From 221a6569e89149022feb03f9538231e0f31e2d47 Mon Sep 17 00:00:00 2001
From: Tyrel Souza <923113+tyrelsouza@users.noreply.github.com>
Date: Thu, 21 Oct 2021 00:26:42 -0400
Subject: [PATCH] vertical scrolling

---
 README.md |  3 +++
 tted.c    | 22 +++++++++++++++++-----
 2 files changed, 20 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index 8ffd262..bace049 100644
--- a/README.md
+++ b/README.md
@@ -1 +1,4 @@
 Tyrel's Text Editor
+
+
+https://viewsourcecode.org/snaptoken/kilo/04.aTextViewer.html#vertical-scrolling
diff --git a/tted.c b/tted.c
index 6585cad..035ada6 100644
--- a/tted.c
+++ b/tted.c
@@ -220,10 +220,20 @@ void abFree(struct abuf *ab) {
 
 
 /** output */
+void editorScroll() {
+    if (E.cy < E.rowoff){
+        E.rowoff = E.cy;
+    }
+    if (E.cy >= E.rowoff + E.screenrows) {
+        E.rowoff = E.cy - E.screenrows +1;
+    }
+}
+
 void editorDrawRows(struct abuf *ab) {
     int y;
     for (y = 0; y < E.screenrows; y++) {
-        if (y >= E.numrows) { // Draw rows after buffer
+        int filerow = y + E.rowoff;
+        if (filerow >= E.numrows) { // Draw rows after buffer
             if (E.numrows == 0 && y == E.screenrows / 3) {
                 char welcome[80];
                 int welcomelen = snprintf(welcome, sizeof welcome, "Tyrel Text Editor Deluxe -- version %s",
@@ -240,9 +250,9 @@ void editorDrawRows(struct abuf *ab) {
                 abAppend(ab, "~", 1);
             }
         } else { // draw buffer
-            int len = E.row[y].size;
+            int len = E.row[filerow].size;
             if (len > E.screencols) len = E.screencols; // truncate if further than columns
-            abAppend(ab, E.row[y].chars, len);
+            abAppend(ab, E.row[filerow].chars, len);
         }
 
         abAppend(ab, "\x1b[K", 3);
@@ -253,13 +263,15 @@ void editorDrawRows(struct abuf *ab) {
 }
 
 void editorRefreshScreen() {
+    editorScroll();
+
     struct abuf ab = ABUF_INIT;
     abAppend(&ab, "\x1b[?25l", 6);
     abAppend(&ab, "\x1b[H", 3); // Cursor Position home
     editorDrawRows(&ab);
 
     char buf[32];
-    snprintf(buf, sizeof(buf), "\x1b[%d;%dH", E.cy + 1, E.cx + 1);
+    snprintf(buf, sizeof(buf), "\x1b[%d;%dH", (E.cy - E.rowoff) +1, E.cx + 1);
     abAppend(&ab, buf, strlen(buf));
     abAppend(&ab, "\x1b[?25h", 6);
 
@@ -287,7 +299,7 @@ void editorMoveCursor(int key) {
             }
             break;
         case ARROW_DOWN:
-            if (E.cy != E.screenrows - 1) {
+            if (E.cy != E.numrows) {
                 E.cy++;
             }
             break;