Add GET handler for diagram generation

internal/http.go

@@ -5,6 +5,8 @@ import (
 	"fmt"
 	"io/ioutil"
 	"net/http"
+	"net/url"
+	"strings"
 )
 
 func writeJSON(rw http.ResponseWriter, value interface{}, status int) {
@@ -33,31 +35,76 @@ func writeErr(rw http.ResponseWriter, err error, status int) {
 	}, status)
 }
 
+// URLParam is the URL parameter getDiagramFromGET uses to look for data.
+const URLParam = "data"
+
+func getDiagramFromGET(rw http.ResponseWriter, r *http.Request) *Diagram {
+	if r.Method != http.MethodGet {
+		writeErr(rw, fmt.Errorf("expected HTTP method GET"), http.StatusBadRequest)
+		return nil
+	}
+
+	queryVal := strings.TrimSpace(r.URL.Query().Get(URLParam))
+	if queryVal == "" {
+		writeErr(rw, fmt.Errorf("missing data"), http.StatusBadRequest)
+		return nil
+	}
+	data, err := url.QueryUnescape(queryVal)
+	if err != nil {
+		writeErr(rw, fmt.Errorf("could not read query param: %s", err), http.StatusBadRequest)
+		return nil
+	}
+
+	// Create a diagram from the description
+	d := NewDiagram([]byte(data))
+	return d
+}
+
+func getDiagramFromPOST(rw http.ResponseWriter, r *http.Request) *Diagram {
+	if r.Method != http.MethodPost {
+		writeErr(rw, fmt.Errorf("expected HTTP method POST"), http.StatusBadRequest)
+		return nil
+	}
+	// Get description from request body
+	bytes, err := ioutil.ReadAll(r.Body)
+	if err != nil {
+		writeErr(rw, fmt.Errorf("could not read body: %s", err), http.StatusInternalServerError)
+		return nil
+	}
+
+	// Create a diagram from the description
+	d := NewDiagram(bytes)
+	return d
+}
+
+// GenerateHTTPHandler returns a HTTP handler used to generate a diagram.
 func GenerateHTTPHandler(generator Generator) func(rw http.ResponseWriter, r *http.Request) {
 	return func(rw http.ResponseWriter, r *http.Request) {
-		if r.Method != http.MethodPost {
-			writeErr(rw, fmt.Errorf("expected HTTP method POST"), http.StatusBadRequest)
-			return
-		}
-		// Get description from request body
-		bytes, err := ioutil.ReadAll(r.Body)
-		if err != nil {
-			writeErr(rw, fmt.Errorf("could not read body: %s", err), http.StatusInternalServerError)
-			return
-		}
+		var diagram *Diagram
 
-		// Create a diagram from the description
-		d := NewDiagram(bytes)
+		switch r.Method {
+		case http.MethodGet:
+			diagram = getDiagramFromGET(rw, r)
+		case http.MethodPost:
+			diagram = getDiagramFromPOST(rw, r)
+		default:
+			writeErr(rw, fmt.Errorf("unexpected HTTP method %s", r.Method), http.StatusBadRequest)
+			return
+		}
+		if diagram == nil {
+			writeErr(rw, fmt.Errorf("could not create diagram"), http.StatusInternalServerError)
+			return
+		}
 
 		// Generate the diagram
-		if err := generator.Generate(d); err != nil {
+		if err := generator.Generate(diagram); err != nil {
 			writeErr(rw, fmt.Errorf("could not generate diagram: %s", err), http.StatusInternalServerError)
 			return
 		}
 
 		// Output the diagram as an SVG.
 		// We assume generate always generates an SVG at this point in time.
-		diagramBytes, err := ioutil.ReadFile(d.Output)
+		diagramBytes, err := ioutil.ReadFile(diagram.Output)
 		if err != nil {
 			writeErr(rw, fmt.Errorf("could not read diagram bytes: %s", err), http.StatusInternalServerError)
 			return