bigint.hpp

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#ifndef __POWGEN_BIGINT_HPP__
#define __POWGEN_BIGINT_HPP__
 
#include <vector>
 
#include <cstdint>
#include <cstddef>
 
struct Power
{
    uint64_t hi;
    uint64_t lo;
};
 
class BigInt
{
public:
    explicit BigInt (uint64_t value = 0)
    {
        if (value == 0)
        {
            data.push_back (0);
        }
        else
        {
            data.push_back (static_cast <uint32_t> (value));
            uint32_t high = static_cast <uint32_t> (value >> 32);
            if (high != 0)
            {
                data.push_back (high);
            }
        }
    }
 
    void multiply (uint32_t factor)
    {
        uint64_t carry = 0;
        for (size_t i = 0; i < data.size (); ++i)
        {
            uint64_t product = static_cast <uint64_t> (data[i]) * factor + carry;
            data[i] = static_cast <uint32_t> (product);
            carry = product >> 32;
        }
        if (carry != 0)
        {
            data.push_back (static_cast <uint32_t> (carry));
        }
    }
 
    void divide (uint32_t divisor)
    {
        uint64_t remainder = 0;
        for (int i = static_cast <int> (data.size ()) - 1; i >= 0; --i)
        {
            uint64_t current = (remainder << 32) | data[i];
            data[i] = static_cast <uint32_t> (current / divisor);
            remainder = current % divisor;
        }
        removeLeadingZeros ();
    }
 
    void shiftLeft (int shift)
    {
        if (shift == 0) return;
 
        int blockShift = shift / 32;
        int bitShift = shift % 32;
 
        if (blockShift > 0)
        {
            data.insert (data.begin (), blockShift, 0);
        }
 
        if (bitShift > 0)
        {
            uint32_t carry = 0;
            for (size_t i = blockShift; i < data.size (); ++i)
            {
                uint64_t temp = (static_cast <uint64_t> (data[i]) << bitShift) | carry;
                data[i] = static_cast <uint32_t> (temp);
                carry = static_cast <uint32_t> (temp >> 32);
            }
            if (carry != 0)
            {
                data.push_back (carry);
            }
        }
    }
 
    static BigInt powerOfTwo (int shift)
    {
        BigInt result;
        result.data.clear ();
        
        int chunks = shift / 32;
        int bit = shift % 32;
        
        result.data.resize (chunks, 0);
        result.data.push_back (1U << bit);
        
        return result;
    }
 
    Power getTop128 () const
    {
        if (data.empty () || (data.size () == 1 && data[0] == 0))
        {
            return {0, 0};
        }
 
        int msbBlock = static_cast <int> (data.size ()) - 1;
        uint32_t msbValue = data[msbBlock];
 
        int msbBit = 31;
        while (msbBit > 0 && ((msbValue >> msbBit) & 1) == 0)
        {
            --msbBit;
        }
 
        int startBit = msbBlock * 32 + msbBit;
 
        uint64_t hi = 0;
        uint64_t lo = 0;
 
        for (int i = 0; i < 64; ++i)
        {
            int bitPos = startBit - i;
            if (bitPos >= 0)
            {
                int block = bitPos / 32;
                if (block < static_cast <int> (data.size ()))
                {
                    int bit = bitPos % 32;
                    uint64_t bitValue = (data[block] >> bit) & 1;
                    hi |= (bitValue << (63 - i));
                }
            }
        }
 
        for (int i = 0; i < 64; ++i)
        {
            int bitPos = startBit - 64 - i;
            if (bitPos >= 0)
            {
                int block = bitPos / 32;
                if (block < static_cast <int> (data.size ()))
                {
                    int bit = bitPos % 32;
                    uint64_t bitValue = (data[block] >> bit) & 1;
                    lo |= (bitValue << (63 - i));
                }
            }
        }
 
        return {hi, lo};
    }
 
private:
    void removeLeadingZeros ()
    {
        while (data.size () > 1 && data.back () == 0)
        {
            data.pop_back ();
        }
    }
 
    std::vector <uint32_t> data;
};
 
#endif