// 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 (
	"encoding/json"
	"fmt"
	"os"
	"regexp"
	"strings"
)

// 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"`
	Portal  PortalConfig  `json:"portal"`
	LLM     LLMConfig     `json:"llm"`
}

// ResolveConfig tunes the matching engine.
type ResolveConfig struct {
	FuzzyThreshold  float64 `json:"fuzzy_threshold"`
	AmbiguityMargin float64 `json:"ambiguity_margin"`
	MaxCandidates   int     `json:"max_candidates"`
}

// PortalConfig configures the external outages portal (ArcGIS). When URL is
// empty the "events by house" module is disabled and only /v1/resolve works.
type PortalConfig struct {
	URL            string `json:"url"`
	TimeoutSeconds int    `json:"timeout_seconds"`
	Retries        int    `json:"retries"`
}

// 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 {
	return Config{
		Addr:     ":6171",
		LogLevel: "info",
		Resolve: ResolveConfig{
			FuzzyThreshold:  0.82,
			AmbiguityMargin: 0.03,
			MaxCandidates:   5,
		},
		Portal: PortalConfig{
			TimeoutSeconds: 2,
			Retries:        1,
		},
		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))))
	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
}

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.LLM.Enabled && c.LLM.URL == "" {
		return fmt.Errorf("config: llm.url is required when llm.enabled is true")
	}
	// portal.url is optional: without it the "events by house" module is
	// disabled and only /v1/resolve is served.
	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)
	})
}