DRY up and document magic constant used for hash maps

modules/ripple_basics/types/ripple_HashMaps.h

@@ -15,6 +15,28 @@
 class HashMaps // : beast::Uncopayble
 {
 public:
+    /** Golden ratio constant used in hashing functions.
+
+        The magic number is supposed to be 32 random bits, where each is
+        equally likely to be 0 or 1, and with no simple correlation between
+        the bits. A common way to find a string of such bits is to use the
+        binary expansion of an irrational number; in this case, that number
+        is the reciprocal of the golden ratio:
+
+        @code
+
+        phi = (1 + sqrt(5)) / 2
+        2^32 / phi = 0x9e3779b9
+
+        @endcode
+
+        References:
+
+            http://stackoverflow.com/questions/4948780/magic-number-in-boosthash-combine
+            http://burtleburtle.net/bob/hash/doobs.html
+    */
+    static std::size_t const goldenRatio = 0x9e3779b9;
+
     /** Retrieve the singleton.
 
         @return The global instance of the singleton.

modules/ripple_data/crypto/ripple_Base58Data.cpp

@@ -109,16 +109,8 @@ bool CBase58Data::operator> (const CBase58Data& b58) const { return CompareTo(b5
 
 std::size_t hash_value(const CBase58Data& b58)
 {
-    // VFALCO: TODO, figure out what this is for and whether or
-    //               not it affects the protocol specification.
-    //
-    //         NOTE, this constant is used elsewhere as well.
-    //               should it be DRY?
-    //
-    const std::size_t mysteriousConstant = 0x9e3779b9;
-
     std::size_t seed = HashMaps::getInstance ().getNonce <size_t> ()
-                       + (b58.nVersion * mysteriousConstant);
+                       + (b58.nVersion * HashMaps::goldenRatio);
 
     boost::hash_combine (seed, b58.vchData);
 	

src/cpp/ripple/SHAMap.cpp

@@ -26,21 +26,13 @@ void SHAMapNode::setMHash() const
 {
     using namespace std;
 
-    // VFALCO: TODO, figure out what this is for and whether or
-    //               not it affects the protocol specification.
-    //
-    //         NOTE, this constant is used elsewhere as well.
-    //               should it be DRY?
-    //
-    const std::size_t mysteriousConstant = 0x9e3779b9;
-
     std::size_t h = HashMaps::getInstance ().getNonce <std::size_t> ()
-                    + (mDepth * mysteriousConstant);
+                    + (mDepth * HashMaps::goldenRatio);
 
     const unsigned int *ptr = reinterpret_cast <const unsigned int *>(mNodeID.begin());
 	
     for (int i = (mDepth + 7) / 8; i != 0; --i)
-		h = (h * mysteriousConstant) ^ *ptr++;
+		h = (h * HashMaps::goldenRatio) ^ *ptr++;
 
     mHash = h;
 }