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+;; -*- mode: common-lisp -*-
+#|
+clasm-6502: An assembler for the 6502 written in Common Lisp.
+Copyright (C) 2024  Aleksei Eaves
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License as published by the Free Software Foundation; either
+version 2.1 of the License, or (at your option) any later version.
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
+|#
+
+(defun interpret-character (s x)
+  "Interpret meanings of characters."
+  (cond
+	((>= x (length s))
+	 'end-of-line)
+	(t
+	 ;; Have character, interpret
+	 (let ((c (char s x)))
+	   (cond
+		 ((eq c #\;) ; Comment
+		  'comment)
+		 ((eq c #\ ) ; Space
+		  'whitespace)
+		 (t
+		  'normal))))))
+
+(defun extract-word (line-string index)
+  "Extract a single word from line-string, starting at index."
+  (cond
+	((member (interpret-character line-string index)
+			 `(end-of-line comment))
+	 'end-of-line)
+	((equal (interpret-character line-string index)
+			'whitespace)
+	 'whitespace)
+	(t
+	 (subseq
+	  line-string
+	  index
+	  (do ((end index (incf end)))
+		  ((not (equal 'normal
+					   (interpret-character line-string end)))
+		   end))))))
+
+(defun extract-words (line-string &optional (index 0))
+  "Return a list the constituent word strings of line-string."
+  ;; Word Processing Logic
+  (let ((word (extract-word line-string index)))
+	(cond
+	  ;; Terminate execution at end-of-line.
+	  ((equal word 'end-of-line)
+	   nil)
+	  ;; If empty, continue on.
+	  ((equal word 'whitespace)
+	   (extract-words line-string (+ index 1)))
+	  ;; Otherwise, extract word and move ahead.
+	  (t
+	   (cons word
+			 (extract-words line-string
+							(+ index
+							   (length word))))))))
+
+(defun program (source-filename)
+  "Turns an assembly source file into a list, prepended with line numbers."
+  (let ((source-file (open source-filename))
+		(source-line-number 0))
+	(labels
+		((recurse ()
+		   (let ((line (read-line source-file nil 'EOF)))
+			 (if (equal line 'EOF)
+				 nil
+				 (let ((line-list (extract-words line)))
+				   (if (equal line-list nil)
+					   (progn
+						 (incf source-line-number)
+						 (recurse))
+					   (cons
+						(list (incf source-line-number)
+							  (extract-words line))
+						(recurse))))))))
+	  (recurse))))