#!/usr/bin/perl -W


#subroutines for calculations

use strict;

sub distanceAB($$$$$$) {
	my ($x1,$y1,$z1,$x2,$y2,$z2) = @_;
	$distance = sqrt (($x2-$x1)**2 + ($y2 -$y1)**2 + ($z2-$z1)**2);
	return sprintf("%4.2f", $distance);
	}

#open files for calculations and modify distance cutoff for target residues

$input=$ARGV[0];

open(IN, "$input");

$num = 0;
$count = 0; 

my $xyz_fmt = '@30a8 @38a8 @46a8';

while ($pdbline = <IN>) {
	if ($pdbline =~ m/ZN1  LG1 X/) 
	{
		@ZN = unpack $xyz_fmt, $pdbline;
		next;
	}
	    
	#find xyz coordinates for other atoms and store in array
	if ($pdbline =~ /^ATOM.*(OD2 ASP|NE2 HIS)/)
	{
		push $Atoms[$num], [unpack $xyz_fmt, $pdbline]; 
		$resname[$num] = substr($pdbline, 16, 3);
		$resnumber[$num] = substr($pdbline, 22, 3);
			
		++$num;
		}
	}		

#count number of potential coordinating atoms
	if ($pdbline =~ /OD2 ASP/ or $pdbline =~ /NE2 HIS/)	{
		++$count;
	}
		
	#calculate Zn to each atom distance
	foreach $i (0..$count) {
	$Zndistance=distanceAB($x1[$ZN[0]],$y1[$ZN[1]],$z1[$ZN[2]],$x2[$Atoms[$i][0]],$y2[$Atoms[$i][1]],$z2[$Atoms[$i][2]]);
	if ($Zndistance < 2.5) {
	$distanceAB=$Zndistance;
	print "$distanceAB \n";
	print "$resname[$i] $resnumber[$i] \n";
	}
	}