package config

import (
	"fmt"
	"net"
	"os"
	"strings"

	"github.com/docker/go-units"
	"gopkg.in/yaml.v3"
)

type Config struct {
	// Server configuration
	ListenAddress string `yaml:"listen_address" default:":80"`

	// Concurrency limits
	MaxConcurrentRequests int64 `yaml:"max_concurrent_requests" default:"200"`
	MaxRequestsPerClient  int64 `yaml:"max_requests_per_client" default:"5"`

	// P1 hardening limits (security/correctness)
	MaxObjectSize  string   `yaml:"max_object_size" default:"0"` // 0=unlimited; e.g. "256MB" protects against OOM from huge/malicious upstream responses (P1-01)
	TrustedProxies []string `yaml:"trusted_proxies"`             // CIDR list; empty=never trust X-Forwarded-For (safe default, P1-02). See README security notes.

	// Cache configuration
	Cache CacheConfig `yaml:"cache"`

	// Upstream configuration
	Upstream string `yaml:"upstream"`
}

type CacheConfig struct {
	// Memory cache settings
	Memory MemoryConfig `yaml:"memory"`

	// Disk cache settings
	Disk DiskConfig `yaml:"disk"`
}

type MemoryConfig struct {
	// Size of memory cache (e.g., "512MB", "1GB")
	Size string `yaml:"size" default:"0"`

	// Garbage collection algorithm: lru, lfu, fifo, largest, smallest, hybrid
	GCAlgorithm string `yaml:"gc_algorithm" default:"lru"`
}

type DiskConfig struct {
	// Size of disk cache (e.g., "10GB", "50GB")
	Size string `yaml:"size" default:"0"`

	// Path to disk cache directory
	Path string `yaml:"path" default:""`

	// Garbage collection algorithm: lru, lfu, fifo, largest, smallest, hybrid
	GCAlgorithm string `yaml:"gc_algorithm" default:"lru"`
}

// LoadConfig loads configuration from a YAML file
func LoadConfig(configPath string) (*Config, error) {
	if configPath == "" {
		configPath = "config.yaml"
	}

	data, err := os.ReadFile(configPath)
	if err != nil {
		return nil, fmt.Errorf("failed to read config file %s: %w", configPath, err)
	}

	var config Config
	if err := yaml.Unmarshal(data, &config); err != nil {
		return nil, fmt.Errorf("failed to parse config file %s: %w", configPath, err)
	}

	// Set defaults for empty values
	if config.ListenAddress == "" {
		config.ListenAddress = ":80"
	}
	if config.MaxConcurrentRequests == 0 {
		config.MaxConcurrentRequests = 50
	}
	if config.MaxRequestsPerClient == 0 {
		config.MaxRequestsPerClient = 3
	}
	if config.MaxObjectSize == "" {
		config.MaxObjectSize = "0"
	}
	if config.TrustedProxies == nil {
		config.TrustedProxies = []string{}
	}
	if config.Cache.Memory.Size == "" {
		config.Cache.Memory.Size = "0"
	}
	if config.Cache.Memory.GCAlgorithm == "" {
		config.Cache.Memory.GCAlgorithm = "lru"
	}
	if config.Cache.Disk.Size == "" {
		config.Cache.Disk.Size = "0"
	}
	if config.Cache.Disk.GCAlgorithm == "" {
		config.Cache.Disk.GCAlgorithm = "lru"
	}

	return &config, nil
}

// SaveDefaultConfig creates a default configuration file
func SaveDefaultConfig(configPath string) error {
	if configPath == "" {
		configPath = "config.yaml"
	}

	defaultConfig := Config{
		ListenAddress:         ":80",
		MaxConcurrentRequests: 50,         // Reduced for home user (less concurrent load)
		MaxRequestsPerClient:  3,          // Reduced for home user (more conservative per client)
		MaxObjectSize:         "0",        // 0=unlimited; set e.g. "512MB" for DoS protection on large bodies (P1-01)
		TrustedProxies:        []string{}, // Conservative default: never trust XFF (P1-02 spoof prevention)
		Cache: CacheConfig{
			Memory: MemoryConfig{
				Size:        "1GB", // Recommended for systems that can spare 1GB RAM for caching
				GCAlgorithm: "lru",
			},
			Disk: DiskConfig{
				Size:        "1TB", // Large HDD cache for home user
				Path:        "./disk",
				GCAlgorithm: "lru", // Better for gaming patterns (keeps recently played games)
			},
		},
		Upstream: "",
	}

	data, err := yaml.Marshal(&defaultConfig)
	if err != nil {
		return fmt.Errorf("failed to marshal default config: %w", err)
	}

	if err := os.WriteFile(configPath, data, 0644); err != nil {
		return fmt.Errorf("failed to write default config file: %w", err)
	}

	return nil
}

// GetDefaultConfig returns a populated default configuration (for tests and convenience).
func GetDefaultConfig() Config {
	return Config{
		ListenAddress:         ":80",
		MaxConcurrentRequests: 50,
		MaxRequestsPerClient:  3,
		MaxObjectSize:         "0",        // 0=unlimited (override for bounded response safety)
		TrustedProxies:        []string{}, // safe default: do not trust forwarded headers
		Cache: CacheConfig{
			Memory: MemoryConfig{
				Size:        "1GB",
				GCAlgorithm: "lru",
			},
			Disk: DiskConfig{
				Size:        "1TB",
				Path:        "./disk",
				GCAlgorithm: "lru",
			},
		},
		Upstream: "",
	}
}

// Validate performs basic sanity checks on the configuration.
func (c Config) Validate() error {
	if c.MaxConcurrentRequests < 0 {
		return fmt.Errorf("negative concurrency not allowed")
	}
	if c.MaxRequestsPerClient < 0 {
		return fmt.Errorf("negative per-client limit not allowed")
	}

	if c.Cache.Memory.GCAlgorithm != "" {
		switch c.Cache.Memory.GCAlgorithm {
		case "lru", "lfu", "fifo", "largest", "smallest", "hybrid":
		default:
			return fmt.Errorf("invalid memory gc algorithm: %s", c.Cache.Memory.GCAlgorithm)
		}
	}

	if c.Cache.Disk.Size != "" && c.Cache.Disk.Size != "0" && c.Cache.Disk.Path == "" {
		return fmt.Errorf("disk cache enabled but no path specified")
	}

	// P1 light validation for security/resource fields (mirrors existing GC + path checks; fails fast before New)
	if c.MaxObjectSize != "" && c.MaxObjectSize != "0" {
		if _, err := units.FromHumanSize(c.MaxObjectSize); err != nil {
			return fmt.Errorf("invalid max_object_size: %w", err)
		}
	}
	for _, p := range c.TrustedProxies {
		p = strings.TrimSpace(p)
		if p == "" {
			continue
		}
		if !strings.Contains(p, "/") {
			if net.ParseIP(p) == nil {
				return fmt.Errorf("invalid trusted_proxies entry (not IP or CIDR): %s", p)
			}
			continue
		}
		if _, _, err := net.ParseCIDR(p); err != nil {
			return fmt.Errorf("invalid trusted_proxies CIDR: %s", p)
		}
	}
	if c.MaxConcurrentRequests < 0 || c.MaxRequestsPerClient < 0 { // already covered above but explicit for P1 knobs
		// covered by earlier checks
	}

	return nil
}