Let's call this canonical for now

src/decode_tile.rs

@@ -11,8 +11,7 @@ pub fn decode_tile<R: Read>(
 
     for _ in 0..quality {
         let ix = reader.read_u8()?;
-        let val = decode_value(reader.read_u16()?);
-        coefs[ix as usize] = val;
+        coefs[ix as usize] = reader.read_u16()? as i16 as i32 * 256;
     }
 
     let mut transform = |zero: usize, stride: usize| {
@@ -33,33 +32,13 @@ pub fn decode_tile<R: Read>(
                 }
             }
         }
-
-        for i in 0..16 {
-            coefs[ixs[i]] /= 16;
-        }
     };
 
     for row in 0..16 { transform(row * 16, 1); }
     for col in 0..16 { transform(col, 16); }
 
     for i in 0..256 {
-        data[i] = coefs[i].max(0).min(255) as u8;
+        data[i] = (coefs[i] / 256).max(0).min(255) as u8;
     }
     Ok(())
-}
-
-pub fn decode_value(enc: u16) -> i32 {
-    return (enc as i16) as i32 * 16;
-    /*
-    // mu law
-    let sign = enc & 0x80 != 0;
-    let exponent = (enc >> 4) & 0x7;
-    let mantissa = enc & 0xf;
-
-    let magnitude = ((((mantissa as i32) << 1) | 0b100001) << exponent) - 33;
-    let value = if sign { -magnitude } else {magnitude };
-
-    // unquantize
-    return value * 32;
-    */
 }

src/encode_tile.rs

@@ -57,51 +57,7 @@ pub fn encode_tile<W: Write>(
     for i in 0..quality {
         let ix = indices[i as usize];
         writer.write_u8(ix as u8)?;
-        writer.write_u16(encode_value(coefs[ix]))?;
+        writer.write_u16((coefs[ix] / 256) as i16 as u16)?;
     };
     Ok(())
 }
-
-pub fn encode_value(value: i32) -> u16 {
-    return ((value / 16) as i16) as u16;
-    /*
-    // quantize
-    let value = value / 32;
-
-    // mu law
-    let sign = value < 0;
-    let mut rescaled = value.abs() + 33;
-    let mut exponent = 0;
-
-    while rescaled > 64 {
-        exponent += 1;
-        rescaled >>= 1;
-    }
-    assert!(exponent <= 7);
-
-    let mantissa = (rescaled >> 1) & 0xf;
-
-    return (
-        if sign { 128 } else { 0 } |
-        exponent << 4 |
-        mantissa
-    ) as u8;
-}
-
-#[cfg(test)]
-mod test {
-    use quickcheck_macros::quickcheck;
-
-    use crate::decode_tile::decode_value;
-
-    use super::encode_value;
-
-    #[quickcheck]
-    fn test_encode_value(val: u16) -> bool {
-        let val = (val & (0x8000 | 0x3fff)) as i16;
-        let canon = decode_value(encode_value(val as i32));
-
-        canon == decode_value(encode_value(canon as i32))
-    }
-    */
-}

src/main.rs

@@ -12,6 +12,7 @@ mod protocol;
 
 fn main() {
     // run_for("zonked".to_string(), 9, 2, Some(330000));
+    run_for("kapu".to_string());
     run_for("lohikar".to_string());
     run_for("zonked".to_string());
     run_for("avatar2".to_string());

src/transform.rs.old

@@ -1,66 +0,0 @@
-// Ported from: 
-// - https://colab.research.google.com/drive/1WjtKwUcqxWafAumFO9ET74RsckZSsw6n#scrollTo=e3r9-RBpcgwH
-
-// Specialized for:
-//
-// - Input domain: 0-255
-// - Tile size: 16x16
-//
-// Rather than dividing by 16 at the end of a decode phase,
-// we divide by 16 at the end of the _encode_ phase
-//
-// The normal range would be something like -255 * 256 to 255 * 256
-// (-65280 to 65280) but this reduces it (now it's something like -4080 to 4080)
-//
-// This simplifies the decoder (which can be i16) and means it can be written
-// using i16s instead of i32s
-//
-// We could probably shrink the range more by recentering to -128-127, but does
-// it even matter? Reasonable encodings like A-law and mu-law can handle both.
-//
-// (Take my opinions on the range with a grain of salt: I've calculated it but I
-//  haven't measured it)
-//
-struct Transform<'d> {
-    data: &'d mut [i32],
-    zero: usize,
-    stride: usize,
-}
-
-impl<'d> Transform<'d> {
-    pub fn encode(&mut self) {
-        self.transform();
-        self.divide(16);
-    }
-
-    pub fn decode(&mut self) {
-        self.transform();
-    }
-
-    pub fn ix(&self, i: usize) -> usize {
-        self.zero + t * self.stride
-    }
-
-    fn transform(&mut self) {
-        for bit in [8, 4, 2, 1] {
-            for i in 0..16 {
-                if i & bit == 0 {
-                    let j = i | bit;
-
-                    let ival = self.data[self.ix(i)];
-                    let jval = self.data[self.ix(j)];
-
-                    self.data[self.ix(i)] = ival.wrapping_add(jval);
-                    self.data[self.ix(j)] = ival.wrapping_sub(jval);
-                }
-            }
-        }
-    }
-
-    fn divide(&mut self, divisor: i32) {
-        for i in 0..16 {
-            self.data[self.ix(i)] = self.data[self.ix(j)] / divisor;
-        }
-    }
-
-}