Java Generics for Compare

Java Generics for Compare

I’ve been developing with Java 5+ for quite a while now. Not all developers are this lucky, some are still stuck with 1.4… some even with 1.3! But my clients all made the excellent step forward to Java 5 (some even to 6). The problem is, they moved the runtime/JDK but forget to move their developers!

In Java 5 the language brings some good improvements, the for-loop is easy to understand, and almost all the developers are using this by now. The problem starts with generics. There is a part most developers understand, the Collections API. Almost all programmers use lists now as: List instead of a plain old List. This is a good start, but it must not end here! First, I must admit, generics in Java can sometimes be hard and confusing (when using <? extends X> and <? super X>). So I'm not going to talk about any of this 'hard stuff'. Its the use of 'easy' generics that can our lifes so much easier.

For example the piece of code below:

public class LabelPlaceholderComparator implements Comparator { 

    /**{@inheritDoc */
    public int compare(Object o1, Object o2) {
        LabelPlaceholder p1 = (LabelPlaceholder) o1;
        LabelPlaceholder p2 = (LabelPlaceholder) o2;
        return p1.getLabel().compareTo(p2.getLabel());
    }
}

Of course, there seems to be not much wrong with the code, I see it all the time. Yes, the code breaks if you put something else in the comparator, but hey… the Javadoc says it only accepts LabelPlaceholders! So lets use this code:

List holders = fillList();
Collections.sort(holders, new LabelPlaceholderComparator());

Done! Its working and no problems right? Not quite, your IDE (Eclipse in my case) complains about this code.
For example Eclipse says:

Type safety: The expression of type LabelPlaceholderComparator needs unchecked conversion to conform to Comparator<? super T>

At this point, most programmers at the company I work for now will just ignore this warning. They might even add:
@SuppressWarnings(“unchecked”)

What a shame… Lets just examine this warning, what is Eclipse trying to tell us here? The compiler doesn’t know we created the Comparator with only LabelPlaceholders in mind. But the compiler does know (with generics) that the List only contains LabelPlaceholders. So the warning is (in understandable English):

I’ve got a list here of T (LabelPlaceholders) and a Comparator for Objects, this can go wrong! I’d rather have a specific Comparator for this job. Do you have one for me?

The solution to this problem is very simple, but most neglect to use it:

public class LabelPlaceholderComparator implements Comparator<LabelPlaceholder> { 

    /**{@inheritDoc */
    public int compare(LabelPlaceholder p1, LabelPlaceholder p2) {
        return p1.getLabel().compareTo(p2.getLabel());
    }
}

As you can see, the code is much smaller. The interface is now generified, it knows we are going to compare LabelPlaceholders now, nothing more, nothing less. Also, we don’t have to cast anymore, because of the generics you can’t put anything else in there.

So, lets go to the conclusion: Why is the latter code better code?

  1. As you can see, the code is smaller!
  2. There are no casts, the code is safer (no ClassCastException or eleborate class checks)
  3. If somebody uses your code, he/she knows what kind of objects the Comparator can handle. You don’t have to read the Javadoc or the code to see what it does.

Throughout the projects I encounter I keep finding examples of places where generics would have made the code smaller/safer/more understandable. For some reason the programmers still only use generics on collections. So, even though generics aren’t perfect, please use them where/when you can, it’ll always add clarity to the code, and most of the time it’ll also make your code safer, and in some cases the code gets smaller because you can leave away casts and class-checks.

Don’t ever let me see public int compare(Object o1, Object o2); again!

(You see, it is possible for me to have a discussion about Java generics without mentioning reified generics!)


Son Of Darts

Son Of Darts

Another thing I’ve been very busy with lately is AZsPCs (Al Zimmermanns Programming Competition). The current contest is called Son of Darts.

The idea behind these contests are that they are easy to grasp, but very hard to master.

Lets take three darts. You have to throw them to a dartboard which is divided into 4 regions. For example, the values on these regions are: 1,2,4,6.
The first question is: What is the lowest value you can’t throw with these three darts?

This is easy to calculate:
Can we throw one? Yes: 1 dart in the 1
Can we throw two? Yes: 1 dart in the 2, or 2 darts in the 1
etc etc
Can we throw nine? Yes: 6,2,1
Can we throw ten? Yes: 6,4
Can we throw eleven? Yes: 6,4,1
Can we throw twelve? Yes: 6,6
Can we throw thirteen? Yes: 6,6,1
Can we throw fourteen? Yes: 6,6,2
Can we throw fifteen? Err… no, sorry…

So the score is: 15 points.

The main question: Can you think of better values for the regions of the dartboard to get a higher topscore??

This is what the competition is about. But not only for a dartboard consisting of 4 regions, but up to 40 regions. And not only for three darts, but also 4, 5 and even 6 darts.

If you can create a good solver its pretty easy to bruteforce up to a certain point, but the problem is, you quickly get more and more options for which you have to check the scores… It is an exponential function…!

Actually, this is not a new puzzle. Its been around of quite a long time. But its more commenly known as the local postage stamp problem (LPSP). Formulated just a little bit different, instead of a dartboard with regions you have a postcard with room for H stamps. What is the lowest value you can’t create with stamps Nh? Also check out Wolfram’s description of the problem.

This problem has been proven to be NP-hard, so bruteforcing won’t be an option, you’ll need to use something different. Put on your thinking-caps and create some good innovative heuristics.


Quine - McCluskey

Quine - McCluskey

About a week ago I had a discussion with a fellow programmer about some boolean logic. We had three parameters, something like:

  1. personHasInsurence (A)
  2. personNeedsInsurence (B)
  3. personIsKnownAtThisAgency (C)

We also had two particulair cases for an insurance page:

Case 1:
Person has insurence and isn’t yet known at this agency

Case 2:
Person doesn’t have insurence, needs insurence and is known at this agency

Case 3:
Person doesn’t have insurence, doesn’t need insurence and is known at this agency

So the view-logic was a bit complex:

if( (A && !C) || (!A && B && C) || (!A && !B && C ) ) {
    showPage();
}

Then I remebered something I learned at school some time ago. So called karnaugh maps. I’ve completely forgotten how to use them, but I knew it was possible to calculate the shortest form to comply to the logic rules. When looking further I found the so called “Quine McCluskey“-algorithm, and I decided to implement it (just to learn how it works).

Quine - McCluskey algorithm

First of, lets go through a couple of terms.

Minterm: A small boolean function which has all the different input variables, once. So for example, a minterm using the above variables would be: ABC (A and B and C), or A’BC (not A and B and C).

The first thing you do using this algorithm is that you find so called “prime implicants”. An implicant is a combination of one (or more) minterms, and a prime implicant is a implicant which can’t be combined with other implicants (for more details, read the wikipage with examples)!

After combining all the minterm of your case(s), you’ll end up with a “prime implicant chart”. This is a chart with all the prime implicants and the fields they cover. Sometimes its easy to spot “essential prime implicants”. That is, implicants which are unique in covering a field. You have to use these implicants in the final logic.

When you have multiple options left to combine to cover all the fields, you can use Petrick’s Method to select the best/smallest option.

Using the above example, if you minimize, you’ll come down to:

if((!A && C) || (A && !C)) {
    showPage();
}

The algorithm is pretty fun to program, and its a bit different from most algorithms I’ve seen lately!

And if you need something even faster, try out Espresso!

ps. I love browsing this page: List of algorithms
But OTOH, its kind of depressing I still have soo much to learn.


Corewars - Brainf*ck

Corewars - Brainf*ck

I’ve implemented a Brainfuck interpreter!

Yes, you heard it right, Brainfuck (BF).
BF is an esoteric programming language. More information on BF can be found here.

The language itself is pretty simple, and most of it was implemented rather quickly in Redcode. The only big problem was navigating back to the correct closing brackets in a loop. This is now solved by counting the amount of open/close brackets in between.

So here is the code:

;----------------------------------------------
;redcode-94nop 
;name Brainfuck Interpreter (v0.3) 
;author Roy van Rijn 
;strategy Brainfuck Interpreter with self-reproduction program (a quine) 
;strategy It takes 7244366 cycles (runs slow!)
;assert 1 


LT      equ     60 ;<
GT      equ     62 ;> 
INC     equ     43 ;+ 
DEC     equ     45 ;- 
OUT     equ     46 ;. 
IN      equ     44 ;, 
OP      equ     91 ;[ 
CL      equ     93 ;] 


lt      equ     dat LT  , cLT-START 
gt      equ     dat GT  , cGT-START 
inc     equ     dat INC , cINC-START 
dec     equ     dat DEC , cDEC-START 
out     equ     dat OUT , cOUT-START 
in      equ     dat IN  , cIN-START 
op      equ     dat OP  , cOP-START 
cl      equ     dat CL  , cCL-START 


;Use switch-table 
cINC    add     #2              , @PTR          ;add 2 min 1 
cDEC    sub     #1              , @PTR          ;min 1 


START   mov.b   }PTR            , #0 
       jmp     @-1 


cLT     jmp     START           , PTR 


cOUT    mov.b   @PTR            , <6000         ;sts.b  @PTR 
       jmp     START 
cIN     mov.b   }INPUT          , @PTR          ;now using INPUT-table instead or streams 
       jmp     START 


;Optimize by using a stack? remembering the layer+position? 
cOP     jmn     START           , @PTR 
opCheck sne.ab  *PTR            , #OP 
       add     #1              , CNT1 ;Count nested brackets 
       seq.ab  }PTR            , #CL 
       jmp     opCheck 
CNT1    djn.b   opCheck         , #1 
       jmp     START           , >CNT1 


cCL     jmz     START           , @PTR 
       nop     {PTR            , >CNT2 
clCheck sne.ab  {PTR            , #CL 
       add     #1              , CNT2 ;Count nested brackets 
       seq.ab  *PTR            , #OP 
       jmp     clCheck 
CNT2    djn.b   clCheck         , #0 
       jmp     START 


;Used to mimic keypresses (when not using exmars_streams) 
INPUT   dat     0               , -1 


PROGRAM 


;Brainfuck Quine, unsure who wrote the original.... (not me anyway!) 


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EOF     dat     0               , -10           ; Halt 
PTR     dat     #PROGRAM        , #1            ; A: Execution pointer, B: Data pointer 


end START 


;Translated Brainfuck "Hello World" program 
; 
;The original is: 
;++++++++++[>+++++++>++++++++++>+++>+<<<<-] 
;>++.>+.+++++++..+++.>++.<<+++++++++++++++.> 
;.+++.------.--------.>+.>. 


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EOF     dat     0               , -10           ; Halt 
PTR     dat     #PROGRAM        , #1            ; A: Execution pointer, B: Data pointer 


end START

Corewars - SUBLEQ interpreter

Corewars - SUBLEQ interpreter

In my previous blogpost I talked about Corewars and the Redcode language. But instead of playing the game, you can do a lot more with the programming language. John Metcalf posted a blog about OISC (One Instruction Set Computers). He decided to implement the RSSB algorithm, so I took on the challenge of implementing SUBLEQ, another single instruction set computer.

And here is the result:

;-----------------------------------------------
;redcode-94odd
;name SUBLEQ Hello World
;author Roy van Rijn
;assert 1
;-----------------------------------------------
;Implementation of the OISC SUBLEQ interpreter.
; All instructions take 3 lines:
; dat A,0
; dat B,0
; dat C,0
;
; The interpreter works as following:
;
; Substract A from B
; If B is less or equal to zero jump to C
; Else go to the next instruction (+3)
;
; If the jump is to HALT (-1) the program will halt
; If the substraction is OUT (-2) the program will output the amount substracted
;-----------------------------------------------
HALT   equ -1
OUT    equ -2

;Pointers used by program:
EQ     dat PROGRAM-1      , PROGRAM
PTR    dat PROGRAM        , PROGRAM+1
RESETV dat PROGRAM-SUBSTR , PROGRAM-SUBSTR ;reset values
NEXTV  dat #3             , #3

;Print to command line:
PRINT  mov.ab *SUBSTR     , #0
       mov.b  -1          , <1000 ;sts.b -1,0 for output

;Reset to absolute values:
NEXT   add.f NEXTV        , PTR
RESET  mov.f RESETV       , SUBSTR

;Main logic:
START  add.a  *PTR        , SUBSTR
       add.ab >PTR        , SUBSTR
       mov.a  @PTR        , NEWPTR

;Check if output is OUT, print and go to next instruction:
       mov.a  <PTR        , 1
       sne.ab #0          , #OUT
       jmp    PRINT

;Substract and determine if the value <= 0
SUBSTR sub.a  PROGRAM     , PROGRAM
       slt.a  #CORESIZE/2 , @-1
       jmn.ba NEXT        , @-2

;Jump to new address:
NEWPTR mov.ab #0          , #0
       mov.f  NEWPTR      , PTR
       add.f  EQ          , PTR
       seq.ab NEWPTR      , #HALT

;Check for HALT (jump to DAT-PROGRAM)
       jmp RESET

LABEL
PROGRAM

;-----------------------------------------------
;Translated version of the Hello World program from:
;http://mozaika.com.au/oleg/subleq/

dat A-LABEL,0
dat A-LABEL,0
dat 3,0
dat p-LABEL,0
dat Z-LABEL ,0
dat 6,0

dat Z-LABEL ,0
dat A-LABEL ,0
dat 9,0

dat Z-LABEL ,0

dat Z-LABEL ,0
dat A-LABEL,0

A dat PROGRAM ,0
dat OUT ,0
dat 15,0

dat m1-LABEL,0
dat p-LABEL,0
dat 18,0

dat A-LABEL ,0
dat A-LABEL ,0
dat 21,0

dat E-LABEL ,0
dat Z-LABEL ,0
dat 24,0

dat Z-LABEL ,0
dat A-LABEL ,0
dat 27,0

dat Z-LABEL,0
dat Z-LABEL,0
dat 30,0

dat b-LABEL,0
dat b-LABEL,0
dat 33,0

dat p-LABEL,0
dat Z-LABEL,0
dat 36,0

dat Z-LABEL ,0
dat b-LABEL,0
dat 39,0

dat Z-LABEL,0
dat Z-LABEL,0
dat 42,0

dat E-LABEL,0
dat b-LABEL,0
dat 48,0 ;?+3

dat Z-LABEL ,0
dat Z-LABEL,0
dat 51,0 ;?+3

dat p-LABEL,0
dat A-LABEL,0
dat 54,0 ;?+3

dat Z-LABEL ,0
dat Z-LABEL ,0
dat 0,0

dat 0 ,0
dat 0 ,0
dat -1,0

;data
p dat H-LABEL,0
Z dat 0,0
m1 dat -1,0
b dat 0,0
H dat 72,0 ;:H
dat 101,0 ; e
dat 108,0 ; l
dat 108,0 ; l
dat 111,0 ; o
dat 32,0 ; space
dat 87,0 ; W
dat 111,0 ; o
dat 114,0 ; r
dat 108,0 ; l
dat 100,0 ; d
E dat E-LABEL,0

end START
;-----------------------------------------------

Corewars - An Introduction to Hostile Programming

Corewars - An Introduction to Hostile Programming

Corewar is a game from the 1980’s, played between computer programs written in Redcode, a language similar to assembly. The programmers design their battle programs to remove opponents from the memory of the MARS virtual computer by any means possible.

Some of the simpler techniques include blindly overwriting memory, searching for the opponent or spawning off new processes. These are commonly known as stone, scissors, paper after the popular playground game. Stone usually wins against scissors, scissors normally defeat paper, and paper mostly beats stone.

Here’s an example of a typical Corewar program:

org   wipe
    
step  equ 5
first equ bomb-10
    
    bomb:mov.i #1,       -1
    
    ptr: sub   #step,    #first
    wipe:jmz.f ptr,      @ptr
    
mov   bomb,     >ptr
djn.f wipe,     {ptr-5
    
end

This simple example of scissors once held a 20 point lead over it’s rivals. The first instruction is never executed, it’s the bomb used to overwrite opponents. The next two instructions form a loop which looks through memory for an opponent, and the final two instructions actually overwrite it.

Corewar is still going strong, and celebrates it’s 25th anniversary in 2009. If you’d like to discover more about Corewar, here are the top resources:

  • The Beginner’s Guide to Redcode will teach you the language of Corewar
  • pMARS is a portable implementation of the Corewar virtual machine
  • Corewar Tutorials exist on virtually every aspect of the game
  • Koenigstuhl is an archive of thousands of published Corewar programs
  • SAL organises a number of on-going king of the hill tournaments
  • sfghoul and impomatic report the latest Corewar news on their blogs
  • #corewars is the official Corewar discussion channel, hosted by irc.freenode.net

What are your experiences with Corewar, have you ever had any success?