Add script to generate parser (#498)

* Add script to automate generation of the parser

* Add license

* Improve error messages

.gitignore

@@ -24,3 +24,10 @@ logger.log
 .metals/
 .vscode/
 project/
+
+# antlr4
+*.interp
+*.tokens
+
+# genparser script
+genparser.config

genparser.sh

@@ -0,0 +1,43 @@
+#!/bin/bash
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+#
+# Copyright (c) 2011-2022 ETH Zurich.
+
+# This script generates a new parser from the antlr4 files stored in this repository.
+# This script MUST NOT be run from a symlink.
+
+##### Constants #####
+RED='\033[0;31m'
+GREEN='\033[0;32m'
+RESET='\033[0m'
+
+# This path is taken to be able to call this script from any directory and have
+# the same consistent behaviour. This was taken from
+# https://stackoverflow.com/questions/24112727/relative-paths-based-on-file-location-instead-of-current-working-directory
+SCRIPT_DIR=$( cd "$(dirname "${BASH_SOURCE[0]}")" || exit 1; pwd -P )
+
+##### Configure if it is the first execution #####
+CONFIGFILE="$SCRIPT_DIR/genparser.config"
+if ! test -f "$CONFIGFILE"; then
+  echo -e "What is the ${RED}ABSOLUTE${RESET} path to the antlr4 .jar file?"
+  read -r ANSWER
+  echo "$ANSWER" > "$CONFIGFILE"
+fi
+
+ANTLR4_PATH=$(cat "$CONFIGFILE")
+
+if ! test -f "$ANTLR4_PATH"; then
+  echo "The antlr4 .jar file was not found ($ANTLR4_PATH). Delete the file $CONFIGFILE to reconfigure this script."
+  exit 2
+fi
+
+echo -e "${GREEN}Generating the lexer:${RESET}"
+java -jar "$ANTLR4_PATH" "$SCRIPT_DIR"/src/main/antlr4/GobraLexer.g4 -package viper.gobra.frontend || { echo -e "${RED}Error while generating the lexer.${RESET}"; exit 3; }
+
+echo -e "${GREEN}Generating the parser:${RESET}"
+java -jar "$ANTLR4_PATH" "$SCRIPT_DIR"/src/main/antlr4/GobraParser.g4 -package viper.gobra.frontend -visitor -no-listener || { echo -e "${RED}Error while generating the parser.${RESET}"; exit 3; }
+
+echo -e "${GREEN}Moving the generated files:${RESET}"
+mv -v "$SCRIPT_DIR"/src/main/antlr4/*.java "$SCRIPT_DIR"/src/main/java/viper/gobra/frontend/