From ad431241afd98a7f466488ea6d1a8174c5266897 Mon Sep 17 00:00:00 2001
From: "carpentry-heartbeat[bot]"
 <carpentry-heartbeat[bot]@users.noreply.github.com>
Date: Mon, 8 Jun 2026 15:55:13 +0200
Subject: [PATCH] Fix O(n^2) apply/revert; add lcs-length, edit-distance,
 similarity

Replace Array.concat in apply and revert with element-wise push-back
to avoid quadratic allocation. Add lcs-length, edit-distance, and
similarity functions for text comparison use cases.
---
 diff.carp       | 35 +++++++++++++++++++++---
 tests/diff.carp | 72 ++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 102 insertions(+), 5 deletions(-)

diff --git a/diff.carp b/diff.carp
index 08f2f96..7462d79 100644
--- a/diff.carp
+++ b/diff.carp
@@ -76,14 +76,17 @@
   (doc deletions "Filters a diff to only the Deletion hunks.")
   (defn deletions [d] (Array.copy-filter &deleted? d))
 
+  (defn- push-all! [res xs]
+    (for [j 0 (Array.length xs)] (Array.push-back! res @(Array.unsafe-nth xs j))))
+
   (doc apply
     "Reconstructs the 'new' array from a diff by collecting Eq and Insertion elements.")
   (defn apply [d]
     (let-do [res []]
       (for [i 0 (Array.length d)]
         (match-ref (Array.unsafe-nth d i)
-          (Eq xs) (set! res (Array.concat &[res (Array.copy xs)]))
-          (Insertion xs) (set! res (Array.concat &[res (Array.copy xs)]))
+          (Eq xs) (push-all! &res xs)
+          (Insertion xs) (push-all! &res xs)
           (Deletion _) ()))
       res))
 
@@ -93,11 +96,35 @@
     (let-do [res []]
       (for [i 0 (Array.length d)]
         (match-ref (Array.unsafe-nth d i)
-          (Eq xs) (set! res (Array.concat &[res (Array.copy xs)]))
+          (Eq xs) (push-all! &res xs)
           (Insertion _) ()
-          (Deletion xs) (set! res (Array.concat &[res (Array.copy xs)]))))
+          (Deletion xs) (push-all! &res xs)))
       res))
 
+  (doc lcs-length
+    "Returns the length of the longest common subsequence of two arrays.")
+  (defn lcs-length [old new]
+    (let-do [d &(diff old new)
+             len 0]
+      (for [i 0 (Array.length d)]
+        (match-ref (Array.unsafe-nth d i)
+          (Eq xs) (set! len (+ len (Array.length xs)))
+          _ ()))
+      len))
+
+  (doc edit-distance
+    "Returns the edit distance (number of insertions plus deletions) between two arrays.")
+  (defn edit-distance [old new]
+    (- (+ (Array.length old) (Array.length new)) (* 2 (lcs-length old new))))
+
+  (doc similarity
+    "Returns a similarity ratio between 0.0 and 1.0 for two arrays, based on their longest common subsequence. Two empty arrays have similarity 1.0.")
+  (defn similarity [old new]
+    (let [total (+ (Array.length old) (Array.length new))]
+      (if (= total 0)
+        1.0
+        (/ (Double.from-int (* 2 (lcs-length old new))) (Double.from-int total)))))
+
   (defn- emit-hunk [output old-start old-count new-start new-count hunk-lines]
     (let [header (String.concat
             &[@"@@ -"
diff --git a/tests/diff.carp b/tests/diff.carp
index 187bdee..cb3cf66 100644
--- a/tests/diff.carp
+++ b/tests/diff.carp
@@ -246,4 +246,74 @@ g
 h
 i
 j
-KK")) "unified generates multiple hunks for distant changes")))
+KK")) "unified generates multiple hunks for distant changes")
+
+    ; lcs-length tests
+    (assert-equal test
+      3
+      (Diff.lcs-length &[1 2 3] &[1 2 3])
+      "lcs-length of identical arrays is their length")
+    (assert-equal test
+      2
+      (Diff.lcs-length &[1 4 3] &[1 2 3])
+      "lcs-length of partially matching arrays")
+    (assert-equal test
+      0
+      (Diff.lcs-length &[1 2 3] &[4 5 6])
+      "lcs-length of completely different arrays is 0")
+    (assert-equal test
+      0
+      (Diff.lcs-length &[] &(the (Array Int) []))
+      "lcs-length of two empty arrays is 0")
+    (assert-equal test
+      0
+      (Diff.lcs-length &[] &[1 2 3])
+      "lcs-length with one empty array is 0")
+    (assert-equal test
+      2
+      (Diff.lcs-length &[1 2] &[1 2 3 4])
+      "lcs-length when one is prefix of other")
+
+    ; edit-distance tests
+    (assert-equal test
+      0
+      (Diff.edit-distance &[1 2 3] &[1 2 3])
+      "edit-distance of identical arrays is 0")
+    (assert-equal test
+      2
+      (Diff.edit-distance &[1 4 3] &[1 2 3])
+      "edit-distance with one substitution is 2 (delete + insert)")
+    (assert-equal test
+      6
+      (Diff.edit-distance &[1 2 3] &[4 5 6])
+      "edit-distance of completely different arrays")
+    (assert-equal test
+      0
+      (Diff.edit-distance &[] &(the (Array Int) []))
+      "edit-distance of two empty arrays is 0")
+    (assert-equal test
+      3
+      (Diff.edit-distance &[] &[1 2 3])
+      "edit-distance with one empty array")
+    (assert-equal test
+      2
+      (Diff.edit-distance &[1 2] &[1 2 3 4])
+      "edit-distance when one is prefix of other")
+
+    ; similarity tests
+    (assert-equal test
+      1.0
+      (Diff.similarity &[1 2 3] &[1 2 3])
+      "similarity of identical arrays is 1.0")
+    (assert-equal test
+      0.0
+      (Diff.similarity &[1 2 3] &[4 5 6])
+      "similarity of completely different arrays is 0.0")
+    (assert-equal test
+      1.0
+      (Diff.similarity &[] &(the (Array Int) []))
+      "similarity of two empty arrays is 1.0")
+    (assert-equal test
+      0.0
+      (Diff.similarity &[] &[1 2 3])
+      "similarity with one empty array is 0.0")))