new -flip arg to rotate 180 degrees

Fixes #4

src/main.zig

@@ -38,6 +38,7 @@ fn usage(args: [][]u8) !void {
 const Options = struct {
     background_filename: [:0]u8,
     device_file: [:0]u8,
+    flip: bool,
 };
 pub fn main() !void {
     defer deinit();
@@ -83,12 +84,15 @@ pub fn main() !void {
             line_inx += 1;
         }
     }
-    const opts = try processArgs(alloc, args, &lines);
-    defer alloc.destroy(opts);
+    const opts = try processArgs(args, &lines);
     if (opts.background_filename.len > 0) try stdout.print("Converting {s}\n", .{opts.background_filename});
     var pixels: [display.WIDTH * display.HEIGHT]u8 = undefined;
     try convertImage(opts.background_filename, &pixels, noTextForLine);
     try addTextToImage(alloc, &pixels, &lines);
+    if (opts.flip) {
+        try stdout.print("Flipping\n", .{});
+        flipImage(&pixels);
+    }
     try bw.flush();
 
     // We should take the linux device file here, then inspect for ttyUSB vs
@@ -114,24 +118,31 @@ fn deinit() void {
     font_arena = null;
 }
 
-fn processArgs(allocator: std.mem.Allocator, args: [][:0]u8, line_array: *[display.LINES]*const [:0]u8) !*Options {
+fn processArgs(args: [][:0]u8, line_array: *[display.LINES]*const [:0]u8) !Options {
     if (args.len < 2) try usage(args);
     const prefix = "/dev/ttyUSB";
-    var opts = try allocator.create(Options);
-    opts.device_file = args[1];
+    var opts = Options{
+        .device_file = args[1],
+        .flip = false,
+        .background_filename = @constCast(""),
+    };
     if (!std.mem.eql(u8, opts.device_file, "-") and !std.mem.startsWith(u8, opts.device_file, prefix)) try usage(args);
 
-    opts.background_filename = @constCast("");
     var is_filename = false;
     var line_number: ?usize = null;
     for (args[1..], 1..) |arg, i| {
+        if (std.mem.eql(u8, "-flip", arg)) {
+            opts.flip = true;
+            continue;
+        }
         if (std.mem.eql(u8, "-bg", arg)) {
             is_filename = true;
             continue;
         }
         if (is_filename) {
+            is_filename = false;
             opts.background_filename = args[i]; // arg capture changes value...
-            break;
+            continue;
         }
         if ((arg[0] == '-' and arg.len > 1) and areDigits(arg[1..])) {
             line_number = try std.fmt.parseInt(usize, arg[1..], 10);
@@ -160,6 +171,15 @@ fn areDigits(bytes: []u8) bool {
     }
     return true;
 }
+fn flipImage(pixels: *[display.WIDTH * display.HEIGHT]u8) void {
+    for (pixels[0..(display.WIDTH * display.HEIGHT / 2)], 0..) |_, i| {
+        const dest_pixel = pixels[pixels.len - 1 - i];
+        const src_pixel = pixels[i];
+
+        pixels[pixels.len - 1 - i] = src_pixel;
+        pixels[i] = dest_pixel;
+    }
+}
 
 fn addTextToImage(allocator: std.mem.Allocator, pixels: *[display.WIDTH * display.HEIGHT]u8, data: []*[]u8) !void {
     var maybe_font_data = try getFontData(allocator, DEFAULT_FONT);