package gcfg

import (
	"fmt"
	"io"
	"reflect"
	"strings"
)

const (
	// Default value string in case a value for a variable isn't provided.
	defaultValue = "true"
)

func fieldFold(v reflect.Value, name string) reflect.Value {
	n := strings.Replace(name, "-", "_", -1)
	return v.FieldByNameFunc(func(fieldName string) bool {
		return strings.EqualFold(n, fieldName)
	})
}

func set(cfg interface{}, sect, sub, name, value string) error {
	vPCfg := reflect.ValueOf(cfg)
	if vPCfg.Kind() != reflect.Ptr || vPCfg.Elem().Kind() != reflect.Struct {
		panic(fmt.Errorf("config must be a pointer to a struct"))
	}
	vCfg := vPCfg.Elem()
	vSect := fieldFold(vCfg, sect)
	if !vSect.IsValid() {
		return fmt.Errorf("invalid section: section %q", sect)
	}
	if vSect.Kind() == reflect.Map {
		vst := vSect.Type()
		if vst.Key().Kind() != reflect.String ||
			vst.Elem().Kind() != reflect.Ptr ||
			vst.Elem().Elem().Kind() != reflect.Struct {
			panic(fmt.Errorf("map field for section must have string keys and "+
				" pointer-to-struct values: section %q", sect))
		}
		if vSect.IsNil() {
			vSect.Set(reflect.MakeMap(vst))
		}
		k := reflect.ValueOf(sub)
		pv := vSect.MapIndex(k)
		if !pv.IsValid() {
			vType := vSect.Type().Elem().Elem()
			pv = reflect.New(vType)
			vSect.SetMapIndex(k, pv)
		}
		vSect = pv.Elem()
	} else if vSect.Kind() != reflect.Struct {
		panic(fmt.Errorf("field for section must be a map or a struct: "+
			"section %q", sect))
	} else if sub != "" {
		return fmt.Errorf("invalid subsection: "+
			"section %q subsection %q", sect, sub)
	}
	vName := fieldFold(vSect, name)
	if !vName.IsValid() {
		return fmt.Errorf("invalid variable: "+
			"section %q subsection %q variable %q", sect, sub, name)
	}
	vAddr := vName.Addr().Interface()
	switch v := vAddr.(type) {
	case *string:
		*v = value
		return nil
	case *bool:
		vAddr = (*gbool)(v)
	}
	// attempt to read an extra rune to make sure the value is consumed
	var r rune
	n, err := fmt.Sscanf(value, "%v%c", vAddr, &r)
	switch {
	case n < 1 || n == 1 && err != io.EOF:
		return fmt.Errorf("failed to parse %q as %#v: parse error %v", value,
			vName.Type(), err)
	case n > 1:
		return fmt.Errorf("failed to parse %q as %#v: extra characters", value,
			vName.Type())
	case n == 1 && err == io.EOF:
		return nil
	}
	panic("never reached")
}