Fnv hasher with default capacity

Cargo.toml

@@ -8,6 +8,7 @@ serde_json = "1.0.13"
 failure = "0.1.1"
 serde = "1.0.53"
 serde_derive = "1.0.53"
+fnv = "1.0.6"
 
 [dependencies.pyo3]
 version = "0.4.0"

src/lib.rs

@@ -12,14 +12,18 @@ extern crate serde_derive;
 extern crate pyo3;
 extern crate serde_json;
 
-use std::collections::BTreeMap;
+extern crate fnv;
+
+use fnv::FnvHashMap;
 use std::fmt;
 use std::marker::PhantomData;
 
 use pyo3::prelude::*;
 use serde::de::{self, DeserializeSeed, Deserializer, MapAccess, SeqAccess, Visitor};
 use serde::ser::{self, Serialize, SerializeMap, SerializeSeq, Serializer};
 
+const DEFAULT_HASHMAP_CAPACITY: usize = 10;
+
 #[derive(Debug, Fail)]
 pub enum HyperJsonError {
     #[fail(display = "Conversion error: {}", error)]
@@ -538,7 +542,7 @@ impl<'de, 'a> Visitor<'de> for HyperJsonValue<'a> {
     where
         A: MapAccess<'de>,
     {
-        let mut entries = BTreeMap::new();
+        let mut entries = FnvHashMap::with_capacity_and_hasher(DEFAULT_HASHMAP_CAPACITY, Default::default());
 
         while let Some((key, value)) = map.next_entry_seed(PhantomData::<String>, self)? {
             entries.insert(key, value);