Fix cost for ReplacedComment

This needs to be 2x novel nodes, or we prefer it far too often.

CHANGELOG.md

@@ -4,6 +4,12 @@
 
 Updated Scala parser.
 
+### Diffing
+
+Fixed an issue with the cost model for comment replacement, leading
+difftastic to prefer modified comments even when exact comment matches
+are possible.
+
 ### Display
 
 Improved word highlighting in comments when they contain numbers.

sample_files/comma_and_comment_after.js

@@ -0,0 +1 @@
+funName(1 /* kinda like bar */ , /* foo */)

sample_files/comma_and_comment_before.js

@@ -0,0 +1 @@
+funName(1 /* foo */ , /* bar */)

sample_files/compare.expected

@@ -22,6 +22,9 @@ b2cace14b01c272217eec27d16adddbe  -
 sample_files/clojure_before.clj sample_files/clojure_after.clj
 b8e17b8eb649ba0b8d29b57a23e4ac81  -
 
+sample_files/comma_and_comment_before.js sample_files/comma_and_comment_after.js
+78be73c9caf82b3aeee81d8b8e7cf3c6  -
+
 sample_files/comma_before.js sample_files/comma_after.js
 e615c73ff860651f7749dc13cb3f110f  -
 

src/diff/graph.rs

@@ -311,9 +311,6 @@ impl Edge {
             // Matching an outer delimiter is good.
             EnterUnchangedDelimiter { depth_difference } => 100 + min(40, depth_difference),
 
-            // Replacing a comment is better than treating it as novel.
-            ReplacedComment { levenshtein_pct } => 150 + u32::from(100 - levenshtein_pct),
-
             // Otherwise, we've added/removed a node.
             NovelAtomLHS {
                 contiguous,
@@ -348,6 +345,12 @@ impl Edge {
                 }
                 cost
             }
+            // Replacing a comment is better than treating it as
+            // novel. However, since ReplacedComment is an alternative
+            // to NovelAtomLHS nad NovelAtomRHS, we need to be
+            // slightly less than 2 * (300 + NOT_CONTIGUOUS_PENALTY),
+            // so less than 700.
+            ReplacedComment { levenshtein_pct } => 600 + u32::from(100 - levenshtein_pct),
         }
     }
 }