package config

import (
	"errors"
	"fmt"
	"net/netip"
	"os"
	"path/filepath"
	"strconv"
	"strings"
	"time"

	"gopkg.in/yaml.v3"
)

const (
	defaultListenAddr        = ":8080"
	defaultMaxUploadSize     = "100MB"
	defaultTTL               = "1h"
	defaultDataDir           = "./data"
	defaultCleanupInterval   = "1m"
	defaultRateLimitEnabled  = true
	defaultRequestsPerMinute = 30
	defaultRateLimitBurst    = 10
	maxDefaultTTLLimit       = 24 * time.Hour
	minCleanupInterval       = 10 * time.Second
)

type Config struct {
	Server   ServerConfig   `yaml:"server"`
	Limits   LimitsConfig   `yaml:"limits"`
	Storage  StorageConfig  `yaml:"storage"`
	Security SecurityConfig `yaml:"security"`
}

type ServerConfig struct {
	ListenAddr string `yaml:"listen_addr"`
}

type LimitsConfig struct {
	MaxUploadSize      string        `yaml:"max_upload_size"`
	DefaultTTL         string        `yaml:"default_ttl"`
	MaxUploadSizeBytes int64         `yaml:"-"`
	DefaultTTLDuration time.Duration `yaml:"-"`
}

type StorageConfig struct {
	DataDir               string        `yaml:"data_dir"`
	CleanupInterval       string        `yaml:"cleanup_interval"`
	CleanupIntervalParsed time.Duration `yaml:"-"`
}

type SecurityConfig struct {
	AllowedIPs        []string        `yaml:"allowed_ips"`
	TrustProxyHeaders bool            `yaml:"trust_proxy_headers"`
	RateLimit         RateLimitConfig `yaml:"rate_limit"`
	AllowedPrefixes   []netip.Prefix  `yaml:"-"`
}

type RateLimitConfig struct {
	Enabled           bool `yaml:"enabled"`
	RequestsPerMinute int  `yaml:"requests_per_minute"`
	Burst             int  `yaml:"burst"`
}

func Load(path string) (Config, error) {
	cfg := defaults()
	if path != "" {
		b, err := os.ReadFile(path)
		if err != nil {
			return Config{}, fmt.Errorf("read config: %w", err)
		}
		if err := yaml.Unmarshal(b, &cfg); err != nil {
			return Config{}, fmt.Errorf("parse config yaml: %w", err)
		}
	}

	if err := cfg.Validate(); err != nil {
		return Config{}, err
	}
	return cfg, nil
}

func defaults() Config {
	return Config{
		Server: ServerConfig{
			ListenAddr: defaultListenAddr,
		},
		Limits: LimitsConfig{
			MaxUploadSize: defaultMaxUploadSize,
			DefaultTTL:    defaultTTL,
		},
		Storage: StorageConfig{
			DataDir:         defaultDataDir,
			CleanupInterval: defaultCleanupInterval,
		},
		Security: SecurityConfig{
			RateLimit: RateLimitConfig{
				Enabled:           defaultRateLimitEnabled,
				RequestsPerMinute: defaultRequestsPerMinute,
				Burst:             defaultRateLimitBurst,
			},
		},
	}
}

func (c *Config) Validate() error {
	if strings.TrimSpace(c.Server.ListenAddr) == "" {
		return errors.New("server.listen_addr is required")
	}

	sizeBytes, err := parseHumanSize(c.Limits.MaxUploadSize)
	if err != nil {
		return fmt.Errorf("limits.max_upload_size invalid: %w", err)
	}
	c.Limits.MaxUploadSizeBytes = sizeBytes

	ttl, err := time.ParseDuration(strings.TrimSpace(c.Limits.DefaultTTL))
	if err != nil {
		return fmt.Errorf("limits.default_ttl invalid: %w", err)
	}
	if ttl <= 0 || ttl > maxDefaultTTLLimit {
		return fmt.Errorf("limits.default_ttl must be in (0, %s]", maxDefaultTTLLimit)
	}
	c.Limits.DefaultTTLDuration = ttl

	cleanupInterval, err := time.ParseDuration(strings.TrimSpace(c.Storage.CleanupInterval))
	if err != nil {
		return fmt.Errorf("storage.cleanup_interval invalid: %w", err)
	}
	if cleanupInterval < minCleanupInterval {
		return fmt.Errorf("storage.cleanup_interval must be at least %s", minCleanupInterval)
	}
	c.Storage.CleanupIntervalParsed = cleanupInterval

	if strings.TrimSpace(c.Storage.DataDir) == "" {
		return errors.New("storage.data_dir is required")
	}
	if err := ensureWritableDir(c.Storage.DataDir); err != nil {
		return fmt.Errorf("storage.data_dir not writable: %w", err)
	}

	prefixes, err := parseAllowedPrefixes(c.Security.AllowedIPs)
	if err != nil {
		return fmt.Errorf("security.allowed_ips invalid: %w", err)
	}
	c.Security.AllowedPrefixes = prefixes

	if c.Security.RateLimit.RequestsPerMinute <= 0 {
		return errors.New("security.rate_limit.requests_per_minute must be > 0")
	}
	if c.Security.RateLimit.Burst <= 0 {
		return errors.New("security.rate_limit.burst must be > 0")
	}

	return nil
}

func ensureWritableDir(dir string) error {
	if err := os.MkdirAll(dir, 0o755); err != nil {
		return err
	}

	probe := filepath.Join(dir, ".write_probe")
	if err := os.WriteFile(probe, []byte("ok"), 0o644); err != nil {
		return err
	}
	return os.Remove(probe)
}

func parseAllowedPrefixes(values []string) ([]netip.Prefix, error) {
	if len(values) == 0 {
		return nil, nil
	}

	out := make([]netip.Prefix, 0, len(values))
	for _, value := range values {
		item := strings.TrimSpace(value)
		if item == "" {
			continue
		}

		if strings.Contains(item, "/") {
			prefix, err := netip.ParsePrefix(item)
			if err != nil {
				return nil, fmt.Errorf("parse cidr %q: %w", item, err)
			}
			out = append(out, prefix)
			continue
		}

		ip, err := netip.ParseAddr(item)
		if err != nil {
			return nil, fmt.Errorf("parse ip %q: %w", item, err)
		}
		out = append(out, netip.PrefixFrom(ip, ip.BitLen()))
	}
	return out, nil
}

func parseHumanSize(raw string) (int64, error) {
	value := strings.TrimSpace(strings.ToUpper(raw))
	if value == "" {
		return 0, errors.New("size is empty")
	}

	idx := 0
	for idx < len(value) && (value[idx] == '.' || (value[idx] >= '0' && value[idx] <= '9')) {
		idx++
	}
	if idx == 0 {
		return 0, fmt.Errorf("size %q does not start with a number", raw)
	}

	numPart := value[:idx]
	unitPart := strings.TrimSpace(value[idx:])
	if unitPart == "" {
		unitPart = "B"
	}

	number, err := strconv.ParseFloat(numPart, 64)
	if err != nil {
		return 0, fmt.Errorf("parse number %q: %w", numPart, err)
	}
	if number <= 0 {
		return 0, errors.New("size must be > 0")
	}

	multipliers := map[string]float64{
		"B":   1,
		"KB":  1_000,
		"MB":  1_000_000,
		"GB":  1_000_000_000,
		"TB":  1_000_000_000_000,
		"KIB": 1024,
		"MIB": 1024 * 1024,
		"GIB": 1024 * 1024 * 1024,
		"TIB": 1024 * 1024 * 1024 * 1024,
	}

	multiplier, ok := multipliers[unitPart]
	if !ok {
		return 0, fmt.Errorf("unsupported unit %q", unitPart)
	}

	size := int64(number * multiplier)
	if size <= 0 {
		return 0, errors.New("calculated size must be > 0")
	}
	return size, nil
}