Eliminate double file read in diff mode

main.go

@@ -128,15 +128,6 @@ var openFile = func(name string) (file, error) {
 	return os.OpenFile(name, os.O_RDWR, 0666)
 }
 
-func readFile(path string) ([]byte, error) {
-	f, err := openFile(path)
-	if err != nil {
-		return nil, err
-	}
-	defer f.Close()
-	return io.ReadAll(f)
-}
-
 func processFile(path string, rewrite, doDiff bool) (foundDiff bool, err error) {
 	if filepath.Ext(path) != ".md" {
 		return false, fmt.Errorf("not a markdown file")
@@ -148,17 +139,19 @@ func processFile(path string, rewrite, doDiff bool) (foundDiff bool, err error)
 	}
 	defer f.Close()
 
+	var original bytes.Buffer
+	var r io.Reader = f
+	if doDiff {
+		r = io.TeeReader(f, &original)
+	}
+
 	buf := new(bytes.Buffer)
-	if err := embedmd.Process(buf, f, embedmd.WithBaseDir(filepath.Dir(path))); err != nil {
+	if err := embedmd.Process(buf, r, embedmd.WithBaseDir(filepath.Dir(path))); err != nil {
 		return false, err
 	}
 
 	if doDiff {
-		f, err := readFile(path)
-		if err != nil {
-			return false, fmt.Errorf("could not read %s for diff: %v", path, err)
-		}
-		data, err := diff(string(f), buf.String())
+		data, err := diff(original.String(), buf.String())
 		if err != nil || len(data) == 0 {
 			return false, err
 		}

main_test.go

@@ -99,7 +99,7 @@ func TestEmbedFiles(t *testing.T) {
 		{name: "diffing a single file",
 			in:  "one\ntwo\nthree",
 			d:   true,
-			out: "@@ -1 +1,4 @@\n+one\n+two\n+three\n \n",
+			out: "@@ -1,3 +1,4 @@\n one\n two\n three\n+\n",
 		},
 	}