package config

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

// durationType is compared against to tell a Duration from a plain int64.
var durationType = reflect.TypeOf(time.Duration(0))

// ApplyEnv overwrites every field carrying an `env` tag whose variable is set
// to a non-empty value. lookup is usually os.Getenv; tests pass a fake.
//
// Supported kinds: string, bool, all ints, time.Duration and []string
// (comma-separated, whitespace trimmed, empty elements dropped).
func ApplyEnv(cfg *Config, lookup func(string) string) error {
	return applyEnv(reflect.ValueOf(cfg).Elem(), lookup)
}

func applyEnv(v reflect.Value, lookup func(string) string) error {
	t := v.Type()
	for i := range t.NumField() {
		field := t.Field(i)
		if !field.IsExported() {
			continue
		}
		fv := v.Field(i)

		if fv.Kind() == reflect.Struct && field.Type != durationType {
			if err := applyEnv(fv, lookup); err != nil {
				return err
			}
			continue
		}

		name := field.Tag.Get("env")
		if name == "" {
			continue
		}
		raw := strings.TrimSpace(lookup(name))
		if raw == "" {
			continue
		}
		if err := setFromString(fv, raw); err != nil {
			return fmt.Errorf("%s: %w", name, err)
		}
	}
	return nil
}

func setFromString(fv reflect.Value, raw string) error {
	if fv.Type() == durationType {
		d, err := time.ParseDuration(raw)
		if err != nil {
			return fmt.Errorf("invalid duration %q", raw)
		}
		fv.SetInt(int64(d))
		return nil
	}

	switch fv.Kind() {
	case reflect.String:
		fv.SetString(raw)
	case reflect.Bool:
		b, err := strconv.ParseBool(raw)
		if err != nil {
			return fmt.Errorf("invalid boolean %q (want true or false)", raw)
		}
		fv.SetBool(b)
	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		n, err := strconv.ParseInt(raw, 10, 64)
		if err != nil {
			return fmt.Errorf("invalid integer %q", raw)
		}
		fv.SetInt(n)
	case reflect.Slice:
		if fv.Type().Elem().Kind() != reflect.String {
			return fmt.Errorf("unsupported slice type %s", fv.Type())
		}
		fv.Set(reflect.ValueOf(splitList(raw)))
	default:
		return fmt.Errorf("unsupported field type %s", fv.Type())
	}
	return nil
}

// splitList parses a comma-separated env value. A lone "-" clears the list,
// which is the only way to turn a YAML list back into an empty one from the
// environment.
func splitList(raw string) []string {
	if raw == "-" {
		return []string{}
	}
	parts := strings.Split(raw, ",")
	out := make([]string, 0, len(parts))
	for _, p := range parts {
		if p = strings.TrimSpace(p); p != "" {
			out = append(out, p)
		}
	}
	return out
}