Avoid rebuilding the sketch if the sketch file is unchanged.

Author: cmaglie

internal/arduino/builder/internal/preprocessor/ctags.go

@@ -55,8 +55,20 @@ func PreprocessSketchWithCtags(
 
 	stdout, stderr := &bytes.Buffer{}, &bytes.Buffer{}
 
-	// Run GCC preprocessor
+	// Check if the preprocessed file is already up-to-date
+	unpreprocessedSourceFile := buildPath.Join("sketch", sketch.MainFile.Base()+".cpp.merged")
 	sourceFile := buildPath.Join("sketch", sketch.MainFile.Base()+".cpp")
+	if unpreprocessedStat, err := unpreprocessedSourceFile.Stat(); err != nil {
+		return nil, fmt.Errorf("%s: %w", i18n.Tr("unable to open unpreprocessed source file"), err)
+	} else if sourceStat, err := sourceFile.Stat(); err != nil {
+		return nil, fmt.Errorf("%s: %w", i18n.Tr("unable to open source file"), err)
+	} else if unpreprocessedStat.ModTime().Before(sourceStat.ModTime()) {
+		fmt.Fprintln(stdout, i18n.Tr("Sketch is unchanged, skipping preprocessing."))
+		res := &runner.Result{Stdout: stdout.Bytes(), Stderr: stderr.Bytes()}
+		return res, nil
+	}
+
+	// Run GCC preprocessor
 	result := GCC(sourceFile, ctagsTarget, includes, buildProperties).Run(ctx)
 	stdout.Write(result.Stdout)
 	stderr.Write(result.Stderr)

internal/arduino/builder/sketch.go

@@ -50,9 +50,21 @@ func (b *Builder) prepareSketchBuildPath() error {
 		return err
 	}
 
-	destFile := b.sketchBuildPath.Join(b.sketch.MainFile.Base() + ".cpp")
-	if err := destFile.WriteFile([]byte(mergedSource)); err != nil {
-		return err
+	// Save the unpreprocessed merged source to a file named sketch.cpp.merged.
+	destFileUnpreprocessed := b.sketchBuildPath.Join(b.sketch.MainFile.Base() + ".cpp.merged")
+	oldUnpreprocessedSource, _ := destFileUnpreprocessed.ReadFile()
+
+	// If the merged source is unchanged, skip writing it.
+	// This avoids unnecessary rebuilds and keeps the build path clean.
+	if !bytes.Equal(oldUnpreprocessedSource, []byte(mergedSource)) {
+		if err := destFileUnpreprocessed.WriteFile([]byte(mergedSource)); err != nil {
+			return err
+		}
+
+		destFile := b.sketchBuildPath.Join(b.sketch.MainFile.Base() + ".cpp")
+		if err := destFileUnpreprocessed.CopyTo(destFile); err != nil {
+			return err
+		}
 	}
 
 	if err := b.sketchCopyAdditionalFiles(b.sketchBuildPath, b.sourceOverrides); err != nil {