add 8x8 cell partial update encoding

encoder/src/enc.rs

@@ -1,5 +1,43 @@
 use crate::*;
 
+pub fn cell_diff_8_horizontal(prev_frame: &Frame, frame: &Frame) -> EncodedFrame {
+	let cells_x = WIDTH / 8;
+	let cells_y = HEIGHT / 8;
+	let loss = 0;
+
+	let mut bitmap: u32 = 0;
+	let mut changed_pixels = Vec::new();
+
+	for celly in 0..cells_y {
+		for cellx in 0..cells_x {
+			let mut changed = 0;
+			let mut cell_contents = Vec::new();
+			// todo try encoding the cells in frame space instead of one cell at a time
+			for dx in 0..8 {
+				for dy in 0..8 {
+					let x = cellx * 8 + dx;
+					let y = celly * 8 + dy;
+					let pixel = frame[x][y];
+					cell_contents.push(pixel);
+					if pixel != prev_frame[x][y] {
+						changed += 1;
+					}
+				}
+			}
+			if changed > loss {
+				bitmap |= 1 << (cellx + cells_x * celly);
+				changed_pixels.extend_from_slice(&cell_contents);
+			}
+		}
+	}
+	let mut data = vec![(bitmap >> 16) as u8, (bitmap >> 8) as u8, bitmap as u8];
+	data.extend_from_slice(&rle_255_encode(&changed_pixels));
+	EncodedFrame {
+		encoding: Encoding::CellDiff8H,
+		data,
+	}
+}
+
 pub fn bg_strips_horizontal(_prev_frame: &Frame, frame: &Frame) -> EncodedFrame {
 	let bg = most_common_pixel(frame);
 	fn pack_strip(x: usize, y: usize, width: usize) -> [u8; 2] {