lazyhandler/inject.go

package lazyhandler

import (
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"net/http"
	"reflect"

	"git.jeffthecoder.xyz/public/lazyhandler/magic"
	"git.jeffthecoder.xyz/public/lazyhandler/util"
)

type ErrArgumentIsNotExtractable int

func (err ErrArgumentIsNotExtractable) Error() string {
	return fmt.Sprintf("argument %v is not extractable", int(err))
}

type ErrReturnValueNotConvertableIntoResponsePart int

func (err ErrReturnValueNotConvertableIntoResponsePart) Error() string {
	return fmt.Sprintf("return value %v is not convertable into response part", int(err))
}

var (
	ErrNotAFunc                         = errors.New("not a function")
	ErrDuplicateResponseWriterExtractor = errors.New("duplicate response writer extractor")
	ErrResponseWriterCannotBeExtracted  = errors.New("http.ResponseWriter extractor must not exists if function has return value")
)

func canConvert[T any](o any) bool {
	t := reflect.TypeOf((*T)(nil)).Elem()
	if reflectValue, ok := o.(reflect.Value); ok {
		return reflectValue.CanConvert(t)
	}
	if reflectType, ok := o.(reflect.Type); ok {
		return reflectType.ConvertibleTo(t)
	}
	return reflect.ValueOf(o).CanConvert(t)
}

func MagicHandler(fn any) (http.Handler, error) {
	funcValue := reflect.ValueOf(fn)
	t := funcValue.Type()

	if t.Kind() != reflect.Func {
		return nil, ErrNotAFunc
	}

	responseWriterExtracted := -1

	extractors := make([]func(http.ResponseWriter, *http.Request) (any, error), t.NumIn())
	for idx := 0; idx < t.NumIn(); idx++ {
		in := t.In(idx)

		extractor, isTakeResponseWriter := magic.GetExtractor(in)
		if extractor == nil {
			return nil, ErrArgumentIsNotExtractable(idx)
		}
		if isTakeResponseWriter {
			if responseWriterExtracted >= 0 {
				return nil, ErrDuplicateResponseWriterExtractor
			}
			responseWriterExtracted = idx
		}
		extractors[idx] = extractor
	}

	// http.ResponseWriter extractor must not exists if function has return value
	if responseWriterExtracted >= 0 && t.NumOut() > 0 {
		return nil, ErrResponseWriterCannotBeExtracted
	}

	for idx := 0; idx < t.NumOut(); idx++ {
		out := t.Out(idx)

		// int(status) || string(body)
		if out.Kind() == reflect.Int || out.Kind() == reflect.String {
			continue
		}

		// []byte(body)
		if out.Kind() == reflect.Slice && out.Elem().Kind() == reflect.Uint8 {
			continue
		}

		// [T] map[string]T(header)
		if out.Kind() == reflect.Map && out.Key().Kind() == reflect.String {
			continue
		}

		if _, ok := util.Implements[io.Reader](out); ok {
			continue
		}

		if _, ok := util.Implements[magic.RespondWriter](out); ok {
			continue
		}

		// last is error
		if _, ok := util.Implements[error](out); ok && idx == t.NumOut()-1 {
			continue
		}

		return nil, ErrReturnValueNotConvertableIntoResponsePart(idx)
	}

	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		var closers []io.Closer
		defer func() {
			for _, closer := range closers {
				closer.Close()
			}
		}()
		in := make([]reflect.Value, len(extractors))
		for idx, extractor := range extractors {
			v, err := extractor(w, r)
			if err != nil {
				if errResponse, ok := err.(magic.ErrorResponse); ok {
					errResponse.WriteResponse(w)
				} else {
					w.WriteHeader(http.StatusBadRequest)
					json.NewEncoder(w).Encode(magic.Map{
						"error": err.Error(),
					})
				}
				return
			}
			if closer, ok := v.(io.Closer); ok {
				defer closer.Close()
			}
			in[idx] = reflect.ValueOf(v)
		}
		values := funcValue.Call(in)

		if numValues := len(values); numValues > 0 {
			lastValue := values[numValues-1].Interface()
			if err, isError := lastValue.(error); isError {
				if err != nil {
					if errResponse, ok := lastValue.(magic.ErrorResponse); ok {
						errResponse.WriteResponse(w)
					} else {
						w.WriteHeader(http.StatusBadRequest)
						json.NewEncoder(w).Encode(magic.Map{
							"error": err.Error(),
						})
					}
					return
				}
				values = values[:numValues-1]
			}
		}

		for _, value := range values {
			obj := value.Interface()

			if value.Kind() == reflect.Int {
				w.WriteHeader(int(value.Int()))
			} else if value.Kind() == reflect.String {
				w.Write([]byte(value.String()))
			} else if canConvert[[]byte](value) {
				w.Write(value.Bytes())
			} else if reader, ok := util.Implements[io.Reader](value); ok {
				io.Copy(w, reader)
			} else if responseWriter, ok := util.Implements[magic.RespondWriter](value); ok {
				if responseWriter == nil {
					continue
				}
				responseWriter.WriteResponse(w)
			} else if headers, ok := obj.(http.Header); ok { // not
				for name, values := range headers {
					for idx, value := range values {
						if idx == 0 {
							w.Header().Set(name, value)
						} else {
							w.Header().Add(name, value)
						}
					}
				}
			} else if value.Kind() == reflect.Map {
				for _, key := range value.MapKeys() {
					value := value.MapIndex(key).Interface()
					w.Header().Set(key.String(), fmt.Sprint(value))
				}
			}
		}
	}), nil
}

func MustMagicHandler(fn any) http.Handler {
	if handler, err := MagicHandler(fn); err != nil {
		panic("can not be converted to http.Handler: " + fmt.Sprintf("%T", fn) + ": " + err.Error())
	} else {
		return handler
	}

}