danielebarchiesi@0: <?php
danielebarchiesi@0: 
danielebarchiesi@0: /*
danielebarchiesi@0: ================================================================================
danielebarchiesi@0: 
danielebarchiesi@0: ctools_math_expr - PHP Class to safely evaluate math expressions
danielebarchiesi@0: Copyright (C) 2005 Miles Kaufmann <http://www.twmagic.com/>
danielebarchiesi@0: 
danielebarchiesi@0: ================================================================================
danielebarchiesi@0: 
danielebarchiesi@0: NAME
danielebarchiesi@0:     ctools_math_expr - safely evaluate math expressions
danielebarchiesi@0: 
danielebarchiesi@0: SYNOPSIS
danielebarchiesi@0:       include('ctools_math_expr.class.php');
danielebarchiesi@0:       $m = new ctools_math_expr;
danielebarchiesi@0:       // basic evaluation:
danielebarchiesi@0:       $result = $m->evaluate('2+2');
danielebarchiesi@0:       // supports: order of operation; parentheses; negation; built-in functions
danielebarchiesi@0:       $result = $m->evaluate('-8(5/2)^2*(1-sqrt(4))-8');
danielebarchiesi@0:       // create your own variables
danielebarchiesi@0:       $m->evaluate('a = e^(ln(pi))');
danielebarchiesi@0:       // or functions
danielebarchiesi@0:       $m->evaluate('f(x,y) = x^2 + y^2 - 2x*y + 1');
danielebarchiesi@0:       // and then use them
danielebarchiesi@0:       $result = $m->evaluate('3*f(42,a)');
danielebarchiesi@0: 
danielebarchiesi@0: DESCRIPTION
danielebarchiesi@0:     Use the ctools_math_expr class when you want to evaluate mathematical expressions
danielebarchiesi@0:     from untrusted sources.  You can define your own variables and functions,
danielebarchiesi@0:     which are stored in the object.  Try it, it's fun!
danielebarchiesi@0: 
danielebarchiesi@0: METHODS
danielebarchiesi@0:     $m->evalute($expr)
danielebarchiesi@0:         Evaluates the expression and returns the result.  If an error occurs,
danielebarchiesi@0:         prints a warning and returns false.  If $expr is a function assignment,
danielebarchiesi@0:         returns true on success.
danielebarchiesi@0: 
danielebarchiesi@0:     $m->e($expr)
danielebarchiesi@0:         A synonym for $m->evaluate().
danielebarchiesi@0: 
danielebarchiesi@0:     $m->vars()
danielebarchiesi@0:         Returns an associative array of all user-defined variables and values.
danielebarchiesi@0: 
danielebarchiesi@0:     $m->funcs()
danielebarchiesi@0:         Returns an array of all user-defined functions.
danielebarchiesi@0: 
danielebarchiesi@0: PARAMETERS
danielebarchiesi@0:     $m->suppress_errors
danielebarchiesi@0:         Set to true to turn off warnings when evaluating expressions
danielebarchiesi@0: 
danielebarchiesi@0:     $m->last_error
danielebarchiesi@0:         If the last evaluation failed, contains a string describing the error.
danielebarchiesi@0:         (Useful when suppress_errors is on).
danielebarchiesi@0: 
danielebarchiesi@0: AUTHOR INFORMATION
danielebarchiesi@0:     Copyright 2005, Miles Kaufmann.
danielebarchiesi@0: 
danielebarchiesi@0: LICENSE
danielebarchiesi@0:     Redistribution and use in source and binary forms, with or without
danielebarchiesi@0:     modification, are permitted provided that the following conditions are
danielebarchiesi@0:     met:
danielebarchiesi@0: 
danielebarchiesi@0:     1   Redistributions of source code must retain the above copyright
danielebarchiesi@0:         notice, this list of conditions and the following disclaimer.
danielebarchiesi@0:     2.  Redistributions in binary form must reproduce the above copyright
danielebarchiesi@0:         notice, this list of conditions and the following disclaimer in the
danielebarchiesi@0:         documentation and/or other materials provided with the distribution.
danielebarchiesi@0:     3.  The name of the author may not be used to endorse or promote
danielebarchiesi@0:         products derived from this software without specific prior written
danielebarchiesi@0:         permission.
danielebarchiesi@0: 
danielebarchiesi@0:     THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
danielebarchiesi@0:     IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
danielebarchiesi@0:     WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
danielebarchiesi@0:     DISCLAIMED. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT,
danielebarchiesi@0:     INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
danielebarchiesi@0:     (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
danielebarchiesi@0:     SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
danielebarchiesi@0:     HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
danielebarchiesi@0:     STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
danielebarchiesi@0:     ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
danielebarchiesi@0:     POSSIBILITY OF SUCH DAMAGE.
danielebarchiesi@0: 
danielebarchiesi@0: */
danielebarchiesi@0: 
danielebarchiesi@0: class ctools_math_expr {
danielebarchiesi@0:     var $suppress_errors = false;
danielebarchiesi@0:     var $last_error = null;
danielebarchiesi@0: 
danielebarchiesi@0:     var $v = array('e'=>2.71,'pi'=>3.14); // variables (and constants)
danielebarchiesi@0:     var $f = array(); // user-defined functions
danielebarchiesi@0:     var $vb = array('e', 'pi'); // constants
danielebarchiesi@0:     var $fb = array(  // built-in functions
danielebarchiesi@0:         'sin','sinh','arcsin','asin','arcsinh','asinh',
danielebarchiesi@0:         'cos','cosh','arccos','acos','arccosh','acosh',
danielebarchiesi@0:         'tan','tanh','arctan','atan','arctanh','atanh',
danielebarchiesi@0:         'pow', 'exp',
danielebarchiesi@0:         'sqrt','abs','ln','log',
danielebarchiesi@0:         'time', 'ceil', 'floor', 'min', 'max', 'round');
danielebarchiesi@0: 
danielebarchiesi@0:     function ctools_math_expr() {
danielebarchiesi@0:         // make the variables a little more accurate
danielebarchiesi@0:         $this->v['pi'] = pi();
danielebarchiesi@0:         $this->v['e'] = exp(1);
danielebarchiesi@0:         drupal_alter('ctools_math_expression_functions', $this->fb);
danielebarchiesi@0:     }
danielebarchiesi@0: 
danielebarchiesi@0:     function e($expr) {
danielebarchiesi@0:         return $this->evaluate($expr);
danielebarchiesi@0:     }
danielebarchiesi@0: 
danielebarchiesi@0:     function evaluate($expr) {
danielebarchiesi@0:         $this->last_error = null;
danielebarchiesi@0:         $expr = trim($expr);
danielebarchiesi@0:         if (substr($expr, -1, 1) == ';') $expr = substr($expr, 0, strlen($expr)-1); // strip semicolons at the end
danielebarchiesi@0:         //===============
danielebarchiesi@0:         // is it a variable assignment?
danielebarchiesi@0:         if (preg_match('/^\s*([a-z]\w*)\s*=\s*(.+)$/', $expr, $matches)) {
danielebarchiesi@0:             if (in_array($matches[1], $this->vb)) { // make sure we're not assigning to a constant
danielebarchiesi@0:                 return $this->trigger("cannot assign to constant '$matches[1]'");
danielebarchiesi@0:             }
danielebarchiesi@0:             if (($tmp = $this->pfx($this->nfx($matches[2]))) === false) return false; // get the result and make sure it's good
danielebarchiesi@0:             $this->v[$matches[1]] = $tmp; // if so, stick it in the variable array
danielebarchiesi@0:             return $this->v[$matches[1]]; // and return the resulting value
danielebarchiesi@0:         //===============
danielebarchiesi@0:         // is it a function assignment?
danielebarchiesi@0:         } elseif (preg_match('/^\s*([a-z]\w*)\s*\(\s*([a-z]\w*(?:\s*,\s*[a-z]\w*)*)\s*\)\s*=\s*(.+)$/', $expr, $matches)) {
danielebarchiesi@0:             $fnn = $matches[1]; // get the function name
danielebarchiesi@0:             if (in_array($matches[1], $this->fb)) { // make sure it isn't built in
danielebarchiesi@0:                 return $this->trigger("cannot redefine built-in function '$matches[1]()'");
danielebarchiesi@0:             }
danielebarchiesi@0:             $args = explode(",", preg_replace("/\s+/", "", $matches[2])); // get the arguments
danielebarchiesi@0:             if (($stack = $this->nfx($matches[3])) === false) return false; // see if it can be converted to postfix
danielebarchiesi@0:             for ($i = 0; $i<count($stack); $i++) { // freeze the state of the non-argument variables
danielebarchiesi@0:                 $token = $stack[$i];
danielebarchiesi@0:                 if (preg_match('/^[a-z]\w*$/', $token) and !in_array($token, $args)) {
danielebarchiesi@0:                     if (array_key_exists($token, $this->v)) {
danielebarchiesi@0:                         $stack[$i] = $this->v[$token];
danielebarchiesi@0:                     } else {
danielebarchiesi@0:                         return $this->trigger("undefined variable '$token' in function definition");
danielebarchiesi@0:                     }
danielebarchiesi@0:                 }
danielebarchiesi@0:             }
danielebarchiesi@0:             $this->f[$fnn] = array('args'=>$args, 'func'=>$stack);
danielebarchiesi@0:             return true;
danielebarchiesi@0:         //===============
danielebarchiesi@0:         } else {
danielebarchiesi@0:             return $this->pfx($this->nfx($expr)); // straight up evaluation, woo
danielebarchiesi@0:         }
danielebarchiesi@0:     }
danielebarchiesi@0: 
danielebarchiesi@0:     function vars() {
danielebarchiesi@0:         $output = $this->v;
danielebarchiesi@0:         unset($output['pi']);
danielebarchiesi@0:         unset($output['e']);
danielebarchiesi@0:         return $output;
danielebarchiesi@0:     }
danielebarchiesi@0: 
danielebarchiesi@0:     function funcs() {
danielebarchiesi@0:         $output = array();
danielebarchiesi@0:         foreach ($this->f as $fnn=>$dat)
danielebarchiesi@0:             $output[] = $fnn . '(' . implode(',', $dat['args']) . ')';
danielebarchiesi@0:         return $output;
danielebarchiesi@0:     }
danielebarchiesi@0: 
danielebarchiesi@0:     //===================== HERE BE INTERNAL METHODS ====================\\
danielebarchiesi@0: 
danielebarchiesi@0:     // Convert infix to postfix notation
danielebarchiesi@0:     function nfx($expr) {
danielebarchiesi@0: 
danielebarchiesi@0:         $index = 0;
danielebarchiesi@0:         $stack = new ctools_math_expr_stack;
danielebarchiesi@0:         $output = array(); // postfix form of expression, to be passed to pfx()
danielebarchiesi@0:         $expr = trim(strtolower($expr));
danielebarchiesi@0: 
danielebarchiesi@0:         $ops   = array('+', '-', '*', '/', '^', '_');
danielebarchiesi@0:         $ops_r = array('+'=>0,'-'=>0,'*'=>0,'/'=>0,'^'=>1); // right-associative operator?
danielebarchiesi@0:         $ops_p = array('+'=>0,'-'=>0,'*'=>1,'/'=>1,'_'=>1,'^'=>2); // operator precedence
danielebarchiesi@0: 
danielebarchiesi@0:         $expecting_op = false; // we use this in syntax-checking the expression
danielebarchiesi@0:                                // and determining when a - is a negation
danielebarchiesi@0: 
danielebarchiesi@0:         if (preg_match("/[^\w\s+*^\/()\.,-]/", $expr, $matches)) { // make sure the characters are all good
danielebarchiesi@0:             return $this->trigger("illegal character '{$matches[0]}'");
danielebarchiesi@0:         }
danielebarchiesi@0: 
danielebarchiesi@0:         while(1) { // 1 Infinite Loop ;)
danielebarchiesi@0:             $op = substr($expr, $index, 1); // get the first character at the current index
danielebarchiesi@0:             // find out if we're currently at the beginning of a number/variable/function/parenthesis/operand
danielebarchiesi@0:             $ex = preg_match('/^([a-z]\w*\(?|\d+(?:\.\d*)?|\.\d+|\()/', substr($expr, $index), $match);
danielebarchiesi@0:             //===============
danielebarchiesi@0:             if ($op == '-' and !$expecting_op) { // is it a negation instead of a minus?
danielebarchiesi@0:                 $stack->push('_'); // put a negation on the stack
danielebarchiesi@0:                 $index++;
danielebarchiesi@0:             } elseif ($op == '_') { // we have to explicitly deny this, because it's legal on the stack
danielebarchiesi@0:                 return $this->trigger("illegal character '_'"); // but not in the input expression
danielebarchiesi@0:             //===============
danielebarchiesi@0:             } elseif ((in_array($op, $ops) or $ex) and $expecting_op) { // are we putting an operator on the stack?
danielebarchiesi@0:                 if ($ex) { // are we expecting an operator but have a number/variable/function/opening parethesis?
danielebarchiesi@0:                     $op = '*'; $index--; // it's an implicit multiplication
danielebarchiesi@0:                 }
danielebarchiesi@0:                 // heart of the algorithm:
danielebarchiesi@0:                 while($stack->count > 0 and ($o2 = $stack->last()) and in_array($o2, $ops) and ($ops_r[$op] ? $ops_p[$op] < $ops_p[$o2] : $ops_p[$op] <= $ops_p[$o2])) {
danielebarchiesi@0:                     $output[] = $stack->pop(); // pop stuff off the stack into the output
danielebarchiesi@0:                 }
danielebarchiesi@0:                 // many thanks: http://en.wikipedia.org/wiki/Reverse_Polish_notation#The_algorithm_in_detail
danielebarchiesi@0:                 $stack->push($op); // finally put OUR operator onto the stack
danielebarchiesi@0:                 $index++;
danielebarchiesi@0:                 $expecting_op = false;
danielebarchiesi@0:             //===============
danielebarchiesi@0:             } elseif ($op == ')' and $expecting_op) { // ready to close a parenthesis?
danielebarchiesi@0:                 while (($o2 = $stack->pop()) != '(') { // pop off the stack back to the last (
danielebarchiesi@0:                     if (is_null($o2)) return $this->trigger("unexpected ')'");
danielebarchiesi@0:                     else $output[] = $o2;
danielebarchiesi@0:                 }
danielebarchiesi@0:                 if (preg_match("/^([a-z]\w*)\($/", $stack->last(2), $matches)) { // did we just close a function?
danielebarchiesi@0:                     $fnn = $matches[1]; // get the function name
danielebarchiesi@0:                     $arg_count = $stack->pop(); // see how many arguments there were (cleverly stored on the stack, thank you)
danielebarchiesi@0:                     $output[] = $stack->pop(); // pop the function and push onto the output
danielebarchiesi@0:                     if (in_array($fnn, $this->fb)) { // check the argument count
danielebarchiesi@0:                         if($arg_count > 1)
danielebarchiesi@0:                             return $this->trigger("too many arguments ($arg_count given, 1 expected)");
danielebarchiesi@0:                     } elseif (array_key_exists($fnn, $this->f)) {
danielebarchiesi@0:                         if ($arg_count != count($this->f[$fnn]['args']))
danielebarchiesi@0:                             return $this->trigger("wrong number of arguments ($arg_count given, " . count($this->f[$fnn]['args']) . " expected)");
danielebarchiesi@0:                     } else { // did we somehow push a non-function on the stack? this should never happen
danielebarchiesi@0:                         return $this->trigger("internal error");
danielebarchiesi@0:                     }
danielebarchiesi@0:                 }
danielebarchiesi@0:                 $index++;
danielebarchiesi@0:             //===============
danielebarchiesi@0:             } elseif ($op == ',' and $expecting_op) { // did we just finish a function argument?
danielebarchiesi@0:                 while (($o2 = $stack->pop()) != '(') {
danielebarchiesi@0:                     if (is_null($o2)) return $this->trigger("unexpected ','"); // oops, never had a (
danielebarchiesi@0:                     else $output[] = $o2; // pop the argument expression stuff and push onto the output
danielebarchiesi@0:                 }
danielebarchiesi@0:                 // make sure there was a function
danielebarchiesi@0:                 if (!preg_match("/^([a-z]\w*)\($/", $stack->last(2), $matches))
danielebarchiesi@0:                     return $this->trigger("unexpected ','");
danielebarchiesi@0:                 $stack->push($stack->pop()+1); // increment the argument count
danielebarchiesi@0:                 $stack->push('('); // put the ( back on, we'll need to pop back to it again
danielebarchiesi@0:                 $index++;
danielebarchiesi@0:                 $expecting_op = false;
danielebarchiesi@0:             //===============
danielebarchiesi@0:             } elseif ($op == '(' and !$expecting_op) {
danielebarchiesi@0:                 $stack->push('('); // that was easy
danielebarchiesi@0:                 $index++;
danielebarchiesi@0:                 $allow_neg = true;
danielebarchiesi@0:             //===============
danielebarchiesi@0:             } elseif ($ex and !$expecting_op) { // do we now have a function/variable/number?
danielebarchiesi@0:                 $expecting_op = true;
danielebarchiesi@0:                 $val = $match[1];
danielebarchiesi@0:                 if (preg_match("/^([a-z]\w*)\($/", $val, $matches)) { // may be func, or variable w/ implicit multiplication against parentheses...
danielebarchiesi@0:                     if (in_array($matches[1], $this->fb) or array_key_exists($matches[1], $this->f)) { // it's a func
danielebarchiesi@0:                         $stack->push($val);
danielebarchiesi@0:                         $stack->push(1);
danielebarchiesi@0:                         $stack->push('(');
danielebarchiesi@0:                         $expecting_op = false;
danielebarchiesi@0:                     } else { // it's a var w/ implicit multiplication
danielebarchiesi@0:                         $val = $matches[1];
danielebarchiesi@0:                         $output[] = $val;
danielebarchiesi@0:                     }
danielebarchiesi@0:                 } else { // it's a plain old var or num
danielebarchiesi@0:                     $output[] = $val;
danielebarchiesi@0:                 }
danielebarchiesi@0:                 $index += strlen($val);
danielebarchiesi@0:             //===============
danielebarchiesi@0:             } elseif ($op == ')') { // miscellaneous error checking
danielebarchiesi@0:                 return $this->trigger("unexpected ')'");
danielebarchiesi@0:             } elseif (in_array($op, $ops) and !$expecting_op) {
danielebarchiesi@0:                 return $this->trigger("unexpected operator '$op'");
danielebarchiesi@0:             } else { // I don't even want to know what you did to get here
danielebarchiesi@0:                 return $this->trigger("an unexpected error occured");
danielebarchiesi@0:             }
danielebarchiesi@0:             if ($index == strlen($expr)) {
danielebarchiesi@0:                 if (in_array($op, $ops)) { // did we end with an operator? bad.
danielebarchiesi@0:                     return $this->trigger("operator '$op' lacks operand");
danielebarchiesi@0:                 } else {
danielebarchiesi@0:                     break;
danielebarchiesi@0:                 }
danielebarchiesi@0:             }
danielebarchiesi@0:             while (substr($expr, $index, 1) == ' ') { // step the index past whitespace (pretty much turns whitespace
danielebarchiesi@0:                 $index++;                             // into implicit multiplication if no operator is there)
danielebarchiesi@0:             }
danielebarchiesi@0: 
danielebarchiesi@0:         }
danielebarchiesi@0:         while (!is_null($op = $stack->pop())) { // pop everything off the stack and push onto output
danielebarchiesi@0:             if ($op == '(') return $this->trigger("expecting ')'"); // if there are (s on the stack, ()s were unbalanced
danielebarchiesi@0:             $output[] = $op;
danielebarchiesi@0:         }
danielebarchiesi@0:         return $output;
danielebarchiesi@0:     }
danielebarchiesi@0: 
danielebarchiesi@0:     // evaluate postfix notation
danielebarchiesi@0:     function pfx($tokens, $vars = array()) {
danielebarchiesi@0: 
danielebarchiesi@0:         if ($tokens == false) return false;
danielebarchiesi@0: 
danielebarchiesi@0:         $stack = new ctools_math_expr_stack;
danielebarchiesi@0: 
danielebarchiesi@0:         foreach ($tokens as $token) { // nice and easy
danielebarchiesi@0:             // if the token is a binary operator, pop two values off the stack, do the operation, and push the result back on
danielebarchiesi@0:             if (in_array($token, array('+', '-', '*', '/', '^'))) {
danielebarchiesi@0:                 if (is_null($op2 = $stack->pop())) return $this->trigger("internal error");
danielebarchiesi@0:                 if (is_null($op1 = $stack->pop())) return $this->trigger("internal error");
danielebarchiesi@0:                 switch ($token) {
danielebarchiesi@0:                     case '+':
danielebarchiesi@0:                         $stack->push($op1+$op2); break;
danielebarchiesi@0:                     case '-':
danielebarchiesi@0:                         $stack->push($op1-$op2); break;
danielebarchiesi@0:                     case '*':
danielebarchiesi@0:                         $stack->push($op1*$op2); break;
danielebarchiesi@0:                     case '/':
danielebarchiesi@0:                         if ($op2 == 0) return $this->trigger("division by zero");
danielebarchiesi@0:                         $stack->push($op1/$op2); break;
danielebarchiesi@0:                     case '^':
danielebarchiesi@0:                         $stack->push(pow($op1, $op2)); break;
danielebarchiesi@0:                 }
danielebarchiesi@0:             // if the token is a unary operator, pop one value off the stack, do the operation, and push it back on
danielebarchiesi@0:             } elseif ($token == "_") {
danielebarchiesi@0:                 $stack->push(-1*$stack->pop());
danielebarchiesi@0:             // if the token is a function, pop arguments off the stack, hand them to the function, and push the result back on
danielebarchiesi@0:             } elseif (preg_match("/^([a-z]\w*)\($/", $token, $matches)) { // it's a function!
danielebarchiesi@0:                 $fnn = $matches[1];
danielebarchiesi@0:                 if (in_array($fnn, $this->fb)) { // built-in function:
danielebarchiesi@0:                     if (is_null($op1 = $stack->pop())) return $this->trigger("internal error");
danielebarchiesi@0:                     $fnn = preg_replace("/^arc/", "a", $fnn); // for the 'arc' trig synonyms
danielebarchiesi@0:                     if ($fnn == 'ln') $fnn = 'log';
danielebarchiesi@0:                     eval('$stack->push(' . $fnn . '($op1));'); // perfectly safe eval()
danielebarchiesi@0:                 } elseif (array_key_exists($fnn, $this->f)) { // user function
danielebarchiesi@0:                     // get args
danielebarchiesi@0:                     $args = array();
danielebarchiesi@0:                     for ($i = count($this->f[$fnn]['args'])-1; $i >= 0; $i--) {
danielebarchiesi@0:                         if (is_null($args[$this->f[$fnn]['args'][$i]] = $stack->pop())) return $this->trigger("internal error");
danielebarchiesi@0:                     }
danielebarchiesi@0:                     $stack->push($this->pfx($this->f[$fnn]['func'], $args)); // yay... recursion!!!!
danielebarchiesi@0:                 }
danielebarchiesi@0:             // if the token is a number or variable, push it on the stack
danielebarchiesi@0:             } else {
danielebarchiesi@0:                 if (is_numeric($token)) {
danielebarchiesi@0:                     $stack->push($token);
danielebarchiesi@0:                 } elseif (array_key_exists($token, $this->v)) {
danielebarchiesi@0:                     $stack->push($this->v[$token]);
danielebarchiesi@0:                 } elseif (array_key_exists($token, $vars)) {
danielebarchiesi@0:                     $stack->push($vars[$token]);
danielebarchiesi@0:                 } else {
danielebarchiesi@0:                     return $this->trigger("undefined variable '$token'");
danielebarchiesi@0:                 }
danielebarchiesi@0:             }
danielebarchiesi@0:         }
danielebarchiesi@0:         // when we're out of tokens, the stack should have a single element, the final result
danielebarchiesi@0:         if ($stack->count != 1) return $this->trigger("internal error");
danielebarchiesi@0:         return $stack->pop();
danielebarchiesi@0:     }
danielebarchiesi@0: 
danielebarchiesi@0:     // trigger an error, but nicely, if need be
danielebarchiesi@0:     function trigger($msg) {
danielebarchiesi@0:         $this->last_error = $msg;
danielebarchiesi@0:         if (!$this->suppress_errors) trigger_error($msg, E_USER_WARNING);
danielebarchiesi@0:         return false;
danielebarchiesi@0:     }
danielebarchiesi@0: }
danielebarchiesi@0: 
danielebarchiesi@0: // for internal use
danielebarchiesi@0: class ctools_math_expr_stack {
danielebarchiesi@0: 
danielebarchiesi@0:     var $stack = array();
danielebarchiesi@0:     var $count = 0;
danielebarchiesi@0: 
danielebarchiesi@0:     function push($val) {
danielebarchiesi@0:         $this->stack[$this->count] = $val;
danielebarchiesi@0:         $this->count++;
danielebarchiesi@0:     }
danielebarchiesi@0: 
danielebarchiesi@0:     function pop() {
danielebarchiesi@0:         if ($this->count > 0) {
danielebarchiesi@0:             $this->count--;
danielebarchiesi@0:             return $this->stack[$this->count];
danielebarchiesi@0:         }
danielebarchiesi@0:         return null;
danielebarchiesi@0:     }
danielebarchiesi@0: 
danielebarchiesi@0:     function last($n=1) {
danielebarchiesi@0:         return !empty($this->stack[$this->count-$n]) ? $this->stack[$this->count-$n] : NULL;
danielebarchiesi@0:     }
danielebarchiesi@0: }
danielebarchiesi@0: