nit on naming

src/proto/flow_control_state.rs

@@ -0,0 +1,83 @@
+use proto::WindowSize;
+
+#[derive(Clone, Copy, Debug)]
+pub struct WindowUnderflow;
+
+pub const DEFAULT_INITIAL_WINDOW_SIZE: WindowSize = 65_535;
+
+#[derive(Copy, Clone, Debug)]
+pub struct FlowControlState {
+    /// Amount that may be claimed.
+    window_size: WindowSize,
+    /// Amount to be removed by future increments.
+    underflow: WindowSize,
+    /// The amount that has been incremented but not yet advertised (to the application or
+    /// the remote).
+    next_window_update: WindowSize,
+}
+
+impl Default for FlowControlState {
+    fn default() -> Self {
+        Self::new(DEFAULT_INITIAL_WINDOW_SIZE)
+    }
+}
+
+impl FlowControlState {
+    pub fn new(window_size: WindowSize) -> FlowControlState {
+        FlowControlState {
+            window_size,
+            underflow: 0,
+            next_window_update: 0,
+        }
+    }
+
+    /// Reduce future capacity of the window.
+    ///
+    /// This accomodates updates to SETTINGS_INITIAL_WINDOW_SIZE.
+    pub fn shrink_window(&mut self, decr: WindowSize) {
+        self.underflow += decr;
+    }
+
+    /// Claims the provided amount from the window, if there is enough space.
+    ///
+    /// Fails when `take_window_update()` hasn't returned at least `sz` more bytes than
+    /// have been previously claimed.
+    pub fn claim_window(&mut self, sz: WindowSize) -> Result<(), WindowUnderflow> {
+        if self.window_size < sz {
+            return Err(WindowUnderflow);
+        }
+
+        self.window_size -= sz;
+        Ok(())
+    }
+
+    /// Applies a window increment immediately.
+    pub fn grow_window(&mut self, sz: WindowSize) {
+        if sz <= self.underflow {
+            self.underflow -= sz;
+            return;
+        }
+
+        let added = sz - self.underflow;
+        self.window_size += added;
+        self.next_window_update += added;
+        self.underflow = 0;
+    }
+
+    /// Obtains and clears an unadvertised window update.
+    pub fn take_window_update(&mut self) -> Option<WindowSize> {
+        if self.next_window_update == 0 {
+            return None;
+        }
+
+        let incr = self.next_window_update;
+        self.next_window_update = 0;
+        Some(incr)
+    }
+}
+
+#[test]
+fn test() {
+    let mut fc = FlowControlState::new(65_535);
+
+}