// Package sqlite provides the blocked_issues_cache optimization for GetReadyWork performance.
//
// # Performance Impact
//
// GetReadyWork originally used a recursive CTE to compute blocked issues on every query,
// taking ~752ms on a 10K issue database. With the cache, queries complete in ~29ms
// (25x speedup) by using a simple NOT EXISTS check against the materialized cache table.
//
// # Cache Architecture
//
// The blocked_issues_cache table stores issue_id values for all issues that are currently
// blocked. An issue is blocked if:
//   - It has a 'blocks' dependency on an open/in_progress/blocked issue (direct blocking)
//   - It has a 'blocks' dependency on an external:* reference (cross-project blocking, bd-om4a)
//   - Its parent is blocked and it's connected via 'parent-child' dependency (transitive blocking)
//
// The cache is maintained automatically by invalidating and rebuilding whenever:
//   - A 'blocks' or 'parent-child' dependency is added or removed
//   - Any issue's status changes (affects whether it blocks others)
//   - An issue is closed (closed issues don't block others)
//
// Related and discovered-from dependencies do NOT trigger cache invalidation since they
// don't affect blocking semantics.
//
// # Cache Invalidation Strategy
//
// On any triggering change, the entire cache is rebuilt from scratch (DELETE + INSERT).
// This full-rebuild approach is chosen because:
//   - Rebuild is fast (<50ms even on 10K databases) due to optimized CTE logic
//   - Simpler implementation than incremental updates
//   - Dependency changes are rare compared to reads
//   - Guarantees consistency - no risk of partial/stale updates
//
// The rebuild happens within the same transaction as the triggering change, ensuring
// atomicity and consistency. The cache can never be in an inconsistent state visible
// to queries.
//
// # Transaction Safety
//
// All cache operations support both transaction and direct database execution:
//   - rebuildBlockedCache accepts optional *sql.Tx parameter
//   - If tx != nil, uses transaction; otherwise uses direct db connection
//   - Cache invalidation during CreateIssue/UpdateIssue/AddDependency happens in their tx
//   - Ensures cache is always consistent with the database state
//
// # Performance Characteristics
//
// Query performance (GetReadyWork):
//   - Before cache: ~752ms (recursive CTE on 10K issues)
//   - With cache: ~29ms (NOT EXISTS check)
//   - Speedup: 25x
//
// Write overhead:
//   - Cache rebuild: <50ms (full DELETE + INSERT)
//   - Only triggered on dependency/status changes (rare operations)
//   - Trade-off: slower writes for much faster reads
//
// # Edge Cases Handled
//
// 1. Parent-child transitive blocking:
//    - Children of blocked parents are automatically marked as blocked
//    - Propagates through arbitrary depth hierarchies (limited to depth 50)
//
// 2. Multiple blockers:
//    - Issue blocked by multiple open issues stays blocked until all are closed
//    - DISTINCT in CTE ensures issue appears once in cache
//
// 3. Status changes:
//    - Closing a blocker removes all blocked descendants from cache
//    - Reopening a blocker adds them back
//
// 4. Dependency removal:
//    - Removing last blocker unblocks the issue
//    - Removing parent-child link unblocks orphaned subtree
//
// 5. Foreign key cascades:
//    - Cache entries automatically deleted when issue is deleted (ON DELETE CASCADE)
//    - No manual cleanup needed
//
// # Future Optimizations
//
// If rebuild becomes a bottleneck in very large databases (>100K issues):
//   - Consider incremental updates for specific dependency types
//   - Add indexes to dependencies table for CTE performance
//   - Implement dirty tracking to avoid rebuilds when cache is unchanged
//
// However, current performance is excellent for realistic workloads.
package sqlite

import (
	"context"
	"database/sql"
	"fmt"
)

// execer is an interface for types that can execute SQL queries
// Both *sql.DB and *sql.Tx implement this interface
type execer interface {
	ExecContext(ctx context.Context, query string, args ...interface{}) (sql.Result, error)
}

// rebuildBlockedCache completely rebuilds the blocked_issues_cache table
// This is used during cache invalidation when dependencies change
func (s *SQLiteStorage) rebuildBlockedCache(ctx context.Context, exec execer) error {
	// Use direct db connection if no execer provided
	if exec == nil {
		exec = s.db
	}

	// Clear the cache
	if _, err := exec.ExecContext(ctx, "DELETE FROM blocked_issues_cache"); err != nil {
		return fmt.Errorf("failed to clear blocked_issues_cache: %w", err)
	}

	// Rebuild using the recursive CTE logic
	// Only includes local blockers (open issues) - external refs are resolved
	// lazily at query time by GetReadyWork (bd-zmmy supersedes bd-om4a)
	query := `
		INSERT INTO blocked_issues_cache (issue_id)
		WITH RECURSIVE
		  -- Step 1: Find issues blocked directly by LOCAL dependencies
		  -- External refs (external:*) are excluded - they're resolved lazily by GetReadyWork
		  blocked_directly AS (
		    SELECT DISTINCT d.issue_id
		    FROM dependencies d
		    JOIN issues blocker ON d.depends_on_id = blocker.id
		    WHERE d.type = 'blocks'
		      AND blocker.status IN ('open', 'in_progress', 'blocked', 'deferred')
		  ),

		  -- Step 2: Propagate blockage to all descendants via parent-child
		  blocked_transitively AS (
		    -- Base case: directly blocked issues
		    SELECT issue_id, 0 as depth
		    FROM blocked_directly

		    UNION ALL

		    -- Recursive case: children of blocked issues inherit blockage
		    SELECT d.issue_id, bt.depth + 1
		    FROM blocked_transitively bt
		    JOIN dependencies d ON d.depends_on_id = bt.issue_id
		    WHERE d.type = 'parent-child'
		      AND bt.depth < 50
		  )
		SELECT DISTINCT issue_id FROM blocked_transitively
	`

	if _, err := exec.ExecContext(ctx, query); err != nil {
		return fmt.Errorf("failed to rebuild blocked_issues_cache: %w", err)
	}

	return nil
}

// invalidateBlockedCache rebuilds the blocked issues cache
// Called when dependencies change or issue status changes
func (s *SQLiteStorage) invalidateBlockedCache(ctx context.Context, exec execer) error {
	return s.rebuildBlockedCache(ctx, exec)
}