1 files changed, 40 insertions, 0 deletions

diff --git a/cursor.lisp b/cursor.lisp
new file mode 100644
index 0000000..321ec3b
--- /dev/null
+++ b/cursor.lisp
@@ -0,0 +1,40 @@
+(in-package #:monparser)
+
+(defclass text ()
+  ((index :type (unsigned-byte 44) :initarg :index :accessor index :initform 0)
+   (data :type simple-string :initarg :data :reader data :initform "")))
+
+(defun has-data? (cursor)
+  (< (index cursor) (length (data cursor))))
+
+(defun peek (cursor)
+  (char (data cursor)
+        (index cursor)))
+
+(defun advance (cursor)
+  (make-instance 'text
+                 :data (data cursor)
+                 :index (+ (index cursor) 1)))
+
+(defun distance (from to)
+  (- (index to)
+     (index from)))
+
+(defun context-window (str index &key (side-length 20))
+  (let ((begin (max (- index side-length) 0))
+        (end (min (+ index side-length) (length str)))
+        (result '()))
+    (push (subseq str (1+ index) end) result)
+    (push (elt str index) result)
+    (push (subseq str begin index) result)
+    result))
+
+(defmethod print-object ((obj text) stream)
+  (print-unreadable-object (obj stream :type t)
+    (let ((str (if (has-data? obj)
+                 (format nil "~{~a~a~a~}"
+                         (context-window (data obj)
+                                         (index obj)
+                                         :side-length 10))
+                 "END OF DATA")))
+      (format stream "~s" (substitute #\~ #\Newline str)))))