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Fast Double Parse

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FastDoubleParse.cs
C#
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public static bool TryParseDoubleFastStream(string s, int begin, int end, out double result)
{
result = 0;
char c = s[begin];
int sign = 0;
int start = begin;
 
if (c == '-')
{
sign = -1;
start = begin + 1;
}
else if (c > 57 || c < 48)
{
if (StandardFunctions.IgnoreChar(c))
{
do
{
++start;
}
while (start < end && StandardFunctions.IgnoreChar(c = s[start]));
 
if (start >= end)
{
return false;
}
 
if (c == '-')
{
sign = -1;
++start;
}
else
{
sign = 1;
}
}
else
{
result = 0;
return false;
}
}
else
{
start = begin + 1;
result = 10 * result + (c - 48);
sign = 1;
}
 
int i = start;
 
for (; i < end; ++i)
{
c = s[i];
if (c > 57 || c < 48)
{
if (c == '.')
{
++i;
goto DecimalPoint;
}
else
{
result = 0;
return false;
}
}
 
result = 10 * result + (c - 48);
}
 
result *= sign;
return true;
 
DecimalPoint:
 
long temp = 0;
int length = i;
double exponent = 0;
 
for (; i < end; ++i)
{
c = s[i];
if (c > 57 || c < 48)
{
if (!StandardFunctions.IgnoreChar(c))
{
if (c == 'e' || c == 'E')
{
length = i - length;
goto ProcessExponent;
}
 
result = 0;
return false;
}
else
{
length = i - length;
goto ProcessFraction;
}
}
temp = 10 * temp + (c - 48);
}
length = i - length;
 
ProcessFraction:
 
double fraction = (double)temp;
 
if (length < _powLookup.Length)
{
fraction = fraction / _powLookup[length];
}
else
{
fraction = fraction / _powLookup[_powLookup.Length - 1];
}
 
result += fraction;
 
result *= sign;
 
if (exponent > 0)
{
result *= exponent;
}
else if(exponent < 0)
{
result /= -exponent;
}
 
return true;
 
ProcessExponent:
 
int expSign = 1;
int exp = 0;
 
for (++i; i < end; ++i)
{
c = s[i];
if (c > 57 || c < 48)
{
if (c == '-')
{
expSign = -1;
continue;
}
}
 
exp = 10 * exp + (c - 48);
}
 
exponent = _doubleExpLookup[exp] * expSign;
 
goto ProcessFraction;
}
 
private static readonly long[] _powLookup = new[]
{
1, // 10^0
10, // 10^1
100, // 10^2
1000, // 10^3
10000, // 10^4
100000, // 10^5
1000000, // 10^6
10000000, // 10^7
100000000, // 10^8
1000000000, // 10^9,
10000000000, // 10^10,
100000000000, // 10^11,
1000000000000, // 10^12,
10000000000000, // 10^13,
100000000000000, // 10^14,
1000000000000000, // 10^15,
10000000000000000, // 10^16,
100000000000000000, // 10^17,
};
 
private static readonly double[] _doubleExpLookup = GetDoubleExponents();
 
private static double[] GetDoubleExponents()
{
var max = 309;
 
var exps = new double[max];
 
for (var i = 0; i < max; i++)
{
exps[i] = Math.Pow(10, i);
}
 
return exps;
}

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