show names on tabs in tabbed layout

src/shm.zig

@@ -1,8 +1,8 @@
 //! Minimal wl_shm buffer pool for the tab bar.
 //!
 //! The tab bar is the only thing att_wm draws itself, and it draws nothing but
-//! solid rectangles, so this deliberately stops at "memfd, mmap, fill" rather
-//! than pulling in pixman or a font stack.
+//! solid rectangles and text in the bitmap font from `font.zig`, so this
+//! deliberately stops at "memfd, mmap, fill" rather than pulling in pixman.
 
 const std = @import("std");
 const posix = std.posix;
@@ -12,6 +12,7 @@ const sys = @import("sys.zig");
 const wayland = @import("wayland");
 const wl = wayland.client.wl;
 
+const font = @import("font.zig");
 const layout = @import("layout.zig");
 const Box = layout.Box;
 
@@ -58,6 +59,51 @@ pub const Buffer = struct {
         }
     }
 
+    /// Fill a rectangle, clipped to `clip` as well as to the buffer.
+    pub fn fillClipped(self: *Buffer, rect: Box, clip: Box, argb: u32) void {
+        const x0 = @max(rect.x, clip.x);
+        const y0 = @max(rect.y, clip.y);
+        const x1 = @min(rect.x + rect.width, clip.x + clip.width);
+        const y1 = @min(rect.y + rect.height, clip.y + clip.height);
+        if (x1 <= x0 or y1 <= y0) return;
+        self.fill(.{ .x = x0, .y = y0, .width = x1 - x0, .height = y1 - y0 }, argb);
+    }
+
+    /// Draw text with the top left of its first cell at (x, y), clipped to
+    /// `clip`, and return the x the next cell would start at. The font has no
+    /// antialiasing, so lit pixels are simply written: nothing to blend.
+    pub fn drawText(self: *Buffer, text: []const u8, x: i32, y: i32, scale: i32, argb: u32, clip: Box) i32 {
+        var pen = x;
+        var it: font.Iterator = .{ .bytes = text };
+        while (it.next()) |cp| {
+            // Stop as soon as the pen leaves the clip rather than walking the
+            // rest of a long title a pixel at a time.
+            if (pen >= clip.x + clip.width) break;
+            self.drawGlyph(cp, pen, y, scale, argb, clip);
+            pen += font.advance * scale;
+        }
+        return pen;
+    }
+
+    pub fn drawGlyph(self: *Buffer, cp: u21, x: i32, y: i32, scale: i32, argb: u32, clip: Box) void {
+        const rows = font.glyph(cp);
+        for (rows, 0..) |bits, row| {
+            if (bits == 0) continue;
+            const py = y + @as(i32, @intCast(row)) * scale;
+            var col: u3 = 0;
+            while (col < font.cell_width) : (col += 1) {
+                const mask = @as(u8, 1) << @intCast(font.cell_width - 1 - col);
+                if (bits & mask == 0) continue;
+                self.fillClipped(.{
+                    .x = x + @as(i32, col) * scale,
+                    .y = py,
+                    .width = scale,
+                    .height = scale,
+                }, clip, argb);
+            }
+        }
+    }
+
     fn deinit(self: *Buffer, gpa: Allocator) void {
         self.wl_buffer.destroy();
         posix.munmap(self.data);