Example of how to use std::hash_map

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libethereum/BlockChain.cpp

@@ -271,7 +271,7 @@ bytesConstRef BlockChain::block(h256 _hash) const
 
 BlockDetails const& BlockChain::details(h256 _h) const
 {
-	std::map<h256, BlockDetails>::const_iterator it;
+	BlockDetailsHash::const_iterator it;
 	bool fetchRequired;
 	{
 		lock_guard<mutex> l(m_lock);

libethereum/BlockChain.h

@@ -48,6 +48,8 @@ struct BlockDetails
 	h256s children;
 };
 
+typedef std::hash_map<h256, BlockDetails, H256Hash> BlockDetailsHash;
+
 static const BlockDetails NullBlockDetails;
 static const h256s NullH256s;
 
@@ -101,7 +103,7 @@ private:
 	void checkConsistency();
 
 	/// Get fully populated from disk DB.
-	mutable std::map<h256, BlockDetails> m_details;
+	mutable BlockDetailsHash m_details;
 	mutable std::map<h256, std::string> m_cache;
 	mutable std::mutex m_lock;
 

libethereum/Common.h

@@ -40,6 +40,7 @@
 #include <chrono>
 #include <array>
 #include <map>
+#include <hash_map>
 #include <unordered_map>
 #include <set>
 #include <array>
@@ -181,6 +182,43 @@ using HexMap = std::map<bytes, std::string>;
 static const u256 Invalid256 = ~(u256)0;
 static const bytes NullBytes;
 
+// This is the helper class for the std::hash_map lookup; it converts the input 256bit hash into a size_t sized hash value
+// and does an exact comparison of two hash entries.
+class H256Hash
+{
+public:
+	static const size_t bucket_size = 4;
+	static const size_t min_buckets = 8;
+
+	// Compute the size_t hash of this hash
+	size_t operator()(const h256 &index) const
+	{
+		const uint64_t *data = (const uint64_t *)index.data();
+		uint64_t hash = data[0];
+		hash ^= data[1];
+		hash ^= data[2];
+		hash ^= data[3];
+		return (size_t)hash;
+	}
+
+	// Return true if hash s1 is less than hash s2
+	bool operator()(const h256 &s1, const h256 &s2) const
+	{
+		const uint64_t *hash1 = (const uint64_t *)s1.data();
+		const uint64_t *hash2 = (const uint64_t *)s2.data();
+
+		if (hash1[0] < hash2[0]) return true;
+		if (hash1[0] > hash2[0]) return false;
+
+		if (hash1[1] < hash2[1]) return true;
+		if (hash1[1] > hash2[1]) return false;
+
+		if (hash1[2] < hash2[2]) return true;
+		if (hash1[2] > hash2[2]) return false;
+
+		return hash1[3] < hash2[3];
+	}
+};
 
 /// Logging
 class NullOutputStream