forms/forms.go

package forms

import (
	"encoding"
	"reflect"
	"strconv"
	"strings"
	"unicode"
)

type DecodeError struct {
	Field string
	Type  string
	Value string
}

func (de DecodeError) Error() string {
	return `Failed to decode "` + de.Value + `" into field ` + de.Field + ` of type ` + de.Type + `.`
}

type Values = map[string][]string

func Decode(form Values, v interface{}) error {
	rv := reflect.ValueOf(v)
	if rv.Kind() != reflect.Ptr {
		panic(`Input is not a pointer`)
	}

	return decode(form, rv.Elem(), ``)
}

func decode(form Values, rv reflect.Value, prefix string) error {
	for i := 0; i < rv.NumField(); i++ {
		ft, fv := rv.Type().Field(i), rv.Field(i)

		if !unicode.IsUpper(rune(ft.Name[0])) {
			continue
		}

		fe := ft.Type
		if fe.Kind() == reflect.Ptr {
			fe = fe.Elem()
		}

		if tu, ok := fv.Addr().Interface().(encoding.TextUnmarshaler); ok {
			v, ok := form[getName(ft, prefix)]
			if !ok || len(v) == 0 {
				continue
			}

			err := tu.UnmarshalText([]byte(v[0]))
			if err != nil {
				return err
			}

			continue
		}

		if fe.Kind() == reflect.Struct {
			if fv.Kind() == reflect.Ptr {
				found := false
				for k, _ := range form {
					if strings.HasPrefix(k, getPrefix(ft, prefix)) {
						found = true
						break
					}
				}
				if !found {
					continue
				}

				fv.Set(reflect.New(ft.Type.Elem()))

				fv = fv.Elem()
			}

			err := decode(form, fv, getPrefix(ft, prefix))
			if err != nil {
				return err
			}
			continue
		}

		fieldName := getName(ft, prefix)
		v, ok := form[fieldName]
		if !ok || len(v) == 0 {
			continue
		}

		if fv.Kind() != reflect.Ptr {
			fv = fv.Addr()
		} else if fv.IsNil() {
			fv.Set(reflect.New(ft.Type.Elem()))
		}

		err := setValue(fieldName, v, fv)
		if err != nil {
			return err
		}
	}

	return nil
}

func setValue(fieldName string, values []string, fv reflect.Value) error {
	if fv.Elem().Type().Kind() != reflect.Slice {
		return parse(fieldName, values[0], fv)
	}

	slice := reflect.MakeSlice(fv.Elem().Type(), len(values), len(values))
	val := reflect.New(slice.Type().Elem())
	for i, v := range values {
		err := parse(fieldName, v, val)
		if err != nil {
			return err
		}
		slice.Index(i).Set(val.Elem())
	}
	fv.Elem().Set(slice)

	return nil
}

func parse(fieldName, v string, fv reflect.Value) error {
	var err error

	switch f := fv.Interface().(type) {
	case *string:
		*f = v
	case *int:
		*f, err = strconv.Atoi(v)
	case *float32:
		var f64 float64
		f64, err = strconv.ParseFloat(v, 32)
		*f = float32(f64)
	case *float64:
		*f, err = strconv.ParseFloat(v, 64)
	}

	if err != nil {
		return DecodeError{fieldName, fv.Elem().Type().Kind().String(), v}
	}

	return nil
}

func getPrefix(ft reflect.StructField, prefix string) string {
	if ft.Anonymous {
		return prefix
	}

	if prefix != `` {
		prefix += `.`
	}

	return prefix + ft.Name
}

func getName(ft reflect.StructField, prefix string) string {
	name := ft.Tag.Get(`form`)
	if name == `` {
		name = ft.Name
	}

	if prefix == `` {
		return name
	}

	return prefix + `.` + name
}