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Some advice on a "simple" thing...

From: Sean Charles
Subject: Some advice on a "simple" thing...
Date: Wed, 9 Apr 2014 00:02:37 +0100

Hello list,

I have almost completed a predicate called ‘jread’ which parses a Java .class file into a term that will allow me to create a database of all of the methods, fields, superclass and interfaces for that class and ultimately create a database for a complete “android.jar” file of any required API level.

I am, for educational purposes and my own learning, trying to build a system like Hoogle/Hayoo for Android. I do a lot of Android and I wanted to create a sytem I can wire into Emacs/IntelliJ or anything for that matter, a simple HTTP server that can supply a list of methods that have a certain type signature using a si mple query language.  Searching by types it very very useful when using Haskell and I wanted to improve my Prolog so I figured why not do something like that in GNU Prolog? I have already started a simple HTTP library in pure GNU Prolog as well but this comes first now.

Progress so far is good…if I run it like this from a folder containing the unpacked android.jar file...

| ?- jread('javax/net/SocketFactory.class',X).

X = javaclass(super('java/lang/Object'),class('javax/net/SocketFactory'),implements([]),
returns('(Ljava/net/InetAddress;ILjava/net/InetAddress;I)Ljava/net/Socket;'),[attr(18,[0,1,0,19])])])) ? 

The place I am at now is decoding the bit flags for the class (and eventually the methods etc) into a term.

A typical value would be “1057” decimal, 0x421, this contains the flags:

ACC_PUBLIC 0x0001 Declared public; may be accessed from outside its package.
ACC_FINAL 0x0010 Declared final; no subclasses allowed.
ACC_SUPER 0x0020 Treat superclass methods specially when invoked by the invokespecial instruction.
ACC_INTERFACE 0x0200 Is an interface, not a class.
ACC_ABSTRACT 0x0400 Declared abstract; must not be instantiated.
ACC_SYNTHETIC 0x1000 Declared synthetic; not present in the source code.
ACC_ANNOTATION 0x2000 Declared as an annotation type.
ACC_ENUM 0x4000 Declared as an enum type.

So you can see that 0x421 means “ACC_PUBLIC, ACC_SUPER and ACC_ABSTRACT”. I would like to produce a term something like this:


It’s not that I don’t know how to do it or that I can’t do it but I am not sure what is the *most elegant* way to do it in Prolog! Heaven knows I have written bit shifting loops to test for flags in a dozen languages but not in Prolog.

So, there’s the challenge, what is the most elegant way to turn a list of bits into a list of atom terms. Ideally I would make the list of atoms a parameter so that I can reuse it for the other flag based values.

Consider the gauntlet thrown!   In the meantime I am continuing to research the possibilities for myself. I will of course put it all on GitHub along with my Redis client. I may even create a Redis database with the information!


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