := e.db.QueryContext(ctx,
		`SELECT id, from_table_id, to_table_id, player_id
		 FROM balance_suggestions
		 WHERE tournament_id = ? AND status = 'pending'`,
		tournamentID,
	)
	if err != nil {
		return fmt.Errorf("query pending suggestions: %w", err)
	}
	defer rows.Close()

	type pending struct {
		id          int
		fromTableID int
		toTableID   int
		playerID    sql.NullString
	}

	var pendings []pending
	for rows.Next() {
		var p pending
		if err := rows.Scan(&p.id, &p.fromTableID, &p.toTableID, &p.playerID); err != nil {
			return fmt.Errorf("scan suggestion: %w", err)
		}
		pendings = append(pendings, p)
	}
	if err := rows.Err(); err != nil {
		return err
	}

	// Get current table counts
	counts, err := e.getTableCounts(ctx, tournamentID)
	if err != nil {
		return err
	}
	countMap := make(map[int]int)
	for _, tc := range counts {
		countMap[tc.TableID] = tc.PlayerCount
	}

	for _, p := range pendings {
		fromCount := countMap[p.fromTableID]
		toCount := countMap[p.toTableID]

		stale := false

		// Check: does this move still make sense?
		if fromCount-toCount < 2 {
			stale = true
		}

		// Check: is the suggested player still at the source table?
		if !stale && p.playerID.Valid {
			var playerTableID sql.NullInt64
			err := e.db.QueryRowContext(ctx,
				`SELECT seat_table_id FROM tournament_players
				 WHERE tournament_id = ? AND player_id = ? AND status IN ('registered', 'active')`,
				tournamentID, p.playerID.String,
			).Scan(&playerTableID)
			if err != nil || !playerTableID.Valid || int(playerTableID.Int64) != p.fromTableID {
				stale = true
			}
		}

		if stale {
			_, _ = e.db.ExecContext(ctx,
				`UPDATE balance_suggestions SET status = 'expired', resolved_at = unixepoch()
				 WHERE id = ?`, p.id)
			e.broadcast(tournamentID, "balance.suggestion_expired", map[string]interface{}{
				"suggestion_id": p.id,
			})
		}
	}

	return nil
}

// GetPendingSuggestions returns all pending balance suggestions for a tournament.
func (e *BalanceEngine) GetPendingSuggestions(ctx context.Context, tournamentID string) ([]BalanceSuggestion, error) {
	rows, err := e.db.QueryContext(ctx,
		`SELECT bs.id, bs.from_table_id, ft.name, bs.to_table_id, tt.name,
		        bs.player_id, p.name, bs.status, bs.created_at
		 FROM balance_suggestions bs
		 JOIN tables ft ON ft.id = bs.from_table_id
		 JOIN tables tt ON tt.id = bs.to_table_id
		 LEFT JOIN players p ON p.id = bs.player_id
		 WHERE bs.tournament_id = ? AND bs.status = 'pending'
		 ORDER BY bs.created_at`,
		tournamentID,
	)
	if err != nil {
		return nil, fmt.Errorf("query suggestions: %w", err)
	}
	defer rows.Close()

	var suggestions []BalanceSuggestion
	for rows.Next() {
		var s BalanceSuggestion
		var pid, pname sql.NullString
		if err := rows.Scan(&s.ID, &s.FromTableID, &s.FromTableName,
			&s.ToTableID, &s.ToTableName, &pid, &pname, &s.Status, &s.CreatedAt); err != nil {
			return nil, fmt.Errorf("scan suggestion: %w", err)
		}
		if pid.Valid {
			s.PlayerID = &pid.String
		}
		if pname.Valid {
			s.PlayerName = &pname.String
		}
		suggestions = append(suggestions, s)
	}
	return suggestions, rows.Err()
}

// ---------- Helpers ----------

// getTableCounts returns player counts per active table.
func (e *BalanceEngine) getTableCounts(ctx context.Context, tournamentID string) ([]TableCount, error) {
	rows, err := e.db.QueryContext(ctx,
		`SELECT t.id, t.name,
		        (SELECT COUNT(*) FROM tournament_players tp
		         WHERE tp.seat_table_id = t.id AND tp.tournament_id = t.tournament_id
		           AND tp.status IN ('registered', 'active')) AS player_count
		 FROM tables t
		 WHERE t.tournament_id = ? AND t.is_active = 1
		 ORDER BY t.name`,
		tournamentID,
	)
	if err != nil {
		return nil, fmt.Errorf("get table counts: %w", err)
	}
	defer rows.Close()

	var counts []TableCount
	for rows.Next() {
		var tc TableCount
		if err := rows.Scan(&tc.TableID, &tc.TableName, &tc.PlayerCount); err != nil {
			return nil, fmt.Errorf("scan table count: %w", err)
		}
		counts = append(counts, tc)
	}
	return counts, rows.Err()
}

// pickPlayerToMove selects a player to move from a table for balancing.
// Prefers the player in the worst position relative to the dealer button
// (fairness per TDA rules).
func (e *BalanceEngine) pickPlayerToMove(ctx context.Context, tournamentID string, tableID int) (*string, *string) {
	// Get dealer button position
	var btnPos sql.NullInt64
	var seatCount int
	_ = e.db.QueryRowContext(ctx,
		`SELECT dealer_button_position, seat_count FROM tables
		 WHERE id = ? AND tournament_id = ?`,
		tableID, tournamentID,
	).Scan(&btnPos, &seatCount)

	// Get all players at this table
	rows, err := e.db.QueryContext(ctx,
		`SELECT tp.player_id, p.name, tp.seat_position
		 FROM tournament_players tp
		 JOIN players p ON p.id = tp.player_id
		 WHERE tp.tournament_id = ? AND tp.seat_table_id = ?
		   AND tp.status IN ('registered', 'active') AND tp.seat_position IS NOT NULL
		 ORDER BY tp.seat_position`,
		tournamentID, tableID,
	)
	if err != nil {
		return nil, nil
	}
	defer rows.Close()

	type playerSeat struct {
		id       string
		name     string
		position int
	}
	var players []playerSeat
	for rows.Next() {
		var ps playerSeat
		if err := rows.Scan(&ps.id, &ps.name, &ps.position); err != nil {
			continue
		}
		players = append(players, ps)
	}

	if len(players) == 0 {
		return nil, nil
	}

	// If no button set, pick the last player (arbitrary but deterministic)
	if !btnPos.Valid {
		p := players[len(players)-1]
		return &p.id, &p.name
	}

	btn := int(btnPos.Int64)

	// Pick the player farthest from the button in clockwise direction
	// (i.e. the player who will wait longest for the button)
	// The player immediately after the button (SB) waits the longest for
	// their next button.
	// We want to move the player who is "worst off" = closest clockwise AFTER
	// the button (big blind or later positions are less penalized by a move).
	//
	// Simplification: pick the player in the first position clockwise after
	// the button (the small blind position) — that player benefits least from
	// staying.
	best := players[0]
	bestDist := clockwiseDist(btn, players[0].position, seatCount)
	for _, ps := range players[1:] {
		d := clockwiseDist(btn, ps.position, seatCount)
		if d < bestDist {
			bestDist = d
			best = ps
		}
	}

	return &best.id, &best.name
}

// clockwiseDist returns the clockwise distance from 'from' to 'to' on a
// table with seatCount seats.
func clockwiseDist(from, to, seatCount int) int {
	d := to - from
	if d <= 0 {
		d += seatCount
	}
	return d
}

func (e *BalanceEngine) recordAudit(ctx context.Context, tournamentID, action, targetType, targetID string, prevState, newState interface{}) {
	if e.audit == nil {
		return
	}
	var prev, next json.RawMessage
	if prevState != nil {
		prev, _ = json.Marshal(prevState)
	}
	if newState != nil {
		next, _ = json.Marshal(newState)
	}
	tid := tournamentID
	e.audit.Record(ctx, audit.AuditEntry{
		TournamentID:  &tid,
		Action:        action,
		TargetType:    targetType,
		TargetID:      targetID,
		PreviousState: prev,
		NewState:      next,
	})
}

func (e *BalanceEngine) broadcast(tournamentID, msgType string, data interface{}) {
	if e.hub == nil {
		return
	}
	payload, err := json.Marshal(data)
	if err != nil {
		return
	}
	e.hub.Broadcast(tournamentID, msgType, payload)
}