package config

import (
	"errors"
	"fmt"
	"os"
	"strings"

	"github.com/spf13/viper"
)

// Config is the root configuration for vecna.
type Config struct {
	Server  ServerConfig  `mapstructure:"server"`
	Metrics MetricsConfig `mapstructure:"metrics"`
	Forward ForwardConfig `mapstructure:"forward"`
	Adapter AdapterConfig `mapstructure:"adapter"`
}

// ServerConfig controls the HTTP listener and inbound auth.
type ServerConfig struct {
	Port    int      `mapstructure:"port"`
	Host    string   `mapstructure:"host"`
	APIKeys []string `mapstructure:"api_keys"`
}

// MetricsConfig controls Prometheus metrics exposure.
type MetricsConfig struct {
	Enabled bool   `mapstructure:"enabled"`
	Path    string `mapstructure:"path"`
	APIKey  string `mapstructure:"api_key"`
}

// ForwardConfig holds all named forwarding targets.
type ForwardConfig struct {
	Default string                   `mapstructure:"default"`
	Targets map[string]ForwardTarget `mapstructure:"targets"`
}

// ForwardTarget is a named backing embedding model with one or more endpoints.
type ForwardTarget struct {
	Endpoints        []EndpointConfig `mapstructure:"endpoints"`
	Model            string           `mapstructure:"model"`
	APIKey           string           `mapstructure:"api_key"`
	APIType          string           `mapstructure:"api_type"`
	TimeoutSecs      int              `mapstructure:"timeout_secs"`
	CooldownSecs     int              `mapstructure:"cooldown_secs"`
	PriorityDecay    int              `mapstructure:"priority_decay"`
	PriorityRecovery int              `mapstructure:"priority_recovery"`
}

// EndpointConfig is a single URL within a ForwardTarget.
type EndpointConfig struct {
	URL      string `mapstructure:"url"`
	Priority int    `mapstructure:"priority"`
	APIKey   string `mapstructure:"api_key"`
}

// AdapterConfig selects and tunes the dimension adapter.
type AdapterConfig struct {
	Type         string `mapstructure:"type"`
	SourceDim    int    `mapstructure:"source_dim"`
	TargetDim    int    `mapstructure:"target_dim"`
	TruncateMode string `mapstructure:"truncate_mode"`
	PadMode      string `mapstructure:"pad_mode"`
	Seed         int64  `mapstructure:"seed"`
	MatrixFile   string `mapstructure:"matrix_file"`
}

// extensions viper will detect automatically.
var extensions = []string{"json", "yaml", "toml"}

// ResolveFile returns the config file path that would be used by Load.
// If cfgFile is non-empty it is returned as-is.
// Otherwise the default search paths are checked; if no existing file is found,
// the preferred default (~/.vecna.json) is returned so callers can create it.
func ResolveFile(cfgFile string) string {
	if cfgFile != "" {
		return cfgFile
	}
	home, _ := os.UserHomeDir()
	dirs := []string{".", home, home + "/.config/vecna"}
	for _, dir := range dirs {
		for _, ext := range extensions {
			path := dir + "/vecna." + ext
			if _, err := os.Stat(path); err == nil {
				return path
			}
		}
	}
	return home + "/vecna.json"
}

// Load reads configuration from the given file path (empty = search defaults),
// environment variables (prefix VECNA_), and applies built-in defaults.
func Load(cfgFile string) (*Config, error) {
	v := viper.New()

	// Defaults
	v.SetDefault("server.port", 8080)
	v.SetDefault("server.host", "0.0.0.0")
	v.SetDefault("metrics.enabled", false)
	v.SetDefault("metrics.path", "/metrics")
	v.SetDefault("adapter.type", "truncate")
	v.SetDefault("adapter.truncate_mode", "from_end")
	v.SetDefault("adapter.pad_mode", "at_end")

	// Environment
	v.SetEnvPrefix("VECNA")
	v.SetEnvKeyReplacer(strings.NewReplacer(".", "_"))
	v.AutomaticEnv()

	// Config file
	if cfgFile != "" {
		v.SetConfigFile(cfgFile)
	} else {
		home, _ := os.UserHomeDir()
		v.SetConfigName("vecna")
		// No SetConfigType — viper detects format from file extension (json, yaml, toml, etc.)
		v.AddConfigPath(".")
		v.AddConfigPath(home)
		v.AddConfigPath(home + "/.config/vecna")
	}

	if err := v.ReadInConfig(); err != nil {
		// Missing config file is acceptable when all required values come from flags/env
		var notFound viper.ConfigFileNotFoundError
		if !errors.As(err, &notFound) {
			return nil, fmt.Errorf("load config: %w", err)
		}
	}

	var cfg Config
	if err := v.Unmarshal(&cfg); err != nil {
		return nil, fmt.Errorf("unmarshal config: %w", err)
	}

	applyForwardDefaults(&cfg)

	return &cfg, nil
}

// applyForwardDefaults fills in zero-value fields on ForwardTarget entries.
func applyForwardDefaults(cfg *Config) {
	for name, t := range cfg.Forward.Targets {
		if t.TimeoutSecs == 0 {
			t.TimeoutSecs = 30
		}
		if t.CooldownSecs == 0 {
			t.CooldownSecs = 60
		}
		if t.PriorityDecay == 0 {
			t.PriorityDecay = 2
		}
		if t.PriorityRecovery == 0 {
			t.PriorityRecovery = 5
		}
		for i, ep := range t.Endpoints {
			if ep.Priority == 0 {
				t.Endpoints[i].Priority = 10
			}
			if ep.APIKey == "" && t.APIKey != "" {
				t.Endpoints[i].APIKey = t.APIKey
			}
		}
		cfg.Forward.Targets[name] = t
	}
}