arity

/*
 *  call-seq:
 *     prc.arity -> fixnum
 *  
 *  Returns the number of arguments that would not be ignored. If the block
 *  is declared to take no arguments, returns 0. If the block is known
 *  to take exactly n arguments, returns n. If the block has optional
 *  arguments, return -n-1, where n is the number of mandatory
 *  arguments. A <code>proc</code> with no argument declarations
 *  is the same a block declaring <code>||</code> as its arguments.
 *     
 *     Proc.new {}.arity          #=>  0
 *     Proc.new {||}.arity        #=>  0
 *     Proc.new {|a|}.arity       #=>  1
 *     Proc.new {|a,b|}.arity     #=>  2
 *     Proc.new {|a,b,c|}.arity   #=>  3
 *     Proc.new {|*a|}.arity      #=> -1
 *     Proc.new {|a,*b|}.arity    #=> -2
 */

static VALUE
proc_arity(proc)
    VALUE proc;
{
    struct BLOCK *data;
    NODE *list;
    int n;

    Data_Get_Struct(proc, struct BLOCK, data);
    if (data->var == 0) {
        if (data->body && nd_type(data->body) == NODE_IFUNC &&
            data->body->nd_cfnc == bmcall) {
            return method_arity(data->body->nd_tval);
        }
        return INT2FIX(-1);
    }
    if (data->var == (NODE*)1) return INT2FIX(0);
    if (data->var == (NODE*)2) return INT2FIX(0);
    switch (nd_type(data->var)) {
      default:
        return INT2FIX(1);
      case NODE_MASGN:
        list = data->var->nd_head;
        n = 0;
        while (list) {
            n++;
            list = list->nd_next;
        }
        if (data->var->nd_args) return INT2FIX(-n-1);
        return INT2FIX(n);
    }
}