Basic scrolling up or down a file

editor/draw.go

@@ -12,6 +12,8 @@ import (
 // ----------------------------------------------------------------------------
 
 func (e *editor) RefreshScreen() {
+	e.scroll()
+
 	ab := newBuffer()
 
 	ab.append(terminal.HideCursor)
@@ -25,12 +27,24 @@ func (e *editor) RefreshScreen() {
 	terminal.Write(ab.toString())
 }
 
+func (e *editor) scroll() {
+	if e.cursor.y < e.offset.y {
+		e.offset.y = e.cursor.y
+	}
+
+	if e.cursor.y >= e.offset.y + e.screen.Rows {
+		e.offset.y = e.cursor.y - e.screen.Rows + 1
+	}
+}
+
 func (e *editor) drawRows(ab *buffer) {
 	for y :=0; y < e.screen.Rows; y++ {
-		if y >= len(e.rows) {
+		fileRow := y + e.offset.y
+
+		if fileRow >= len(e.rows) {
 			e.drawPlaceholderRow(y, ab)
 		} else {
-			row := truncateString(string(e.rows[y].chars), e.screen.Cols)
+			row := truncateString(string(e.rows[fileRow].chars), e.screen.Cols)
 			ab.append(row)
 		}
 

editor/editor.go

@@ -11,23 +11,30 @@ import (
 // !Editor
 // ----------------------------------------------------------------------------
 
-type cursor struct {
+type point struct {
 	x int
 	y int
 }
 
 type editor struct {
 	screen *terminal.Screen
-	cursor *cursor
+	cursor *point
+	offset *point
 	rows []*row
 }
 
 func New() *editor {
 	screen := terminal.Size()
-	cursor := &cursor { 0, 0 }
+	cursor := &point { 0, 0 }
+	offset := &point { 0, 0 }
 	var rows []*row
 
-	return &editor{screen, cursor, rows }
+	return &editor{
+		screen,
+		cursor,
+		offset,
+		rows,
+	}
 }
 
 func (e *editor) Open(filename string) {
@@ -92,7 +99,7 @@ func (e *editor) moveCursor (key string) {
 			e.cursor.y -= 1
 		}
 	case keyDown:
-		if e.cursor.y != e.screen.Rows-1 {
+		if e.cursor.y < len(e.rows) {
 			e.cursor.y += 1
 		}
 	case keyPageUp:
@@ -106,6 +113,11 @@ func (e *editor) moveCursor (key string) {
 	}
 }
 
+func (e *editor) appendRow(s string) {
+	newRow := NewRow(s)
+	e.rows = append(e.rows, newRow)
+}
+
 // Convert the raw ANSI escape sequences to the type of key input
 func parseEscapeSequence () string {
 	var runes []rune
@@ -169,8 +181,3 @@ func parseEscapeSequence () string {
 
 	return string('\x1b')
 }
-
-func (e *editor) appendRow(s string) {
-	newRow := NewRow(s)
-	e.rows = append(e.rows, newRow)
-}