Add an allocator for packet IDs.

Cargo.lock

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Cargo.toml

 rmp-serde = "0.14"
 fswatcher = { git = "https://github.com/mgottschlag/fswatcher-rs.git" }
 url = "*"
+range-set = "*"
 
 [dev-dependencies]
 criterion = "0.3"

src/network/idalloc.rs

+use range_set::RangeSet;
+use std::ops::RangeInclusive;
+
+/// Allocator for packet IDs.
+pub struct IdAllocator {
+    free_ids: RangeSet<[RangeInclusive<u32>; 4]>,
+}
+
+impl IdAllocator {
+    /// Creates a new allocator for IDs in the range between 0 and 1<<31-1.
+    pub fn new() -> IdAllocator {
+        IdAllocator {
+            free_ids: RangeSet::<[RangeInclusive<u32>; 4]>::from(0..=(1 << 31) - 1),
+        }
+    }
+
+    /// Allocates an ID.
+    ///
+    /// The ID needs to be manually freed by calling `free(id)` once the server
+    /// has sent a response..
+    pub fn alloc(&mut self) -> u32 {
+        assert!(!self.free_ids.is_empty());
+        let id = self.free_ids.iter().next().unwrap();
+        self.free_ids.remove(id);
+        id
+    }
+
+    /// Frees an ID to allow reuse.
+    pub fn free(&mut self, id: u32) {
+        assert!(id < 1 << 31);
+        let inserted = self.free_ids.insert(id);
+        if !inserted {
+            panic!("free() called for invalid packet ID.");
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_alloc() {
+        let mut ids = IdAllocator::new();
+        assert_eq!(ids.alloc(), 0);
+        assert_eq!(ids.alloc(), 1);
+        assert_eq!(ids.alloc(), 2);
+        assert_eq!(ids.alloc(), 3);
+        assert_eq!(ids.alloc(), 4);
+        ids.free(2);
+        ids.free(3);
+        assert_eq!(ids.alloc(), 2);
+        assert_eq!(ids.alloc(), 3);
+        assert_eq!(ids.alloc(), 5);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_invalid_free() {
+        let mut ids = IdAllocator::new();
+        ids.free(42);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_invalid_free2() {
+        let mut ids = IdAllocator::new();
+        ids.free(1 << 31);
+    }
+}

src/network/mod.rs

 pub mod client;
+pub mod idalloc;
 pub mod packet;
 pub mod server;
 pub mod websocket;