Fix incorrect RLE4 padding checking (#50)

* fix: incorrect padding behaviour

* chore: CHANGELOG.md entry

* chore: add test

* chore: reformat changelog entry a bit

* Update CHANGELOG.md

Co-authored-by: Ralf Fuest <mail@rfuest.de>

* chore: improve readability of `has_padding` calculation for 4bit rle

* chore: tidy up tests

---------

Co-authored-by: Ralf Fuest <mail@rfuest.de>

Author: Ddystopia

CHANGELOG.md

@@ -6,6 +6,10 @@
 
 ## [Unreleased] - ReleaseDate
 
+### Fixed
+
+- [#50](https://github.com/embedded-graphics/tinybmp/pull/50) Fixed handling of padding bytes in absolute mode for RLE4 compressed files.
+
 ### Changed
 
 - **(breaking)** [#49](https://github.com/embedded-graphics/tinybmp/pull/41) Use 1.81 as the MSRV.

src/raw_iter.rs

@@ -413,13 +413,14 @@ impl<'a> Iterator for Rle4Pixels<'a> {
                                     // Delta encoding is unsupported.
                                     return None;
                                 }
-                                _ => {
+                                num_pixels => {
+                                    let num_bytes = num_pixels.div_ceil(2);
                                     // Absolute mode
                                     self.rle_state = RleState::Absolute {
                                         remaining: param.saturating_sub(1),
                                         is_odd: (param % 2) != 0,
                                         // padding if the number of *bytes* is odd
-                                        has_padding: ((param >> 1) % 2) != 0,
+                                        has_padding: num_bytes & 1 != 0,
                                     };
                                 }
                             }

tests/embedded_graphics.rs

@@ -1,7 +1,7 @@
 use embedded_graphics::{
-    image::Image,
+    image::{Image, ImageRawLE},
     mock_display::{ColorMapping, MockDisplay},
-    pixelcolor::{Gray8, Rgb555, Rgb565, Rgb888},
+    pixelcolor::{Bgr888, Gray8, Rgb555, Rgb565, Rgb888},
     prelude::*,
     primitives::Rectangle,
 };
@@ -149,3 +149,21 @@ fn issue_8_height_is_negative() {
     bottom_up_display.assert_pattern(&["WK", "KK"]);
     top_down_display.assert_eq(&bottom_up_display);
 }
+
+/// Test for PR #50
+#[test]
+fn pr_50_rle4_padding() {
+    let bmp = Bmp::<Bgr888>::from_slice(include_bytes!("pr_50_rle4_padding.bmp")).unwrap();
+    let raw = ImageRawLE::<Bgr888>::new(include_bytes!("pr_50_rle4_padding.raw"), 20);
+
+    assert_eq!(raw.size(), Size::new(20, 20));
+    assert_eq!(bmp.size(), Size::new(20, 20));
+
+    let mut display_bmp = MockDisplay::new();
+    let mut display_raw = MockDisplay::new();
+
+    bmp.draw(&mut display_bmp).unwrap();
+    raw.draw(&mut display_raw).unwrap();
+
+    display_bmp.assert_eq(&display_raw);
+}