Restructure project

We previously didn't really had any structure in our project apart
from creating a new folder for each package in our project root.
Now that we have accumulated some packages, we use the well-known
Golang project layout in order to clearly communicate our intent
with packages. See https://github.com/golang-standards/project-layout

internal/api/helpers.go

@@ -0,0 +1,43 @@
+package api
+
+import (
+	"encoding/json"
+	"fmt"
+	"gitlab.hpi.de/codeocean/codemoon/poseidon/pkg/dto"
+	"net/http"
+)
+
+func writeInternalServerError(writer http.ResponseWriter, err error, errorCode dto.ErrorCode) {
+	sendJSON(writer, &dto.InternalServerError{Message: err.Error(), ErrorCode: errorCode}, http.StatusInternalServerError)
+}
+
+func writeBadRequest(writer http.ResponseWriter, err error) {
+	sendJSON(writer, &dto.ClientError{Message: err.Error()}, http.StatusBadRequest)
+}
+
+func writeNotFound(writer http.ResponseWriter, err error) {
+	sendJSON(writer, &dto.ClientError{Message: err.Error()}, http.StatusNotFound)
+}
+
+func sendJSON(writer http.ResponseWriter, content interface{}, httpStatusCode int) {
+	writer.Header().Set("Content-Type", "application/json")
+	writer.WriteHeader(httpStatusCode)
+	response, err := json.Marshal(content)
+	if err != nil {
+		// cannot produce infinite recursive loop, since json.Marshal of dto.InternalServerError won't return an error
+		writeInternalServerError(writer, err, dto.ErrorUnknown)
+		return
+	}
+	if _, err = writer.Write(response); err != nil {
+		log.WithError(err).Error("Could not write JSON response")
+		http.Error(writer, err.Error(), http.StatusInternalServerError)
+	}
+}
+
+func parseJSONRequestBody(writer http.ResponseWriter, request *http.Request, structure interface{}) error {
+	if err := json.NewDecoder(request.Body).Decode(structure); err != nil {
+		writeBadRequest(writer, err)
+		return fmt.Errorf("error parsing JSON request body: %w", err)
+	}
+	return nil
+}