prolog-shell: Simple command line Prolog interpreter

Since Prolog isn't the most popular language, define a small
interactive interpreter that users or site administartors can play
around with by downloading the Gerrit WAR file and executing:

  java -jar gerrit.war prolog-shell

Change-Id: I0474b59e6ace24e6e6bfda58f574add3b2bf79e0

gerrit-pgm/src/main/java/com/google/gerrit/pgm/PrologShell.java

@@ -0,0 +1,93 @@
+// Copyright (C) 2011 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package com.google.gerrit.pgm;
+
+import com.google.gerrit.pgm.util.AbstractProgram;
+
+import com.googlecode.prolog_cafe.lang.BufferingPrologControl;
+import com.googlecode.prolog_cafe.lang.HaltException;
+import com.googlecode.prolog_cafe.lang.Prolog;
+import com.googlecode.prolog_cafe.lang.PrologClassLoader;
+import com.googlecode.prolog_cafe.lang.PrologMain;
+import com.googlecode.prolog_cafe.lang.SymbolTerm;
+
+import org.kohsuke.args4j.Option;
+
+import java.io.File;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.EnumSet;
+import java.util.List;
+
+public class PrologShell extends AbstractProgram {
+  @Option(name = "-s", multiValued = true, metaVar = "FILE.pl", usage = "file to load")
+  private List<String> fileName = new ArrayList<String>();
+
+  @Override
+  public int run() {
+    banner();
+
+    BufferingPrologControl pcl = new BufferingPrologControl();
+    pcl.setPrologClassLoader(new PrologClassLoader(getClass().getClassLoader()));
+    pcl.setEnabled(EnumSet.allOf(Prolog.Feature.class), false);
+    pcl.setEnabled(Prolog.Feature.IO, true);
+    pcl.setEnabled(Prolog.Feature.STATISTICS_RUNTIME, true);
+
+    pcl.initialize(Prolog.BUILTIN);
+    pcl.execute(Prolog.BUILTIN, "set_prolog_flag",
+        SymbolTerm.intern("print_stack_trace"),
+        SymbolTerm.intern("on"));
+
+    for (String file : fileName) {
+      String path;
+      try {
+        path = new File(file).getCanonicalPath();
+      } catch (IOException e) {
+        path = new File(file).getAbsolutePath();
+      }
+      pcl.execute(Prolog.BUILTIN, "consult", SymbolTerm.create(path));
+      System.err.println();
+      System.err.flush();
+    }
+
+    try {
+      pcl.execute(Prolog.BUILTIN, "cafeteria");
+      write("% halt\n");
+      return 0;
+    } catch (HaltException halt) {
+      write("% halt(" + halt.getStatus() + ")\n");
+      return halt.getStatus();
+    }
+  }
+
+  private void banner() {
+    System.err.format("Gerrit Code Review %s - Interactive Prolog Shell",
+        com.google.gerrit.common.Version.getVersion());
+    System.err.println();
+    System.err.println("based on " + PrologMain.VERSION);
+    System.err.println("         " + PrologMain.COPYRIGHT);
+    System.err.println("(type Ctrl-D or \"halt.\" to exit,"
+        + " \"['path/to/file.pl'].\" to load a file)");
+    System.err.println();
+    System.err.flush();
+  }
+
+  private void write(String msg) {
+    System.out.flush();
+    System.err.flush();
+    System.out.println(msg);
+    System.out.flush();
+  }
+}