feat: create a method that projects text diffs

Author: eugen-shulimov

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

@@ -82,4 +82,26 @@ public static <T extends Label> String variationDiffUnparser(VariationDiff<T> di
     public static String variationDiffUnparser(VariationDiff<DiffLinesLabel> diff) throws IOException {
         return variationDiffUnparser(diff, DiffLinesLabel::withInvalidLineNumbers);
     }
+
+    public static String undiff(String text, Time time) {
+        StringBuilder result = new StringBuilder();
+        String[] textSplit = text.split("\n");
+        if (Time.AFTER == time) {
+            for (String line : textSplit) {
+                if (line.charAt(0) != '-') {
+                    result.append(line.substring(1));
+                    result.append("\n");
+                }
+            }
+            return result.substring(0, result.length() - 1);
+        } else {
+            for (String line : textSplit) {
+                if (line.charAt(0) != '+') {
+                    result.append(line.substring(1));
+                    result.append("\n");
+                }
+            }
+            return result.substring(0, result.length() - 1);
+        }
+    }
 }