recursion.erl

%% ITESM CEM, August 31, 2010.
%% Erlang Source File
%% Activity: Recursive Functions
%% Author: Juliana Pena, 1165536

-module(recursion).
-export([but_last/1, merge/2, insert/2, sort/1, binary/1, bcd/1, prime_factors/1, compress/1, encode/1, decode/1]).

but_last([_]) -> [];
but_last([H|T]) -> [H|but_last(T)].

insert(A, []) -> [A];
insert(A, [H|T]) ->
	if
		H >= A -> [A, H | T];
		true   -> [H | insert(A, T)]
	end.

merge(A, []) -> A;
merge([], B) -> B;
merge([Ha|Ta], [Hb|Tb]) ->
	if
		Ha < Hb -> [Ha | merge(Ta, [Hb|Tb])];
		true    -> [Hb | merge([Ha|Ta], Tb)]
	end.

sort([]) -> [];
sort([H|T]) ->	insert(H, sort(T)).

binary(0) -> [];
binary(N) -> binary(N div 2) ++ [N rem 2].

bcd(N) when N < 10 ->
	if 
		N == 0 -> ["0000"];
		N == 1 -> ["0001"];
		N == 2 -> ["0010"];
		N == 3 -> ["0011"];
		N == 4 -> ["0100"];
		N == 5 -> ["0101"];
		N == 6 -> ["0110"];
		N == 7 -> ["0111"];
		N == 8 -> ["1000"];
		N == 9 -> ["1001"]
	end;
bcd(N) ->
	bcd(N div 10) ++ bcd(N rem 10).

prime_factors(N) -> prime_helper(N, 2).
	
prime_helper(N, Factor) ->
	if
		N == 1 -> [];
		Factor == N -> [N];
		N rem Factor == 0 -> [Factor | prime_helper(N div Factor, Factor)];
		true -> prime_helper(N, Factor + 1)
	end.

compress([])  -> [];
compress([A]) -> [A];
compress([First | T]) ->
	[Second | _] = T,
	if
		First == Second -> compress(T);
		true -> [First | compress(T)]
	end.

encode([]) -> [];
encode([A]) -> [A];
encode([H|T]) -> encode_helper({1, H}, T).

encode_helper({Count, Element}, []) -> [{Count, Element}];
encode_helper({Count, Element}, [H | T]) ->
	if
		Element == H -> encode_helper({Count + 1, Element}, T);
		true -> 
			if 
				Count > 1 -> [{Count, Element} | encode_helper({1, H}, T)];
				true -> [Element | encode_helper({1, H}, T)]
			end
	end.

decode([]) -> [];
decode([{Count, Element} | T]) ->
	decode_helper({Count, Element}) ++ decode(T);
decode([A | T]) ->
	[A | decode(T)].
	
decode_helper({0, _}) -> [];
decode_helper({Count, Element}) ->
	[Element | decode_helper({Count - 1, Element})].