#!/usr/bin/perl -w
# Derived from code by Nathan Torkington.
use strict;
use integer;

my ($word, $file) = @ARGV;
open (FILE, $file) or die "Can't open $file: $!";
my $position = binary_search_file(\*FILE, $word);

if (defined $position) { print "$word occurs at position $position\n" }
else                   { print "$word does not occur in $file.\n" }

sub binary_search_file {
    my ( $file, $word ) = @_;
    my ( $high, $low, $mid, $mid2, $line );
    $low  = 0;                  # Guaranteed to be the start of a line.
    $high = (stat($file))[7];   # Might not be the start of a line.
    $word =~ s/\W//g;           # Remove punctuation from $word.
    $word = lc($word);          # Convert $word to lower case.

    while ($high != $low) {
        $mid = ($high+$low)/2;
        seek($file, $mid, 0) || die "Couldn't seek : $!\n";

        # $mid is probably in the middle of a line, so read the rest
        # and set $mid2 to that new position.
        $line = <$file>;
        $mid2 = tell($file);

        if ($mid2 < $high) {    # We're not near file's end, so read on.
            $mid  = $mid2;
            $line = <$file>;
        } else {   # $mid plunked us in the last line, so linear search.
            seek($file, $low, 0) || die "Couldn't seek: $!\n";
            while ( defined( $line = <$file> ) ) {
                last if compare( $line, $word ) >= 0;
                $low = tell($file);
            }
            last;
        }

        if (compare($line, $word) < 0) { $low  = $mid }
        else                           { $high = $mid }
    }

    return if compare( $line, $word );
    return $low;
}

sub compare {   # $word1 needs to be lowercased; $word2 doesn't.
    my ($word1, $word2) = @_;
    $word1 =~ s/\W//g; $word1 = lc($word1);
    return $word1 cmp $word2;
}