pseudocode-compiler

A utility and CGI program which translates AQA's GCSE pseudocode specification (course code 8525) to Javascript, ready for execution in a browser or by Node.JS/Spidermonkey.

I have written a series of articles about this project, which can be found at https://learnmoderncpp.com:

• 1. Setting the scene
• 2. Abstract syntax tree
• 3. Generating a scanner
• 4. Generating a parser
• 5. Rules, statements and expressions
• 6. Compiler driver frontend

Building under Windows

Note:

Both 32 and 64-bit executables for Windows are available from the releases page:

https://github.com/cpp-tutor/pseudocode-compiler/releases

Prerequisites:

• Visual Studio 2019 or 2022 with C++ compiler supporting /std:c++20

• Both win_flex.exe and win_bison.exe from https://github.com/lexxmark/winflexbison

Method:

• Unzip the contents of the release zipfile to a new directory and place all of win_flex.exe, FlexLexer.h win_bison.exe and data into the same directory

• Open a Visual Studio command prompt and navigate to this directory

• Run nmake /f Makefile.nmake and locate generated psc.exe

• (Optional) Use pscexamples.exe to generate pscexamples.js

Building under Linux

Prerequisites:

• GNU C++ compiler (g++) supporting -std=c++20

• Both flex and bison, tested with flex 2.6.4 and bison 3.8.1

Method:

• Unzip the contents of the release zipfile to a user directory and navigate to it

• Run make to generate psc

• (Optional) Use ./pscexamples to generate pscexamples.js

Running the compiler

Simply run psc.exe or ./psc in a console/command prompt window to read from the keyboard and output to the console, redirections using < and > work equally well too. (Note: to end input under Linux use Ctrl-D, under Windows Ctrl-Z followed by Enter.) See the example programs in the progs directory.

Alternatively, specify an input filename, and optionally an output filename, to avoid the need for redirections:

./psc myprog.psc            # outputs to myprog.js
./psc input.psc output.js   # outputs to output.js

The Javascript output is compatible with the jsNN executable from Mozilla (supplied by the Debian packages libmozjs-NN-dev), and with NodeJS (see option -n below). This output is not meant to be interesting but should always be correct, in case of an input (syntax) error, no output is generated.

Input options (immediately after psc and before any redirections or filenames):

• -h (or /? under Windows) outputs a help text and quits

• -v (or /v under Windows) outputs the version number and quits

• -n (or /n under Windows) prepends a small support library to the output program, which supports NodeJS. Note: you may need to run npm install readline-sync (once) to allow support for NodeJS.

Use as a CGI program

The supplied pscweb.html web-page together with pscweb.css and pscweb.js can be used as a front-end to the compiler utility. Under Linux/Apache these files would need to be copied to /var/www/html/ or /srv/http/, or under Windows to C:\inetpub\wwwroot\.

Permissions would need to be given to the utility to run as a CGI program, under Linux/Apache it would typically need to be copied to /usr/lib/cgi-bin/ or /srv/http/cgi-bin/, or under Windows to the same directory as before.

The minimum setup under Debian/Ubuntu is:

$ sudo apt install apache2
$ sudo a2enmod cgi
$ sudo cp pscweb.* /var/www/html/
$ sudo cp psc /usr/lib/cgi-bin/

(Possibly with sudo a2ensite ... and restarting the apache2 service too.) See your individual distro's documentation for how to install and set up a webserver.

Windows needs Internet Information Services (IIS) Manager to be enabled and set up together with a permissions rule for the executable binary, see documentation for your version of Windows (tested under Windows 10). Note that some editing of pscweb.js is necessary in function run(): /cgi-bin/psc? needs to be changed to ./psc.exe? (note the .), this is assuming that the utility is located in the same directory as the web-page.

Note that inputs to the running program have to be entered into the text field bottom-right before the program is run, these being separated by commas. Also the user is prompted to clear the output window when it is full. These are the only significant differences to running the Javascript in a console window.

License

This software is released under the highly permissive MIT License. If you do modify or improve parts of this software, especially the web front-end, I hope you would want to share your efforts, but this is not a requirement.

For this software to be useful to school students, setting up the web interface would be of great advantage, as the edit/compile/run cycle necessary for using the console has an associated learning curve.

Please note that this software is not either endorsed or supported by AQA Exam Board, and also its author is not affiliated to them in any way.

Resources

Pseudocode Specification as an unrestricted download from the AQA website:

https://filestore.aqa.org.uk/resources/computing/AQA-8525-NG-PC.PDF

Note that the only deliberate changes from the specification are:

• SUBROUTINE parameters in a definition needing to be followed by : Type (as for RECORD fields), but not where the SUBROUTINE is called with arguments.

• Initializing a variable from a RECORD type needs curly braces { and } and not plain parentheses, this is due to the earlier design of the compiler which means it cannot otherwise distinguish from a SUBROUTINE call (with the name of the RECORD)

Updates:

• Compound assignment of variables from RECORDs (in addition to field assignment) now appears in the specification above, as does OUTPUTting of a comma-separated list of StringExps. Support for both of these is provided.

• Version 1.00 released! Other release downloads have been removed as they contained bugs, but sources for these are still available via tags.