z80 branch, but with original (slightly more accurate) prob update

compare-variants

@@ -2,7 +2,9 @@
 
 configs = [
     [:master, '-b'],
-    [:z80, '-b']
+    [:z80, '-b'],
+    [:z80, ['-b', '-r']],
+    ['old-prob-update', '-b']
 ]
 
 files = Dir[ARGV[0] + '/*'].select {|f| !(f =~ /\.txt$/) }
@@ -34,8 +36,8 @@ for config in configs
     config_results = []
     results << config_results
     for file in files
-        if system('cargo', 'run', '--release', 'pack', '-l', '9', config[1], file, '/tmp/out.upk') &&
-            system('cargo', 'run', '--release', 'unpack', config[1], '/tmp/out.upk', '/tmp/out.bin') &&
+        if system(*['cargo', 'run', '--release', 'pack', '-l', '9', config[1], file, '/tmp/out.upk'].flatten) &&
+            system(*['cargo', 'run', '--release', 'unpack', config[1], '/tmp/out.upk', '/tmp/out.bin'].flatten) &&
             File.read(file) == File.read('/tmp/out.bin')
             size = File.size('/tmp/out.upk')
             config_results << size

src/context_state.rs

@@ -33,13 +33,12 @@ impl<'a> Context<'a> {
 
     pub fn update(&mut self, bit: bool) {
         let old = self.state.contexts[self.index];
-        let offset = if !bit {
-            ONE_PROB as i32 >> UPDATE_RATE
+        if bit {
+            self.state.contexts[self.index] =
+                old - ((old as i32 + UPDATE_ADD) >> UPDATE_RATE) as u8;
         } else {
-            0
-        };
-
-        self.state.contexts[self.index] =
-            (offset + old as i32 - ((old as i32 + UPDATE_ADD) >> UPDATE_RATE)) as u8;
+            self.state.contexts[self.index] =
+                old + (((ONE_PROB as i32 - old as i32) + UPDATE_ADD) >> UPDATE_RATE) as u8;
+        }
     }
 }