(defmacro let1 (var val &body body)
  `(let ((,var ,val))
     ,@body))

(defmacro split (val yes no)
  (let1 g (gensym)
  `(let1 ,g ,val
     (if ,g
       (let ((head (car ,g))
             (tail (cdr ,g)))
         ,yes)
         ,no))))

(defun pairs (lst)
    (labels ((f (lst acc)
                (split lst
                  (if tail
                      (f (cdr tail) (cons (cons head (car tail)) acc))
                      (reverse acc))
                  (reverse acc))))
      (f lst nil)))

(defun print-tag (name alst closingp)
  (princ #\<)
  (when closingp
    (princ #\/))
  (princ (string-downcase name))
  (mapc (lambda (att)
          (format t " ~a=\"~a\"" (string-downcase (car att)) (cdr att)))
	alst)
  (princ #\>))

(defmacro tag (name atts &body body)
  `(progn (print-tag ',name
                     (list ,@(mapcar (lambda (x)
                                       `(cons ',(car x) ,(cdr x)))
                                     (pairs atts)))
                     nil)
          ,@body
          (print-tag ',name nil t)))

(defmacro svg (&body body)
  `(tag svg (xmlns "http://www.w3.org/2000/svg" 
             "xmlns:xlink" "http://www.w3.org/1999/xlink")
	,@body))

(defun brightness (col amt)
  (mapcar (lambda (x)
	    (min 255 (max 0 (+ x amt))))
	  col))

(defun svg-style (color)
  (format nil 
          "~{fill:rgb(~a,~a,~a);stroke:rgb(~a,~a,~a)~}" 
          (append color 
                  (brightness color -100))))

(defun polygon (points color)
  (tag polygon (points (format nil 
			       "~{~a,~a ~}" 
			       (mapcan (lambda (tp)
					 (list (car tp) (cdr tp)))
				       points))
		style (svg-style color))))