$cache = proximityCache(\%hash, $range, $step);
$value = getCached($cache, $key);


sub proximityCache {
  my ($mi,$span,$step) = @_;
  my ($min, $max);
  
  while (my ($m) = each %$mi) {
    $min = $m if not defined($min) or $min > $m;
    $max = $m if not defined($max) or $max < $m;
  }
  
  my $latest = $min - $span;
  my %cache = ();
  
  for (my $m = $min - $span; $m <= $max + $span; $m += $step) {
    
    $latest = $m - $span if $m - $span > $latest;

    for my $d ($latest .. ($m + $span)) {
      $cache{$m} = [ $mi->{$d}, $latest = $d ]
        if exists $mi->{$d} and (
          not($cache{$m}) or
          $cache{$m}[0] < $mi->{$d}
        );
    }
  }
  
  return \%cache;
}


sub getCached {
   my ($cache, $key) = @_;
   return $cache->{$key}[0];
}
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