mergeExternal

fn mergeExternal(
    comptime T: type,
    items: []T,
    A: Range,
    B: Range,
    cache: []T,
    context: anytype,
    comptime lessThan: fn (@TypeOf(context), lhs: T, rhs: T) bool,
) void {
    // A fits into the cache, so use that instead of the internal buffer
    var A_index: usize = 0;
    var B_index: usize = B.start;
    var insert_index: usize = A.start;
    const A_last = A.length();
    const B_last = B.end;

    if (B.length() > 0 and A.length() > 0) {
        while (true) {
            if (!lessThan(context, items[B_index], cache[A_index])) {
                items[insert_index] = cache[A_index];
                A_index += 1;
                insert_index += 1;
                if (A_index == A_last) break;
            } else {
                items[insert_index] = items[B_index];
                B_index += 1;
                insert_index += 1;
                if (B_index == B_last) break;
            }
        }
    }

    // copy the remainder of A into the final array
    const cache_a = cache[A_index..A_last];
    @memcpy(items[insert_index..][0..cache_a.len], cache_a);
}