Add formatter removing enclosing brackets

Readd functionality "Remove double braces around BibTeX fields when
loading" (which was removed in c2928d2)
as a formatter.

src/main/java/net/sf/jabref/logic/formatter/bibtexfields/RemoveBracesFormatter.java

@@ -0,0 +1,62 @@
+package net.sf.jabref.logic.formatter.bibtexfields;
+
+import net.sf.jabref.logic.formatter.Formatter;
+
+import java.util.Objects;
+
+public class RemoveBracesFormatter implements Formatter {
+
+    @Override
+    public String getName() {
+        return "Remove enclosing double braces";
+    }
+
+    @Override
+    public String getKey() {
+        return "RemoveBracesFormatter";
+    }
+
+    @Override
+    public String format(String value) {
+        Objects.requireNonNull(value);
+
+        String formatted = value;
+        while (formatted.length() >= 2 && formatted.charAt(0) == '{' && formatted.charAt(formatted.length() - 1)
+                == '}') {
+            String trimmed = formatted.substring(1, formatted.length() - 1);
+
+            // It could be that the removed braces were not matching
+            // For example, "{A} test {B}" results in "A} test {B"
+            // In this case, trimmed has a closing } without an opening { before that
+            if(hasNegativeBraceCount(trimmed)) {
+                return formatted;
+            } else {
+                formatted = trimmed;
+            }
+        }
+        return formatted;
+    }
+
+    /**
+     * Check if a string at any point has had more ending } braces than opening { ones.
+     * Will e.g. return true for the string "DNA} blahblal {EPA"
+     *
+     * @param value The string to check.
+     * @return true if at any index the brace count is negative.
+     */
+    private boolean hasNegativeBraceCount(String value) {
+        int braceCount = 0;
+        for (int index = 0; index < value.length(); index++) {
+            char charAtIndex = value.charAt(index);
+            if (charAtIndex == '{') {
+                braceCount++;
+            } else if (charAtIndex == '}') {
+                braceCount--;
+            }
+            if (braceCount < 0) {
+                return true;
+            }
+        }
+        return false;
+    }
+}

src/test/java/net/sf/jabref/logic/formatter/FormatterTest.java

@@ -84,7 +84,8 @@ public static Collection<Object[]> instancesToTest() {
                 new Object[]{new TrimFormatter()},
                 new Object[]{new HTMLConverter()},
                 new Object[]{new SuperscriptFormatter()},
-                new Object[]{new UnitFormatter()}
+                new Object[]{new UnitFormatter()},
+                new Object[]{new RemoveBracesFormatter()}
         );
     }
 }

src/test/java/net/sf/jabref/logic/formatter/bibtexfields/RemoveBracesFormatterTest.java

@@ -0,0 +1,70 @@
+package net.sf.jabref.logic.formatter.bibtexfields;
+
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+
+public class RemoveBracesFormatterTest {
+
+    @Test
+    public void formatRemovesSingleEnclosingBraces() {
+        RemoveBracesFormatter formatter = new RemoveBracesFormatter();
+        assertEquals("test", formatter.format("{test}"));
+    }
+
+    @Test
+    public void formatKeepsUnmatchedBracesAtBeginning() {
+        RemoveBracesFormatter formatter = new RemoveBracesFormatter();
+        assertEquals("{test", formatter.format("{test"));
+    }
+
+    @Test
+    public void formatKeepsUnmatchedBracesAtEnd() {
+        RemoveBracesFormatter formatter = new RemoveBracesFormatter();
+        assertEquals("test}", formatter.format("test}"));
+    }
+
+    @Test
+    public void formatKeepsShortString() {
+        RemoveBracesFormatter formatter = new RemoveBracesFormatter();
+        assertEquals("t", formatter.format("t"));
+    }
+
+    @Test
+    public void formatKeepsEmptyString() {
+        RemoveBracesFormatter formatter = new RemoveBracesFormatter();
+        assertEquals("", formatter.format(""));
+    }
+
+    @Test
+    public void formatRemovesDoubleEnclosingBraces() {
+        RemoveBracesFormatter formatter = new RemoveBracesFormatter();
+        assertEquals("test", formatter.format("{{test}}"));
+    }
+
+    @Test
+    public void formatRemovesTripleEnclosingBraces() {
+        RemoveBracesFormatter formatter = new RemoveBracesFormatter();
+        assertEquals("test", formatter.format("{{{test}}}"));
+    }
+
+    @Test
+    public void formatKeepsNonMatchingBraces() {
+        RemoveBracesFormatter formatter = new RemoveBracesFormatter();
+        assertEquals("{A} and {B}", formatter.format("{A} and {B}"));
+    }
+
+    @Test
+    public void formatRemovesOnlyMatchingBraces() {
+        RemoveBracesFormatter formatter = new RemoveBracesFormatter();
+        assertEquals("{A} and {B}", formatter.format("{{A} and {B}}"));
+    }
+
+    @Test
+    public void formatDoesNotRemoveBracesInBrokenString() {
+        RemoveBracesFormatter formatter = new RemoveBracesFormatter();
+        // We opt here for a conservative approach although one could argue that "A} and {B}" is also a valid return
+        assertEquals("{A} and {B}}", formatter.format("{A} and {B}}"));
+    }
+}