Tile-level RLE

src/compression.rs

@@ -11,6 +11,7 @@ struct CoefTile {
     coefs: [i16; TILE_SZ2]
 }
 
+#[derive(Clone, Copy)]
 struct QuantTile {
     quants: [i16; TILE_SZ2]
 }
@@ -53,8 +54,10 @@ pub fn compress<W: Write>(
     for t in tiles.iter() {
         t.write_zero(writer)?;
     }
-    for t in tiles.iter() {
-        t.write_rest(writer)?;
+    for ti in 0..tiles.len() {
+        let prev = if ti > 0 { Some(tiles[ti - 1]) } else { None };
+        let t = tiles[ti];
+        t.write_rest(prev, writer)?;
     }
 
     Ok(())
@@ -116,9 +119,28 @@ impl QuantTile {
         Ok(())
     }
 
-    fn write_rest<W: Write>(&self, writer: &mut ProtocolWriter<W>) -> ProtocolWriterResult<()> {
-        for i in 1..self.quants.len() {
+    fn write_rest<W: Write>(&self, prev: Option<QuantTile>, writer: &mut ProtocolWriter<W>) -> ProtocolWriterResult<()> {
+        if let Some(p) = prev {
+            if self.quants[1..] == p.quants[1..] {
+                // 255 zeroes can also encode "copy the rest from the previous tile"
+                writer.write_u8_wide(0xff)?;  
+                return Ok(());
+            }
+        }
+
+        let mut i = 0;
+        loop {
+            let mut zeros: u8 = 0;
+            while i < TILE_SZ2 && self.quants[i] == 0 {
+                zeros += 1;
+                i += 1;
+            }
+            writer.write_u8_wide(zeros)?;
+
+            if i >= TILE_SZ2 { break; }
+
             writer.write_i16_packed(self.quants[i])?;
+            i += 1;
         }
         Ok(())
     }

src/decompression.rs

@@ -12,7 +12,7 @@ struct CoefTile {
     coefs: [i16; TILE_SZ2]
 }
 
-#[derive(Clone)]
+#[derive(Clone, Copy)]
 struct QuantTile {
     quants: [i16; TILE_SZ2]
 }
@@ -42,7 +42,11 @@ pub fn decompress<R: Read>(
         tiles[i].load_zero(reader)?;
     }
     for i in 0..n_tiles {
-        tiles[i].load_rest(reader)?;
+        let prev = if i > 0 { Some(tiles[i - 1]) } else { None };
+        tiles[i].load_rest(
+            prev,
+            reader
+        )?;
     }
 
     let mut tile_i = 0;
@@ -120,10 +124,28 @@ impl QuantTile {
         Ok(())
     }
 
-    fn load_rest<R: Read>(&mut self, reader: &mut ProtocolReader<R>) -> ProtocolReaderResult<()> {
-        for i in 1..self.quants.len() {
-            self.quants[i] = reader.read_i16_packed()?;
+    fn load_rest<R: Read>(&
+        mut self, 
+        prev: Option<QuantTile>,
+        reader: &mut ProtocolReader<R>
+    ) -> ProtocolReaderResult<()> {
+        let mut i: usize = 0;
+        loop {
+            let cmd = reader.read_u8_wide()? as usize;
+            if let Some(p) = prev {
+                if cmd == 0xff {
+                    self.quants[1..].copy_from_slice(&p.quants[1..]);
+                    return Ok(());
                 }
+            }
+
+            i += cmd;
+            if i >= TILE_SZ2 { break; }
+
+            self.quants[i] = reader.read_i16_packed()?;
+            i += 1;
+        }
+ 
         Ok(())
     }
     

src/protocol.rs

@@ -65,6 +65,10 @@ impl<W: Write> ProtocolWriter<W> {
         self.writer.write_all(&value.to_le_bytes())?;
         Ok(())
     }
+    pub fn write_u8_wide(&mut self, value: u8) -> ProtocolWriterResult<()> {
+        self.writer.write_all(&value.to_le_bytes())?;
+        Ok(())
+    }
     pub fn write_i16_packed(&mut self, value: i16) -> ProtocolWriterResult<()> {
         // We reserve i8::MAX and i16::MAX as signal values that `value` is too big
         // for each respective type
@@ -110,6 +114,13 @@ impl<R: Read> ProtocolReader<R> {
         Ok(i16::from_le_bytes(i16_buf))
     }
 
+    pub fn read_u8_wide(&mut self) -> ProtocolReaderResult<u8> {
+        let mut u8_buf = [0; 1];
+        self.read_exact(&mut u8_buf)?;
+
+        Ok(u8::from_le_bytes(u8_buf))
+    }
+
     pub fn read_i16_packed(&mut self) -> ProtocolReaderResult<i16> {
         let mut i8_buf = [0; 1];
         self.read_exact(&mut i8_buf)?;