package config

import (
	"fmt"
	"os"
	"strconv"
	"time"

	"github.com/joho/godotenv"
)

// Config represents the complete application configuration structure.
// This is the root configuration object that holds all subsystem configurations
// (server, database, JWT, cookies, Redis, NATS, and storage). By organizing
// configuration into this hierarchical structure, we achieve:
// 1. Clear separation of concerns - each subsystem has its own config struct
// 2. Easy maintainability - adding new config is as simple as adding a new field
// 3. Type safety - all config values are properly typed
// 4. Centralized validation - one place to validate all configuration
type Config struct {
	//Server
	Server   ServerConfig
	Database DatabaseConfig
	JWT      JWTConfig
	Cookie   CookieConfig
	Redis    RedisConfig
	NATS     NATSConfig
	Storage  StorageConfig
}

// ServerConfig holds HTTP server-specific configuration settings.
// These settings control how the Echo web server behaves:
// - Port determines which port the server listens on
// - Environment determines the runtime mode (affects logging, error detail, etc.)
// - Timeouts prevent slow clients from holding connections indefinitely
type ServerConfig struct {
	Port         string        // HTTP port to listen on (e.g., "8080", ":3000")
	Environment  string        // Runtime environment: "development", "staging", "production", "test", "UAT"
	ReadTimeout  time.Duration // Maximum duration for reading the entire request, including the body. Prevents slow-read attacks
	WriteTimeout time.Duration // Maximum duration before timing out writes of the response. Prevents slow-write attacks
}

// DatabaseConfig contains PostgreSQL connection settings and connection pool configuration.
// The connection pool settings are critical for performance:
// - MaxOpenConns limits total connections to prevent overwhelming the database
// - MaxIdleConns keeps connections warm for reuse, improving response times
// - ConnMaxLifetime recycles connections to prevent stale connection issues
type DatabaseConfig struct {
	Host            string        // Database server hostname or IP address (e.g., "localhost", "db.example.com")
	Port            string        // Database port number (default PostgreSQL port is "5432")
	User            string        // Database username for authentication
	Password        string        // Database password for authentication
	DBName          string        // Name of the specific database to connect to
	SSLMode         string        // SSL/TLS mode: "disable" (no encryption), "require" (encrypted but no cert verification), "verify-full" (encrypted with full cert verification)
	MaxOpenConns    int           // Maximum number of open connections to the database. Limits total concurrent connections
	MaxIdleConns    int           // Maximum number of idle connections kept in the pool. Higher values = faster connection reuse but more resources
	ConnMaxLifetime time.Duration // Maximum amount of time a connection may be reused. Forces connection refresh to prevent issues with stale connections
}

// JWTConfig contains JWT (JSON Web Token) authentication settings.
// We use two types of tokens for security:
// 1. Access tokens: Short-lived (15 minutes), used for API requests
// 2. Refresh tokens: Long-lived (7 days), used to obtain new access tokens
// This dual-token approach balances security (short access token lifetime) with
// user experience (long refresh token means less frequent re-authentication).
// The secrets MUST be different to prevent token type confusion attacks.
type JWTConfig struct {
	AccessSecret  string        // Secret key for signing access tokens. MUST be cryptographically random and kept secret
	RefreshSecret string        // Secret key for signing refresh tokens. MUST differ from AccessSecret to prevent token substitution
	AccessExpiry  time.Duration // How long access tokens remain valid (typically 15 minutes). Shorter = more secure but more token refreshes
	RefreshExpiry time.Duration // How long refresh tokens remain valid (typically 7 days). Longer = better UX but higher risk if stolen
	Issuer        string        // Token issuer claim (iss). Identifies which application/service issued the token for validation
}

// CookieConfig contains HTTP cookie settings for token storage.
// These settings control how authentication tokens are stored in browser cookies:
// - Domain controls which domains can access the cookie
// - Secure ensures cookies only sent over HTTPS in production
// - SameSite prevents CSRF attacks by controlling when cookies are sent
type CookieConfig struct {
	CookieDomain   string // Domain scope for cookies (e.g., "example.com" allows *.example.com to access). Use "localhost" for local development
	CookieSecure   bool   // If true, cookies only sent over HTTPS. MUST be true in production to prevent token theft over unencrypted connections
	CookieSameSite string // SameSite policy: "strict" (never sent cross-site), "lax" (sent on top-level navigation), "none" (always sent, requires Secure=true)
}

// RedisConfig contains Redis connection settings.
// Redis is used for caching and session storage to improve performance.
// Key uses include:
// - Session storage for distributed systems
// - Caching frequently accessed data
// - Rate limiting counters
// - Real-time analytics
type RedisConfig struct {
	Host     string // Redis server hostname or IP address (e.g., "localhost", "redis.example.com")
	Port     string // Redis port number (default is "6379")
	Password string // Redis password for authentication. Leave empty ("") if Redis is running without auth (not recommended for production)
	DB       int    // Redis database number (0-15 by default). Allows logical separation of data within a single Redis instance
}

// NATSConfig contains NATS messaging system settings.
// NATS is a message broker used for asynchronous communication between services:
// - Decouples services (sender doesn't need to know about receivers)
// - Enables event-driven architecture
// - Provides reliable message delivery
// - Supports pub/sub, request/reply, and queue patterns
type NATSConfig struct {
	URL string // NATS server connection URL (e.g., "nats://localhost:4222" or "nats://user:pass@host:4222")
}

// StorageConfig contains MinIO (S3-compatible) object storage settings.
// MinIO provides distributed object storage for files like:
// - User-uploaded documents and images
// - Generated reports and exports
// - Backup files
// - Any binary data that shouldn't go in the database
type StorageConfig struct {
	Endpoint        string // MinIO server endpoint URL without protocol (e.g., "localhost:9000", "minio.example.com:9000")
	AccessKeyID     string // Access key for MinIO authentication (similar to AWS access key)
	SecretAccessKey string // Secret key for MinIO authentication (similar to AWS secret key)
	BucketName      string // Name of the bucket to use for storing objects. Must be created before use
	UseSSL          bool   // If true, use HTTPS for MinIO connections. Should be true in production for security
}

// Load reads configuration from environment variables and returns a fully populated Config struct.
// Configuration loading follows this priority:
// 1. Environment variables (highest priority - allows override in deployment)
// 2. .env file (for local development)
// 3. Default values (fallback to sensible defaults)
//
// The loading process:
// 1. In non-production environments, attempts to load .env file (fails gracefully if missing)
// 2. Reads each config value using getEnv() which checks environment then falls back to defaults
// 3. Parses string values into appropriate types (durations, ints, bools)
// 4. Validates the complete configuration
// 5. Returns error if validation fails, otherwise returns the populated Config
//
// Why this approach?
// - .env files make local development easy (no need to set environment variables manually)
// - Environment variables are standard for containerized deployments (Docker, Kubernetes)
// - Defaults prevent the application from crashing if non-critical config is missing
// - Validation ensures critical config is present before the app starts
func Load() (*Config, error) {
	// In non-production environments, try to load .env file
	// godotenv.Load() reads key=value pairs from .env file and sets them as environment variables
	// We only do this in non-production because:
	// 1. Production should use real environment variables (set by infrastructure)
	// 2. .env files shouldn't exist in production (security risk if committed to version control)
	if os.Getenv("APP_ENV") != "production" {
		if err := godotenv.Load(); err != nil {
			// Warning only - we continue even if .env doesn't exist
			// This allows the app to work purely with environment variables if needed
			fmt.Println("Warning: .env file not found, using environment variables")
		}
	}

	// Build the configuration struct by reading from environment variables
	// Each field uses getEnv() which provides a fallback default value
	cfg := &Config{
		// Server configuration with sensible defaults for local development
		Server: ServerConfig{
			Port:         getEnv("SERVER_PORT", "8080"),                        // Default to port 8080 (common for APIs)
			Environment:  getEnv("APP_ENV", "development"),                     // Default to development mode
			ReadTimeout:  parseDuration(getEnv("SERVER_READ_TIMEOUT", "10s")),  // 10 seconds is reasonable for most API requests
			WriteTimeout: parseDuration(getEnv("SERVER_WRITE_TIMEOUT", "10s")), // 10 seconds handles most response sizes
		},

		// Database configuration with defaults suitable for local PostgreSQL
		Database: DatabaseConfig{
			Host:            getEnv("DB_HOST", "localhost"),                            // Assume PostgreSQL is running locally
			Port:            getEnv("DB_PORT", "5432"),                                 // 5432 is PostgreSQL's default port
			User:            getEnv("DB_USER", "aurganize"),                            // Default username matches project name
			Password:        getEnv("DB_PASSWORD", ""),                                 // Empty by default (will require setting in production)
			DBName:          getEnv("DB_NAME", "aruganize_db_1"),                       // Default database name
			SSLMode:         getEnv("DB_SSLMODE", "disable"),                           // SSL disabled for local development (enable in production!)
			MaxOpenConns:    parseInt(getEnv("DB_MAX_OPEN_CONNECTIONS", "25")),         // 25 is a good starting point for connection pool
			MaxIdleConns:    parseInt(getEnv("DB_MAX_IDLE_CONNECTIONS", "5")),          // Keep 5 connections warm for quick reuse
			ConnMaxLifetime: parseDuration(getEnv("DB_CONNECTION_MAX_LIFETIME", "5m")), // Refresh connections every 5 minutes
		},

		// JWT configuration - secrets MUST be set via environment variables (no defaults for security)
		JWT: JWTConfig{
			AccessSecret:  getEnv("JWT_ACCESS_SECRET", ""),                     // Empty default forces explicit configuration
			RefreshSecret: getEnv("JWT_REFRESH_SECRET", ""),                    // Empty default forces explicit configuration
			AccessExpiry:  parseDuration(getEnv("JWT_ACCESS_EXPIRY", "15m")),   // 15 minutes is secure but requires frequent refresh
			RefreshExpiry: parseDuration(getEnv("JWT_REFRESH_EXPIRY", "168h")), // 168 hours = 7 days for good user experience
			Issuer:        getEnv("JWT_ISSUER", "aurganize-v62"),               // Identifies this application as token issuer
		},

		// Cookie configuration for storing tokens in browser
		Cookie: CookieConfig{
			CookieDomain: getEnv("COOKIE_DOMAIN", "localhost"), // localhost for development
			// CookieSecure is true only in production (requires HTTPS)
			// This line checks ENV (not APP_ENV) to determine if we're in production
			CookieSecure:   getEnv("ENV", "development") == "production",
			CookieSameSite: getEnv("COOKIE_SAMESITE", "lax"), // "lax" is a good balance between security and usability
		},

		// Redis configuration for caching and sessions
		Redis: RedisConfig{
			Host:     getEnv("REDIST_HOST", "localhost"), // Note: typo in env var name (REDIST vs REDIS)
			Port:     getEnv("REDIS_PORT", "6379"),       // 6379 is Redis default port
			Password: getEnv("REDIS_PASSWORD", ""),       // Empty for local development (no auth)
			DB:       parseInt(getEnv("REDIS_DB", "0")),  // Use database 0 by default
		},

		// NATS configuration for message queuing
		NATS: NATSConfig{
			URL: getEnv("NATS_URL", "nats://localhost:4222"), // Standard NATS URL format
		},

		// MinIO configuration for object storage
		Storage: StorageConfig{
			Endpoint:        getEnv("MINIO_ENDPOINT", "localhost:9000"),   // MinIO default port is 9000
			AccessKeyID:     getEnv("MINIO_ACCESS_KEY", "minioadmin"),     // Default MinIO credentials
			SecretAccessKey: getEnv("MINIO_SECRET_KEY", "miniosecretkey"), // Default MinIO credentials (change in production!)
			BucketName:      getEnv("MINIO_BUCKET", "aurganize_bucket_1"), // Default bucket name
			UseSSL:          parseBool(getEnv("MINIO_USE_SSL", "false")),  // No SSL for local development
		},
	}

	// Validate the entire configuration before returning
	// This catches configuration errors at startup rather than during runtime
	if err := cfg.Validate(); err != nil {
		// Wrap the error with context for better debugging
		return nil, fmt.Errorf("configuration validation failure [%w]", err)
	}

	return cfg, nil
}

// Validate checks if all required configuration values are present and valid.
// This method enforces configuration rules that prevent the application from
// starting with invalid or insecure settings. Validation happens at startup
// so errors are caught immediately rather than during runtime.
//
// Validation rules:
// 1. Database password required in production (prevents accidental no-auth deployments)
// 2. JWT secrets must be set (no default secrets for security)
// 3. JWT secrets must be different (prevents token substitution attacks)
// 4. Environment must be valid (ensures proper behavior for the deployment type)
//
// Why validate at startup?
// - Fail fast: Better to crash at startup than fail during a user request
// - Clear errors: Validation errors explain exactly what's wrong
// - Security: Prevents running with insecure configuration
func (c *Config) Validate() error {
	// Validate database password in production
	// In production, an empty password likely means configuration was forgotten
	// This prevents accidentally deploying with an open database connection
	if c.Database.Password == "" && c.Server.Environment == "production" {
		return fmt.Errorf("DB_PASSWORD is required in production")
	}

	// JWT access secret is always required (no environment exceptions)
	// Without this, we cannot sign or verify access tokens
	if c.JWT.AccessSecret == "" {
		return fmt.Errorf("JWT_ACCESS_SECRET is required")
	}

	// JWT refresh secret is always required (no environment exceptions)
	// Without this, we cannot sign or verify refresh tokens
	if c.JWT.RefreshSecret == "" {
		return fmt.Errorf("JWT_REFRESH_SECRET is required")
	}

	// JWT secrets must be different to prevent token type confusion
	// If they're the same, an attacker could use a refresh token as an access token
	// This is a security vulnerability that could allow privilege escalation
	if c.JWT.AccessSecret == c.JWT.RefreshSecret {
		return fmt.Errorf("JWT_ACCESS_SECRET and JWT_REFRESH_SECRET must be different")
	}

	// Validate environment value against allowed list
	// This prevents typos in environment configuration (e.g., "prod" instead of "production")
	// Each environment may have different behaviors (logging, error detail, etc.)
	validEnvs := map[string]bool{
		"development": true, // Local development with debug logging
		"test":        true, // Automated testing environment
		"staging":     true, // Pre-production testing environment
		"UAT":         true, // User Acceptance Testing environment
		"production":  true, // Live production environment
	}
	if !validEnvs[c.Server.Environment] {
		return fmt.Errorf("invalid environment configured in enviroment")
	}

	return nil
}

// Helper Functions

// getEnv retrieves an environment variable value or returns a default if not set.
// This is a wrapper around os.Getenv that adds default value support.
//
// How it works:
// 1. Checks if environment variable exists and has a non-empty value
// 2. If yes, returns that value
// 3. If no, returns the provided default value
//
// Why use this instead of os.Getenv directly?
// - Provides sensible defaults for non-critical configuration
// - Reduces repetitive if-else checks throughout the code
// - Makes configuration more resilient (app still works if some vars missing)
// - Documents expected configuration by showing default values
func getEnv(key, defaultValue string) string {
	// os.Getenv returns empty string if variable doesn't exist
	if value := os.Getenv(key); value != "" {
		return value
	}
	return defaultValue
}

// parseDuration converts a string duration to time.Duration type.
// Durations are expected in Go's duration format: "10s", "5m", "2h", etc.
//
// Examples of valid formats:
// - "10s" = 10 seconds
// - "5m" = 5 minutes
// - "2h" = 2 hours
// - "1h30m" = 1 hour 30 minutes
//
// If parsing fails (invalid format), returns 0 duration rather than crashing.
// This makes the app more resilient but means invalid config might be silently ignored.
// Consider adding logging here to warn about parsing failures.
func parseDuration(s string) time.Duration {
	d, err := time.ParseDuration(s)
	if err != nil {
		// Returns zero duration on error - might want to log this
		return 0
	}
	return d
}

// parseInt converts a string to an integer.
// If conversion fails (non-numeric string), returns 0 rather than crashing.
//
// Why return 0 on error?
// - Makes configuration more resilient (app starts even with bad config)
// - 0 is often a reasonable default for numeric config
// - Drawback: Invalid config might be silently ignored
//
// Consider: Adding logging to warn when parsing fails
func parseInt(s string) int {
	i, err := strconv.Atoi(s)
	if err != nil {
		// Returns 0 on error - might want to log this
		return 0
	}
	return i
}

// parseBool converts a string to a boolean.
// Accepts: "true", "false", "1", "0", "t", "f", "T", "F", "TRUE", "FALSE" (case-insensitive)
//
// If conversion fails, returns false rather than crashing.
//
// Why return false on error?
// - Makes configuration more resilient
// - false is typically the "safe" default for boolean flags
// - Drawback: Invalid config might be silently ignored
//
// Consider: Adding logging to warn when parsing fails
func parseBool(s string) bool {
	b, err := strconv.ParseBool(s)
	if err != nil {
		// Returns false on error - might want to log this
		return false
	}
	return b
}

// DatabaseDSN constructs and returns the PostgreSQL Data Source Name (connection string).
// DSN format: "host=X port=Y user=Z password=W dbname=N sslmode=M"
//
// The DSN is used by database drivers to establish a connection to PostgreSQL.
// Each component tells the driver:
// - Where to connect (host and port)
// - How to authenticate (user and password)
// - Which database to use (dbname)
// - Security requirements (sslmode)
//
// Note: There's a typo in the format string - "post" should be "port"
// This will cause connection failures! Should be fixed to: "host=%s port=%s ..."
func (c *Config) DatabaseDSN() string {
	return fmt.Sprintf(
		"host=%s port=%s user=%s password=%s dbname=%s sslmode=%s", // BUG: "post" should be "port"
		c.Database.Host,
		c.Database.Port,
		c.Database.User,
		c.Database.Password,
		c.Database.DBName,
		c.Database.SSLMode,
	)
}

// RedisDSN constructs and returns the Redis connection string.
// Format: "host:port" (e.g., "localhost:6379")
//
// This simple format is used by most Redis clients to connect.
// Authentication (if needed) is typically handled separately via
// Redis client options rather than being part of the DSN.
func (c *Config) RedisDSN() string {
	return fmt.Sprintf("%s:%s", c.Redis.Host, c.Redis.Port)
}