Ejemplo n.º 1
0
        // Reads a DiyFp from the buffer.
        // The returned DiyFp is not necessarily normalized.
        // If remaining_decimals is zero then the returned DiyFp is accurate.
        // Otherwise it has been rounded and has error of at most 1/2 ulp.
        static void ReadDiyFp(Vector buffer,
                              out DiyFp result,
                              out int remaining_decimals)
        {
            int      read_digits;
            uint64_t significand = ReadUint64(buffer, out read_digits);

            if (buffer.length() == read_digits)
            {
                result             = new DiyFp(significand, 0);
                remaining_decimals = 0;
            }
            else
            {
                // Round the significand.
                if (buffer[read_digits] >= '5')
                {
                    significand++;
                }
                // Compute the binary exponent.
                int exponent = 0;
                result             = new DiyFp(significand, exponent);
                remaining_decimals = buffer.length() - read_digits;
            }
        }
Ejemplo n.º 2
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        public static void GetCachedPowerForDecimalExponent(int requested_exponent,
                                                            out DiyFp power,
                                                            out int found_exponent)
        {
            int         index        = (requested_exponent + kCachedPowersOffset) / kDecimalExponentDistance;
            CachedPower cached_power = kCachedPowers[index];

            power          = new DiyFp(cached_power.significand, cached_power.binary_exponent);
            found_exponent = cached_power.decimal_exponent;
        }
Ejemplo n.º 3
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        public static void GetCachedPowerForBinaryExponentRange(
            int min_exponent,
            int max_exponent,
            out DiyFp power,
            out int decimal_exponent)
        {
            int    kQ    = DiyFp.kSignificandSize;
            double k     = Math.Ceiling((min_exponent + kQ - 1) * kD_1_LOG2_10);
            int    foo   = kCachedPowersOffset;
            int    index = (foo + (int)(k) - 1) / kDecimalExponentDistance + 1;

            CachedPower cached_power = kCachedPowers[index];

            // (void)max_exponent;  // Mark variable as used.
            decimal_exponent = cached_power.decimal_exponent;
            power            = new DiyFp(cached_power.significand, cached_power.binary_exponent);
        }
Ejemplo n.º 4
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        // Computes the two boundaries of this.
        // The bigger boundary (m_plus) is normalized. The lower boundary has the same
        // exponent as m_plus.
        // Precondition: the value encoded by this Single must be greater than 0.
        public void NormalizedBoundaries(out DiyFp out_m_minus, out DiyFp out_m_plus)
        {
            DiyFp v      = this.AsDiyFp();
            var   __     = new DiyFp((v.f << 1) + 1, v.e - 1);
            DiyFp m_plus = DiyFp.Normalize(ref __);
            DiyFp m_minus;

            if (LowerBoundaryIsCloser())
            {
                m_minus = new DiyFp((v.f << 2) - 1, v.e - 2);
            }
            else
            {
                m_minus = new DiyFp((v.f << 1) - 1, v.e - 1);
            }
            m_minus.f   = (m_minus.f << (m_minus.e - m_plus.e));
            m_minus.e   = (m_plus.e);
            out_m_plus  = m_plus;
            out_m_minus = m_minus;
        }
Ejemplo n.º 5
0
        public static ulong DiyFpToUint64(DiyFp diy_fp)
        {
            ulong significand = diy_fp.f;
            int   exponent    = diy_fp.e;

            while (significand > kHiddenBit + kSignificandMask)
            {
                significand >>= 1;
                exponent++;
            }
            if (exponent >= kMaxExponent)
            {
                return(kInfinity);
            }
            if (exponent < kDenormalExponent)
            {
                return(0);
            }
            while (exponent > kDenormalExponent && (significand & kHiddenBit) == 0)
            {
                significand <<= 1;
                exponent--;
            }
            ulong biased_exponent;

            if (exponent == kDenormalExponent && (significand & kHiddenBit) == 0)
            {
                biased_exponent = 0;
            }
            else
            {
                biased_exponent = (ulong)(exponent + kExponentBias);
            }
            return((significand & kSignificandMask) |
                   (biased_exponent << kPhysicalSignificandSize));
        }
Ejemplo n.º 6
0
        // If the function returns true then the result is the correct double.
        // Otherwise it is either the correct double or the double that is just below
        // the correct double.
        static bool DiyFpStrtod(Vector buffer,
                                int exponent,
                                out double result)
        {
            DiyFp input;
            int   remaining_decimals;

            ReadDiyFp(buffer, out input, out remaining_decimals);
            // Since we may have dropped some digits the input is not accurate.
            // If remaining_decimals is different than 0 than the error is at most
            // .5 ulp (unit in the last place).
            // We don't want to deal with fractions and therefore keep a common
            // denominator.
            const int kDenominatorLog = 3;
            const int kDenominator    = 1 << kDenominatorLog;

            // Move the remaining decimals into the exponent.
            exponent += remaining_decimals;
            uint64_t error = (ulong)(remaining_decimals == 0 ? 0 : kDenominator / 2);

            int old_e = input.e;

            input.Normalize();
            error <<= old_e - input.e;

            if (exponent < PowersOfTenCache.kMinDecimalExponent)
            {
                result = 0.0;
                return(true);
            }
            DiyFp cached_power;
            int   cached_decimal_exponent;

            PowersOfTenCache.GetCachedPowerForDecimalExponent(exponent,
                                                              out cached_power,
                                                              out cached_decimal_exponent);

            if (cached_decimal_exponent != exponent)
            {
                int   adjustment_exponent = exponent - cached_decimal_exponent;
                DiyFp adjustment_power    = AdjustmentPowerOfTen(adjustment_exponent);
                input.Multiply(ref adjustment_power);
                if (kMaxUint64DecimalDigits - buffer.length() >= adjustment_exponent)
                {
                    // The product of input with the adjustment power fits into a 64 bit
                    // integer.
                }
                else
                {
                    // The adjustment power is exact. There is hence only an error of 0.5.
                    error += kDenominator / 2;
                }
            }

            input.Multiply(ref cached_power);
            // The error introduced by a multiplication of a*b equals
            //   error_a + error_b + error_a*error_b/2^64 + 0.5
            // Substituting a with 'input' and b with 'cached_power' we have
            //   error_b = 0.5  (all cached powers have an error of less than 0.5 ulp),
            //   error_ab = 0 or 1 / kDenominator > error_a*error_b/ 2^64
            int error_b     = kDenominator / 2;
            int error_ab    = (error == 0 ? 0 : 1); // We round up to 1.
            int fixed_error = kDenominator / 2;

            error += (ulong)(error_b + error_ab + fixed_error);

            old_e = input.e;
            input.Normalize();
            error <<= old_e - input.e;

            // See if the double's significand changes if we add/subtract the error.
            int order_of_magnitude         = DiyFp.kSignificandSize + input.e;
            int effective_significand_size = Double.SignificandSizeForOrderOfMagnitude(order_of_magnitude);
            int precision_digits_count     = DiyFp.kSignificandSize - effective_significand_size;

            if (precision_digits_count + kDenominatorLog >= DiyFp.kSignificandSize)
            {
                // This can only happen for very small denormals. In this case the
                // half-way multiplied by the denominator exceeds the range of an uint64.
                // Simply shift everything to the right.
                int shift_amount = (precision_digits_count + kDenominatorLog) -
                                   DiyFp.kSignificandSize + 1;
                input.f = (input.f >> shift_amount);
                input.e = (input.e + shift_amount);
                // We add 1 for the lost precision of error, and kDenominator for
                // the lost precision of input.f().
                error = (error >> shift_amount) + 1 + kDenominator;
                precision_digits_count -= shift_amount;
            }
            // We use uint64_ts now. This only works if the DiyFp uses uint64_ts too.
            uint64_t one64 = 1;
            uint64_t precision_bits_mask = (one64 << precision_digits_count) - 1;
            uint64_t precision_bits      = input.f & precision_bits_mask;
            uint64_t half_way            = one64 << (precision_digits_count - 1);

            precision_bits *= kDenominator;
            half_way       *= kDenominator;
            DiyFp rounded_input = new DiyFp(input.f >> precision_digits_count, input.e + precision_digits_count);

            if (precision_bits >= half_way + error)
            {
                rounded_input.f = (rounded_input.f + 1);
            }
            // If the last_bits are too close to the half-way case than we are too
            // inaccurate and round down. In this case we return false so that we can
            // fall back to a more precise algorithm.

            result = new Double(rounded_input).value();
            if (half_way - error < precision_bits && precision_bits < half_way + error)
            {
                // Too imprecise. The caller will have to fall back to a slower version.
                // However the returned number is guaranteed to be either the correct
                // double, or the next-lower double.
                return(false);
            }
            else
            {
                return(true);
            }
        }
Ejemplo n.º 7
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 public Double(DiyFp d)
 {
     d64_ = DiyFpToUint64(d);
 }