package config

import (
	"fmt"
	"os"
	"reflect"
	"strconv"
	"strings"
)

// decodeValue populates the struct pointed to by v. Precedence, highest first:
// process environment variable, the parsed .env file, then the default tag.
func decodeValue(v reflect.Value, fromEnv map[string]string) error {
	if v.Kind() != reflect.Ptr || v.Elem().Kind() != reflect.Struct {
		return fmt.Errorf("config: decode expects a pointer to a struct")
	}
	return decodeStruct(v.Elem(), fromEnv)
}

func decodeStruct(sv reflect.Value, fromEnv map[string]string) error {
	st := sv.Type()
	for i := 0; i < st.NumField(); i++ {
		field := st.Field(i)
		if !field.IsExported() {
			continue
		}
		fv := sv.Field(i)

		envTag, hasEnv := field.Tag.Lookup("env")
		if !hasEnv {
			// Untagged struct fields are nested config groups.
			if fv.Kind() == reflect.Struct {
				if err := decodeStruct(fv, fromEnv); err != nil {
					return err
				}
			}
			continue
		}

		name := envTagName(envTag)
		raw := lookupRaw(name, field.Tag.Get("default"), fromEnv)
		if err := setField(fv, raw); err != nil {
			return fmt.Errorf("config: %s: %w", name, err)
		}
	}
	return nil
}

// envTagName extracts the variable name from an env tag (dropping any
// comma-separated options that may be added later).
func envTagName(tag string) string {
	name, _, _ := strings.Cut(tag, ",")
	return strings.TrimSpace(name)
}

// lookupRaw returns the effective raw value for key: process env, then .env,
// then default.
func lookupRaw(key, def string, fromEnv map[string]string) string {
	if v, ok := os.LookupEnv(key); ok {
		return v
	}
	if v, ok := fromEnv[key]; ok {
		return v
	}
	return def
}

func setField(fv reflect.Value, raw string) error {
	switch fv.Kind() {
	case reflect.String:
		fv.SetString(raw)
		return nil
	case reflect.Bool:
		return setBool(fv, raw)
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return setInt(fv, raw)
	case reflect.Slice:
		return setStringSlice(fv, raw)
	default:
		return fmt.Errorf("unsupported kind %s", fv.Kind())
	}
}

func setBool(fv reflect.Value, raw string) error {
	if raw == "" {
		fv.SetBool(false)
		return nil
	}
	b, err := strconv.ParseBool(strings.ToLower(raw))
	if err != nil {
		return fmt.Errorf("invalid boolean %q", raw)
	}
	fv.SetBool(b)
	return nil
}

func setInt(fv reflect.Value, raw string) error {
	if raw == "" {
		fv.SetInt(0)
		return nil
	}
	n, err := strconv.ParseInt(raw, 10, 64)
	if err != nil {
		return fmt.Errorf("invalid integer %q", raw)
	}
	fv.SetInt(n)
	return nil
}

func setStringSlice(fv reflect.Value, raw string) error {
	if fv.Type().Elem().Kind() != reflect.String {
		return fmt.Errorf("unsupported slice element kind %s", fv.Type().Elem().Kind())
	}
	if strings.TrimSpace(raw) == "" {
		fv.Set(reflect.MakeSlice(fv.Type(), 0, 0))
		return nil
	}
	parts := strings.Split(raw, ",")
	out := make([]string, 0, len(parts))
	for _, p := range parts {
		if t := strings.TrimSpace(p); t != "" {
			out = append(out, t)
		}
	}
	fv.Set(reflect.ValueOf(out))
	return nil
}