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 #ifndef BOOST_UUID_DETAIL_CHACHA20_HPP_INCLUDED
 #define BOOST_UUID_DETAIL_CHACHA20_HPP_INCLUDED
 
 // Copyright 2024 Peter Dimov
 // Distributed under the Boost Software License, Version 1.0.
 // https://www.boost.org/LICENSE_1_0.txt
 
 #include <limits>
 #include <cstdint>
 #include <cstddef>
 
 namespace boost {
 namespace uuids {
 namespace detail {
 
 class chacha20_12
 {
 private:
 
     std::uint32_t state_[ 16 ];
     std::uint32_t block_[ 16 ];
     std::size_t index_;
 
 private:
 
     static inline std::uint32_t rotl( std::uint32_t x, int n ) noexcept
     {
         return ( x << n ) | ( x >> (32 - n) );
     }
 
     static inline void quarter_round( std::uint32_t (&x)[ 16 ], int a, int b, int c, int d ) noexcept
     {
         x[ a ] += x[ b ]; x[ d ] = rotl( x[d] ^ x[a], 16 );
         x[ c ] += x[ d ]; x[ b ] = rotl( x[b] ^ x[c], 12 );
         x[ a ] += x[ b ]; x[ d ] = rotl( x[d] ^ x[a],  8 );
         x[ c ] += x[ d ]; x[ b ] = rotl( x[b] ^ x[c],  7 );
     }
 
     void get_next_block() noexcept
     {
         for( int i = 0; i < 16; ++i )
         {
             block_[ i ] = state_[ i ];
         }
 
         for( int i = 0; i < 6; ++i )
         {
             quarter_round( block_, 0, 4,  8, 12 );
             quarter_round( block_, 1, 5,  9, 13 );
             quarter_round( block_, 2, 6, 10, 14 );
             quarter_round( block_, 3, 7, 11, 15 );
             quarter_round( block_, 0, 5, 10, 15 );
             quarter_round( block_, 1, 6, 11, 12 );
             quarter_round( block_, 2, 7,  8, 13 );
             quarter_round( block_, 3, 4,  9, 14 );
         }
 
         for( int i = 0; i < 16; ++i )
         {
             block_[ i ] += state_[ i ];
         }
 
         if( ++state_[ 12 ] == 0 ) ++state_[ 13 ];
     }
 
 public:
 
     using result_type = std::uint32_t;
 
     chacha20_12() noexcept: index_( 16 )
     {
         state_[ 0 ] = 0x61707865;
         state_[ 1 ] = 0x3320646e;
         state_[ 2 ] = 0x79622d32;
         state_[ 3 ] = 0x6b206574;
 
         for( int i = 4; i < 16; ++i )
         {
             state_[ i ] = 0;
         }
     }
 
     chacha20_12( std::uint32_t const (&key)[ 8 ], std::uint32_t const (&nonce)[ 2 ] ) noexcept: index_( 16 )
     {
         state_[ 0 ] = 0x61707865;
         state_[ 1 ] = 0x3320646e;
         state_[ 2 ] = 0x79622d32;
         state_[ 3 ] = 0x6b206574;
 
         for( int i = 0; i < 8; ++i )
         {
             state_[ i + 4 ] = key[ i ];
         }
 
         state_[ 12 ] = 0;
         state_[ 13 ] = 0;
 
         state_[ 14 ] = nonce[ 0 ];
         state_[ 15 ] = nonce[ 1 ];
     }
 
     // only needed because basic_random_generator looks for it
     void seed() noexcept
     {
         index_ = 16;
 
         for( int i = 4; i < 16; ++i )
         {
             state_[ i ] = 0;
         }
     }
 
     template<class Seq> void seed( Seq& seq )
     {
         index_ = 16;
 
         seq.generate( state_ + 4, state_ + 16 );
 
         // reset counter
         state_[ 12 ] = 0;
         state_[ 13 ] = 0;
     }
 
     // perturbs the generator state so that it no longer generates
     // the same sequence; useful for e.g. moved from objects
     void perturb() noexcept
     {
         index_ = 16;
 
         for( int i = 12; i < 16; ++i )
         {
             ++state_[ i ];
         }
     }
 
     static constexpr result_type min() noexcept
     {
         return std::numeric_limits<result_type>::min();
     }
 
     static constexpr result_type max() noexcept
     {
         return std::numeric_limits<result_type>::max();
     }
 
     result_type operator()() noexcept
     {
         if( index_ == 16 )
         {
             get_next_block();
             index_ = 0;
         }
 
         return block_[ index_++ ];
     }
 };
 
 }}} // namespace boost::uuids::detail
 
 #endif // #ifndef BOOST_UUID_DETAIL_SHA1_HPP_INCLUDED

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