sha256: Add method to return approx log as double

include/fc/crypto/sha256.hpp

@@ -93,6 +93,7 @@ class sha256
     uint32_t approx_log_32()const;
 
     void set_to_inverse_approx_log_32( uint32_t x );
+    static double inverse_approx_log_32_double( uint32_t x );
 
     uint64_t _hash[4];
 };

src/crypto/sha256.cpp

@@ -3,6 +3,7 @@
 #include <fc/fwd_impl.hpp>
 #include <openssl/sha.h>
 #include <string.h>
+#include <cmath>
 #include <fc/crypto/sha256.hpp>
 #include <fc/variant.hpp>
 #include <fc/exception/exception.hpp>
@@ -150,6 +151,16 @@ namespace fc {
       return;
    }
 
+   double sha256::inverse_approx_log_32_double( uint32_t x )
+   {
+      uint8_t nzbits = uint8_t( x >> 0x18 );
+      if( nzbits == 0 )
+         return 0.0;
+      uint32_t b = 1 << 0x18;
+      uint32_t y = (x & (b-1)) | b;
+      return std::ldexp( y, int( nzbits ) - 0x18 );
+   }
+
    uint16_t sha256::clz()const
    {
       const uint8_t* my_bytes = (uint8_t*) data();

tests/crypto/log_test.cpp

@@ -1,4 +1,6 @@
 
+#include <boost/multiprecision/cpp_int.hpp>
+
 #include <fc/crypto/sha256.hpp>
 #include <fc/exception/exception.hpp>
 
@@ -55,12 +57,23 @@ int main(int argc, char**argv, char** envp)
          std::cerr << "got error on log(" << str_h << ")" << std::endl;
          ++errors;
       }
+      double d_ilog_h_test = h.inverse_approx_log_32_double( ref_log );
       h.set_to_inverse_approx_log_32( ref_log );
       if( ref_log != h.approx_log_32() )
       {
          std::cerr << "got error on ilog(" << ref_log << ")" << std::endl;
          ++errors;
       }
+
+      std::string str_ilog_h = h.str();
+      boost::multiprecision::uint256_t u256_ilog_h( "0x" + str_ilog_h );
+      double d_ilog_h_ref = u256_ilog_h.template convert_to<double>();
+      if( d_ilog_h_ref != d_ilog_h_test )
+      {
+         std::cerr << "got error on d_ilog(" << ref_log << ")" << std::endl;
+         ++errors;
+      }
+
       if( h != fc::sha256() )
       {
          fc::sha256 h_before = h;