How to make this code simpler, clearer and "more lispy"? - common-lisp

Question

I want to parse the text line from the Wavefront OBJ file. Currently I am interested in "V" and "F" types only.
My algorithm is as follows:
check if line is not nil (otherwise step 2 would fail)
drop comment after "#" and trim spaces
drop prefix "v " or "f "
split string to the list of elements where each element
is split to the list if it is symbol like |34/76/23|
is converted from the list: I take one element only, the first by default
or coerced to the given type if it is atomic number already.
Here is the code:
(defun parse-line (line prefix &key (type 'single-float))
(declare (optimize (debug 3)))
(labels ((rfs (what)
(read-from-string (concatenate 'string "(" what ")")))
(unpack (str &key (char #\/) (n 0))
(let ((*readtable* (copy-readtable)))
(when char ;; we make the given char a delimiter (space)
(set-syntax-from-char char #\Space))
(typecase str
;; string -> list of possibly symbols.
;; all elements are preserved by (map). nil's are dropped
(string (delete-if #'null
(map 'list
#'unpack
(rfs str))))
;; symbol -> list of values
(symbol (unpack (rfs (symbol-name str))))
;; list -> value (only the requested one)
(list (unpack (nth n str)))
;; value -> just coerce to type
(number (coerce str type))))))
(and line
(setf line (string-trim '(#\Space #\Tab)
(subseq line 0 (position #\# line))))
(< (length prefix) (length line))
(string= line prefix :end1 (length prefix) :end2 (length prefix))
(setf line (subseq line (length prefix)))
(let ((value (unpack line :char nil)))
(case (length value)
(3 value)
(4 (values (subseq value 0 3) ;; split quad 0-1-2-3 on tri 0-1-2 + tri 0-2-3
(list (nth 0 value)
(nth 2 value)
(nth 3 value)))))))))
Step four (label "unpack") is kind of recursive. It is one function and can call itself three times.
Anyway, this solution seems to be clunky.
My question is: how should one solve this task with shorter and clearer code?

Answer 1

I would approach this in a more structured manner.
You want to parse an obj file into some sort of data structure:
(defun parse-obj-file (filespec)
;; todo
)
You need to think about how the data structure returned should look. For now, let us return a list of two lists, one of the vertices, one of the faces. The parser will go through each line, determine whether it is either a vertex or a face, and then collect it into the appropriate list:
(defun parse-obj-file (filespec)
(with-open-file (in-stream filespec
:direction :input)
(loop for line = (read-line in-stream nil)
while line
when (cl-ppcre:scan "^v " line)
collect (parse-vertex line) into vertices
when (cl-ppcre:scan "^f " line)
collect (parse-face line) into faces
finally (return (list vertices faces)))))
I used the cl-ppcre library here, but you could also use mismatch or search. You will then need to define parse-vertex and parse-face, for which cl-ppcre:split should come in quite handy.
It would perhaps also be useful to define classes for vertices and faces.
Update: This is how I would approach vertices:
(defclass vertex ()
((x :accessor x :initarg :x)
(y :accessor y :initarg :y)
(z :accessor z :initarg :z)
(w :accessor w :initarg :w)))
(defun parse-vertex (line)
(destructuring-bind (label x y z &optional w)
(cl-ppcre:split "\\s+" (remove-comment line))
(declare (ignorable label))
(make-instance 'vertex
:x (parse-number x)
:y (parse-number y)
:z (parse-number z)
:w (parse-number w))))
Parse-number is from the parse-number library. It is better than using read.
Update 2: (Sorry for making this a run-on story; I have to interlace some work.) A face consists of a list of face-points.
(defclass face-point ()
((vertex-index :accessor vertex-index :initarg :vertex-index)
(texture-coordinate :accessor texture-coordinate
:initarg :texture-coordinate)
(normal :accessor normal :initarg :normal)))
(defun parse-face (line)
(destructuring-bind (label &rest face-points)
(cl-ppcre:split "\\s+" (remove-comment line))
(declare (ignorable label))
(mapcar #'parse-face-point face-points)))
(defun parse-face-point (string)
(destructuring-bind (vertex-index &optional texture-coordinate normal)
(cl-ppcre:split "/" string)
(make-instance 'face-point
:vertex-index vertex-index
:texture-coordinate texture-coordinate
:normal normal)))
Remove-comment simply throws away everything after the first #:
(defun remove-comment (line)
(subseq line 0 (position #\# line)))