#!/usr/bin/perl -w

###########################################################################
# long-lat v1.0.1
###########################################################################
#
# Parses coordinates selected from a cylindrical projection
# and returns longitude and latitude.
#
# Output
# ######
#
# Returns longitude latitude with appropriate number of decimals
# for the size of the xmax and ymax you input, 
# ie roughly calculates precision.
# This is up to a maximum of 5 decimal places but can be easily extended if
# anyone needs it (by editting the script of course!).
#
# Gives only positive/negative values, not E/W/N/W.
#
# N,E = +   W,S = -
#
# Output values seperated by space.
#
# Input
# #####
#
# long-lat Xmax,Ymax Xgmt X,Y        Expects integer input values.
#
# Xmax and Ymax are the largest values of X and Y respectively
# for the coordinate grid you are using.
# eg max X and Y size of image.
#
# Xgmt is the X value of the zero meridian known as GMT.
#
# X and Y are the coordinates you want to know the longitude and latitude for.
#
# Your values should start at 0, so if they instead start at 1, then
# a simple solution would be to subtract 1 from all values before
# running them through this script.
#
# I could do a whole lot of fancy maths in this script, but half the
# time I think the input would be given this way anyway, so why bother?
#
# ie Make sure that the maximum X you could supply is Xmax, not Xmax - 1
# and make sure the minimum X you could supply is 0, not 1.
# Do the same for Y.
#
# eg if you are using a map which is 300 pixels high, then in fact Ymax = 299,
# because you count the fist pixel as 0, not 1. Get it?
#
# Known bugs: yes, already!
#
# 1
#
# If you give it X = 0 you get -180 if you give it X = Xmax
# you get 180. For pixels on an image this is incorrect,
# there should be a small discrepency, eg x = 0 should give
# -179.5. Who cares?
#
# Let me know if this matters to you and I will fix it!
#
# 2
#
# An extra space is preserved in the output when there is no '-' sign.
#
# This should be seen as a feature, since a) who cares, b) it could be
# nifty if you generated a list of coordinates, they would all line up.
#
##########################################################################
# 21 Nov 2002 Simeon Scott simeonscott@iname.com
# GPL License applies - see http://opensource.org/licenses/gpl-license.php
##########################################################################

use strict;
use Math::Trig;

# Get input and do sanity checking:
unless ($ARGV[0] =~ /^([0-9]+),([0-9]+)$/)
	{die "$0 coords format incorrect.\nPlease use format $0 Xmax,Ymax Xgmt X,Y\n";}
my $xmax = $1;
my $ymax = $2;

unless ($ARGV[1] =~ /^[0-9]+$/)
	{die "$0 coords format incorrect.\nPlease use format $0 Xmax,Ymax Xgmt X,Y\n";}
my $xgmt = $ARGV[1];
unless (0 <= $xgmt)
	{die "$0 coords format incorrect.\nPlease use format $0 Xmax,Ymax Xgmt X,Y\n";}
unless ($xgmt <= $xmax)
	{die "$0 coords format incorrect.\nPlease use format $0 Xmax,Ymax Xgmt X,Y\n";}

unless ($ARGV[2] =~ /^([0-9]+),([0-9]+)$/)
	{die "$0 coords format incorrect.\nPlease use format $0 Xmax,Ymax Xgmt X,Y\n";}
my $x = $1;
my $y = $2;

unless (0 <= $x)
	{die "$0 coords format incorrect.\nPlease use format $0 Xmax,Ymax Xgmt X,Y\n";}
unless ($x <= $xmax)
	{die "$0 coords format incorrect.\nPlease use format $0 Xmax,Ymax Xgmt X,Y\n";}
unless (0 <= $y)
	{die "$0 coords format incorrect.\nPlease use format $0 Xmax,Ymax Xgmt X,Y\n";}
unless ($y <= $ymax)
	{die "$0 coords format incorrect.\nPlease use format $0 Xmax,Ymax Xgmt X,Y\n";}


# Calculate the longitude from the X coordinate value:
my $long = ($x - $xgmt)*360/$xmax;

# Calculate $long precision,
# ie How many longitude degrees in one X pixel?
my $longprec = 360/$xmax;

my $lat;
# Calculate the latitude from Y coordinate value:
	if ($y == $ymax/2)
		{$lat = 0;}

	elsif ($y < $ymax/2)
		{$lat = 90*($ymax - 2*$y)/$ymax;}

	else 
		{$lat = -90*(2*$y - $ymax)/$ymax;}

# Calculate $lat precision,
# ie How many latitude degrees in one Y pixel?
my $latprec = 180/$ymax;


# Decimal precision formatting for $long:
my $longfmt = '@#';
my $multiplyer = 10;
if (abs($long) >= $multiplyer)
{
	$longfmt .= '#';
	$multiplyer = 100;
	if (abs($long) >= $multiplyer)
		{$longfmt .= '#';}
}
$multiplyer = 0.4;
if ($longprec <= $multiplyer)
{
	$longfmt .= '.#';
	$multiplyer = 0.04;
	while ($longprec <= $multiplyer)
	{
		$longfmt .= '#';
		$multiplyer *= 0.1;
	}
}

# Decimal precision formatting for $lat:
my $latfmt = '@#';
$multiplyer = 10;
if (abs($lat) >= $multiplyer)
{
	$latfmt .= '#';
	$multiplyer = 100;
	if (abs($lat) >= $multiplyer)
		{$latfmt .= '#';}
}
$multiplyer = 0.4;
if ($latprec <= $multiplyer)
{
	$latfmt .= '.#';
	$multiplyer = 0.04;
	while ($latprec <= $multiplyer)
	{
		$latfmt .= '#';
		$multiplyer *= 0.1;
	}
}


# Restate $long and $lat as E,W and N,S instead of +,-.
#my ($longdir, $latdir);
#if ($long < 0)
#{
#	$long = -1*$long;
#	$longdir = "W";
#}
#else
#	{$longdir = "E";}
#
#if ($lat < 0)
#{
#	$lat = -1*$lat;
#	$latdir = "S";
#}
#else
#	{$latdir = "N";}


# Generate formatting for output:
my $format = "format STDOUT =\n"
. $longfmt . " " . $latfmt . "\n"
#. '$long,$longdir,$lat,$latdir' . "\n"
. '$long,$lat' . "\n"
. ".\n";
 
eval $format;
die $@ if $@;


# Output the results to STDOUT:
write;