Hi BrowserUk,

this is my attempt, using a hash for the breakpoint lookup and an array of sorted keys of that hash for a binary search. I also stored the calculation in an anonymous closure to avoid repeating the sorting of the keys and so on for each call (if this is not an option under JS, maybe you can use static variables or even global variables, or whatever, I haven't used JS for about 10 years).

use strict;
use warnings;

my $lookup = create_lookup();

printf "%14d : %14d \n", $_, $lookup->($_) for (qw /24000 25000 51000 
+85000000 670000 260000000 78893999998 157788000001/);

sub create_lookup {
    my %lookup_h = qw /
               25000        2500
               50000        5000
              150000       12500
              225000       25000
              300000       37500
              600000       60000
             1200000      120000
             3600000      300000
             5400000      600000
            10800000      900000
            21600000     1800000
            43200000     3600000
            64800000     7200000
           129600000    10800000
           216000000    21600000
           432000000    43200000
           864000000    86400000
          1728000000   172800000
          3024000000   345600000
          6048000000   604800000
         12096000000  1209600000
         31557600000  2629800000
         63115200000  5259600000
         78894000000  7889400000
        157788000000 15778800000 /;
    
    my @keys = sort {$a <=> $b} keys %lookup_h;
    my $size = scalar @keys;
    my $ceil = $keys[-1];
    my $ceil_val = = $lookup_h{$ceil};
    
    return sub {
        my $v = shift;
        return $ceil_val if $v >= $ceil;
        return $lookup_h{$keys[0]} if $v < $keys[0];
        my ($min, $max) = (0, $size);
        while (1) {
            my $mid = int (($min + $max)/2);
            return $lookup_h{$keys[$max]} if $max - $min <= 1;
            $min = $mid and next if $v > $keys[$mid];    
            $max = $mid;
        }
    }
}
[download]

This prints (correctly, I think):

$ perl lookup.pl
         24000 :           2500
         25000 :           5000
         51000 :          12500
      85000000 :       10800000
        670000 :         120000
     260000000 :       43200000
   78893999998 :     7889400000
  157788000001 :    15778800000
[download]

but I have not thoroughly tested edge cases.

Update: fixed a couple of errors, especially when the input value is smaller than the first value in the hash.

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