Improve memory usage for large files

Test:
* Open a 35MB text file
* Search it for some string it doesn't contain

Memory usage before this change: 2.3GB
Memory usage after this change:  0.5GB

The trick is to only provide cells from Lines on demand, since we only
need a few of them when scrolling.

And not storing all of those cells is where the gain in memory usage
comes from.

m/ansiTokenizer.go

@@ -21,10 +21,8 @@ var sgrSequencePattern = regexp.MustCompile("\x1b\\[([0-9;]*m)")
 
 // A Line represents a line of text that can / will be paged
 type Line struct {
-	done  bool
 	raw   *string
 	plain *string
-	cells []twin.Cell
 }
 
 // NewLine creates a new Line from a (potentially ANSI / man page formatted) string
@@ -32,14 +30,13 @@ func NewLine(raw string) *Line {
 	return &Line{
 		raw:   &raw,
 		plain: nil,
-		cells: nil,
 	}
 }
 
 // Tokens returns a representation of the string split into styled tokens
 func (line *Line) Tokens() []twin.Cell {
-	line.parse()
-	return line.cells
+	cells, _ := cellsFromString(*line.raw)
+	return cells
 }
 
 // Plain returns a plain text representation of the initial string
@@ -49,13 +46,12 @@ func (line *Line) Plain() string {
 }
 
 func (line *Line) parse() {
-	if line.done {
+	if line.plain != nil {
 		// Already done
 		return
 	}
 
-	line.cells, line.plain = cellsFromString(*line.raw)
-	line.done = true
+	_, line.plain = cellsFromString(*line.raw)
 }
 
 // SetManPageFormatFromEnv parses LESS_TERMCAP_xx environment variables and

m/embed-api.go

@@ -2,6 +2,7 @@ package m
 
 import (
 	"fmt"
+
 	"github.com/walles/moar/twin"
 )
 
@@ -18,7 +19,7 @@ func (p *Pager) Page() error {
 			return
 		}
 
-		// FIXME: Consider moving this logic into the twin package.
+		// FIXME: Consider moving this logic into the twin package, or into Pager.
 		_, height := p.screen.Size()
 		lines := p.reader.GetLines(p.firstLineOneBased, height-1).lines
 		for _, line := range lines {