:- use_module(library(lists), [reverse/2]).
:- use_module(contestlib, [write_elements/1, writeN/2]).

triangle(Chars) :-
	length(Chars,Spaces),
	Chars = [Middle|RestChars],
	End = [],
	Spaces1 is 2*Spaces,
	part1(Middle,End,RestChars,Spaces1).
triangle(Chars) :-
	length(Chars,Spaces),
	reverse(Chars,[_|Post]),
	part2(Post,5,Spaces).
triangle(_).

part1(Middle,End,RestChars,Spaces) :-
	writeN(Spaces,' '),
	reverse_write_elements(End),
	write_elements([Middle|End]),
	nl,
	Spaces1 is Spaces - 1,
	RestChars = [NewMiddle|NewRestChars],
	part1(NewMiddle,[Middle|End],NewRestChars,Spaces1).

part2([X|Cs],Len,Spaces) :-
	writeN(Spaces,' '),
	reverse_write_elements(Cs),
	writeN(Len,X),
	write_elements(Cs),
	nl,
	Spaces1 is Spaces - 1,
	Len1 is Len + 4,
	part2(Cs,Len1,Spaces1).

reverse_write_elements(L) :-
	reverse(L,RL),
	write_elements(RL).