first version, only very simple greedy packer

src/context_state.rs

@@ -0,0 +1,39 @@
+const INIT_PROB: u16 = 0x8000;
+const UPDATE_RATE: u32 = 4;
+
+#[derive(Clone)]
+pub struct ContextState {
+    contexts: Vec<u16>,
+}
+
+pub struct Context<'a> {
+    state: &'a mut ContextState,
+    index: usize,
+}
+
+impl ContextState {
+    pub fn new(size: usize) -> ContextState {
+        ContextState {
+            contexts: vec![INIT_PROB; size],
+        }
+    }
+
+    pub fn context_mut(&mut self, index: usize) -> Context {
+        Context { state: self, index }
+    }
+}
+
+impl<'a> Context<'a> {
+    pub fn prob(&self) -> u16 {
+        self.state.contexts[self.index]
+    }
+
+    pub fn update(&mut self, bit: bool) {
+        let old = self.state.contexts[self.index];
+        self.state.contexts[self.index] = if bit {
+            old + (((1 << 16) - old as u32) >> UPDATE_RATE) as u16
+        } else {
+            old - (old >> UPDATE_RATE)
+        };
+    }
+}