Fibonacci numbers in D

fib.d

import std.stdio;
import std.bigint;
import core.checkedint;

void main() {
	writefln("%d", fib_iter(72));
	writefln("%d", fib_binet(72));
}

auto fib_iter (int n) {
	int k;
	ulong a = 0;
	ulong b = 1;
	bool overflow = false;
	if (!overflow) {
		foreach (number; 0..n) {
			// should overflow at n = 92
			ulong c = a;
			a = b;
			b = addu(a, c, overflow);
			if (overflow) {
				k = number + 1;
				break;
			}
		}
	}
	else {
		// overflow; continue from k with bigints
		BigInt ab = a;
		BigInt bb = b;
		foreach (number; k..n) {
			BigInt cb = ab;
			ab = bb;
			bb = ab + cb;
		}
		return ab;
	}
	// we need to upgrade to bigint
	return BigInt(a);
}

auto fib_binet (int n) {
	real phi = (1 + 5.0^^0.5) / 2;
	// should lose precision at n = 72
	return cast(ulong)((phi^^n - (1 - phi)^^n) / 5.0^^0.5);
}