The Weekly Challenge ‐ Perl and Raku

CY's Seek for Equilibrium (related to TWC 160)

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This is a POST-official-deadline blogpost.

I would like to talk about my solution on the Week 160 Task 2.

As I can remember, that was a busy week for me and I left the task solving late, completed at 11pm (HKT ≡ GMT+8:00) Sunday. While I was writing the code, I noticed a sequence containing zeros will make my algorithm unable to work, and I saw from the official examples, all items are positive integers. Then I made an ASSUMPTION: "This task concerns only positive integers."

During writing the lengthy algorithm, I made a check and found most of the time "the equilibrium index" laying around the middle of the dataset.

I thought, would we need something like binary search? I used some large numbers to test and it seemed that the values were close to the middle of the sequences.

Hence I wrote this algorithm proudly; it starts from the middle index, and moves one step each time:

    my $success = $hint_lower == $hint_upper;
    return $ind if $success;

    while (!$success) {
        return -1 if $ind < 1 or $ind > $#n-1;

        my $cur_val = $n[$ind];

        # Modify the two portions
        if ($step == +1) {
            $hint_lower = $hint_lower + $n[$expired_ind];
            $hint_upper = $hint_upper - $cur_val;
        elsif ($step == -1) {
            $hint_lower = $hint_lower - $cur_val;
            $hint_upper = $hint_upper + $n[$expired_ind];

        # Decide which direction going to move
        if ($hint_lower+$cur_val < $hint_upper) {
            $new_ind = $ind+1;
        elsif ($hint_lower > $hint_upper+$cur_val) {
            $new_ind = $ind-1;
        elsif ($hint_lower+$cur_val == $hint_upper) {
            $new_ind = $ind+$step;  # follow the previous direction
        elsif ($hint_lower == $hint_upper+$cur_val) {
            $new_ind = $ind+$step;  # follow the previous direction
        else {
            if ($hint_lower+$cur_val > $hint_upper) {
                $new_ind = $ind+1;
            if ($hint_lower < $hint_upper+$cur_val) {
                $new_ind = $ind-1;

        # Prepare for the next loop block, or, stop
        $step = $new_ind-$ind;
        $success = $hint_lower == $hint_upper;
        return $ind if $success;
        return -1 if $guessed_step == -$step; # No back and fro!
        $guessed_step = $step;
        ($expired_ind, $ind) = ($ind, $new_ind);

On Monday, I compared my solution with teammates'. My solution was the fastest! Then up to some points, I noticed something strange... And I realized the task statement hasn't restricted the numbers to positive integers only. So I misinterpreted a task again.

13 points. Data generated by:

for my $case (1..500) { my @n_temp;
$n_temp[$_] = 1 + int rand(40) for (0..79);
my $ans = ei(@n_temp);
next if $ans == -1; say $ans; }

152 points. Data generated by: (cases of more than one equilibrium indices plotted as other cases of a single equilibrium index)

for my $case (1..500) { my @n_temp;
$n_temp[$_] = 20 - int rand(40) for (0..79);
my @as = ei_simple(@n_temp);
next if scalar @as == 0; say join ", ", @as;}

Note: ei() is the subroutine of finding the equilibrium index/indices.

Therefore I submitted a post-deadline solution. At that time I still thought there might be some concentration behavior towards the middle indices. Just now I plotted the above graphs, the second one do not.

A summary for the behavior of "the equilibrium index/indices":

link for CY's full code: (randomized steps),

Contact on twitter: @e7_87.

Discuss via GitHub issues: here.

Email: fungcheokyin at

Created Date: 19th April, 2022.

Release Date: 28th April, 2022.