#!perl -w
use strict;
my %round;
my @lowend;
my $prevend = 0;

# depends on input being sorted
while (<DATA>) {
  if (/(\d+)\s*-\s*(\d+)\s*:\s*(\d+)/) {
    my ($start, $end, $target) = ($1, $2, $3);
    if ($start != $prevend + 1) {
      print STDERR "Warning: prevend=$prevend input=$_"
    }
    $prevend=$end;
    push @lowend, $start;
    $round{$start} = $target;
  }
}


sub customround {
  my ($val) = $_[0];
  my $low = 0;
  my $high = $#lowend;
  # binary search
  while ($low < $high) {
    my $mid = int(($low + $high)/2);
    if ($lowend[$mid] < $val) {
      $low = $mid + 1;
    } elsif ($lowend[$mid] > $val) {
      $high = $mid - 1;
    } else {
      # found exact
      return $round{$lowend[$mid]};
    }
  }

  $low++ unless $low == $#lowend;
  while ($low > 0 && $lowend[$low] > $val) {
    $low--
  }
  return $round{$lowend[$low]};
}


# test
print "customround($_) = ", customround($_), "\n" 
 for qw(0 1 5 6 7 10 14 15 16 19 20 99 100 124 125 126 1499 1500 1501)
+;

__DATA__
# Less than 10
1 - 5:  5
6 - 9: 10

# 10 to 100
10 - 14: 15
15 - 19: 20
20 - 24: 25
etc.

# 100 to 1000
100 - 124: 125
125 - 149: 150
150 - 174: 175
etc.

# 1000 and beyond
1000 - 1249: 1250
1250 - 1549: 1500
1500 - 1749: 1750
etc.
[download]