From b9f9aec88b8a3a3b478f34932a834a94e889f769 Mon Sep 17 00:00:00 2001
From: Aaron Fischer <aaron.fischer@marbis.net>
Date: Fri, 21 Sep 2018 17:05:22 +0200
Subject: [PATCH] Add vertical scolling

---
 kilo.c | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/kilo.c b/kilo.c
index 60bfd7a..bf0bbca 100644
--- a/kilo.c
+++ b/kilo.c
@@ -42,6 +42,7 @@ typedef struct erow {
 
 struct editorConfig {
   int cx, cy;
+  int rowoff;
   int screenrows;
   int screencols;
   int numrows;
@@ -222,10 +223,20 @@ 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) {
+    int filerow = y + E.rowoff;
+    if (filerow >= E.numrows) {
       if (E.numrows == 0 && y == E.screenrows / 3) {
         char welcome[80];
         int welcomelen = snprintf(welcome, sizeof(welcome),
@@ -242,9 +253,9 @@ void editorDrawRows(struct abuf *ab) {
         abAppend(ab, "~", 1);
       }
     } else {
-      int len = E.row[y].size;
+      int len = E.row[filerow].size;
       if (len > E.screencols) len = E.screencols;
-      abAppend(ab, E.row[y].chars, len);
+      abAppend(ab, E.row[filerow].chars, len);
     }
 
     abAppend(ab, "\x1b[K", 3); // Erase one line
@@ -255,6 +266,8 @@ void editorDrawRows(struct abuf *ab) {
 }
 
 void editorRefreshScreen() {
+  editorScroll();
+
   struct abuf ab = ABUF_INIT;
 
   abAppend(&ab, "\x1b[?25l", 6); // Hide cursor
@@ -264,7 +277,7 @@ void editorRefreshScreen() {
 
   // Update cursor position
   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); // Show cursor
@@ -293,7 +306,7 @@ void editorMoveCursor(int key) {
       }
       break;
     case ARROW_DOWN:
-      if (E.cy != E.screenrows -1) {
+      if (E.cy < E.numrows) {
         E.cy++;
       }
       break;
@@ -342,6 +355,7 @@ void editorProcessKeypress() {
 void initEditor() {
   E.cx = 0;
   E.cy = 0;
+  E.rowoff = 0;
   E.numrows = 0;
   E.row = NULL;