implement simple grammar parsing from correct file

src/main/java/parser/grammar/Grammar.java

@@ -1,5 +1,11 @@
 package parser.grammar;
 
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Arrays;
+import java.util.HashSet;
+import java.util.List;
 import java.util.Set;
 
 public class Grammar {
@@ -21,6 +27,52 @@ public class Grammar {
         this.rules = rules;
     }
 
+    public static Grammar fromFile(Path path) throws IOException {
+        List<String> lines = Files.readAllLines(path);
+
+        try {
+            String startSymbol = lines.get(0).split(" ")[1];
+            String epsilonSymbol = lines.get(1).split(" ")[1];
+
+            String[] term = lines.get(2).split(" ");
+            term = Arrays.copyOfRange(term, 1, term.length);
+            Set<String> terminals = new HashSet<>(Arrays.asList(term));
+
+            String[] nterm = lines.get(3).split(" ");
+            nterm = Arrays.copyOfRange(nterm, 1, nterm.length);
+            Set<String> nonterminals = new HashSet<>(Arrays.asList(nterm));
+
+            Set<GrammarRule> rules = new HashSet<>();
+            for (int i = 5; i < lines.size(); i++) {
+                // "S -> E T2 | EPS" wird zu leftside = "S" und rightside = "E T2 | epsilon"
+                String[] split = lines.get(i)
+                                      .replaceAll("EPS", epsilonSymbol)
+                                      .split("->");
+
+                String leftside = split[0].trim();
+                String rightside = split[1].trim();
+
+                // "E T2 | epsilon" wird zu prods[0] = "E T2" und prods[1] = "epsilon"
+                String[] prods = rightside.split("\\|");
+
+                for (String prod : prods) {
+                    GrammarRule rule = new GrammarRule(leftside, prod.split(" "));
+                    rules.add(rule);
+                }
+            }
+
+            return new Grammar(terminals, nonterminals,
+                               startSymbol, epsilonSymbol,
+                               rules);
+        } catch (Exception e) {
+            System.out.println("Die Grammatik kann nicht gelesen werden!");
+            System.out.println(path);
+            e.printStackTrace();
+        }
+
+        return null;
+    }
+
     public Set<String> getTerminals() {
         return this.terminals;
     }