107 lines
2.7 KiB
C++
107 lines
2.7 KiB
C++
#pragma once
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#include <boost/multiprecision/integer.hpp>
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namespace fc {
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/**
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* Always returns 0. Useful for testing.
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*/
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class nullary_rng
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{
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public:
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nullary_rng() {}
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virtual ~nullary_rng() {}
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template< typename T > T operator()( T max )
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{ return T(0); }
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} ;
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/**
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* The hash_ctr_rng generates bits using a hash function in counter (CTR)
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* mode.
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*/
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template<class HashClass, int SeedLength>
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class hash_ctr_rng
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{
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public:
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hash_ctr_rng( const char* seed, uint64_t counter = 0 )
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: _counter( counter ), _current_offset( 0 )
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{
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memcpy( _seed, seed, SeedLength );
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_reset_current_value();
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return;
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}
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virtual ~hash_ctr_rng() {}
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uint64_t get_bits( uint8_t count )
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{
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uint64_t result = 0;
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uint64_t mask = 1;
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// grab the requested number of bits
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while( count > 0 )
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{
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result |=
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(
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(
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(
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_current_value.data()[ (_current_offset >> 3) & 0x1F ]
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& ( 1 << (_current_offset & 0x07) )
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)
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!= 0
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) ? mask : 0
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);
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mask += mask;
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--count;
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++_current_offset;
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if( _current_offset == (_current_value.data_size() << 3) )
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{
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_counter++;
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_current_offset = 0;
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_reset_current_value();
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}
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}
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return result;
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}
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uint64_t operator()( uint64_t bound )
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{
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if( bound <= 1 )
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return 0;
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uint8_t bitcount = boost::multiprecision::detail::find_msb( bound ) + 1;
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// probability of loop exiting is >= 1/2, so probability of
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// running N times is bounded above by (1/2)^N
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while( true )
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{
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uint64_t result = get_bits( bitcount );
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if( result < bound )
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return result;
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}
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}
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// convenience method which does casting for types other than uint64_t
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template< typename T > T operator()( T bound )
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{ return (T) ( (*this)(uint64_t( bound )) ); }
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void _reset_current_value()
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{
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// internal implementation detail, called to update
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// _current_value when _counter changes
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typename HashClass::encoder enc;
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enc.write( _seed , SeedLength );
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enc.write( (char *) &_counter, 8 );
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_current_value = enc.result();
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return;
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}
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uint64_t _counter;
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char _seed[ SeedLength ];
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HashClass _current_value;
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uint16_t _current_offset;
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static const int seed_length = SeedLength;
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};
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} // end namespace fc
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