// Package config loads the addrkit configuration from a JSON file, expanding
// ${VAR} placeholders from the environment. It fails closed: missing required
// values are an error, never a silent default.
package config

import (
	"bytes"
	"encoding/json"
	"fmt"
	"os"
	"regexp"
	"strings"

	"git.totmin.ru/en2zmax/addrkit/internal/domain/address"
	"git.totmin.ru/en2zmax/addrkit/internal/domain/match"
)

// Config is the whole service configuration.
type Config struct {
	// Addr is the listen address of the HTTP server, e.g. ":6171".
	Addr string `json:"addr"`
	// DataPath is the SQLite file produced by the seed pipeline.
	DataPath string `json:"data_path"`
	// LogLevel is one of debug|info|warn|error.
	LogLevel string `json:"log_level"`

	Resolve ResolveConfig `json:"resolve"`

	// Cities is the dictionary of cities this deployment understands. A
	// single-city deployment lists one city and may set DefaultCity so that
	// queries like "ленина 5" work without naming it.
	Cities      []address.City `json:"cities"`
	DefaultCity string         `json:"default_city"`

	LLM LLMConfig `json:"llm"`
}

// ResolveConfig tunes the matching engine. Defaults come from
// match.DefaultOptions so there is exactly one source of truth.
type ResolveConfig struct {
	FuzzyThreshold  float64 `json:"fuzzy_threshold"`
	AmbiguityMargin float64 `json:"ambiguity_margin"`
	MaxCandidates   int     `json:"max_candidates"`
}

// LLMConfig configures the optional, disabled-by-default query rewriter.
type LLMConfig struct {
	Enabled        bool   `json:"enabled"`
	URL            string `json:"url"`
	Token          string `json:"token"`
	Model          string `json:"model"`
	TimeoutSeconds int    `json:"timeout_seconds"`
	// MaxTokens bounds the rewriter's reply. Thinking models need enough room
	// for the final JSON even when thinking is disabled.
	MaxTokens int `json:"max_tokens"`
	// DisableThinking turns off chain-of-thought for models that support it
	// (Qwen3 via chat_template_kwargs). A rewriter only needs the JSON answer.
	DisableThinking bool `json:"disable_thinking"`
	// SystemPrompt overrides the built-in rewriter prompt inline. When empty,
	// the prompt embedded in the binary is used.
	SystemPrompt string `json:"system_prompt"`
	// SystemPromptFile loads the rewriter prompt from a file. When set it takes
	// precedence over SystemPrompt, which keeps long prompts out of the config.
	SystemPromptFile string `json:"system_prompt_file"`
}

// Default returns a config with sane defaults for the non-required fields.
func Default() Config {
	resolve := match.DefaultOptions()
	return Config{
		Addr:     ":6171",
		LogLevel: "info",
		Resolve: ResolveConfig{
			FuzzyThreshold:  resolve.FuzzyThreshold,
			AmbiguityMargin: resolve.AmbiguityMargin,
			MaxCandidates:   resolve.MaxCandidates,
		},
		LLM: LLMConfig{
			TimeoutSeconds:  3,
			MaxTokens:       512,
			DisableThinking: true,
		},
	}
}

// Load reads, expands and validates a config file.
func Load(path string) (Config, error) {
	raw, err := os.ReadFile(path)
	if err != nil {
		return Config{}, fmt.Errorf("read config: %w", err)
	}
	cfg := Default()
	dec := json.NewDecoder(strings.NewReader(expandEnv(string(raw))))
	// A typo in a key (fuzzy_treshold, data_pth, portal, ...) must not silently
	// fall back to a default. Fail closed and let the operator fix the config.
	dec.DisallowUnknownFields()
	if err := dec.Decode(&cfg); err != nil {
		return Config{}, fmt.Errorf("parse config: %w", err)
	}
	if cfg.LLM.SystemPromptFile != "" {
		data, err := os.ReadFile(cfg.LLM.SystemPromptFile)
		if err != nil {
			return Config{}, fmt.Errorf("read llm.system_prompt_file: %w", err)
		}
		cfg.LLM.SystemPrompt = string(data)
	}
	if err := cfg.validate(); err != nil {
		return Config{}, err
	}
	return cfg, nil
}

// LoadCitiesFile reads a standalone JSON array of cities. It is used by the
// seed CLI so that seeding and serving share the same city vocabulary.
func LoadCitiesFile(path string) ([]address.City, error) {
	raw, err := os.ReadFile(path)
	if err != nil {
		return nil, fmt.Errorf("read cities: %w", err)
	}
	var cities []address.City
	dec := json.NewDecoder(bytes.NewReader(raw))
	dec.DisallowUnknownFields()
	if err := dec.Decode(&cities); err != nil {
		return nil, fmt.Errorf("parse cities: %w", err)
	}
	if err := validateCities(cities, ""); err != nil {
		return nil, err
	}
	return cities, nil
}

func (c Config) validate() error {
	if c.Addr == "" {
		return fmt.Errorf("config: addr is required")
	}
	if c.DataPath == "" {
		return fmt.Errorf("config: data_path is required")
	}
	if c.Resolve.FuzzyThreshold <= 0 || c.Resolve.FuzzyThreshold > 1 {
		return fmt.Errorf("config: resolve.fuzzy_threshold must be in (0, 1]")
	}
	if c.Resolve.AmbiguityMargin < 0 || c.Resolve.AmbiguityMargin > 1 {
		return fmt.Errorf("config: resolve.ambiguity_margin must be in [0, 1]")
	}
	if c.Resolve.MaxCandidates < 1 {
		return fmt.Errorf("config: resolve.max_candidates must be at least 1")
	}
	if c.LLM.Enabled && c.LLM.URL == "" {
		return fmt.Errorf("config: llm.url is required when llm.enabled is true")
	}
	return validateCities(c.Cities, c.DefaultCity)
}

// validateCities checks the city dictionary for the mistakes that would make
// matching silently ambiguous: empty names, duplicate cities and an alias
// claimed by two different cities.
func validateCities(cities []address.City, defaultCity string) error {
	seenNames := make(map[string]string)
	seenAliases := make(map[string]string)

	for _, city := range cities {
		name := strings.TrimSpace(city.Name)
		key := address.NormalizeText(name)
		if key == "" {
			return fmt.Errorf("config: cities[].name must not be empty")
		}
		if prev, ok := seenNames[key]; ok {
			return fmt.Errorf("config: duplicate city %q and %q", prev, name)
		}
		seenNames[key] = name

		aliases := append([]string{name}, city.Aliases...)
		for _, alias := range aliases {
			norm := address.NormalizeText(alias)
			if norm == "" {
				continue
			}
			if owner, ok := seenAliases[norm]; ok && owner != key {
				return fmt.Errorf("config: alias %q is claimed by both %q and %q", alias, owner, key)
			}
			seenAliases[norm] = key
		}
	}
	if defaultCity = strings.TrimSpace(defaultCity); defaultCity != "" && len(cities) > 0 {
		if _, ok := seenNames[address.NormalizeText(defaultCity)]; !ok {
			return fmt.Errorf("config: default_city %q is not listed in cities", defaultCity)
		}
	}
	return nil
}

// envRef matches ${VAR} and $VAR placeholders.
var envRef = regexp.MustCompile(`\$\{([A-Za-z_][A-Za-z0-9_]*)\}|\$([A-Za-z_][A-Za-z0-9_]*)`)

// expandEnv replaces ${VAR} / $VAR with the environment value. An unset
// variable expands to the empty string; validation then rejects configs that
// relied on it (fail closed).
func expandEnv(s string) string {
	return envRef.ReplaceAllStringFunc(s, func(m string) string {
		sub := envRef.FindStringSubmatch(m)
		name := sub[1]
		if name == "" {
			name = sub[2]
		}
		return os.Getenv(name)
	})
}