Monotone cubic interpolation

http://en.wikipedia.org/wiki/Monotone_cubic_interpolation

We have implemented it using the formula from Wikipedia :

```
public class MonotoneCubicSplineInterpolation
{
public static double[] Calc(double[] xs, double[] ys, double[] x_interp)
{
var length = xs.Length;

// Deal with length issues
if (length != ys.Length)
{
IPDevLoggerWrapper.Error("Need an equal count of xs and ys");
throw new Exception("Need an equal count of xs and ys");
}
if (length == 0)
{
return null;
}
if (length == 1)
{
return new double[] {ys[0]};
}

// Get consecutive differences and slopes
var delta = new double[length - 1];
var m = new double[length];

for (int i = 0; i 0)
{
m[i] = (delta[i - 1] + delta[i]) / 2;
}
}
var toFix = new List();
for (int i = 1; i 0 && delta[i - 1] 0))
{
toFix.Add(i);
}
}
foreach (var val in toFix)
{
m[val] = 0;
}

m[0] = delta[0];
m[length - 1] = delta[length - 2];

toFix.Clear();
for (int i = 0; i 9)
{
toFix.Add(i);
}
}

foreach (var val in toFix)
{
m[val] = tau[val] alpha[val] delta[val];
m[val + 1] = tau[val] beta[val] delta[val];
}

var y_interp = new double[x_interp.Length];
int ind = 0;

foreach (var x in x_interp)
{
int i;
for (i = xs.Length - 2; i >= 0; --i)
{
if (xs[i] <= x)

Don't throw Exception

throw new Exception("Need an equal count of xs and ys");

It forces client code to catch any subclass of Exception. In this case I would throw an ArgumentException.

The continue at the end of the last loop is redundant.

Here you're using double.Equals

if (delta[i] == 0)

From MSDN


The Equals method should be used with caution, because two apparently
equivalent values can be unequal due to the differing precision of the
two values.

That link covers two techniques for dealing with this.

As far as I can tell, toFix can be removed. For example,

var toFix = new List();
for (int i = 1; i  0 && delta[i - 1]  0))
    {
        toFix.Add(i);
    }
}
foreach (var val in toFix)
{
    m[val] = 0;
}

Can be rewritten as

for (int i = 1; i  0 && delta[i - 1]  0))
    {
        m[i] = 0;
    }
}

StackExchange Code Review Q#73622, answer score: 4