refactor: use char instead of String in DiffSymbol

This makes the assumption of single character `DiffSymbol`s explicit.

Author: ibbem

src/main/java/org/variantsync/diffdetective/variation/VariationUnparser.java

@@ -44,12 +44,11 @@ public static <L extends Label> String unparseDiff(VariationDiff<L> diff) throws
      * @return the state before or after the diff
      */
     public static String undiff(String diff, Time time) {
-        String excludedDiffSymbol = DiffType.thatExistsOnlyAt(time.other()).symbol;
+        char excludedDiffSymbol = DiffType.thatExistsOnlyAt(time.other()).symbol;
 
         String result = diff
             .lines()
-            .filter(line -> !line.startsWith(excludedDiffSymbol))
-            // TODO assumes that all diff symbols are exactly 1 char long
+            .filter(line -> line.isEmpty() || line.charAt(0) != excludedDiffSymbol)
             .map(line -> line.isEmpty() ? "" : line.substring(1))
             .collect(Collectors.joining("\n"));
 

src/main/java/org/variantsync/diffdetective/variation/diff/DiffType.java

@@ -12,13 +12,13 @@
  * These values correspond to the domain of the Delta function from our paper.
  */
 public enum DiffType {
-    ADD("+"),
-    REM("-"),
-    NON(" ");
+    ADD('+'),
+    REM('-'),
+    NON(' ');
 
-    public final String symbol;
+    public final char symbol;
 
-    DiffType(String symbol) {
+    DiffType(char symbol) {
         this.symbol = symbol;
     }
 
@@ -139,15 +139,20 @@ public DiffType inverse() {
      * @return The type of edit of <code>line</code> or null if its an invalid diff line.
      */
     public static DiffType ofDiffLine(String line) {
-        if (line.startsWith(ADD.symbol)) {
-            return ADD;
-        } else if (line.startsWith(REM.symbol)) {
-            return REM;
-        } else if (line.startsWith(NON.symbol) || line.isEmpty()) {
+        if (line.isEmpty()) {
             // Diff lines should ideally have at least one character specifying a line's type (i.e., one of the diff
             // type characters: '+', '-', or ' '). However, this is not necessarily the case and unchanged lines may
             // be empty. We thus treat empty lines in a diff as unchanged (i.e., NON),
             return NON;
+        }
+
+        final char diffSymbol = line.charAt(0);
+        if (diffSymbol == ADD.symbol) {
+            return ADD;
+        } else if (diffSymbol == REM.symbol) {
+            return REM;
+        } else if (diffSymbol == NON.symbol) {
+            return NON;
         } else {
             return null;
         }