NAME

       Imager::Expr - implements expression parsing and compilation for the expression evaluation
       engine used by Imager::transform2()

SYNOPSIS

       my $code = Imager::Expr->new({rpnexpr=>$someexpr})
         or die "Cannot compile $someexpr: ",Imager::Expr::error();

DESCRIPTION

       This module is used internally by the Imager::transform2() function.  You shouldn't have
       much need to use it directly, but you may want to extend it.

       To create a new Imager::Expr object, call:

        my %options;
        my $expr = Imager::Expr->new(\%options)
          or die Imager::Expr::error();

       You will need to set an expression value and you may set any of the following:

       •   constants

           A hashref defining extra constants for expression parsing.  The names of the constants
           must be valid identifiers (/[^\W\d]\w*/) and the values must be valid numeric
           constants (that Perl recognizes in scalars).

           Imager::Expr may define it's own constants (currently just pi.)

       •   variables

           A reference to an array of variable names.  These are allocated numeric registers
           starting from register zero.

       By default you can define a "rpnexpr" key (which emulates RPN) or "expr" (an infix
       expression).  It's also possible to write other expression parsers that will use other
       keys.  Only one expression key should be defined.

   Instance methods
       The Imager::Expr::error() method is used to retrieve the error if the expression object
       cannot be created.

   Methods
       Imager::Expr provides only a few simple methods meant for external use:

       Imager::Expr->type_registered($keyword)
           Returns true if the given expression type is available.  The parameter is the key
           supplied to the new() method.

             if (Imager::Expr->type_registered('expr')) {
               # use infix expressions
             }

       $expr->code()
           Returns the compiled code.

       $expr->nregs()
           Returns a reference to the array of numeric registers.

       $expr->cregs()
           Returns a reference to the array of color registers.

       $expr->dumpops()
           Returns a string with the generated VM "machine code".

       $expr->dumpcode()
           Returns a string with the disassembled VM "machine code".

   Creating a new parser
       I'll write this one day.

       Methods used by parsers:

       compile
           This is the main method you'll need to implement in a parser.  See the existing
           parsers for a guide.

           It's supplied the following parameters:

           •   $expr - the expression to be parsed

           •   $options - the options hash supplied to transform2.

           Return an array ref of array refs containing opcodes and operands.

       @vars = $self->_variables()
           A list (not a reference) of the input variables.  This should be used to allocate as
           many registers as there are variable as input registers.

       $self->error($message)
           Set the return value of Imager::Expr::error()

       @ops = $self->stack_to_reg(@stack_ops)
           Converts marginally parsed RPN to register code.

       assemble()
           Called to convert op codes into byte code.

       numre()
           Returns a regular expression that matches floating point numbers.

       optimize()
           Optimizes the assembly code, including attempting common subexpression elimination and
           strength reducing division by a constant into multiplication by a constant.

       register_type()
           Called by a new expression parser implementation to register itself, call as:

             YourClassName->register_type('type code');

           where type code is the parameter that will accept the expression.

   Future compatibility
       Try to avoid doing your own optimization beyond literal folding - if we add some sort of
       jump, the existing optimizer will need to be rewritten, and any optimization you perform
       may well be broken too (well, your code generation will probably be broken anyway <sigh>).

AUTHOR

       Tony Cook <tonyc@cpan.org>, Arnar M. Hrafnkelsson