#!/usr/bin/perl -w
my $usage="
merch_match <FName1> <MID Col> <FName2> <MID Col>

Prints all Merchant IDs found in File 1 that are also
contained in File 2.
";

my %MIDS;
my $FName1 = shift or die $usage . "Missing FName1";
my $MIDcol1 = shift or die $usage . "Missing MID COL #1";
my $FName2 = shift or die $usage . "Missing FName1";
my $MIDcol2 = shift or die $usage . "Missing MID COL #2";

# substr uses 0-based string offsets
$MIDcol1--;
$MIDcol2--;

# Gather MID numbers from File 1
open(INF,'<',$FName1) or die $usage . "Can't open " . $FName1;
while (<INF>) {
    my $M = substr($_, $MIDcol1, 16);
    $MIDS{$M}=0;
}
close(INF);

# Gather MIDs from File 2
open(INF,'<',$FName2) or die $usage . "Can't open " . $FName2;
while (<INF>) {
    my $M = substr($_, $MIDcol2, 16);
    ++$MIDS{$M} if exists $MIDS{$M};
}
close(INF);

# Print match list
for my $M (sort keys %MIDS) {
    print $M if exists $MIDS{$M};
}