From cdbfa453e870756dc32785b23a934b37e28d071c Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Juan=20Manuel=20Tom=C3=A1s?= <jtomas1815@gmail.com>
Date: Sun, 4 Dec 2022 23:57:17 -0300
Subject: First advances to lookahead implementation

---
 input.lisp  | 15 ++++++++++-----
 parser.lisp | 26 ++++++++++++++++++++++----
 2 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/input.lisp b/input.lisp
index 3e646b4..4f803ec 100644
--- a/input.lisp
+++ b/input.lisp
@@ -5,17 +5,22 @@
   (file nil :read-only t)
   (data nil :read-only t))
 
-(defun has-data? (input)
-  (< (input-cursor input)
+(defun has-data? (input &optional (window-size 1))
+  (< (+ window-size -1 (input-cursor input))
      (length (input-data input))))
 
-(defun element (input)
+(defun peek-1 (input)
   (char (input-data input)
         (input-cursor input)))
 
-(defun advance (input)
+(defun peek-n (input window-size)
+  (subseq (input-data input)
+          (input-cursor input)
+          window-size))
+
+(defun advance (input &optional (amount 1))
   (let ((new-input (copy-structure input)))
-    (incf (input-cursor new-input))
+    (incf (input-cursor new-input) amount)
     new-input))
 
 (declaim (ftype (function (simple-string) (values input &optional)) from-string))
diff --git a/parser.lisp b/parser.lisp
index 04d3f2b..a1ef1a8 100644
--- a/parser.lisp
+++ b/parser.lisp
@@ -33,7 +33,7 @@
   (lambda (input)
     (make-critical-failure :place input :message message)))
 
-(defun either (first-parser &rest other-parsers)
+(defun either (first-parser second-parser &rest other-parsers)
   (lambda (input)
     (labels ((either-rec (body)
                          (if (cdr body)
@@ -42,17 +42,26 @@
                                (either-rec (cdr body))
                                r))
                            (funcall (car body) input))))
-      (either-rec (cons first-parser other-parsers)))))
+      (either-rec (cons first-parser (cons second-parser other-parsers))))))
 
 (defun unit (&optional (predicate #'characterp))
   (lambda (input)
     (if (input::has-data? input)
-      (let ((c (input::element input)))
+      (let ((c (input::peek-1 input)))
         (if (funcall predicate c)
           (make-parsing :tree c :left (input::advance input))
           (make-normal-failure :place input :message "Predicate not satisfied.")))
       (make-normal-failure :place input :message "Reached end of input."))))
 
+(defun literal (predicate)
+  (lambda (input)
+    (if (input::has-data? input (length predicate))
+      (let ((str (input::peek-n input (length predicate))))
+        (if (string= predicate str)
+          (make-parsing :tree str :left (input::advance input (length predicate)))
+          (make-normal-failure :place input :message "Predicate not satisfied.")))
+      (make-normal-failure :place input :message "Reached end of input."))))
+
 (defmacro comp (bindings &body body)
   (if (null bindings)
     `(new (progn ,@body))
@@ -65,7 +74,7 @@
 (defparameter nothing
   (new nil))
 
-(defun zero-or-one (p)
+(defun optional (p)
   (either p nothing))
 
 (defun zero-or-more (p)
@@ -78,3 +87,12 @@
   (comp ((x p)
          (xs (zero-or-more p)))
     (cons x xs)))
+
+(defun separated-list (p separator)
+  (comp ((v p)
+         (sep (optional (the-char separator)))
+         (vn (if sep
+               (either (separated-list p separator)
+                       (fail "Value expected."))
+               nothing)))
+    (cons v vn)))
-- 
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