Scrolling performance improvement.

Every time the user scrolls, DrawLine is called many times. Each time
this happens, termbox.Flush is called, leading to unnecessary cpu usage.
This is eliminated by only calling termbox.Flush at the end of all the
DrawLine calls.

screen.go

@@ -92,7 +92,7 @@ func (screen *Screen) ClearLine(x int, y int) *Screen {
 // Increase the offset for scrolling feature by n
 // Takes number of tickers as max, so not scrolling down forever
 func (screen *Screen) IncreaseOffset(n int, max int) {
-	if screen.offset+n+1 < max {
+	if screen.offset + n +1 < max {
 		screen.offset += n
 	}
 }
@@ -142,6 +142,8 @@ func (screen *Screen) Draw(objects ...interface{}) *Screen {
 		}
 	}
 
+    termbox.Flush()
+
 	return screen
 }
 
@@ -170,6 +172,31 @@ func (screen *Screen) DrawLine(x int, y int, str string) {
 	termbox.Flush()
 }
 
+// Identical to DrawLine, no flush at the end perhaps should be a part
+//  of DrawLine, with a flush parameter, or a wrapper function could be
+//  used
+func (screen *Screen) DrawLineWithoutFlush(x int, y int, str string) {
+	start, column := 0, 0
+
+	for _, token := range screen.markup.Tokenize(str) {
+		// First check if it's a tag. Tags are eaten up and not displayed.
+		if screen.markup.IsTag(token) {
+			continue
+		}
+
+		// Here comes the actual text: display it one character at a time.
+		for i, char := range token {
+			if !screen.markup.RightAligned {
+				start = x + column
+				column++
+			} else {
+				start = screen.width - len(token) + i
+			}
+			termbox.SetCell(start, y, char, screen.markup.Foreground, screen.markup.Background)
+		}
+	}
+}
+
 // Underlying workhorse function that takes multiline string, splits it into
 // lines, and displays them row by row.
 func (screen *Screen) draw(str string, offset bool) {
@@ -197,7 +224,7 @@ func (screen *Screen) draw(str string, offset bool) {
 					strings.Contains(allLines[row], "Change") {
 					drewHeading = true
 					screen.headerLine = row
-					screen.DrawLine(0, row, allLines[row])
+					screen.DrawLineWithoutFlush(0, row, allLines[row])
 					// move on to the point to offset to
 					row += screen.offset
 				}
@@ -205,13 +232,13 @@ func (screen *Screen) draw(str string, offset bool) {
 				// only write the necessary lines
 				if row <= len(allLines) &&
 					row > screen.headerLine {
-					screen.DrawLine(0, row-screen.offset, allLines[row])
+					screen.DrawLineWithoutFlush(0, row-screen.offset, allLines[row])
 				} else if row > len(allLines) {
 					row = len(allLines)
 				}
 			}
 		} else {
-			screen.DrawLine(0, row, allLines[row])
+			screen.DrawLineWithoutFlush(0, row, allLines[row])
 		}
 	}
 	// If the quotes lines in this cycle are shorter than in the previous
@@ -227,8 +254,10 @@ func (screen *Screen) draw(str string, offset bool) {
 	// cycle.  In that case, padding with blank lines would overwrite the
 	// stocks list.)
 	if drewHeading {
-		for i := len(allLines) - 1 - screen.offset; i < screen.height; i++ {
+		for i := len(allLines) - 1 - screen.offset ; i < screen.height; i++ {
-			screen.DrawLine(0, i, blankLine)
+            if i > screen.headerLine {
+                screen.DrawLine(0, i, blankLine)
+            }
 		}
 	}
 }