dotsv2: remove jitters

internal/dotwriter/writer.go

@@ -9,11 +9,9 @@ import (
 	"io"
 )
 
-// ESC is the ASCII code for escape character
-const ESC = 27
-
 // Writer buffers writes until Flush is called. Flush clears previously written
 // lines before writing new lines from the buffer.
+// The main logic is platform specific, see the related files.
 type Writer struct {
 	out       io.Writer
 	buf       bytes.Buffer
@@ -25,19 +23,6 @@ func New(out io.Writer) *Writer {
 	return &Writer{out: out}
 }
 
-// Flush the buffer, writing all buffered lines to out
-func (w *Writer) Flush() error {
-	if w.buf.Len() == 0 {
-		return nil
-	}
-	defer w.hideCursor()()
-	w.clearLines(w.lineCount)
-	w.lineCount = bytes.Count(w.buf.Bytes(), []byte{'\n'})
-	_, err := w.out.Write(w.buf.Bytes())
-	w.buf.Reset()
-	return err
-}
-
 // Write saves buf to a buffer
 func (w *Writer) Write(buf []byte) (int, error) {
 	return w.buf.Write(buf)

internal/dotwriter/writer_posix.go

@@ -4,17 +4,53 @@
 package dotwriter
 
 import (
+	"bytes"
 	"fmt"
-	"strings"
 )
 
-// clear the line and move the cursor up
-var clear = fmt.Sprintf("%c[%dA%c[2K", ESC, 1, ESC)
+// ESC is the ASCII code for escape character
+const ESC = 27
+
+// hide cursor
 var hide = fmt.Sprintf("%c[?25l", ESC)
+
+// show cursor
 var show = fmt.Sprintf("%c[?25h", ESC)
 
-func (w *Writer) clearLines(count int) {
-	_, _ = fmt.Fprint(w.out, strings.Repeat(clear, count))
+// Flush the buffer, writing all buffered lines to out
+func (w *Writer) Flush() error {
+	if w.buf.Len() == 0 {
+		return nil
+	}
+	// Hide cursor during write to avoid it moving around the screen
+	defer w.hideCursor()()
+
+	// Move up to the top of our last output.
+	w.up(w.lineCount)
+	lines := bytes.Split(w.buf.Bytes(), []byte{'\n'})
+	w.lineCount = len(lines) - 1 // Record how many lines we will write for the next Flush()
+	for i, line := range lines {
+		// For each line, write the contents and clear everything else on the line
+		w.out.Write(line)
+		w.clearRest()
+		// Add a newline if this isn't the last line
+		if i != len(lines)-1 {
+			w.out.Write([]byte{'\n'})
+		}
+	}
+	w.buf.Reset()
+	return nil
+}
+
+func (w *Writer) up(count int) {
+	if count == 0 {
+		return
+	}
+	_, _ = fmt.Fprint(w.out, fmt.Sprintf("%c[%dA", ESC, count))
+}
+
+func (w *Writer) clearRest() {
+	_, _ = fmt.Fprint(w.out, fmt.Sprintf("%c[0K", ESC))
 }
 
 // hideCursor hides the cursor and returns a function to restore the cursor back.

internal/dotwriter/writer_windows.go

@@ -35,6 +35,19 @@ type fdWriter interface {
 	Fd() uintptr
 }
 
+// Flush implementation on windows is not ideal; we clear the entire screen before writing, which can result in flashing output
+// Windows likely can adopt the same approach as posix if someone invests some effort
+func (w *Writer) Flush() error {
+	if w.buf.Len() == 0 {
+		return nil
+	}
+	w.clearLines(w.lineCount)
+	w.lineCount = bytes.Count(w.buf.Bytes(), []byte{'\n'})
+	_, err := w.out.Write(w.buf.Bytes())
+	w.buf.Reset()
+	return err
+}
+
 func (w *Writer) clearLines(count int) {
 	f, ok := w.out.(fdWriter)
 	if ok && !isConsole(f.Fd()) {
@@ -71,8 +84,3 @@ func isConsole(fd uintptr) bool {
 	err := windows.GetConsoleMode(windows.Handle(fd), &mode)
 	return err == nil
 }
-
-// This may work on Windows but I am not sure how to do it and its optional. For now, just do nothing.
-func (w *Writer) hideCursor() func() {
-	return func() {}
-}