mergeInto

fn mergeInto(
    comptime T: type,
    from: []T,
    A: Range,
    B: Range,
    into: []T,
    context: anytype,
    comptime lessThan: fn (@TypeOf(context), lhs: T, rhs: T) bool,
) void {
    var A_index: usize = A.start;
    var B_index: usize = B.start;
    const A_last = A.end;
    const B_last = B.end;
    var insert_index: usize = 0;

    while (true) {
        if (!lessThan(context, from[B_index], from[A_index])) {
            into[insert_index] = from[A_index];
            A_index += 1;
            insert_index += 1;
            if (A_index == A_last) {
                // copy the remainder of B into the final array
                const from_b = from[B_index..B_last];
                @memcpy(into[insert_index..][0..from_b.len], from_b);
                break;
            }
        } else {
            into[insert_index] = from[B_index];
            B_index += 1;
            insert_index += 1;
            if (B_index == B_last) {
                // copy the remainder of A into the final array
                const from_a = from[A_index..A_last];
                @memcpy(into[insert_index..][0..from_a.len], from_a);
                break;
            }
        }
    }
}