package sqlite

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

	"github.com/steveyegge/beads/internal/types"
)

// CompactionCandidate represents an issue eligible for compaction
type CompactionCandidate struct {
	IssueID        string
	ClosedAt       time.Time
	OriginalSize   int
	EstimatedSize  int
	DependentCount int
}

// Snapshot represents a saved version of issue content before compaction
type Snapshot struct {
	IssueID            string    `json:"issue_id"`
	CompactionLevel    int       `json:"compaction_level"`
	Description        string    `json:"description"`
	Design             string    `json:"design"`
	Notes              string    `json:"notes"`
	AcceptanceCriteria string    `json:"acceptance_criteria"`
	Title              string    `json:"title"`
	CreatedAt          time.Time `json:"created_at"`
}

// GetTier1Candidates returns issues eligible for Tier 1 compaction.
// Criteria:
// - Status = closed
// - Closed for at least compact_tier1_days
// - No open dependents within compact_tier1_dep_levels depth
// - Not already compacted (compaction_level = 0)
func (s *SQLiteStorage) GetTier1Candidates(ctx context.Context) ([]*CompactionCandidate, error) {
	// Get configuration
	daysStr, err := s.GetConfig(ctx, "compact_tier1_days")
	if err != nil {
		return nil, fmt.Errorf("failed to get compact_tier1_days: %w", err)
	}
	if daysStr == "" {
		daysStr = "30"
	}

	depthStr, err := s.GetConfig(ctx, "compact_tier1_dep_levels")
	if err != nil {
		return nil, fmt.Errorf("failed to get compact_tier1_dep_levels: %w", err)
	}
	if depthStr == "" {
		depthStr = "2"
	}

	query := `
		WITH RECURSIVE
		  -- Find all issues that depend on (are blocked by) other issues
		  dependent_tree AS (
		    -- Base case: direct dependents
		    SELECT 
		      d.depends_on_id as issue_id,
		      i.id as dependent_id,
		      i.status as dependent_status,
		      0 as depth
		    FROM dependencies d
		    JOIN issues i ON d.issue_id = i.id
		    WHERE d.type = 'blocks'
		    
		    UNION ALL
		    
		    -- Recursive case: parent-child relationships
		    SELECT 
		      dt.issue_id,
		      i.id as dependent_id,
		      i.status as dependent_status,
		      dt.depth + 1
		    FROM dependent_tree dt
		    JOIN dependencies d ON d.depends_on_id = dt.dependent_id
		    JOIN issues i ON d.issue_id = i.id
		    WHERE d.type = 'parent-child'
		      AND dt.depth < ?
		  )
		SELECT 
		  i.id,
		  i.closed_at,
		  COALESCE(i.original_size, LENGTH(i.description) + LENGTH(i.design) + LENGTH(i.notes) + LENGTH(i.acceptance_criteria)) as original_size,
		  0 as estimated_size,
		  COUNT(DISTINCT dt.dependent_id) as dependent_count
		FROM issues i
		LEFT JOIN dependent_tree dt ON i.id = dt.issue_id 
		  AND dt.dependent_status IN ('open', 'in_progress', 'blocked')
		  AND dt.depth <= ?
		WHERE i.status = 'closed'
		  AND i.closed_at IS NOT NULL
		  AND i.closed_at <= datetime('now', '-' || CAST(? AS INTEGER) || ' days')
		  AND COALESCE(i.compaction_level, 0) = 0
		  AND dt.dependent_id IS NULL  -- No open dependents
		GROUP BY i.id
		ORDER BY i.closed_at ASC
	`

	rows, err := s.db.QueryContext(ctx, query, depthStr, depthStr, daysStr)
	if err != nil {
		return nil, fmt.Errorf("failed to query tier1 candidates: %w", err)
	}
	defer rows.Close()

	var candidates []*CompactionCandidate
	for rows.Next() {
		var c CompactionCandidate
		if err := rows.Scan(&c.IssueID, &c.ClosedAt, &c.OriginalSize, &c.EstimatedSize, &c.DependentCount); err != nil {
			return nil, fmt.Errorf("failed to scan candidate: %w", err)
		}
		candidates = append(candidates, &c)
	}

	if err := rows.Err(); err != nil {
		return nil, fmt.Errorf("rows iteration error: %w", err)
	}

	return candidates, nil
}

// GetTier2Candidates returns issues eligible for Tier 2 compaction.
// Criteria:
// - Status = closed
// - Closed for at least compact_tier2_days
// - No open dependents within compact_tier2_dep_levels depth
// - Already at compaction_level = 1
// - Either has many commits (compact_tier2_commits) or many dependent issues
func (s *SQLiteStorage) GetTier2Candidates(ctx context.Context) ([]*CompactionCandidate, error) {
	// Get configuration
	daysStr, err := s.GetConfig(ctx, "compact_tier2_days")
	if err != nil {
		return nil, fmt.Errorf("failed to get compact_tier2_days: %w", err)
	}
	if daysStr == "" {
		daysStr = "90"
	}

	depthStr, err := s.GetConfig(ctx, "compact_tier2_dep_levels")
	if err != nil {
		return nil, fmt.Errorf("failed to get compact_tier2_dep_levels: %w", err)
	}
	if depthStr == "" {
		depthStr = "5"
	}

	commitsStr, err := s.GetConfig(ctx, "compact_tier2_commits")
	if err != nil {
		return nil, fmt.Errorf("failed to get compact_tier2_commits: %w", err)
	}
	if commitsStr == "" {
		commitsStr = "100"
	}

	query := `
		WITH event_counts AS (
		  SELECT issue_id, COUNT(*) as event_count
		  FROM events
		  GROUP BY issue_id
		)
		SELECT 
		  i.id,
		  i.closed_at,
		  i.original_size,
		  0 as estimated_size,
		  COALESCE(ec.event_count, 0) as dependent_count
		FROM issues i
		LEFT JOIN event_counts ec ON i.id = ec.issue_id
		WHERE i.status = 'closed'
		  AND i.closed_at IS NOT NULL
		  AND i.closed_at <= datetime('now', '-' || CAST(? AS INTEGER) || ' days')
		  AND i.compaction_level = 1
		  AND COALESCE(ec.event_count, 0) >= CAST(? AS INTEGER)
		  AND NOT EXISTS (
		    -- Check for open dependents
		    SELECT 1 FROM dependencies d
		    JOIN issues dep ON d.issue_id = dep.id
		    WHERE d.depends_on_id = i.id
		      AND d.type = 'blocks'
		      AND dep.status IN ('open', 'in_progress', 'blocked')
		  )
		ORDER BY i.closed_at ASC
	`

	rows, err := s.db.QueryContext(ctx, query, daysStr, commitsStr)
	if err != nil {
		return nil, fmt.Errorf("failed to query tier2 candidates: %w", err)
	}
	defer rows.Close()

	var candidates []*CompactionCandidate
	for rows.Next() {
		var c CompactionCandidate
		if err := rows.Scan(&c.IssueID, &c.ClosedAt, &c.OriginalSize, &c.EstimatedSize, &c.DependentCount); err != nil {
			return nil, fmt.Errorf("failed to scan candidate: %w", err)
		}
		candidates = append(candidates, &c)
	}

	if err := rows.Err(); err != nil {
		return nil, fmt.Errorf("rows iteration error: %w", err)
	}

	return candidates, nil
}

// CheckEligibility checks if a specific issue is eligible for compaction at the given tier.
// Returns (eligible, reason, error).
// If not eligible, reason explains why.
func (s *SQLiteStorage) CheckEligibility(ctx context.Context, issueID string, tier int) (bool, string, error) {
	// Get the issue
	var status string
	var closedAt sql.NullTime
	var compactionLevel int
	
	err := s.db.QueryRowContext(ctx, `
		SELECT status, closed_at, COALESCE(compaction_level, 0)
		FROM issues
		WHERE id = ?
	`, issueID).Scan(&status, &closedAt, &compactionLevel)
	
	if err == sql.ErrNoRows {
		return false, "issue not found", nil
	}
	if err != nil {
		return false, "", fmt.Errorf("failed to get issue: %w", err)
	}

	// Check basic requirements
	if status != "closed" {
		return false, "issue is not closed", nil
	}
	
	if !closedAt.Valid {
		return false, "issue has no closed_at timestamp", nil
	}

	if tier == 1 {
		if compactionLevel != 0 {
			return false, "issue is already compacted", nil
		}
		
		// Check if closed long enough
		daysStr, err := s.GetConfig(ctx, "compact_tier1_days")
		if err != nil {
			return false, "", fmt.Errorf("failed to get compact_tier1_days: %w", err)
		}
		if daysStr == "" {
			daysStr = "30"
		}
		
		// Check if it appears in tier1 candidates
		candidates, err := s.GetTier1Candidates(ctx)
		if err != nil {
			return false, "", fmt.Errorf("failed to get tier1 candidates: %w", err)
		}
		
		for _, c := range candidates {
			if c.IssueID == issueID {
				return true, "", nil
			}
		}
		
		return false, "issue has open dependents or not closed long enough", nil
		
	} else if tier == 2 {
		if compactionLevel != 1 {
			return false, "issue must be at compaction level 1 for tier 2", nil
		}
		
		// Check if it appears in tier2 candidates
		candidates, err := s.GetTier2Candidates(ctx)
		if err != nil {
			return false, "", fmt.Errorf("failed to get tier2 candidates: %w", err)
		}
		
		for _, c := range candidates {
			if c.IssueID == issueID {
				return true, "", nil
			}
		}
		
		return false, "issue has open dependents, not closed long enough, or insufficient events", nil
	}
	
	return false, fmt.Sprintf("invalid tier: %d", tier), nil
}

// CreateSnapshot creates a snapshot of the issue's content before compaction.
// The snapshot includes all text fields and is stored as JSON.
// Multiple snapshots can exist per issue (one per compaction level).
// NOTE: This should be called within the same transaction as the compaction operation.
func (s *SQLiteStorage) CreateSnapshot(ctx context.Context, issue *types.Issue, level int) error {
	if level <= 0 {
		return fmt.Errorf("invalid compaction level %d; must be >= 1", level)
	}

	snapshot := Snapshot{
		IssueID:            issue.ID,
		CompactionLevel:    level,
		Description:        issue.Description,
		Design:             issue.Design,
		Notes:              issue.Notes,
		AcceptanceCriteria: issue.AcceptanceCriteria,
		Title:              issue.Title,
		CreatedAt:          time.Now().UTC(),
	}

	snapshotJSON, err := json.Marshal(snapshot)
	if err != nil {
		return fmt.Errorf("failed to marshal snapshot: %w", err)
	}

	query := `
		INSERT INTO compaction_snapshots (issue_id, compaction_level, snapshot_json, created_at)
		VALUES (?, ?, ?, ?)
	`

	_, err = s.db.ExecContext(ctx, query, issue.ID, level, snapshotJSON, snapshot.CreatedAt)
	if err != nil {
		return fmt.Errorf("failed to insert snapshot: %w", err)
	}

	return nil
}

// RestoreFromSnapshot restores an issue's content from a snapshot.
// Returns the exact original content from the snapshot at the specified level.
// Uses a transaction with optimistic concurrency control to prevent race conditions.
func (s *SQLiteStorage) RestoreFromSnapshot(ctx context.Context, issueID string, level int) error {
	if level <= 0 {
		return fmt.Errorf("invalid level %d; must be >= 1", level)
	}

	tx, err := s.db.BeginTx(ctx, nil)
	if err != nil {
		return fmt.Errorf("begin tx: %w", err)
	}
	defer func() {
		if err != nil {
			_ = tx.Rollback()
		}
	}()

	var snapshotJSON []byte
	err = tx.QueryRowContext(ctx, `
		SELECT snapshot_json
		FROM compaction_snapshots
		WHERE issue_id = ? AND compaction_level = ?
		ORDER BY created_at DESC
		LIMIT 1
	`, issueID, level).Scan(&snapshotJSON)
	if err == sql.ErrNoRows {
		return fmt.Errorf("no snapshot found for issue %s at level %d", issueID, level)
	}
	if err != nil {
		return fmt.Errorf("failed to query snapshot: %w", err)
	}

	var snapshot Snapshot
	if err = json.Unmarshal(snapshotJSON, &snapshot); err != nil {
		return fmt.Errorf("failed to unmarshal snapshot: %w", err)
	}

	if snapshot.IssueID != issueID {
		return fmt.Errorf("snapshot issue_id mismatch: got %s, want %s", snapshot.IssueID, issueID)
	}

	restoreLevel := snapshot.CompactionLevel - 1
	if restoreLevel < 0 {
		return fmt.Errorf("invalid restore level computed: %d", restoreLevel)
	}

	res, err := tx.ExecContext(ctx, `
		UPDATE issues
		SET description = ?,
		    design = ?,
		    notes = ?,
		    acceptance_criteria = ?,
		    title = ?,
		    compaction_level = ?,
		    updated_at = ?
		WHERE id = ? AND COALESCE(compaction_level, 0) = ?
	`,
		snapshot.Description,
		snapshot.Design,
		snapshot.Notes,
		snapshot.AcceptanceCriteria,
		snapshot.Title,
		restoreLevel,
		time.Now().UTC(),
		issueID,
		snapshot.CompactionLevel,
	)
	if err != nil {
		return fmt.Errorf("failed to restore issue content: %w", err)
	}

	rows, _ := res.RowsAffected()
	if rows == 0 {
		return fmt.Errorf("restore conflict: current compaction_level changed or issue not found")
	}

	if err = tx.Commit(); err != nil {
		return fmt.Errorf("commit restore tx: %w", err)
	}

	return nil
}

// GetSnapshots returns all snapshots for an issue, ordered by compaction level and creation time.
// Returns the latest snapshot for each level.
func (s *SQLiteStorage) GetSnapshots(ctx context.Context, issueID string) ([]*Snapshot, error) {
	query := `
		SELECT snapshot_json
		FROM compaction_snapshots
		WHERE issue_id = ?
		ORDER BY compaction_level ASC, created_at DESC
	`

	rows, err := s.db.QueryContext(ctx, query, issueID)
	if err != nil {
		return nil, fmt.Errorf("failed to query snapshots: %w", err)
	}
	defer rows.Close()

	var snapshots []*Snapshot
	for rows.Next() {
		var snapshotJSON []byte
		if err := rows.Scan(&snapshotJSON); err != nil {
			return nil, fmt.Errorf("failed to scan snapshot: %w", err)
		}

		var snapshot Snapshot
		if err := json.Unmarshal(snapshotJSON, &snapshot); err != nil {
			return nil, fmt.Errorf("failed to unmarshal snapshot: %w", err)
		}

		snapshots = append(snapshots, &snapshot)
	}

	if err := rows.Err(); err != nil {
		return nil, fmt.Errorf("rows iteration error: %w", err)
	}

	return snapshots, nil
}

// ApplyCompaction updates the compaction metadata for an issue after successfully compacting it.
// This sets compaction_level, compacted_at, and original_size fields.
func (s *SQLiteStorage) ApplyCompaction(ctx context.Context, issueID string, level int, originalSize int) error {
	now := time.Now().UTC()
	
	_, err := s.db.ExecContext(ctx, `
		UPDATE issues
		SET compaction_level = ?,
		    compacted_at = ?,
		    original_size = ?,
		    updated_at = ?
		WHERE id = ?
	`, level, now, originalSize, now, issueID)
	
	if err != nil {
		return fmt.Errorf("failed to apply compaction metadata: %w", err)
	}
	
	return nil
}