Adding scroll feature

Adding an 'offset' to the quotes draw function in screen.go in order to
allow moving down the list of tickers, using PgUp/PgDown or the Up/Down
arrow keys.

README.md

@@ -37,6 +37,8 @@ For demonstration purposes Mop comes preconfigured with a number of stock ticker
     g       Group stocks by advancing/declining issues.
     f       Set a filtering expression.
     F       Unset a filtering expression.
+    PgDn    Scroll Down, down arrow key also works.
+    PgUp    Scroll up, up arrow key also works.
     ?       Display help screen.
     esc     Quit mop.
 

cmd/mop/main.go

@@ -33,6 +33,8 @@ NO WARRANTIES OF ANY KIND WHATSOEVER. SEE THE LICENSE FILE FOR DETAILS.
    g       Group stocks by advancing/declining issues.
    o       Change column sort order.
    p       Pause market data and stock updates.
+   PgDn    Scroll Down, down arrow key also works.
+   PgUp    Scroll up, up arrow key also works.
    q       Quit mop.
   esc      Ditto.
 
@@ -52,6 +54,7 @@ func mainLoop(screen *mop.Screen, profile *mop.Profile) {
 	marketQueue := time.NewTicker(12 * time.Second)
 	showingHelp := false
 	paused := false
+	pgUpDownLines := 10
 
 	go func() {
 		for {
@@ -92,6 +95,14 @@ loop:
 					} else if event.Ch == '?' || event.Ch == 'h' || event.Ch == 'H' {
 						showingHelp = true
 						screen.Clear().Draw(help)
+					} else if event.Key == termbox.KeyPgdn ||
+						event.Key == termbox.KeyArrowDown {
+						screen.IncreaseOffset(pgUpDownLines, len(profile.Tickers))
+						screen.Clear().Draw(market, quotes)
+					} else if event.Key == termbox.KeyPgup ||
+						event.Key == termbox.KeyArrowUp {
+						screen.DecreaseOffset(pgUpDownLines)
+						screen.Clear().Draw(market, quotes)
 					}
 				} else if lineEditor != nil {
 					if done := lineEditor.Handle(event); done {

screen.go

@@ -22,6 +22,8 @@ type Screen struct {
 	layout     *Layout    // Pointer to layout (gets created by screen).
 	markup     *Markup    // Pointer to markup processor (gets created by screen).
 	pausedAt   *time.Time // Timestamp of the pause request or nil if none.
+	offset     int        // Offset for scolling
+	headerLine int        // Line number of header for scroll feature
 }
 
 // Initializes Termbox, creates screen along with layout and markup, and
@@ -34,6 +36,7 @@ func NewScreen(profile *Profile) *Screen {
 	screen := &Screen{}
 	screen.layout = NewLayout()
 	screen.markup = NewMarkup(profile)
+	screen.offset = 0
 
 	return screen.Resize()
 }
@@ -86,6 +89,23 @@ func (screen *Screen) ClearLine(x int, y int) *Screen {
 	return 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 < max {
+		screen.offset += n
+	}
+}
+
+// Decrease the offset for scrolling feature by n
+func (screen *Screen) DecreaseOffset(n int) {
+	if screen.offset > n {
+		screen.offset -= n
+	} else {
+		screen.offset = 0
+	}
+}
+
 // Draw accepts variable number of arguments and knows how to display the
 // market data, stock quotes, current time, and an arbitrary string.
 func (screen *Screen) Draw(objects ...interface{}) *Screen {
@@ -97,15 +117,15 @@ func (screen *Screen) Draw(objects ...interface{}) *Screen {
 		switch ptr.(type) {
 		case *Market:
 			object := ptr.(*Market)
-			screen.draw(screen.layout.Market(object.Fetch()))
+			screen.draw(screen.layout.Market(object.Fetch()), false)
 		case *Quotes:
 			object := ptr.(*Quotes)
-			screen.draw(screen.layout.Quotes(object.Fetch()))
+			screen.draw(screen.layout.Quotes(object.Fetch()), true)
 		case time.Time:
 			timestamp := ptr.(time.Time).Format(`3:04:05pm ` + zonename)
 			screen.DrawLine(0, 0, `<right><time>`+timestamp+`</></right>`)
 		default:
-			screen.draw(ptr.(string))
+			screen.draw(ptr.(string), false)
 		}
 	}
 
@@ -139,7 +159,7 @@ func (screen *Screen) DrawLine(x int, y int, str string) {
 
 // Underlying workhorse function that takes multiline string, splits it into
 // lines, and displays them row by row.
-func (screen *Screen) draw(str string) {
+func (screen *Screen) draw(str string, offset bool) {
 	if !screen.cleared {
 		screen.Clear()
 	}
@@ -152,13 +172,24 @@ func (screen *Screen) draw(str string) {
 
 	// Write the lines being updated.
 	for row := 0; row < len(allLines); row++ {
-		screen.DrawLine(0, row, allLines[row])
+		if offset {
             // Did we draw the underlined heading row?  This is a crude
             // check, but--see comments below...
+            // --- Heading row only appears for quotes, so offset is true
 			if strings.Contains(allLines[row], "Ticker") &&
 				strings.Contains(allLines[row], "Last") &&
 				strings.Contains(allLines[row], "Change") {
 				drewHeading = true
+				screen.headerLine = row
+				screen.DrawLine(0, row, allLines[row])
+			} else {
+				if row+screen.offset < len(allLines) &&
+					row > screen.headerLine {
+					screen.DrawLine(0, row, allLines[row+screen.offset])
+				}
+			}
+		} else {
+            screen.DrawLine(0, row, allLines[row])
 		}
 	}
 	// If the quotes lines in this cycle are shorter than in the previous