bignum.h   [plain text]

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#ifndef DOUBLE_CONVERSION_BIGNUM_H_
#define DOUBLE_CONVERSION_BIGNUM_H_

#include "utils.h"

namespace WTF {

namespace double_conversion {
    
    class Bignum {
    public:
        // 3584 = 128 * 28. We can represent 2^3584 > 10^1000 accurately.
        // This bignum can encode much bigger numbers, since it contains an
        // exponent.
        static const int kMaxSignificantBits = 3584;
        
        Bignum();
        void AssignUInt16(uint16_t value);
        void AssignUInt64(uint64_t value);
        void AssignBignum(const Bignum& other);
        
        void AssignDecimalString(Vector<const char> value);
        void AssignHexString(Vector<const char> value);
        
        void AssignPowerUInt16(uint16_t base, int exponent);
        
        void AddUInt16(uint16_t operand);
        void AddUInt64(uint64_t operand);
        void AddBignum(const Bignum& other);
        // Precondition: this >= other.
        void SubtractBignum(const Bignum& other);
        
        void Square();
        void ShiftLeft(int shift_amount);
        void MultiplyByUInt32(uint32_t factor);
        void MultiplyByUInt64(uint64_t factor);
        void MultiplyByPowerOfTen(int exponent);
        void Times10() { return MultiplyByUInt32(10); }
        // Pseudocode:
        //  int result = this / other;
        //  this = this % other;
        // In the worst case this function is in O(this/other).
        uint16_t DivideModuloIntBignum(const Bignum& other);
        
        bool ToHexString(char* buffer, int buffer_size) const;
        
        static int Compare(const Bignum& a, const Bignum& b);
        static bool Equal(const Bignum& a, const Bignum& b) {
            return Compare(a, b) == 0;
        }
        static bool LessEqual(const Bignum& a, const Bignum& b) {
            return Compare(a, b) <= 0;
        }
        static bool Less(const Bignum& a, const Bignum& b) {
            return Compare(a, b) < 0;
        }
        // Returns Compare(a + b, c);
        static int PlusCompare(const Bignum& a, const Bignum& b, const Bignum& c);
        // Returns a + b == c
        static bool PlusEqual(const Bignum& a, const Bignum& b, const Bignum& c) {
            return PlusCompare(a, b, c) == 0;
        }
        // Returns a + b <= c
        static bool PlusLessEqual(const Bignum& a, const Bignum& b, const Bignum& c) {
            return PlusCompare(a, b, c) <= 0;
        }
        // Returns a + b < c
        static bool PlusLess(const Bignum& a, const Bignum& b, const Bignum& c) {
            return PlusCompare(a, b, c) < 0;
        }
    private:
        typedef uint32_t Chunk;
        typedef uint64_t DoubleChunk;
        
        static const int kChunkSize = sizeof(Chunk) * 8;
        static const int kDoubleChunkSize = sizeof(DoubleChunk) * 8;
        // With bigit size of 28 we loose some bits, but a double still fits easily
        // into two chunks, and more importantly we can use the Comba multiplication.
        static const int kBigitSize = 28;
        static const Chunk kBigitMask = (1 << kBigitSize) - 1;
        // Every instance allocates kBigitLength chunks on the stack. Bignums cannot
        // grow. There are no checks if the stack-allocated space is sufficient.
        static const int kBigitCapacity = kMaxSignificantBits / kBigitSize;
        
        void EnsureCapacity(int size) {
            if (size > kBigitCapacity) {
                UNREACHABLE();
            }
        }
        void Align(const Bignum& other);
        void Clamp();
        bool IsClamped() const;
        void Zero();
        // Requires this to have enough capacity (no tests done).
        // Updates used_digits_ if necessary.
        // shift_amount must be < kBigitSize.
        void BigitsShiftLeft(int shift_amount);
        // BigitLength includes the "hidden" digits encoded in the exponent.
        int BigitLength() const { return used_digits_ + exponent_; }
        Chunk BigitAt(int index) const;
        void SubtractTimes(const Bignum& other, int factor);
        
        Chunk bigits_buffer_[kBigitCapacity];
        // A vector backed by bigits_buffer_. This way accesses to the array are
        // checked for out-of-bounds errors.
        Vector<Chunk> bigits_;
        int used_digits_;
        // The Bignum's value equals value(bigits_) * 2^(exponent_ * kBigitSize).
        int exponent_;
        
        DISALLOW_COPY_AND_ASSIGN(Bignum);
    };
    
}  // namespace double_conversion

} // namespace WTF

#endif  // DOUBLE_CONVERSION_BIGNUM_H_