testSplatBytePreserve

fn testSplatBytePreserve(options: struct { buf_len: u4, fill_len: u4, preserve: u4, splat_len: u8 }) !void {
    assert(options.fill_len <= options.buf_len);
    assert(options.preserve <= options.buf_len);

    const fill_buf = "abcdefghijklmno";
    const fill = fill_buf[0..options.fill_len];
    var expected_out_buf: [256]u8 = @splat('X');
    @memcpy(expected_out_buf[0..options.fill_len], fill);
    const expected_out = expected_out_buf[0 .. options.fill_len + options.splat_len];
    const expected_preserved = expected_out[expected_out.len -| options.preserve..];

    var out_buf: [256]u8 = undefined;
    var fw: Writer = .fixed(&out_buf);
    var indirect_buffer: [16]u8 = undefined;
    var twi: std.testing.WriterIndirect = .init(&fw, indirect_buffer[0..options.buf_len]);
    const w = &twi.interface;

    try w.writeAll(fill);
    try w.splatBytePreserve(options.preserve, 'X', options.splat_len);

    try std.testing.expectEqualStrings(expected_preserved, w.buffer[w.end -| options.preserve..w.end]);

    try w.flush();

    try std.testing.expectEqualStrings(expected_out, fw.buffer[0..fw.end]);
}