#!/usr/bin/env perl
use v5.36; # implies warnings
no warnings q/experimental::for_list/;
no warnings q/experimental::builtin/;
use builtin    qw/indexed/;
use List::Util qw/sum/;

my $target_weight = shift // die 'need target_weight';

my @weights = ( 20, 3, 25, 10, 3, 24, 25 );

say "Before:\n" . display( \@weights, $target_weight );
shrink( \@weights, $target_weight );
say "After:\n" . display( \@weights, $target_weight );
die if sum(@weights) != $target_weight;

sub shrink ( $bags, $target_weight ) {
    my $curr_weight = sum @$bags;
    return if ( $curr_weight <= $target_weight ); # no shrink req'd
    my @refs = sort { ${$b} <=> ${$a} } map \$_, @$bags;
  BAG:
    for my ($i, $ref) ( indexed @refs ) {
        my $next_wt = $i < $#refs ?
            ${$refs[$i+1]} : 0;
        my $drop = $$ref - $next_wt;
        my $lowered_weight = $curr_weight - $drop * ( $i + 1 );
        if ( $lowered_weight >= $target_weight ) {
            for ( 0 .. $i ) {
                ${$refs[ $_ ]} -= $drop;
            }
            $curr_weight = $lowered_weight;
        }
        else {
            use integer;
            my $target_loss = $curr_weight - $target_weight;
            my $div         = $target_loss / ( 1 + $i );
            my $rem         = $target_loss % ( 1 + $i );

            for ( reverse 0 .. $i ) {
                ${$refs[ $_ ]} -= $div + ( $rem-- > 0 ? 1 : 0 );
            }
            last BAG;
        }
    }
}

sub display ($aref, $target) {
    my $r = '';
    for my ( $i, $wt ) ( indexed @$aref ) {
        $r .= sprintf "  %2s: {%s} (%d)\n",
            "#$i", ( '=' x $wt ), $wt;
    }
    $r .= sprintf "Weight %d, target=%d\n",
        sum(@$aref), $target;
    return $r;
}

##</code><code>##

$ shrink 79
Before:
  #0: {====================} (20)
  #1: {===} (3)
  #2: {=========================} (25)
  #3: {==========} (10)
  #4: {===} (3)
  #5: {========================} (24)
  #6: {=========================} (25)
Weight 110, target=79

After:
  #0: {===============} (15)
  #1: {===} (3)
  #2: {================} (16)
  #3: {==========} (10)
  #4: {===} (3)
  #5: {================} (16)
  #6: {================} (16)
Weight 79, target=79