lazyhandler/magic/form.go

package magic

import (
	"fmt"
	"net/http"
	"reflect"
	"regexp"
	"strconv"
	"strings"
)

var (
	cachedStructFormFields      = make(map[reflect.Type][]structFormFields)
	cachedUnsupportedFormFields = make(map[reflect.Type]structFormFields)
)

type Form[T any] struct {
	Data T
}

type structFormFields struct {
	PathKey  string
	FieldKey string

	Type                 reflect.Type
	MinValues, MaxValues int

	Match *regexp.Regexp
}

type UnsupportedFormType struct {
	inner reflect.Type
}

func (err UnsupportedFormType) Error() string {
	return "unsupported type for Form: " + err.inner.String()
}

type FormValueNotFitIn struct {
	Count int

	field structFormFields
}

func (err FormValueNotFitIn) Error() string {
	return fmt.Sprintf("Form %v is expected to has %v~%v values, got %v", err.field.PathKey, err.field.MinValues, err.field.MaxValues, err.Count)
}

type InvalidFormValue struct {
	Match *regexp.Regexp
	Value string
}

func (err InvalidFormValue) Error() string {
	return fmt.Sprintf("value not matched with regexp %v: %v", err.Match, err.Value)
}

func findReflectFormFields(v reflect.Value) ([]structFormFields, error) {
	if cached, ok := cachedStructFormFields[v.Type()]; ok {
		return cached, nil
	}
	if cached, ok := cachedUnsupportedFormFields[v.Type()]; ok {
		return nil, UnsupportedPathValueType{inner: cached.Type}
	}

	var fields []structFormFields
	if v.Kind() == reflect.Struct {
		t := v.Type()
		for idx := 0; idx < t.NumField(); idx++ {
			field := t.Field(idx)

			pathKey := field.Name
			var match *regexp.Regexp

			if tags, ok := field.Tag.Lookup("form"); ok {
				parts := strings.Split(strings.TrimSpace(tags), ",")
				if len(parts) == 0 {
					// do nothing
				} else {
					if parts[0] != "" {
						pathKey = parts[0]
					}

					for _, part := range parts[1:] {
						part = strings.TrimSpace(part)
						if strings.HasPrefix(part, "match=") {
							part := strings.TrimPrefix(part, "match=")
							if unquoted, err := strconv.Unquote(part); err != nil {
								return nil, err
							} else {
								part = unquoted
							}
							exp, err := regexp.Compile(part)
							if err != nil {
								return nil, err
							}
							match = exp
						}
					}
				}
			}

			fieldType := field.Type
			maxValues, minValues := 1, 1

			if fieldType.Kind() == reflect.Ptr {
				fieldType = fieldType.Elem()
				minValues = 0
			} else if fieldType.Kind() == reflect.Array {
				maxValues, minValues = fieldType.Len(), fieldType.Len()
				fieldType = fieldType.Elem()
			} else if fieldType.Kind() == reflect.Slice {
				fieldType = fieldType.Elem()
				maxValues, minValues = -1, 0
			}

			if pathValueConvertTable[fieldType.Kind()] == nil {
				if _, ok := fieldType.MethodByName("FromString"); !ok {
					cachedUnsupportedPathValueFields[t] = field
					return nil, UnsupportedPathValueType{inner: field.Type}
				}
			}
			fields = append(fields, structFormFields{
				PathKey:  pathKey,
				FieldKey: field.Name,

				Type:      fieldType,
				MaxValues: maxValues,
				MinValues: minValues,

				Match: match,
			})

			cachedStructFormFields[t] = fields
		}
	} else {
		return nil, UnsupportedFormType{inner: v.Type()}
	}

	return fields, nil
}

func (form *Form[T]) FromRequest(r *http.Request) error {
	v := reflect.ValueOf(form).Elem().FieldByName("Data")
	fields, err := findReflectFormFields(v)
	if err != nil {
		return err
	}

	if err := r.ParseMultipartForm(32 << 20); err != nil {
		return err
	}
	q := r.PostForm

	for _, field := range fields {
		values := q[field.PathKey]
		if len(values) < field.MinValues || (field.MaxValues > 0 && len(values) > field.MaxValues) {
			return FormValueNotFitIn{Count: len(values), field: field}
		}

		structField := v.FieldByName(field.FieldKey)

		if structField.Kind() == reflect.Slice && structField.Len() < len(values) {
			if structField.Cap() < len(values) {
				structField.SetCap(len(values))
			}
			structField.SetLen(len(values))
		}

		for idx, value := range values {
			if structField.Kind() != reflect.Array && structField.Kind() != reflect.Slice {
				if idx != len(values)-1 {
					// only the last takes effect
					continue
				}
			}
			// check match regexp
			if field.Match != nil && !field.Match.MatchString(value) {
				return InvalidFormValue{Match: field.Match, Value: value}
			}

			// convert to elem value
			convert := pathValueConvertTable[field.Type.Kind()]
			result, err := convert(value)
			if err != nil {
				return err
			}
			reflectValue := reflect.ValueOf(result)

			if structField.Kind() != reflect.Array && structField.Kind() != reflect.Slice {
				if field.MinValues == 0 {
					newReflectValue := reflect.New(field.Type)
					newReflectValue.Elem().Set(reflectValue)
					reflectValue = newReflectValue
				}
				structField.Set(reflectValue)
			} else {
				structField.Index(idx).Set(reflectValue)
			}
		}
	}

	return nil
}

func (form Form[T]) TakeRequestBody() {}