render: implement double buffered screen for rendering

This lets us efficiently render by only updating cells that have changed
since last render

Signed-off-by: Tim Culverhouse <tim@timculverhouse.com>

examples/main.zig

@@ -39,6 +39,7 @@ pub fn main() !void {
         }
 
         const win = vx.window();
+        win.clear();
         const child = win.initChild(win.width / 2 - msg.len / 2, win.height / 2, .expand, .expand);
         for (msg, 0..) |_, i| {
             const cell: Cell = .{ .char = .{ .grapheme = msg[i .. i + 1] } };

src/Screen.zig

@@ -7,16 +7,16 @@ const log = std.log.scoped(.screen);
 
 const Screen = @This();
 
-width: usize,
-height: usize,
+width: usize = 0,
+height: usize = 0,
 
 buf: []Cell = undefined,
 
-pub fn init() Screen {
-    return Screen{
-        .width = 0,
-        .height = 0,
-    };
+/// sets each cell to the default cell
+pub fn init(self: *Screen) void {
+    for (self.buf, 0..) |_, i| {
+        self.buf[i] = .{};
+    }
 }
 
 pub fn deinit(self: *Screen, alloc: std.mem.Allocator) void {
@@ -27,9 +27,6 @@ pub fn resize(self: *Screen, alloc: std.mem.Allocator, w: usize, h: usize) !void
     log.debug("resizing screen: width={d} height={d}", .{ w, h });
     alloc.free(self.buf);
     self.buf = try alloc.alloc(Cell, w * h);
-    for (self.buf, 0..) |_, i| {
-        self.buf[i] = .{};
-    }
     self.width = w;
     self.height = h;
 }

src/Tty.zig

@@ -173,6 +173,11 @@ pub fn run(
     }
 }
 
+const Writer = std.io.Writer(os.fd_t, os.WriteError, os.write);
+
+pub fn writer(self: *Tty) Writer {
+    return .{ .context = self.fd };
+}
 /// write to the tty
 //
 // TODO: buffer the writes

src/Window.zig

@@ -67,6 +67,16 @@ pub fn writeCell(self: Window, col: usize, row: usize, cell: Cell) void {
     self.screen.writeCell(col + self.x_off, row + self.y_off, cell);
 }
 
+pub fn clear(self: Window) void {
+    var row: usize = self.y_off;
+    while (row < (self.height + self.y_off)) : (row += 1) {
+        var col: usize = self.x_off;
+        while (col < (self.width + self.x_off)) : (col += 1) {
+            self.screen.writeCell(col, row, .{});
+        }
+    }
+}
+
 test "Window size set" {
     var parent = Window{
         .x_off = 0,

src/vaxis.zig

@@ -34,6 +34,9 @@ pub fn Vaxis(comptime T: type) type {
         tty: ?Tty,
 
         screen: Screen,
+        // The last screen we drew. We keep this so we can efficiently update on
+        // the next render
+        screen_last: Screen,
 
         alt_screen: bool,
 
@@ -42,7 +45,8 @@ pub fn Vaxis(comptime T: type) type {
             return Self{
                 .queue = .{},
                 .tty = null,
-                .screen = Screen.init(),
+                .screen = .{},
+                .screen_last = .{},
                 .alt_screen = false,
             };
         }
@@ -59,7 +63,10 @@ pub fn Vaxis(comptime T: type) type {
                 }
                 tty.deinit();
             }
-            if (alloc) |a| self.screen.deinit(a);
+            if (alloc) |a| {
+                self.screen.deinit(a);
+                self.screen_last.deinit(a);
+            }
         }
 
         /// spawns the input thread to start listening to the tty for input
@@ -94,6 +101,10 @@ pub fn Vaxis(comptime T: type) type {
         /// freed when resizing
         pub fn resize(self: *Self, alloc: std.mem.Allocator, winsize: Winsize) !void {
             try self.screen.resize(alloc, winsize.cols, winsize.rows);
+            // we only init our current screen. This has the effect of redrawing
+            // every cell
+            self.screen.init();
+            try self.screen_last.resize(alloc, winsize.cols, winsize.rows);
         }
 
         /// returns a Window comprising of the entire terminal screen
@@ -127,8 +138,30 @@ pub fn Vaxis(comptime T: type) type {
         pub fn render(self: *Self) !void {
             var tty = self.tty orelse return;
 
+            // TODO: optimize writes
+
+            // Send the cursor to 0,0
             _ = try tty.write(ctlseqs.home);
-            for (self.screen.buf) |cell| {
+            var reposition: bool = false;
+            var row: usize = 0;
+            var col: usize = 0;
+            for (self.screen.buf, 0..) |cell, i| {
+                col += 1;
+                if (col == self.screen.width) {
+                    row += 1;
+                    col = 0;
+                }
+                // If cell is the same as our last frame, we don't need to do
+                // anything
+                if (std.meta.eql(cell, self.screen_last.buf[i])) {
+                    reposition = true;
+                    continue;
+                }
+                // Set this cell in the last frame
+                self.screen_last.buf[i] = cell;
+                if (reposition) {
+                    try std.fmt.format(tty.writer(), ctlseqs.cup, .{ row + 1, col + 1 });
+                }
                 _ = try tty.write(cell.char.grapheme);
             }
         }