package config

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

func ReadEnv(c *Config, prefix string) (namesAvailable, namesUsed []string) {
	return structFromEnv(reflect.ValueOf(c), reflect.TypeOf(c), prefix)
}

func structFromEnv(v reflect.Value, t reflect.Type, prefix string) (namesAvailable, namesUsed []string) {

	if v.Kind() == reflect.Ptr {
		v = v.Elem()
		t = t.Elem()
	}

	for i := 0; i < v.NumField(); i++ {
		fv := v.Field(i)
		ft := t.Field(i)
		tag := ft.Tag.Get("env")
		if !ft.IsExported() || tag == "-" {
			continue
		}

		name := ft.Name
		if tag != "" {
			name = tag
		}

		if fv.Kind() == reflect.Struct {
			na, nu := structFromEnv(fv, ft.Type, prefix+"_"+name)
			namesAvailable = append(namesAvailable, na...)
			namesUsed = append(namesUsed, nu...)
		} else if !ft.Anonymous {
			envName := strings.ToUpper(prefix + "_" + name)
			namesAvailable = append(namesAvailable, envName)

			val, ok := os.LookupEnv(envName)
			if !ok {
				continue
			}
			if !fv.CanSet() {
				panic("cannot set field for environment variable " + envName)
			}

			switch fk := fv.Kind(); fk {
			case reflect.String:
				fv.SetString(val)
			case reflect.Int:
				x, err := strconv.ParseInt(val, 10, 0)
				if err != nil {
					panic(err)
				}
				fv.SetInt(x)
			case reflect.Bool:
				switch strings.ToLower(val) {
				case "1", "true", "t", "on":
					fv.SetBool(true)
				case "0", "false", "f", "off":
					fv.SetBool(false)
				default:
					panic(fmt.Sprintf("cannot set field for environment variable %s: cannot parse %s as boolean. Please use one of: 1, true, t, on / 0, false, f, off", envName, fk))
				}
			default:
				panic(fmt.Sprintf("cannot set field for environment variable %s: type %s is not supported", envName, fk))
			}
			namesUsed = append(namesUsed, envName)
		}

	}
	return namesAvailable, namesUsed
}