<?php
/**
 * DependencyModel - Manages ticket dependencies
 */
class DependencyModel {
    private $conn;

    public function __construct($conn) {
        $this->conn = $conn;
    }

    /**
     * Get all dependencies for a ticket
     *
     * @param string $ticketId Ticket ID
     * @return array Dependencies grouped by type
     */
    public function getDependencies($ticketId) {
        $sql = "SELECT d.*, t.title, t.status, t.priority
                FROM ticket_dependencies d
                LEFT JOIN tickets t ON d.depends_on_id = t.ticket_id
                WHERE d.ticket_id = ?
                ORDER BY d.dependency_type, d.created_at DESC";

        $stmt = $this->conn->prepare($sql);
        if (!$stmt) {
            throw new Exception('Prepare failed: ' . $this->conn->error);
        }
        $stmt->bind_param("s", $ticketId);
        if (!$stmt->execute()) {
            throw new Exception('Execute failed: ' . $stmt->error);
        }
        $result = $stmt->get_result();

        $dependencies = [
            'blocks' => [],
            'blocked_by' => [],
            'relates_to' => [],
            'duplicates' => []
        ];

        while ($row = $result->fetch_assoc()) {
            $dependencies[$row['dependency_type']][] = $row;
        }

        $stmt->close();
        return $dependencies;
    }

    /**
     * Get tickets that depend on this ticket
     *
     * @param string $ticketId Ticket ID
     * @return array Dependent tickets
     */
    public function getDependentTickets($ticketId) {
        $sql = "SELECT d.*, t.title, t.status, t.priority
                FROM ticket_dependencies d
                LEFT JOIN tickets t ON d.ticket_id = t.ticket_id
                WHERE d.depends_on_id = ?
                ORDER BY d.dependency_type, d.created_at DESC";

        $stmt = $this->conn->prepare($sql);
        if (!$stmt) {
            throw new Exception('Prepare failed: ' . $this->conn->error);
        }
        $stmt->bind_param("s", $ticketId);
        if (!$stmt->execute()) {
            throw new Exception('Execute failed: ' . $stmt->error);
        }
        $result = $stmt->get_result();

        $dependents = [];
        while ($row = $result->fetch_assoc()) {
            $dependents[] = $row;
        }

        $stmt->close();
        return $dependents;
    }

    /**
     * Add a dependency between tickets
     *
     * @param string $ticketId Source ticket ID
     * @param string $dependsOnId Target ticket ID
     * @param string $type Dependency type
     * @param int $createdBy User ID who created the dependency
     * @return array Result with success status
     */
    public function addDependency($ticketId, $dependsOnId, $type = 'blocks', $createdBy = null) {
        // Validate dependency type
        $validTypes = ['blocks', 'blocked_by', 'relates_to', 'duplicates'];
        if (!in_array($type, $validTypes)) {
            return ['success' => false, 'error' => 'Invalid dependency type'];
        }

        // Prevent self-reference
        if ($ticketId === $dependsOnId) {
            return ['success' => false, 'error' => 'A ticket cannot depend on itself'];
        }

        // Check if dependency already exists
        $checkSql = "SELECT dependency_id FROM ticket_dependencies
                     WHERE ticket_id = ? AND depends_on_id = ? AND dependency_type = ?";
        $checkStmt = $this->conn->prepare($checkSql);
        $checkStmt->bind_param("sss", $ticketId, $dependsOnId, $type);
        $checkStmt->execute();
        $checkResult = $checkStmt->get_result();

        if ($checkResult->num_rows > 0) {
            $checkStmt->close();
            return ['success' => false, 'error' => 'Dependency already exists'];
        }
        $checkStmt->close();

        // Check for circular dependency
        if ($this->wouldCreateCycle($ticketId, $dependsOnId, $type)) {
            return ['success' => false, 'error' => 'This would create a circular dependency'];
        }

        // Insert the dependency
        $sql = "INSERT INTO ticket_dependencies (ticket_id, depends_on_id, dependency_type, created_by)
                VALUES (?, ?, ?, ?)";
        $stmt = $this->conn->prepare($sql);
        $stmt->bind_param("sssi", $ticketId, $dependsOnId, $type, $createdBy);

        if ($stmt->execute()) {
            $dependencyId = $stmt->insert_id;
            $stmt->close();
            return ['success' => true, 'dependency_id' => $dependencyId];
        }

        $error = $stmt->error;
        $stmt->close();
        return ['success' => false, 'error' => $error];
    }

    /**
     * Remove a dependency
     *
     * @param int $dependencyId Dependency ID
     * @return bool Success status
     */
    public function removeDependency($dependencyId) {
        $sql = "DELETE FROM ticket_dependencies WHERE dependency_id = ?";
        $stmt = $this->conn->prepare($sql);
        $stmt->bind_param("i", $dependencyId);
        $result = $stmt->execute();
        $stmt->close();
        return $result;
    }

    /**
     * Remove dependency by ticket IDs and type
     *
     * @param string $ticketId Source ticket ID
     * @param string $dependsOnId Target ticket ID
     * @param string $type Dependency type
     * @return bool Success status
     */
    public function removeDependencyByTickets($ticketId, $dependsOnId, $type) {
        $sql = "DELETE FROM ticket_dependencies
                WHERE ticket_id = ? AND depends_on_id = ? AND dependency_type = ?";
        $stmt = $this->conn->prepare($sql);
        $stmt->bind_param("sss", $ticketId, $dependsOnId, $type);
        $result = $stmt->execute();
        $stmt->close();
        return $result;
    }

    /** Maximum depth for cycle detection to prevent DoS */
    private const MAX_DEPENDENCY_DEPTH = 20;

    /**
     * Check if adding a dependency would create a cycle
     *
     * @param string $ticketId Source ticket ID
     * @param string $dependsOnId Target ticket ID
     * @param string $type Dependency type
     * @return bool True if it would create a cycle
     */
    private function wouldCreateCycle($ticketId, $dependsOnId, $type): bool {
        // Only check for cycles in blocking relationships
        if (!in_array($type, ['blocks', 'blocked_by'])) {
            return false;
        }

        // Check if dependsOnId already has ticketId in its dependency chain
        $visited = [];
        return $this->hasDependencyPath($dependsOnId, $ticketId, $visited, 0);
    }

    /**
     * Check if there's a dependency path from source to target
     *
     * Uses iterative BFS approach with depth limit to prevent stack overflow
     * and DoS attacks from deeply nested or circular dependencies.
     *
     * @param string $source Source ticket ID
     * @param string $target Target ticket ID
     * @param array $visited Already visited tickets (passed by reference for efficiency)
     * @param int $depth Current recursion depth
     * @return bool True if path exists
     */
    private function hasDependencyPath($source, $target, array &$visited, int $depth): bool {
        // Depth limit to prevent DoS and stack overflow
        if ($depth >= self::MAX_DEPENDENCY_DEPTH) {
            error_log("Dependency cycle detection hit max depth ({$depth}) from {$source} to {$target}");
            return false; // Assume no cycle to avoid blocking legitimate operations
        }

        if ($source === $target) {
            return true;
        }

        if (in_array($source, $visited, true)) {
            return false;
        }

        // Limit visited array size to prevent memory exhaustion
        if (count($visited) > 100) {
            error_log("Dependency cycle detection visited too many nodes from {$source} to {$target}");
            return false;
        }

        $visited[] = $source;

        $sql = "SELECT depends_on_id FROM ticket_dependencies
                WHERE ticket_id = ? AND dependency_type IN ('blocks', 'blocked_by')";
        $stmt = $this->conn->prepare($sql);
        $stmt->bind_param("s", $source);
        $stmt->execute();
        $result = $stmt->get_result();

        while ($row = $result->fetch_assoc()) {
            if ($this->hasDependencyPath($row['depends_on_id'], $target, $visited, $depth + 1)) {
                $stmt->close();
                return true;
            }
        }

        $stmt->close();
        return false;
    }

    /**
     * Get all dependencies for multiple tickets (batch)
     *
     * @param array $ticketIds Array of ticket IDs
     * @return array Dependencies indexed by ticket ID
     */
    public function getDependenciesBatch($ticketIds) {
        if (empty($ticketIds)) {
            return [];
        }

        $placeholders = str_repeat('?,', count($ticketIds) - 1) . '?';
        $sql = "SELECT d.*, t.title, t.status, t.priority
                FROM ticket_dependencies d
                JOIN tickets t ON d.depends_on_id = t.ticket_id
                WHERE d.ticket_id IN ($placeholders)
                ORDER BY d.ticket_id, d.dependency_type";

        $stmt = $this->conn->prepare($sql);
        $types = str_repeat('s', count($ticketIds));
        $stmt->bind_param($types, ...$ticketIds);
        $stmt->execute();
        $result = $stmt->get_result();

        $dependencies = [];
        while ($row = $result->fetch_assoc()) {
            $ticketId = $row['ticket_id'];
            if (!isset($dependencies[$ticketId])) {
                $dependencies[$ticketId] = [];
            }
            $dependencies[$ticketId][] = $row;
        }

        $stmt->close();
        return $dependencies;
    }
}