add horizontal strip encoding type

encoder/src/enc.rs

@@ -1,5 +1,58 @@
 use crate::*;
 
+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] {
+		// Y is 0..31 so will only need 5 bits
+		// x is 0..42 so needs 6 bits
+		// 5 bits remain for width
+		// y: 1111100000000000
+		let mut y_x_w: u16 = (y as u16) << 11;
+		// x: 0000011111100000
+		y_x_w |= (x as u16) << 5;
+		// w: 0000000000011111
+		y_x_w |= width as u16;
+		[(y_x_w >> 8) as u8, (y_x_w as u8)]
+	}
+
+	let mut strips = Vec::new();
+	'outer: for y in 0..HEIGHT {
+		let mut strip_start = 0;
+		let mut in_strip = false;
+		for x in 0..WIDTH {
+			let pixel = frame[x][y];
+			if !in_strip && pixel != bg {
+				in_strip = true;
+				strip_start = x;
+			}
+			if in_strip {
+				if pixel == bg {
+					strips.push((strip_start, y, x - strip_start));
+					in_strip = false;
+				} else if x - strip_start == 31 {
+					strips.push((strip_start, y, x - strip_start));
+					strip_start = x;
+				}
+				if strips.len() == 127 {
+					break 'outer;
+				}
+			}
+		}
+		if in_strip {
+			strips.push((strip_start, y, WIDTH - strip_start));
+		}
+	}
+	let mut frame_bytes = Vec::with_capacity(1 + strips.len() * 2);
+	frame_bytes.push(bg << 7 | (strips.len() as u8));
+	for (x, y, width) in strips {
+		frame_bytes.extend_from_slice(&pack_strip(x, y, width));
+	}
+	EncodedFrame {
+		encoding: Encoding::BGStripsH,
+		data: frame_bytes,
+	}
+}
+
 pub fn rle_diff_horizontal(prev_frame: &Frame, frame: &Frame) -> EncodedFrame {
 	let mut pixels = Vec::new();
 	for y in 0..HEIGHT {