xmas.erl

-module(xmas).
-export([tree/1]).

tree(Width, _, _) when Width < 3 orelse Width > 21 ->
	error( { out_of_range, { min, 3 }, { max, 21 }, { input, Width } } );
tree(Width, _, _) when Width rem 2 == 0 ->
	error( { width_is_not_odd, Width } );
tree(Width, Trunk, Leaves) ->
	lists:foreach(
		fun (A) ->
			Indent = (Width - A) div 2,
			io:format("~s~s~n", [
				string:copies(" ", Indent),
				string:copies(Leaves, A)
			] )
		end,
		lists:seq(1, Width, 2)
	),
	Indent = (Width - 3) div 2,
	io:format("~s~s~n", [
		string:copies(" ", Indent),
		string:copies(Trunk, 3)
	] ).

tree(Spec) ->
	[ WidthStr, Trunk, Leaves ] = string:tokens(Spec, " "),
	{ Width, _ } = string:to_integer(WidthStr),
	tree(Width, Trunk, Leaves).
	
% 1> c(xmas).
% {ok,xmas}
% 2> xmas:tree("21 # *").
%           *
%          ***
%         *****
%        *******
%       *********
%      ***********
%     *************
%    ***************
%   *****************
%  *******************
% *********************
%          ###
% ok