List version of reduce

diff --git a/src/sequence.lisp b/src/sequence.lisp
index cda0b41..789a7c3 100644 (file)
--- a/src/sequence.lisp
+++ b/src/sequence.lisp
@@ -179,3 +179,27 @@
 
 (defun copy-seq (sequence)
   (subseq sequence 0))
+
+;;; Based on the SBCL's reduce implementation
+(defun reduce (function sequence &key key from-end (start 0) end (initial-value nil ivp))
+  (let ((key (or key #'identity))
+        (end (or end (length sequence))))
+    (if (= end start)
+        (if ivp initial-value (funcall function))
+        (if from-end
+            (let ((sequence (nthcdr (- (length sequence) end) (reverse sequence))))
+              (do ((count (if ivp start (1+ start))
+                          (1+ count))
+                   (sequence (if ivp sequence (cdr sequence))
+                             (cdr sequence))
+                   (value (if ivp initial-value (funcall key (car sequence)))
+                          (funcall function (funcall key (car sequence)) value)))
+                  ((>= count end) value)))
+            (let ((sequence (nthcdr start sequence)))
+              (do ((count (if ivp start (1+ start))
+                          (1+ count))
+                   (sequence (if ivp sequence (cdr sequence))
+                             (cdr sequence))
+                   (value (if ivp initial-value (funcall key (car sequence)))
+                          (funcall function value (funcall key (car sequence)))))
+                  ((>= count end) value)))))))