add bitstream variant, could be useful on 8bit platforms

src/rans.rs

@@ -1,6 +1,5 @@
 use crate::context_state::Context;
 
-const L_BITS: u32 = 12;
 pub const PROB_BITS: u32 = 8;
 pub const ONE_PROB: u32 = 1 << PROB_BITS;
 
@@ -13,43 +12,75 @@ pub trait EntropyCoder {
     }
 }
 
-pub struct RansCoder(Vec<u16>);
+pub struct RansCoder {
+    bits: Vec<u16>,
+    use_bitstream: bool,
+}
 
 impl EntropyCoder for RansCoder {
     fn encode_bit(&mut self, bit: bool, prob: u16) {
         assert!(prob < 32768);
-        self.0.push(prob | ((bit as u16) << 15));
+        self.bits.push(prob | ((bit as u16) << 15));
     }
 }
 
 impl RansCoder {
-    pub fn new() -> RansCoder {
-        RansCoder(Vec::new())
+    pub fn new(use_bitstream: bool) -> RansCoder {
+        RansCoder {
+            bits: Vec::new(),
+            use_bitstream,
+        }
     }
 
     pub fn finish(self) -> Vec<u8> {
         let mut buffer = vec![];
-        let mut state = 1 << L_BITS;
+        let l_bits: u32 = if self.use_bitstream { 15 } else { 12 };
+        let mut state = 1 << l_bits;
 
-        const MAX_STATE_FACTOR: u32 = 1 << (L_BITS + 8 - PROB_BITS);
-        for step in self.0.into_iter().rev() {
+        let mut byte = 0u8;
+        let mut bit = 8;
+        let mut flush_state: Box<dyn FnMut(&mut u32)> = if self.use_bitstream {
+            Box::new(|state: &mut u32| {
+                bit -= 1;
+                byte |= ((*state & 1) as u8) << bit;
+                if bit == 0 {
+                    buffer.push(byte);
+                    byte = 0;
+                    bit = 8;
+                }
+                *state >>= 1;
+            })
+        } else {
+            Box::new(|state: &mut u32| {
+                buffer.push(*state as u8);
+                *state >>= 8;
+            })
+        };
+
+        let num_flush_bits = if self.use_bitstream { 1 } else { 8 };
+        let max_state_factor: u32 = 1 << (l_bits + num_flush_bits - PROB_BITS);
+        for step in self.bits.into_iter().rev() {
             let prob = step as u32 & 32767;
             let (start, prob) = if step & 32768 != 0 {
                 (0, prob)
             } else {
                 (prob, ONE_PROB - prob)
             };
-            let max_state = MAX_STATE_FACTOR * prob;
+            let max_state = max_state_factor * prob;
             while state >= max_state {
-                buffer.push(state as u8);
-                state >>= 8;
+                flush_state(&mut state);
             }
             state = ((state / prob) << PROB_BITS) + (state % prob) + start;
         }
 
         while state > 0 {
-            buffer.push(state as u8);
-            state >>= 8;
+            flush_state(&mut state);
+        }
+
+        drop(flush_state);
+
+        if self.use_bitstream && byte != 0 {
+            buffer.push(byte);
         }
 
         buffer.reverse();
@@ -99,14 +130,22 @@ impl EntropyCoder for CostCounter {
 pub struct RansDecoder<'a> {
     data: &'a [u8],
     state: u32,
+    use_bitstream: bool,
+    byte: u8,
+    bits_left: u8,
 }
 
 const PROB_MASK: u32 = ONE_PROB - 1;
-const L: u32 = 1 << L_BITS;
 
 impl<'a> RansDecoder<'a> {
-    pub fn new(data: &'a [u8]) -> RansDecoder<'a> {
-        RansDecoder { data, state: 0 }
+    pub fn new(data: &'a [u8], use_bitstream: bool) -> RansDecoder<'a> {
+        RansDecoder {
+            data,
+            state: 0,
+            use_bitstream,
+            byte: 0,
+            bits_left: 0,
+        }
     }
 
     pub fn decode_with_context(&mut self, context: &mut Context) -> bool {
@@ -117,9 +156,22 @@ impl<'a> RansDecoder<'a> {
 
     pub fn decode_bit(&mut self, prob: u16) -> bool {
         let prob = prob as u32;
-        while self.state < L {
-            self.state = (self.state << 8) | self.data[0] as u32;
-            self.data = &self.data[1..];
+        if self.use_bitstream {
+            while self.state < 32768 {
+                if self.bits_left == 0 {
+                    self.byte = self.data[0];
+                    self.data = &self.data[1..];
+                    self.bits_left = 8;
+                }
+                self.state = (self.state << 1) | (self.byte & 1) as u32;
+                self.byte >>= 1;
+                self.bits_left -= 1;
+            }
+        } else {
+            while self.state < 4096 {
+                self.state = (self.state << 8) | self.data[0] as u32;
+                self.data = &self.data[1..];
+            }
         }
 
         let bit = (self.state & PROB_MASK) < prob;