package pgmonitor

import (
	"context"
	"errors"
	"fmt"
	"strings"
	"time"

	"github.com/jackc/pgx/v5"
	"github.com/jackc/pgx/v5/pgconn"
	"github.com/jackc/pgx/v5/pgxpool"
)

func clampLimit(limit, def, max int) int {
	if limit <= 0 {
		return def
	}
	if limit > max {
		return max
	}
	return limit
}

func (s *Service) fetchOverview(ctx context.Context) (Overview, error) {
	now := time.Now().UTC()
	out := Overview{CollectedAt: now}

	var active, idle, total, maxConn int
	err := s.pool.QueryRow(ctx, `
		SELECT
		  count(*) FILTER (WHERE state = 'active'),
		  count(*) FILTER (WHERE state = 'idle'),
		  count(*),
		  (SELECT setting::int FROM pg_settings WHERE name = 'max_connections')
		FROM pg_stat_activity
		WHERE datname = current_database()`).Scan(&active, &idle, &total, &maxConn)
	if err != nil {
		return out, fmt.Errorf("pgmonitor: connections: %w", err)
	}
	out.Connections = Connections{Active: active, Idle: idle, Total: total, MaxConnections: maxConn}

	var cachePct *float64
	err = s.pool.QueryRow(ctx, `
		SELECT numbackends, xact_commit, xact_rollback, deadlocks, blks_hit, blks_read,
		       CASE WHEN blks_hit + blks_read > 0
		            THEN round(100.0 * blks_hit::numeric / (blks_hit + blks_read), 2) END
		FROM pg_stat_database WHERE datname = current_database()`).Scan(
		&out.Database.Backends,
		&out.Database.XactCommit,
		&out.Database.XactRollback,
		&out.Database.Deadlocks,
		&out.Database.BlksHit,
		&out.Database.BlksRead,
		&cachePct,
	)
	if err != nil {
		return out, fmt.Errorf("pgmonitor: database stats: %w", err)
	}
	if cachePct != nil {
		out.Database.CacheHitPct = *cachePct
	}

	_ = s.pool.QueryRow(ctx, `
		SELECT checkpoints_timed, checkpoints_req, buffers_checkpoint, buffers_clean,
		       maxwritten_clean, buffers_backend, buffers_alloc
		FROM pg_stat_bgwriter`).Scan(
		&out.Bgwriter.CheckpointsTimed,
		&out.Bgwriter.CheckpointsReq,
		&out.Bgwriter.BuffersCheckpoint,
		&out.Bgwriter.BuffersClean,
		&out.Bgwriter.MaxWrittenClean,
		&out.Bgwriter.BuffersBackend,
		&out.Bgwriter.BuffersAlloc,
	)

	_ = s.pool.QueryRow(ctx, `SELECT pg_database_size(current_database())`).Scan(&out.SizeBytes)

	_ = s.pool.QueryRow(ctx, `
		SELECT
		  (SELECT setting FROM pg_settings WHERE name = 'shared_buffers'),
		  (SELECT setting FROM pg_settings WHERE name = 'work_mem'),
		  (SELECT setting FROM pg_settings WHERE name = 'effective_cache_size')`).Scan(
		&out.MemorySettings.SharedBuffers,
		&out.MemorySettings.WorkMem,
		&out.MemorySettings.EffectiveCacheSize,
	)

	rows, err := s.pool.Query(ctx, `
		SELECT client_addr::text, state, sync_state,
		       EXTRACT(EPOCH FROM COALESCE(write_lag, flush_lag, replay_lag)) * 1000
		FROM pg_stat_replication`)
	if err == nil {
		defer rows.Close()
		for rows.Next() {
			var peer ReplicationPeer
			var lagMs *float64
			if err := rows.Scan(&peer.ClientAddr, &peer.State, &peer.SyncState, &lagMs); err != nil {
				continue
			}
			if lagMs != nil {
				v := int64(*lagMs)
				peer.LagMs = &v
			}
			out.Replication = append(out.Replication, peer)
		}
	}

	out.StatementsEnabled = s.statementsQueryable(ctx)

	return out, nil
}

func queryLocks(ctx context.Context, pool *pgxpool.Pool) ([]LockRow, error) {
	rows, err := pool.Query(ctx, `
		SELECT l.locktype, l.mode, l.granted, a.pid, COALESCE(a.usename, ''),
		       COALESCE(a.state, ''), COALESCE(left(a.query, 300), ''),
		       NOT l.granted AS blocked
		FROM pg_locks l
		JOIN pg_stat_activity a ON a.pid = l.pid
		WHERE a.datname = current_database()
		  AND (NOT l.granted OR l.mode LIKE '%Exclusive%')
		ORDER BY l.granted ASC, a.query_start NULLS LAST
		LIMIT 200`)
	if err != nil {
		return nil, fmt.Errorf("pgmonitor: locks: %w", err)
	}
	defer rows.Close()
	var out []LockRow
	for rows.Next() {
		var r LockRow
		if err := rows.Scan(&r.Locktype, &r.Mode, &r.Granted, &r.PID, &r.User, &r.State, &r.Query, &r.Blocked); err != nil {
			return nil, err
		}
		out = append(out, r)
	}
	return out, rows.Err()
}

func queryTables(ctx context.Context, pool *pgxpool.Pool, limit int) ([]TableStat, error) {
	limit = clampLimit(limit, 20, 100)
	rows, err := pool.Query(ctx, `
		SELECT t.relname,
		       pg_total_relation_size(t.relid),
		       s.heap_blks_read, s.heap_blks_hit,
		       t.idx_scan, t.seq_scan, t.n_dead_tup, t.last_autovacuum,
		       CASE WHEN t.n_live_tup + t.n_dead_tup > 0
		            THEN round(t.n_dead_tup::numeric / (t.n_live_tup + t.n_dead_tup), 4)
		            ELSE 0 END
		FROM pg_statio_user_tables s
		JOIN pg_stat_user_tables t ON t.relid = s.relid
		WHERE t.schemaname = 'public'
		ORDER BY pg_total_relation_size(t.relid) DESC
		LIMIT $1`, limit)
	if err != nil {
		return nil, fmt.Errorf("pgmonitor: tables: %w", err)
	}
	defer rows.Close()
	var out []TableStat
	for rows.Next() {
		var r TableStat
		var last *time.Time
		if err := rows.Scan(&r.Relname, &r.TotalBytes, &r.HeapBlksRead, &r.HeapBlksHit,
			&r.IdxScan, &r.SeqScan, &r.DeadTuples, &last, &r.BloatRatio); err != nil {
			return nil, err
		}
		r.LastAutovacuum = last
		out = append(out, r)
	}
	return out, rows.Err()
}

// TopQueries loads from pg_stat_statements when available.
func (s *Service) TopQueries(ctx context.Context, limit int) (QueriesResponse, error) {
	if s == nil || s.pool == nil {
		return QueriesResponse{}, errors.New("pgmonitor: postgres not configured")
	}
	limit = clampLimit(limit, 20, 100)
	now := time.Now().UTC()

	if snap, ok, err := s.loadSnapshot(ctx, "slow_queries", 15*time.Minute); err == nil && ok {
		var items []QueryStat
		if err := decodePayload(snap.Payload, &items); err == nil {
			return QueriesResponse{
				CollectedAt:         snap.CollectedAt,
				Source:              "snapshot",
				Items:               items,
				StatementsAvailable: true,
			}, nil
		}
	}

	if !s.statementsQueryable(ctx) {
		return queriesUnavailable(now), nil
	}
	items, err := queryTopStatements(ctx, s.pool, limit)
	if err != nil {
		if isPgStatStatementsUnavailable(err) {
			s.markStatementsUnavailable()
			return queriesUnavailable(now), nil
		}
		return QueriesResponse{}, err
	}
	return QueriesResponse{
		CollectedAt:         now,
		Source:              "live",
		Items:               items,
		StatementsAvailable: true,
	}, nil
}

func queriesUnavailable(at time.Time) QueriesResponse {
	return QueriesResponse{
		CollectedAt:         at,
		Source:              "unavailable",
		Items:               nil,
		StatementsAvailable: false,
		StatementsHint:      statementsUnavailableHint,
	}
}

const statementsUnavailableHint = "pg_stat_statements requires shared_preload_libraries and PostgreSQL restart (see docs/db-diagnostics.md)"

// statementsQueryable returns true only when pg_stat_statements can be queried (not merely installed).
func (s *Service) statementsQueryable(ctx context.Context) bool {
	if s == nil || s.pool == nil {
		return false
	}
	if v, ok := s.cache.get("stmt_queryable"); ok {
		if b, ok := v.(bool); ok {
			return b
		}
	}
	ok := probePgStatStatements(ctx, s.pool)
	s.cache.set("stmt_queryable", ok)
	return ok
}

func (s *Service) markStatementsUnavailable() {
	s.cache.set("stmt_queryable", false)
}

func probePgStatStatements(ctx context.Context, pool *pgxpool.Pool) bool {
	var dummy int64
	err := pool.QueryRow(ctx, `
		SELECT COALESCE(SUM(calls), 0)::bigint FROM pg_stat_statements LIMIT 1`).Scan(&dummy)
	if err == nil {
		return true
	}
	return !isPgStatStatementsUnavailable(err)
}

func queryTopStatements(ctx context.Context, pool *pgxpool.Pool, limit int) ([]QueryStat, error) {
	rows, err := pool.Query(ctx, `
		SELECT queryid, left(query, 500), calls, total_exec_time, mean_exec_time, rows
		FROM pg_stat_statements
		WHERE dbid = (SELECT oid FROM pg_database WHERE datname = current_database())
		ORDER BY mean_exec_time DESC
		LIMIT $1`, limit)
	if err != nil {
		if isPgStatStatementsUnavailable(err) {
			return nil, nil
		}
		return nil, fmt.Errorf("pgmonitor: pg_stat_statements: %w", err)
	}
	defer rows.Close()
	var out []QueryStat
	for rows.Next() {
		var r QueryStat
		if err := rows.Scan(&r.QueryID, &r.Query, &r.Calls, &r.TotalExecMs, &r.MeanExecMs, &r.Rows); err != nil {
			return nil, err
		}
		out = append(out, r)
	}
	return out, rows.Err()
}

// isPgStatStatementsUnavailable reports extension missing or not loaded via shared_preload_libraries.
func isPgStatStatementsUnavailable(err error) bool {
	if err == nil {
		return false
	}
	var pgErr *pgconn.PgError
	if errors.As(err, &pgErr) {
		switch pgErr.Code {
		case "42P01", "42704", "55000":
			return true
		}
		msg := strings.ToLower(pgErr.Message)
		if strings.Contains(msg, "shared_preload_libraries") || strings.Contains(msg, "pg_stat_statements") {
			return true
		}
	}
	low := strings.ToLower(err.Error())
	return strings.Contains(low, "shared_preload_libraries") || strings.Contains(low, "pg_stat_statements")
}

func isSafeIdent(name string) bool {
	if name == "" {
		return true
	}
	for _, r := range name {
		if (r >= 'a' && r <= 'z') || (r >= 'A' && r <= 'Z') || (r >= '0' && r <= '9') || r == '_' {
			continue
		}
		return false
	}
	return true
}

// ExecMaintenance runs VACUUM/ANALYZE/REINDEX with optional dry-run (returns SQL executed or planned).
func ExecMaintenance(ctx context.Context, pool *pgxpool.Pool, kind, table string, dryRun bool) (detail map[string]any, err error) {
	if pool == nil {
		return nil, errors.New("pgmonitor: postgres not configured")
	}
	table = strings.TrimSpace(table)
	if table != "" && !isSafeIdent(table) {
		return nil, errors.New("pgmonitor: invalid table name")
	}
	qual := ""
	if table != "" {
		qual = " " + pgx.Identifier{table}.Sanitize()
	}
	var sql string
	switch kind {
	case "vacuum":
		sql = "VACUUM" + qual
	case "vacuum_analyze":
		sql = "VACUUM ANALYZE" + qual
	case "analyze":
		sql = "ANALYZE" + qual
	case "reindex":
		if table == "" {
			return nil, errors.New("pgmonitor: reindex requires table")
		}
		sql = "REINDEX TABLE" + qual
	default:
		return nil, fmt.Errorf("pgmonitor: unknown maintenance kind %q", kind)
	}
	detail = map[string]any{"sql": sql, "dry_run": dryRun}
	if dryRun {
		return detail, nil
	}
	_, err = pool.Exec(ctx, sql)
	if err != nil {
		return detail, fmt.Errorf("pgmonitor: %s: %w", kind, err)
	}
	detail["executed"] = true
	return detail, nil
}