From c8e9a2e8528eda700ad2c35eca4d461fa458a132 Mon Sep 17 00:00:00 2001
From: John Wellbelove <john.wellbelove@raymarine.com>
Date: Thu, 10 Sep 2015 11:25:17 +0100
Subject: [PATCH] Added 'fold_bits()' to binary.h and used in bloom_filter.h

---
 binary.h       | 29 +++++++++++++++++++++++++++++
 bloom_filter.h | 19 ++-----------------
 2 files changed, 31 insertions(+), 17 deletions(-)

diff --git a/binary.h b/binary.h
index 23e82a40..6f32782c 100644
--- a/binary.h
+++ b/binary.h
@@ -37,6 +37,8 @@ SOFTWARE.
 #include "type_traits.h"
 #include "integral_limits.h"
 #include "static_assert.h"
+#include "log.h"
+#include "power.h"
 
 namespace etl
 {
@@ -421,6 +423,33 @@ namespace etl
     return (0x69966996 >> value) & 1;
   }
 
+  //***************************************************************************
+  /// Fold a binary number down to a set number of bits using XOR.
+  //***************************************************************************
+  template <typename TReturn, const size_t NBITS, typename TValue>
+  TReturn fold_bits(TValue value)
+  {
+    STATIC_ASSERT(integral_limits<TReturn>::bits >= NBITS, "Return type too small to hold result");
+
+    const size_t mask  = etl::power<2, NBITS>::value - 1;
+    const size_t shift = NBITS;
+
+    // Fold the value down to fit the width.
+    TReturn folded_value = 0;
+
+    // Keep shifting down and XORing the lower bits.
+    while (value >= NBITS)
+    {
+      folded_value ^= value & mask;
+      value >>= shift;
+    }
+
+    // Fold the remaining bits.
+    folded_value ^= value;
+
+    return folded_value;
+  }
+
   //***************************************************************************
   /// 8 bit binary constants.
   //***************************************************************************
diff --git a/bloom_filter.h b/bloom_filter.h
index b25f075e..18086b86 100644
--- a/bloom_filter.h
+++ b/bloom_filter.h
@@ -33,6 +33,7 @@ SOFTWARE.
 #include "parameter_type.h"
 #include "bitset.h"
 #include "type_traits.h"
+#include "binary.h"
 #include "log.h"
 #include "power.h"
 
@@ -172,26 +173,10 @@ namespace etl
     template <typename THash>
     size_t get_hash(parameter_t key) const
     {
-      const size_t mask = etl::power_of_2_round_up<WIDTH>::value - 1;
-
       size_t hash = THash()(key);
 
       // Fold the hash down to fit the width.
-      size_t folded_hash = 0;
-
-      const size_t shift = etl::log2<etl::power_of_2_round_up<WIDTH>::value>::value;
-
-      // Keep shifting down and XORing the lower bits.
-      while (hash >= WIDTH)
-      {
-        folded_hash ^= hash & mask;
-        hash >>= shift;
-      }
-
-      // Fold the remaining bits.
-      folded_hash ^= hash;
-
-      return folded_hash;
+      return fold_bits<size_t, etl::log2<WIDTH>::value>(hash);
     }
 
     /// The Bloom filter flags.