Add generated notice at the top of generated file (#55)

* Add generated notice at the top of generated file

* Reword variable name

internal/file/write.go

@@ -1,15 +1,52 @@
 package file
 
-import "os"
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
 
 // WriteAfterTo writes the processed content to the provided filepath.
-func (f *File) WriteAfterTo(filepath string) error {
-	file, err := os.OpenFile(filepath, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
+func (f *File) WriteAfterTo(targetFilePath string) error {
+	file, err := os.OpenFile(targetFilePath, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0644)
 	if err != nil {
 		return err
 	}
 	defer file.Close()
 
-	_, err = file.Write(f.ContentAfter)
+	content := []byte{}
+	content = append(content, f.prepareGeneratedHeader(targetFilePath)...)
+	content = append(content, f.ContentAfter...)
+
+	_, err = file.Write(content)
 	return err
 }
+
+func (f *File) prepareGeneratedHeader(targetFilePath string) []byte {
+	fileType := filepath.Ext(f.FileName)
+
+	comment := `# == %s ==`
+	switch fileType {
+	case ".md":
+		comment = `<!-- == %s == -->`
+	case ".yaml", ".yml":
+		comment = `# == %s ==`
+	default:
+	}
+
+	// Compare directories of each file, rather than file itself, so that we
+	// don't end up with an extra "../".
+	relDir, err := filepath.Rel(filepath.Dir(targetFilePath), filepath.Dir(f.FileName))
+	if err != nil {
+		// This shouldn't happen, but when it does, simply ignore and return an
+		// empty slice.
+		return nil
+	}
+
+	baseFile := fmt.Sprintf("%s/%s", relDir, filepath.Base(f.FileName))
+	note := fmt.Sprintf(`improter-generated-from: %s`, baseFile)
+
+	x := fmt.Sprintf(comment+"\n", note)
+
+	return []byte(x)
+}

testdata/other/demo-generated.md

@@ -0,0 +1,9 @@
+<!-- == improter-generated-from: ../markdown/demo-before.md == -->
+# Markdown Demo
+
+<!-- == imptr: short-description / begin from: ./snippet-description.md#[for-demo] == -->
+This demonstrates how a markdown can import other file content.
+
+Importer is a CLI tool to read and process Importer and Exporter markers.  
+This can be easily integrated into CI/CD and automation setup.
+<!-- == imptr: short-description / end == -->