Archive for May, 2007


Monday, May 28th, 2007

Coupling Cribbage and Erlang into a program sounds like a fun little program to write to aid in learning Erlang while writing a program that brings a game I like in life to the virtual world. Is it the most efficient? Probably not, but you gotta start somewhere. To the code!

The first thing I wanted to do was create a method to calculate points. An ace is a 1, 2-10 are face value, and Jack, Queen, King are 11, 12, 13, respectively. Easy adjustments could be made to allow characters (A, J, Q, K) but for now, I like keeping it simple.

points([]) -> 0;
points(L) ->
    Hand = lists:sort(L),
    fifteens(Hand, 0) + runs(Hand, 0, 0) + pairs(Hand, 1, 0).

The above creates a module called cribbage and exports a function called points/1 which takes one parameter, a list of cards. There are three kinds of scoring in Cribbage: combinations of cards that equal 15, runs of three or more, and pairs (or sets or four of a kind). There is one other kind, but it’s not part of this portion of the game.

cardval(C) when C > 9 -> 10;
cardval(C) -> C.
fifteens(_L, Total) when Total > 15 -> 0;
fifteens(_Hand, Total) when Total =:= 15 -> 2;
fifteens([], _Total) -> 0;
fifteens([H | T], Total) when Total < 15 ->
    fifteens(T, Total) + fifteens(T, Total + cardval(H)).

cardval is a function that converts the value of face cards (11-J,12-Q,13-K) to 10 and leaves other cards unchanged in value. This is useful in finding all combinations of 15 in the hand. When a combo equals 15, two points are added to the score.

Runs were the trickiest of the three to get right. First, I defined a simple function to determine the points for a run of given length.

run(3) -> 3;
run(4) -> 6;
run(5) -> 12;
run(_Length) -> 0.

Some people may play with different values for runs of different lengths, so this allows for easy editing.

Runs come in two flavors: 1) A normal run, and 2) A run where one or two of the cards are doubled. To account for this, I have runs/3 and runs/4. runs/3 handles the first case, and passes control to runs/4 when a run of the second case is encountered. Another special case is when a run has two different cards doubled (e.g. 3,4,4,5,5) where the run of three is doubled and doubled again.

%% two cases for runs
%%   1. A straight run - 4,5,6,7
%%   2. A run with a double in the sequence - 4,4,5,6 or 4,5,5,6
runs([], _Curr, Len) -> run(Len);
runs([H | T], Curr, Len) when H =:= (Curr+1) -> runs(T, H, Len + 1);
runs([H | T], Curr, Len) when H =:= Curr -> runs(T, Curr, Len, {H, 2});
runs([H | T], _Curr, Len) -> run(Len) + runs(T, H, 1).
runs([], _Curr, Len, {_Card, Mult}) -> Mult * run(Len);
runs([H | T], Curr, Len, {Card, Mult}) when H =:= (Curr+1) ->
    runs(T, H, (Len+1), {Card, Mult});
%% needed for special cases where multiple cards are doubled up
%% like 3,4,4,5,5
runs([H | T], Curr, Len, {Card, Mult}) when H =:= Curr, H > Card -> 
    runs(T, Curr, Len, {H, (Mult*2)});
%% handles a triple carding, like 2,2,2,3,4
runs([H | T], Curr, Len, {Card, Mult}) when H =:= Curr ->
    runs(T, Curr, Len, {Card, (Mult+1)});
runs([H | T], _Curr, Len, {_Card, Mult}) ->
    (Mult * run(Len)) + runs(T, H, 1).

For pairs, I do a similar thing: define a pair(Length) function that returns the point value given a number of similar cards. But it’s all pretty straightforward.

pairs([], Pairs, _Curr) -> pair(Pairs);
pairs([H | T], Pairs, Curr) when H =:= Curr -> pairs(T, Pairs+1, Curr);
pairs([H | T], Pairs, _Curr) -> pair(Pairs) + pairs(T, 1, H).
pair(2) -> 2;
pair(3) -> 6;
pair(4) -> 12;
pair(_Length) -> 0.

That’s it for now. Actual game play to come. You can get the code here.

Remove nested arrays in javascript using the prototype library

Tuesday, May 22nd, 2007

I have been playing with some drawing code in javascript, storing coordinates and using them later on in the application. My list of coordinates is of the form [ [id1, x1, y1, width1, height1], [id2, x2, y2, width2, height2],…]. A requirement of the application is that a user can delete a set of coordinates from the list. Using prototype.js, I created a simple function to remove the nested array based on the id.

// remove an array from the list based on the id
function remove(id, list) {
    return $A(list).map( 
        function(arr) {
            if ( $A(arr).first() == id ) { return ; }
            else { return arr; }

In your favorite editor, this function can be a one-liner, but spacing helps here for clarity and formatting on the page. Onward!

So what’s happening? The first thing we do is wrap list with the $A() call to ensure we have access to the extensions prototype gives us for arrays (I’m calling the parameter a list because I’m on an Erlang kick and it has infiltrated my core!). Once extended, we call the map function to iterate through the list and apply the supplied function to each element in the list (in this case it is a list of arrays, so each element passed to the supplied function will be an array as well).

Within the supplied function, we are dealing with a single array of the form [id, x, y, width, height], so $A(arr).first() returns the id of the array. This value is compared to the value of the id parameter and if it matches, returns nothing, or ‘undefined’ in Javascript. If the ids don’t match, it returns the array unaltered. As the map function iterates through the list, a new list is created containing the results of the supplied function. So the return value of the map function call is an array. We then call the compact function on the resulting array, which removes any undefined values from the array, essentially leaving only those arrays that did not have the id passed in.

This function is fairly specialized; the requirements for the function are fairly specific. A more general function could be written but that is an exercise left to the reader.

Recursive FTP

Wednesday, May 9th, 2007

So you want to download some files from an ftp server, but they are contained in more than one subdirectory. With a straight ftp client, you would have to recurse through all of the directories and mget each directory’s contents manually. Never fear, though, there is a little utility that can help – wget.

> wget -r .

If the ftp site allows anonymous logins, you can omit the user:pass portion. This will get everything…it is left as an exercise to the reader to customize the command.