mergeInternal

fn mergeInternal(
    comptime T: type,
    items: []T,
    A: Range,
    B: Range,
    buffer: Range,
    context: anytype,
    comptime lessThan: fn (@TypeOf(context), lhs: T, rhs: T) bool,
) void {
    // whenever we find a value to add to the final array, swap it with the value that's already in that spot
    // when this algorithm is finished, 'buffer' will contain its original contents, but in a different order
    var A_count: usize = 0;
    var B_count: usize = 0;
    var insert: usize = 0;

    if (B.length() > 0 and A.length() > 0) {
        while (true) {
            if (!lessThan(context, items[B.start + B_count], items[buffer.start + A_count])) {
                mem.swap(T, &items[A.start + insert], &items[buffer.start + A_count]);
                A_count += 1;
                insert += 1;
                if (A_count >= A.length()) break;
            } else {
                mem.swap(T, &items[A.start + insert], &items[B.start + B_count]);
                B_count += 1;
                insert += 1;
                if (B_count >= B.length()) break;
            }
        }
    }

    // swap the remainder of A into the final array
    blockSwap(T, items, buffer.start + A_count, A.start + insert, A.length() - A_count);
}