#!/usr/bin/env perl -w

print "Enter a formula, or 'done' to quit\n";
while (($formula = <STDIN>) !~ /^done$/i){
    chomp $formula;
    if ($formula !~ /^\s*              #beginning of string
	            (-?\d+(?:\.\d+)?)  #possible negative, & numbers
	            \s*
	            ([\+\-\*\/%])       #operator
	            \s*
	            (-?\d+(?:\.\d+)?)  #possible negative, & numbers
	            \s*=\s*            #spaces, =, spaces
	            $/x){              #end of string
	print "That is not a valid formula\n";
	print "Enter another formula, or 'done' to quit\n";
	next;
    }
    #formula matched, now assign variables
    ($arg1, $op, $arg2) = ($1, $2, $3); 
    
    #Special case for division
    if ($op eq '/'){
	#Floating point division
	if (($arg1 =~ /\./) || ($arg2 =~ /\./)){
	    $sol = $arg1 / $arg2;
	} 
	#Integer division
	else {
	    $sol = int ($arg1 / $arg2); 
	}
    } else {
	#anything else, just evaluate the mathematical sentence
	$sol = eval ("$arg1 $op $arg2");

	#NOTE:  I realize we did not yet mention the eval() function.
	#This function takes a string and evaluates it as perl code
	#I am using it here for brevity.  However, if you were unaware
	#of this function's existance, it should be a trivial matter
	#to set the $sol using a series of if/elsif/else blocks instead
    }
    #Print the solution, and store the operators and solution in hashes
    print "$sol\n";
    $solutions{$formula} = $sol;
    $operators{$formula} = $op;
}

#Get all the formulas, then sort them.
#In the sort routine, if the first condition returns 0, (that is, the 
#operators are equal to each other, short circuit logic comes into play
#with the 'or' operator, and the second condition is evaluated.  If you 
#are confused by this, please email me.
@all_forms = keys %solutions;
@sorted = sort { 
    ($operators{$a} cmp $operators{$b}) #sort first ASCIIbetically by operator
 or ($solutions{$b} <=> $solutions{$a}) #sort 2nd numerically by solution
 } @all_forms;


#finally, print out all formulas and their solutions.
print "\nAll formulas and their solutions are:\n";
foreach (@sorted){
    print "$_ $solutions{$_}\n";
}