Additional empty line should make a difference

this closes #422

Author: ghostbuster91

core/src/main/scala/com/softwaremill/diffx/instances/DiffForString.scala

@@ -11,12 +11,51 @@ class DiffForString(similarityThreshold: Double = 0.5) extends Diff[String] {
       val rows = generator.generateDiffRows(splitIntoLines(left), splitIntoLines(right))
       val lineResults = processLineDiffs(rows)
       if (lineResults.forall(_.isIdentical)) {
-        IdenticalValue(left)
+        if (left == right) {
+          IdenticalValue(left)
+        } else {
+          val resultsByChar =
+            generator.generateDiffRows[Int](left.map(c => c.toInt).toList, right.map(c => c.toInt).toList)
+          val resultsAsString = resultsByChar.map {
+            case DiffRow.Insert(newLine)          => DiffRow.Insert(newLine.toChar.toString)
+            case DiffRow.Delete(oldLine)          => DiffRow.Delete(oldLine.toChar.toString)
+            case DiffRow.Change(oldLine, newLine) => DiffRow.Change(oldLine.toChar.toString, newLine.toChar.toString)
+            case DiffRow.Equal(oldLine, newLine)  => DiffRow.Equal(oldLine.toChar.toString, newLine.toChar.toString)
+          }
+          DiffResultString(mergeIdentical(resultsAsString))
+        }
       } else {
         DiffResultString(lineResults)
       }
     }
 
+  private def mergeIdentical(rows: List[DiffRow[String]]) = {
+    rows
+      .foldLeft(List[DiffResult](IdenticalValue(""))) { (acc, item) =>
+        acc match {
+          case list @ ::(IdenticalValue(head), next) =>
+            item match {
+              case DiffRow.Insert("\n")             => DiffResultMissing("") :: list
+              case DiffRow.Insert(newLine)          => DiffResultMissing(newLine) :: list
+              case DiffRow.Delete("\n")             => DiffResultAdditional("") :: list
+              case DiffRow.Delete(oldLine)          => DiffResultAdditional(oldLine) :: list
+              case DiffRow.Change(oldLine, newLine) => DiffResultValue(oldLine, newLine) :: list
+              case DiffRow.Equal(oldLine, newLine)  => IdenticalValue(head + oldLine) :: next
+            }
+          case other =>
+            item match {
+              case DiffRow.Insert("\n")             => DiffResultMissing("") :: other
+              case DiffRow.Insert(newLine)          => DiffResultMissing(newLine) :: other
+              case DiffRow.Delete("\n")             => DiffResultAdditional("") :: other
+              case DiffRow.Delete(oldLine)          => DiffResultAdditional(oldLine) :: other
+              case DiffRow.Change(oldLine, newLine) => DiffResultValue(oldLine, newLine) :: other
+              case DiffRow.Equal(oldLine, newLine)  => IdenticalValue(oldLine) :: other
+            }
+        }
+      }
+      .reverse
+  }
+
   private def processLineDiffs(rows: List[DiffRow[String]]) = {
     rows.map {
       case DiffRow.Insert(newLine)     => DiffResultMissing(newLine)
@@ -56,8 +95,10 @@ class DiffForString(similarityThreshold: Double = 0.5) extends Diff[String] {
       .map(_.mkString)
   }
 
-  private def processWordDiffs(words: List[DiffRow[String]]): List[DiffResult] = {
-    words.map {
+  private def processWordDiffs(words: List[DiffRow[String]]): List[DiffResult] = words.map(processWordDiff)
+
+  private def processWordDiff(wd: DiffRow[String]) = {
+    wd match {
       case DiffRow.Insert(newLine)     => DiffResultMissingChunk(newLine)
       case DiffRow.Delete(oldLine)     => DiffResultAdditionalChunk(oldLine)
       case DiffRow.Change("", newLine) => DiffResultMissingChunk(newLine)

core/src/test/scala/com/softwaremill/diffx/test/DiffStringTest.scala

@@ -5,6 +5,7 @@ import com.softwaremill.diffx.{
   DiffResultAdditional,
   DiffResultAdditionalChunk,
   DiffResultChunk,
+  DiffResultMissing,
   DiffResultMissingChunk,
   DiffResultString,
   DiffResultStringLine,
@@ -239,5 +240,16 @@ class DiffStringTest extends AnyFreeSpec with Matchers {
         List(IdenticalValue("bob"), IdenticalValue("and mark"), DiffResultAdditional("alice went to school"))
       )
     }
+
+    "additional empty line should make a difference" in {
+      diffForString("hello", "hello\n") shouldBe DiffResultString(List(IdenticalValue("hello"), DiffResultMissing("")))
+      diffForString("hello\n", "hello") shouldBe DiffResultString(
+        List(IdenticalValue("hello"), DiffResultAdditional(""))
+      )
+      diffForString("\nhello", "hello") shouldBe DiffResultString(
+        List(DiffResultAdditional(""), IdenticalValue("hello"))
+      )
+      diffForString("hello", "\nhello") shouldBe DiffResultString(List(DiffResultMissing(""), IdenticalValue("hello")))
+    }
   }
 }