simple template metaprogramming example

[Deewiant]

Paulo Herrera wrote:
> I've been experimenting with D's template capabilities to speed up some
> number crunching codes.
> In that process I adapted the evaluation of the dot product of two
> arrays, originally in C++ and found at:
> http://osl.iu.edu/~tveldhui/papers/techniques/.
> 
> I think the D version is very concise and simple to understand, so it
> could be added to the examples given in the templates documentation. On
> the other hand, this kind of examples could motivate people working on
> numerical codes to try D.
> 

Nice. Function templates make the code even nicer-looking, IMHO:

import std.stdio;

T dot_product(T, size_t N)(T[N] a, T[N] b) {
	return meta_dot!(N-1, N, T)(a,b);
}

T meta_dot(size_t I, size_t N, T)(T[N] a, T[N] b) {
	static if (I == 0)
		return a[0]*b[0];
	else
		return a[I]*b[I] + meta_dot!(I-1, N, T)(a,b);
}

void main() {
	float[3] a;
	float[3] b;
	a[] = 0.5;
	b[] = 0.5;
	float dot = dot_product(a, b); // note how implicit function template
instantiation means we don't need to specify float and 3
	writefln("dot: %f", dot);
}


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