From d44801de76d95d8351006c1c232d1ffc27e8af5d Mon Sep 17 00:00:00 2001
From: Felix Angell <felixangell@members.fsf.org>
Date: Sun, 29 Apr 2018 01:03:45 +0100
Subject: [PATCH] only render visible characters (i.e. horizontal line culling)

---
 gui/buffer.go | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/gui/buffer.go b/gui/buffer.go
index df026fa..96e8b59 100644
--- a/gui/buffer.go
+++ b/gui/buffer.go
@@ -763,6 +763,13 @@ var (
 	CAPS_LOCK         = false
 )
 
+func min(a, b int) int {
+	if a < b {
+		return a
+	}
+	return b
+}
+
 // TODO(Felix) this is really stupid
 func (b *Buffer) makeTab() string {
 	blah := []rune{}
@@ -929,7 +936,7 @@ func (b *Buffer) renderAt(ctx *strife.Renderer, rx int, ry int) {
 		ctx.Rect(highlightLinePosX, highlightLinePosY, b.w-ex, last_h, strife.Fill)
 	}
 
-	var visibleLines int = 50
+	var visibleLines, visibleChars int = 50, -1
 
 	// HACK
 	// force a render off screen
@@ -949,6 +956,12 @@ func (b *Buffer) renderAt(ctx *strife.Renderer, rx int, ry int) {
 		visibleLines = (int(b.h) / int(last_h)) + 3
 	}
 
+	// calculate how many chars we can fit
+	// on the screen.
+	if int(last_w) > 0 && int(b.w) != 0 {
+		visibleChars = (int(b.w-ex) / int(last_w)) - 3
+	}
+
 	start := b.cam.y
 	upper := b.cam.y + visibleLines
 	if start > len(b.contents) {
@@ -969,6 +982,9 @@ func (b *Buffer) renderAt(ctx *strife.Renderer, rx int, ry int) {
 	for lineNum, rope := range b.contents[start:upper] {
 		currLine := []rune(rope.String())
 
+		// slice the visible characters only.
+		currLine = currLine[:min(visibleChars, len(currLine))]
+
 		// char index => colour
 		var matches map[int]syntaxRuneInfo
 		if b.languageInfo != nil && len(currLine) > 0 {