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+;;; lcs.el --- find out the longest common sequence
+
+;;   Copyright (c) 2002-2003 by Alex Shinn, All rights reserved.
+;;   Copyright (c) 2002-2003 by Shiro Kawai, All rights reserved.
+;;   Copyright (c) 2006, 2012 by Jorgen Schaefer, All rights reserved.
+
+;; Authors: Alex Shinn, Shiro Kawai
+;; Maintainer: Jorgen Schaefer <forcer@forcix.cx>
+;; URL: https://github.com/jorgenschaefer/circe/wiki/lcs
+
+;;   Redistribution and use in source and binary forms, with or without
+;;   modification, are permitted provided that the following conditions
+;;   are met:
+
+;;   1. Redistributions of source code must retain the above copyright
+;;      notice, this list of conditions and the following disclaimer.
+
+;;   2. Redistributions in binary form must reproduce the above copyright
+;;      notice, this list of conditions and the following disclaimer in the
+;;      documentation and/or other materials provided with the distribution.
+
+;;   3. Neither the name of the authors nor the names of its contributors
+;;      may be used to endorse or promote products derived from this
+;;      software without specific prior written permission.
+
+;;   THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+;;   "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+;;   LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+;;   A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+;;   OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+;;   SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
+;;   TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+;;   PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
+;;   LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
+;;   NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
+;;   SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+;;; Commentary:
+
+;; lcs.el is a library for other Emacs Lisp programs not useful by
+;; itself.
+
+;; This library provides functions to find the Longest Common Sequence
+;; (LCS) of two sequences. This is used to create a unified diff of to
+;; two lists. See `lcs-unified-diff' for a useful function to be
+;; called.
+
+;; The code is more or less a literal translation of (part of)
+;; Gauche's util/lcs.scm module to Emacs Lisp.
+
+;;; Code:
+
+(put 'lcs-for 'lisp-indent-function 4)
+(defmacro lcs-for (var from to step &rest body)
+  "A simple FOR loop macro.
+Count VAR from FROM to TO by stepsize STEP. Evaluate BODY in each
+iteration."
+  (let ((sto (make-symbol "to"))
+        (sstep (make-symbol "step")))
+    `(let ((,var ,from)
+           (,sto ,to)
+           (,sstep ,step))
+       (while (<= ,var ,sto)
+         (progn
+           ,@body)
+         (setq ,var (+ ,var ,sstep))))))
+
+(defun lcs-split-at (lis pos)
+  "Return a cons cell of the first POS elements of LIS and the rest."
+  (let ((head nil))
+    (while (> pos 0)
+      (setq head (cons (car lis)
+                       head)
+            pos (- pos 1)
+            lis (cdr lis)))
+    (cons (reverse head)
+          lis)))
+
+(defun lcs-finish (M+N V_l vl V_r vr)
+  "Finalize the LCS algorithm.
+Should be used only by `lcs-with-positions'."
+  (let ((maxl 0)
+        (r '()))
+    (lcs-for i (- M+N) M+N 1
+      (when (> (funcall vl i)
+               maxl)
+        (setq maxl (funcall vl i)
+              r (funcall vr i))))
+    (list maxl (reverse r))))
+
+(defun lcs-with-positions (a-ls b-ls &optional equalp)
+  "Return the longest common subsequence (LCS) of A-LS and B-LS.
+EQUALP can be any procedure which returns non-nil when two
+elements should be considered equal."
+  (let* ((A (vconcat a-ls))
+         (B (vconcat b-ls))
+         (N (length A))
+         (M (length B))
+         (M+N (+ M N))
+         (V_d (make-vector (+ 1 (* 2 M+N))
+                           0))
+         (V_r (make-vector (+ 1 (* 2 M+N))
+                           nil))
+         (V_l (make-vector (+ 1 (* 2 M+N))
+                           0))
+         (vd (lambda (i &optional x)
+               (if x
+                   (aset V_d (+ i M+N) x)
+                 (aref V_d (+ i M+N)))))
+         (vr (lambda (i &optional x)
+               (if x
+                   (aset V_r (+ i M+N) x)
+                 (aref V_r (+ i M+N)))))
+         (vl (lambda (i &optional x)
+               (if x
+                   (aset V_l (+ i M+N) x)
+                 (aref V_l (+ i M+N))))))
+    (when (not equalp)
+      (setq equalp 'equal))
+    (catch 'return
+      (if (= M+N 0)
+          (throw 'return '(0 ()))
+        (lcs-for d 0 M+N 1
+          (lcs-for k (- d) d 2
+            (let ((x nil)
+                  (y nil)
+                  (l nil)
+                  (r nil))
+              (if (or (= k (- d))
+                      (and (not (= k d))
+                           (< (funcall vd (- k 1))
+                              (funcall vd (+ k 1)))))
+                  (setq x (funcall vd (+ k 1))
+                        l (funcall vl (+ k 1))
+                        r (funcall vr (+ k 1)))
+                (setq x (+ 1 (funcall vd (- k 1)))
+                      l (funcall vl (- k 1))
+                      r (funcall vr (- k 1))))
+              (setq y (- x k))
+              (while (and (< x N)
+                          (< y M)
+                          (funcall equalp (aref A x) (aref B y)))
+                (setq r (cons (list (aref A x) x y)
+                              r)
+                      x (+ x 1)
+                      y (+ y 1)
+                      l (+ l 1)))
+              (funcall vd k x)
+              (funcall vr k r)
+              (funcall vl k l)
+              (when (and (>= x N)
+                         (>= y M))
+                (throw 'return(lcs-finish M+N V_l vl V_r vr)))))))
+      (error "Can't happen"))))
+
+(defun lcs-unified-diff (a b &optional equalp)
+  "Return a unified diff of the lists A and B.
+EQUALP should can be a procedure that returns non-nil when two
+elements of A and B should be considered equal. It's `equal' by
+default."
+  (let ((common (cadr (lcs-with-positions a b equalp)))
+        (a a)
+        (a-pos 0)
+        (b b)
+        (b-pos 0)
+        (diff '()))
+    (while common
+      (let* ((elt (car common))
+             (a-off (nth 1 elt))
+             (a-skip (- a-off a-pos))
+             (b-off (nth 2 elt))
+             (b-skip (- b-off b-pos))
+             (a-split (lcs-split-at a a-skip))
+             (a-head (car a-split))
+             (a-tail (cdr a-split))
+             (b-split (lcs-split-at b b-skip))
+             (b-head (car b-split))
+             (b-tail (cdr b-split)))
+        (setq diff (append diff
+                           (mapcar (lambda (a)
+                                     `(- ,a))
+                                   a-head)
+                           (mapcar (lambda (b)
+                                     `(+ ,b))
+                                   b-head)
+                           `((! ,(car elt))))
+
+              common (cdr common)
+              a (cdr a-tail)
+              a-pos (+ a-off 1)
+              b (cdr b-tail)
+              b-pos (+ b-off 1))))
+    (append diff
+            (mapcar (lambda (a)
+                      `(- ,a))
+                    a)
+            (mapcar (lambda (b)
+                      `(+ ,b))
+                    b))))
+
+(provide 'lcs)
+;;; lcs.el ends here