Make the example more interesting

src/main.zig

@@ -1,6 +1,7 @@
-const std = @import("std");
-
 const mfb = @import("minifb");
+const std = @import("std");
+const min = std.math.min;
+const max = std.math.max;
 
 const Width = 800;
 const Height = 600;
@@ -9,38 +10,102 @@ fn grey(value: u8) u32 {
     return mfb.rgb(value, value, value);
 }
 
+const Coords = struct {
+    x: i32,
+    y: i32,
+};
+
+const State = struct {
+    bgValue: u8,
+    increasing: bool,
+};
+
+fn render(buf: Buffer, state: State) void {
+    buf.drawRectangle(.{.x=0,.y=0}, .{.x=Width, .y=Height}, grey(state.bgValue));
+    buf.drawRectangle(.{.x=Width/3, .y=Height/3}, .{.x=2*Width/3, .y=2*Height/3}, mfb.rgb(255,0,0));
+}
+
+const Buffer = struct {
+    alloc: *std.mem.Allocator,
+    slice: []u32,
+    width: u32,
+    height: u32,
+
+    pub fn init(alloc: *std.mem.Allocator, width: u32, height: u32) anyerror!Buffer {
+        const slice = try alloc.alloc(u32, width*height);
+        return Buffer {
+            .alloc=alloc,
+            .slice=slice,
+            .width=width,
+            .height=height,
+        };
+    }
+
+    pub fn deinit(self: Buffer) void {
+        self.alloc.free(self.slice);
+    }
+
+    pub fn setPixel(self: Buffer, coords: Coords, color: u32) void {
+        const x = @intCast(u32, coords.x);
+        const y = @intCast(u32, coords.y);
+        self.slice[y*self.width + x] = color;
+    }
+
+    pub fn drawRectangle(self: Buffer, a: Coords, b: Coords, color: u32) void {
+        var start = Coords{
+            .x=max(0, min(a.x, b.x)),
+            .y=max(0, min(a.y, b.y)),
+        };
+        var end = Coords{
+            .x=min(@intCast(i32, self.width), max(a.x, b.x)),
+            .y=min(@intCast(i32, self.height), max(a.y, b.y)),
+        };
+
+        var y = start.y;
+        while (y < end.y) {
+            var x = start.x;
+            while (x < end.x) {
+                self.setPixel(.{.x=x, .y=y}, color);
+                x += 1;
+            }
+            y += 1;
+        }
+    }
+};
+
 pub fn main() anyerror!void {
+    var gp_allocator = std.heap.GeneralPurposeAllocator(.{}){};
+    const alloc = &gp_allocator.allocator;
+
     std.log.info("All your codebase are belong to us.", .{});
     var win = mfb.Window.open("Hello minifb-zig", Width, Height) catch unreachable;
     mfb.setTargetFPS(60);
 
-    var color: u32 = grey(0);
-    const deltaColor = grey(1);
-    var increasing = true;
-    var rawBuffer: [Width*Height]u32 = undefined;
-    const buffer = rawBuffer[0..rawBuffer.len];
-    std.mem.set(u32, buffer, color);
+    var state = State {
+        .increasing=true,
+        .bgValue=0,
+    };
+    var buf = try Buffer.init(alloc, Width, Height);
+    defer buf.deinit();
 
+    render(buf, state);
     while (win.waitSync()) {
-        if (increasing) {
-            if (color < grey(255)) {
-                color += deltaColor;
+        if (state.increasing) {
+            if (state.bgValue < 255) {
+                state.bgValue += 1;
             } else {
-                increasing = false;
+                state.increasing = false;
             }
         } else {
-            if (color > grey(0)) {
-                color -= deltaColor;
+            if (state.bgValue > 0) {
+                state.bgValue -= 1;
             } else {
-                increasing = true;
+                state.increasing = true;
             }
         }
-        std.mem.set(u32, buffer, color);
-        var i: u32 = 0;
-        while (i < Width) {
-            buffer[i] = mfb.rgb(255,0,0);
-            i += 1;
-        }
-        _ = win.update(buffer) catch unreachable;
+
+        render(buf, state);
+        // TODO Handle mfb state
+        _ = win.update(buf.slice) catch unreachable;
     }
 }