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tombowers/nth-fibonacci.cs

Last active August 29, 2015 14:27
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nth-fibonacci.cs
public long FibonacciNumber(long n)
{
if (n > 92)
throw new ArgumentOutOfRangeException("n", "Fib(>92) will cause a 64-bit integer overflow.");
if (n < -92)
throw new ArgumentOutOfRangeException("n", "Fib(<-92) will cause a 64-bit integer overflow.");
var sqrt5 = Math.Sqrt(5);
var bigPhi = (sqrt5 + 1) / 2;
var miniPhi = bigPhi - 1;
return Convert.ToInt64((Math.Pow(bigPhi, n) - Math.Pow(-miniPhi, n)) / sqrt5);
}
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