Fix Cayley distance calculation by reordering search and assignment

[Copilot]

The Cayley distance implementation had lines 60-61 in the wrong order, causing the search operation to include the just-modified index.

Changes

• src/distances.cpp: Swap lines 60-61 to search for swap positions before modifying the array

Technical Details

The bug:

double tmp1 = tmp2(i);
tmp2(i) = r2(i);                 // Modifies tmp2(i) first
uvec inds = find(tmp2 == r2(i)); // Search finds the just-set value at index i
tmp2.elem(inds).fill(tmp1);      // Incorrectly overwrites tmp2(i)

The fix:

double tmp1 = tmp2(i);
uvec inds = find(tmp2 == r2(i)); // Search before modification
tmp2(i) = r2(i);                 // Then modify
tmp2.elem(inds).fill(tmp1);

This ensures the algorithm finds the correct swap positions in the unmodified array.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Subtle bug?</issue_title>
<issue_description>I wonder if lines 60 and 61 should be swapped to avoid potential compiler issues.

BayesMallows/src/distances.cpp

Lines 52 to 66 in 4b60017

	double CayleyDistance::d(const vec& r1, const vec& r2) {
	double distance{};
	vec tmp2 = r1;
	for(size_t i{}; i < r1.n_elem; ++i){
	if(tmp2(i) != r2(i)) {
	distance += 1;
	double tmp1 = tmp2(i);
	tmp2(i) = r2(i);
	uvec inds = find(tmp2 == r2(i));
	tmp2.elem(inds).fill(tmp1);
	}
	}
	return distance;
	}

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Subtle bug? #442

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