Implement dynamic-programming string alignment

apheleia.el

@@ -22,13 +22,75 @@
 
 ;;; Code:
 
+(require 'cl-lib)
+
 (defgroup apheleia nil
   "Better mode lighter overriding."
   :group 'external
   :link '(url-link :tag "GitHub" "https://github.com/raxod502/apheleia")
   :link '(emacs-commentary-link :tag "Commentary" "apheleia"))
 
-;; TODO
+(cl-defun apheleia--align-strings (s1 s2 &key i1 i2 memo)
+  "Align strings S1 and S2 for minimum edit distance.
+Return a hash table mapping (I1 . I2) to COST, where I1 and I2
+are indices into S1 and S2, and COST is the edit distance between
+the substrings of S1 and S2 starting at I1 and I2 respectively.
+
+Recursively, I1 and I2 are indices as explained above, and MEMO
+is the hash table being filled."
+  (if (null memo)
+      (let ((memo (make-hash-table :test #'equal)))
+        (prog1 memo
+          (apheleia--align-strings s1 s2 :i1 0 :i2 0 :memo memo)))
+    (or
+     (gethash (cons i1 i2) memo)
+     (puthash
+      (cons i1 i2)
+      (cond
+       ((= i1 (length s1))
+        (- (length s2) i2))
+       ((= i2 (length s2))
+        (- (length s1) i1))
+       ((= (aref s1 i1) (aref s2 i2))
+        (apheleia--align-strings s1 s2 :i1 (1+ i1) :i2 (1+ i2) :memo memo))
+       (t
+        (1+
+         (min
+          (apheleia--align-strings s1 s2 :i1 (1+ i1) :i2 (1+ i2) :memo memo)
+          (apheleia--align-strings s1 s2 :i1     i1  :i2 (1+ i2) :memo memo)
+          (apheleia--align-strings s1 s2 :i1 (1+ i1) :i2     i2  :memo memo)))))
+      memo))))
+
+(defun apheleia--align-point (s1 s2 p1)
+  "Given strings S1 and S2 and index P1 in S1, return matching index P2 in S2.
+If S1 and S2 are the same, then P1 and P2 will also be the same.
+Otherwise, the text of S2 surrounding P2 is \"similar\" to the
+text of S1 surrounding P1."
+  (let* ((memo (apheleia--align-strings s1 s2))
+         (i1 0)
+         (i2 0))
+    (while (< i1 p1)
+      (message "currently at %S, %S" i1 i2)
+      (let* ((costs
+              `(,(gethash (cons (1+ i1) (1+ i2)) memo)
+                ;; Replicate the short-circuiting in our dynamic
+                ;; programming implementation; otherwise we will be
+                ;; trying to look up hash table entries that don't
+                ;; exist.
+                ,@(unless (= (aref s1 i1) (aref s2 i2))
+                    (list
+                     (gethash (cons     i1  (1+ i2)) memo)
+                     (gethash (cons (1+ i1)     i2)  memo)))))
+             (min-cost (apply #'min costs)))
+        (cond
+         ((= min-cost (nth 0 costs))
+          (cl-incf i1)
+          (cl-incf i2))
+         ((= min-cost (nth 1 costs))
+          (cl-incf i2))
+         ((= min-cost (nth 2 costs))
+          (cl-incf i1)))))
+    i2))
 
 (provide 'apheleia)