Add a server for checking brackets

app/src/main/java/ch/nuth/zhaw/exbox/BracketServer.java

@@ -0,0 +1,45 @@
+package ch.nuth.zhaw.exbox;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.regex.MatchResult;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+public class BracketServer implements CommandExecutor {
+    private static Pattern bracketPattern = Pattern.compile("(/\\*|\\*/|[(){}\\[\\]])");
+
+    private static Map<String, String> bracketPairs;
+
+    public BracketServer() {
+        super();
+        bracketPairs = new HashMap<>();
+        bracketPairs.put("(", ")");
+        bracketPairs.put("[", "]");
+        bracketPairs.put("{", "}");
+        bracketPairs.put("/*", "*/");
+    }
+
+    @Override
+    public String execute(String command) throws Exception {
+        return Boolean.toString(checkBrackets(command)) + "\n";
+    }
+
+    public boolean checkBrackets(String expression) {
+        ListStack openBrackets = new ListStack();
+        Matcher matcher = bracketPattern.matcher(expression);
+        Iterable<MatchResult> results = () -> matcher.results().iterator();
+
+        for (MatchResult result : results) {
+            if (bracketPairs.keySet().contains(result.group())) {
+                openBrackets.push(result.group());
+            } else if (result.group().equals(bracketPairs.get(openBrackets.peek()))) {
+                openBrackets.pop();
+            } else {
+                return false;
+            }
+        }
+
+        return openBrackets.isEmpty();
+    }
+}