Speed up window drawing.

We were repeatedly calling gb_linecol_to_pos for each cell, which is
linear in the number of lines. gb_linecol_to_pos could be optimized, but
for now just inline the work and do it incrementally.

Closes #54.

Author: isbadawi

draw.c

@@ -313,17 +313,36 @@ static void window_draw_leaf(struct window *window, struct editor *editor) {
   struct syntax syntax;
   struct syntax_token token = {SYNTAX_TOKEN_NONE, 0, 0};
   bool highlight = syntax_init(&syntax, window->buffer);
+
+  size_t line_pos = 0;
+  for (size_t i = 0; i < window->top; ++i) {
+    line_pos += gb->lines->buf[i] + 1;
+  }
+
   for (size_t y = 0; y < rows; ++y) {
     size_t line = y + window->top;
     window_draw_line_number(window, line);
 
-    size_t cols = (size_t) max(0,
-        min((ssize_t) gb_utf8len_line(gb, line) - (ssize_t) window->left, (ssize_t) w));
+    if (y > 0) {
+      line_pos += gb->lines->buf[line - 1] + 1;
+    }
+
     size_t tabs = 0;
+    size_t linelen = gb_utf8len_line(gb, line_pos);
+    size_t cols = (size_t)max(0,
+        min((ssize_t)linelen - (ssize_t)window->left, (ssize_t)w));
+
+    size_t pos = line_pos;
+    for (size_t i = 0; i < window->left; ++i) {
+      pos += gb_utf8len(gb, pos);
+    }
+
     for (size_t x = 0; x < cols; ++x) {
       size_t x_offset = tabs * (tabstop - 1) + numberwidth + x;
-      size_t col = x + window->left;
-      size_t pos = gb_linecol_to_pos(gb, line, col);
+
+      if (x > 0) {
+        pos += gb_utf8len(gb, pos);
+      }
 
       tb_color fg = COLOR_WHITE;
       tb_color bg = COLOR_DEFAULT;

gap.c

@@ -296,8 +296,7 @@ size_t gb_linecol_to_pos(struct gapbuf *gb, size_t line, size_t column) {
   return offset;
 }
 
-size_t gb_utf8len_line(struct gapbuf *gb, size_t line) {
-  size_t pos = gb_linecol_to_pos(gb, line, 0);
+size_t gb_utf8len_line(struct gapbuf *gb, size_t pos) {
   size_t len = 0;
   while (gb_getchar(gb, pos) != '\n') {
     pos += gb_utf8len(gb, pos);

gap.h

@@ -56,7 +56,7 @@ size_t gb_utf8next(struct gapbuf *gb, size_t pos);
 // Returns the offset of the previous utf8 start byte before pos.
 size_t gb_utf8prev(struct gapbuf *gb, size_t pos);
 // Returns the number of the unicode codepoints for the given line.
-size_t gb_utf8len_line(struct gapbuf *gb, size_t line);
+size_t gb_utf8len_line(struct gapbuf *gb, size_t line_pos);
 
 // Reads n characters starting at offset pos.
 struct buf *gb_getstring(struct gapbuf *gb, size_t pos, size_t n);