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+;; Simple number guessing game in Common Lisp
+
+(defmacro while (condition &body body)
+  "While `condition' is not nil, evaluate `body'."
+  `(loop while ,condition do (progn ,@body)))
+
+(defun make-random (max)
+  "Generate a random number up to `max' using a new random state."
+  (let ((*random-state* (make-random-state t)))
+    (random max)))
+
+(defun read-integer ()
+  (parse-integer (read-line *query-io*) :junk-allowed t))
+
+(defun attempt-guess (number)
+  "Run one round of a number guessing game where `number'
+is the number the user is trying to guess."
+  (let ((guess nil)
+        (incorrect t))
+    (loop do
+      (format *query-io* "Enter a number: ")
+      (force-output *query-io*)
+      (setf guess (read-integer))
+      (format t
+              (cond
+                ((< guess number) "The number is larger than ~D~%")
+                ((> guess number) "The number is smaller than ~D~%")
+                (t "Correct! The number was ~D~%"))
+              guess)
+      (when (= guess number)
+        (setf incorrect nil))
+      while incorrect))
+  t)
+
+(defun begin-number-guessing-game (&key one-shot (maximum 100))
+  "If `one-shot' is t, run one round of a number guessing game.
+Otherwise run a chain of games until the user wants to stop.
+
+In both cases, `:maximum' is the maximum possible number."
+  (when one-shot
+    (return-from begin-number-guessing-game
+      (attempt-guess (make-random maximum))))
+
+  (let ((playing nil))
+    (loop do
+          (attempt-guess (make-random maximum))
+          (setf playing (y-or-n-p "Play again?"))
+          (format t "~%")
+          while playing)))