use {Type, Error};

use std::collections::{HashMap, BTreeMap};
use std::hash::Hash;

use std::io::prelude::*;

pub type SizeType = u32;

macro_rules! impl_map_type {
    ( $ty:ident => K: $( $k_pred:ident ),+ ) => {
        impl<K, V> Type for $ty<K, V>
            where K: Type + $( $k_pred +)+,
                  V: Type
        {
            fn read(read: &mut Read) -> Result<Self, Error> {
                let mut map = $ty::new();

                let length = SizeType::read(read)?;

                for _ in 0..length {
                    let key = K::read(read)?;
                    let value = V::read(read)?;

                    map.insert(key, value);
                }

                Ok(map)
            }

            fn write(&self, write: &mut Write) -> Result<(), Error> {
                (self.len() as SizeType).write(write)?;

                for (key, value) in self.iter() {
                    key.write(write)?;
                    value.write(write)?;
                }

                Ok(())
            }
        }
    }
}

impl_map_type!(HashMap => K: Hash, Eq);
impl_map_type!(BTreeMap => K: Ord);


use {Type, Error};

use std::collections::HashMap;
use std::io::prelude::*;
use std;

pub type SizeType = u32;

impl<K, V> Type for HashMap<K, V>
    where K: Type + std::hash::Hash + Eq,
          V: Type
{
    fn read(read: &mut Read) -> Result<Self, Error> {
        let mut map = HashMap::new();

        let length = SizeType::read(read)?;

        for _ in 0..length {
            let key = K::read(read)?;
            let value = V::read(read)?;

            map.insert(key, value);
        }

        Ok(map)
    }

    fn write(&self, write: &mut Write) -> Result<(), Error> {
        (self.len() as SizeType).write(write)?;

        for (key, value) in self.iter() {
            key.write(write)?;
            value.write(write)?;
        }

        Ok(())
    }
}


--- 
+++ 
@@ -1,39 +1,47 @@
 use {Type, Error};
 
-use std::collections::HashMap;
+use std::collections::{HashMap, BTreeMap};
+use std::hash::Hash;
+
 use std::io::prelude::*;
-use std;
 
 pub type SizeType = u32;
 
-impl<K, V> Type for HashMap<K, V>
-    where K: Type + std::hash::Hash + Eq,
-          V: Type
-{
-    fn read(read: &mut Read) -> Result<Self, Error> {
-        let mut map = HashMap::new();
+macro_rules! impl_map_type {
+    ( $ty:ident => K: $( $k_pred:ident ),+ ) => {
+        impl<K, V> Type for $ty<K, V>
+            where K: Type + $( $k_pred +)+,
+                  V: Type
+        {
+            fn read(read: &mut Read) -> Result<Self, Error> {
+                let mut map = $ty::new();
 
-        let length = SizeType::read(read)?;
+                let length = SizeType::read(read)?;
 
-        for _ in 0..length {
-            let key = K::read(read)?;
-            let value = V::read(read)?;
+                for _ in 0..length {
+                    let key = K::read(read)?;
+                    let value = V::read(read)?;
 
-            map.insert(key, value);
+                    map.insert(key, value);
+                }
+
+                Ok(map)
+            }
+
+            fn write(&self, write: &mut Write) -> Result<(), Error> {
+                (self.len() as SizeType).write(write)?;
+
+                for (key, value) in self.iter() {
+                    key.write(write)?;
+                    value.write(write)?;
+                }
+
+                Ok(())
+            }
         }
-
-        Ok(map)
-    }
-
-    fn write(&self, write: &mut Write) -> Result<(), Error> {
-        (self.len() as SizeType).write(write)?;
-
-        for (key, value) in self.iter() {
-            key.write(write)?;
-            value.write(write)?;
-        }
-
-        Ok(())
     }
 }
 
+impl_map_type!(HashMap => K: Hash, Eq);
+impl_map_type!(BTreeMap => K: Ord);
+
Generalize the HashMap Type implementation
