axiom/forms/forms.go

package forms

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

func FormToStruct[T any](r *http.Request) (T, error) {
	var target T

	ct := r.Header.Get("Content-Type")
	if strings.HasPrefix(ct, "multipart/form-data") {
		if err := r.ParseMultipartForm(32 << 20); err != nil {
			return target, fmt.Errorf("error parsing multipart form: %v", err)
		}
	} else {
		if err := r.ParseForm(); err != nil {
			return target, fmt.Errorf("error parsing form: %v", err)
		}
	}

	return target, UrlValuesToStruct(r.Form, &target)
}

func UrlValuesToStruct(form url.Values, dst any) error {
	v := reflect.ValueOf(dst)
	if v.Kind() != reflect.Ptr || v.Elem().Kind() != reflect.Struct {
		return fmt.Errorf("dst must be a pointer to a struct")
	}

	v = v.Elem()
	t := v.Type()
	for i := 0; i < t.NumField(); i++ {
		field := t.Field(i)
		fieldValue := v.Field(i)

		if !fieldValue.CanSet() {
			continue
		}

		key := field.Tag.Get("form")
		required := field.Tag.Get("req") == "1"
		formatters := parseFormatters(field.Tag.Get("fmt"))
		validateTags := parseValidators(field.Tag.Get("validate"))

		values, ok := form[key]
		if !ok || len(values) == 0 {
			if required {
				return fmt.Errorf("missing required form field: %s", key)
			}
			continue
		}

		for i := range values {
			for _, fmtFunc := range formatters {
				values[i] = fmtFunc(values[i])
			}
		}

		fieldKind := fieldValue.Kind()

		if fieldKind == reflect.Slice {
			elemKind := field.Type.Elem().Kind()

			castedSlice, err := castStringSliceToType(values, elemKind)
			if err != nil {
				return fmt.Errorf("field '%s': %v", field.Name, err)
			}

			sliceValue := reflect.MakeSlice(field.Type, len(castedSlice), len(castedSlice))
			for i, val := range castedSlice {
				sliceValue.Index(i).Set(reflect.ValueOf(val).Convert(field.Type.Elem()))
			}
			fieldValue.Set(sliceValue)
		} else {
			if len(values) != 1 {
				return fmt.Errorf("field '%s' expects a single value", field.Name)
			}

			castedVal, err := castStringToType(values[0], fieldKind)
			if err != nil {
				return fmt.Errorf("field '%s': %v", field.Name, err)
			}
			fieldValue.Set(reflect.ValueOf(castedVal).Convert(field.Type))
		}

		var finalValue any
		if fieldKind == reflect.Slice {
			finalValue = fieldValue.Interface()
		} else {
			finalValue = fieldValue.Interface()
		}

		for _, validator := range validateTags {
			if fn, ok := Validators[validator.Name]; ok {
				if err := fn(field.Name, finalValue, validator.Param); err != nil {
					return err
				}
			}
		}
	}

	return nil
}

func castStringSliceToType(input []string, kind reflect.Kind) ([]any, error) {
	var output []any

	for _, value := range input {
		cast, err := castStringToType(value, kind)
		if err != nil {
			return nil, err
		}

		output = append(output, cast)
	}

	return output, nil
}

func castStringToType(value string, kind reflect.Kind) (any, error) {
	switch kind {
	case reflect.String:
		return value, nil

	case reflect.Int, reflect.Int64:
		new, err := strconv.Atoi(value)
		if err != nil {
			return nil, fmt.Errorf("failed to cast string to integer: %v", err)
		}
		return new, nil

	case reflect.Float32, reflect.Float64:
		new, err := strconv.ParseFloat(value, 64)
		if err != nil {
			return nil, fmt.Errorf("failed to cast string to float64: %v", err)
		}
		return new, nil

	case reflect.Bool:
		new, err := strconv.ParseBool(value)
		if err != nil {
			return nil, fmt.Errorf("failed to cast string to boolean: %v", err)
		}
		return new, nil

	default:
		return nil, fmt.Errorf("unsupported kind: %s", kind)
	}
}

func parseFormatters(tag string) []func(string) string {
	if tag == "" {
		return nil
	}

	var fns []func(string) string

	parts := strings.SplitSeq(tag, ",")
	for p := range parts {
		if fn, ok := Formatters[strings.TrimSpace(p)]; ok {
			fns = append(fns, fn)
		}
	}

	return fns
}

func parseValidators(tag string) []struct {
	Name  string
	Param string
} {
	if tag == "" {
		return nil
	}

	var result []struct {
		Name  string
		Param string
	}

	parts := strings.SplitSeq(tag, ",")
	for part := range parts {
		pair := strings.SplitN(part, ":", 2)
		if len(pair) == 2 {
			result = append(result, struct{ Name, Param string }{pair[0], pair[1]})
		} else {
			result = append(result, struct{ Name, Param string }{pair[0], ""})
		}
	}

	return result
}