Use xdot (if available) as graph viewer

Update install instructions accordingly.

README.md

@@ -20,9 +20,12 @@ machine instructions.
 
 * The [ply](https://www.dabeaz.com/ply/) Python library is used for lexical analysis and parsing.
 
+Optional:
+* It is recommended to use the [xdot](https://github.com/jrfonseca/xdot.py#readme) program to view the abstract syntax tree. If `xdot` isn't available, the PDF viewer `evince` is used as a fallback.
+
 ### Fedora
 
-	dnf install -y python-ply
+	dnf install -y python-ply python-xdot evince
 
 ### Debian/Ubuntu
 
@@ -72,7 +75,7 @@ Here is a full example using the interpreter:
 	10946
 	-- Stack Frame --
 	Constants: {'K': 20}
-	Variables: {'m': 17711, 'n': 28657, 'k': 17711, 'count': 21}
+	Variables: {'count': 21, 'k': 17711, 'm': 17711, 'n': 28657}
 	Procedures: {}
 
 Example of using the compiler, assembler and virtual machine:
@@ -104,11 +107,14 @@ Example of using the compiler, assembler and virtual machine:
 	Stack: []
 	Offset: -1
 
-If you want to see a abstract syntax tree of your program, use the pl0_graphviz.py command:
+To see the result of lexical analysis and parsing:
+
+	python pl0_lexer.py < examples/fibonacci.pl0
+	python pl0_parser.py < examples/fibonacci.pl0
 
-	./pl0_graphviz.py < examples/fibonacci.pl0
+To get a graphical view of the abstract syntax tree:
 
-A sample graph is included in the `examples` directory.
+	python pl0_graphviz.py < examples/fibonacci.pl0
 
 For more advanced usage, including documentation on individual components, please see the [online documentation](http://programming.dojo.net.nz/study/pl0-language-tools/index).
 

pl0_graphviz.py

@@ -163,11 +163,12 @@ def accept_term(self, node):
     visitor = GraphPrinter()
     contents = visitor.generate_graph(program)
 
+    print("Generating Graph...")
     with open('graph.dot', 'w') as f:
         f.write(contents)
 
-    print("Generating Graph...")
-    os.system("dot -v -Tpdf -ograph.pdf graph.dot")
-
-    print("Opening Graph...")
-    os.system("open graph.pdf")
+    # To view, use 'xdot' if available, else the PDF viewer 'evince'.
+    if (os.system("xdot graph.dot") >> 8):
+        print("Generating PDF...")
+        os.system("dot -T pdf -o graph.pdf graph.dot")
+        os.system("evince graph.pdf")

test

@@ -10,9 +10,8 @@ rm -rf err.txt
 ./pl0_interpreter.py < examples/fibonacci.pl0 > expect/fibonacci.run 2>>err.txt
 
 # the graphviz output is messy so just ignore it. we'll test the .dot file only.
-./pl0_graphviz.py < examples/fibonacci.pl0 2>>/dev/null
-mv graph.dot expect/fibonacci.dot
-rm -f graph.pdf
+./pl0_graphviz.py < examples/fibonacci.pl0
+(sleep 1; mv graph.dot expect/fibonacci.dot; rm -f graph.pdf) &
 
 ## retro tests ( should be easy to update for other backends )
 for f in tests/*.pl0 ; do