spiral.erl

-module(spiral).
-compile(export_all).

%% -- if accumulator is empty, put first row into acc and continue
%% -- if last accum value is first value of row 1, put row one into accum and continue
%% -- if last accum value is last value of row one, collect all last row values into accum and continue
%% -- if last accum value is last value of last row, collect last row into accum and continue
%% -- if last accume value is first value of last row, collect first values into accum and continue

follow2() ->
	A = [
		[01, 02, 03, 04, 05],
		[14, 15, 16, 17, 06],
		[13, 20, 19, 18, 07],
		[12, 11, 10, 09, 08]
	],
	follow2(A, [0]).

follow2([FirstRow | Matrix], []) ->
	follow2(Matrix, FirstRow);

follow2([], Acc) -> Acc;

follow2(Matrix, Acc) ->
	LastVal = lists:max(Acc) + 1,
	case LastVal == first_of_first(Matrix) of
		true ->
			[Row | Tail] = Matrix,
			follow2(Tail, Acc ++ Row);
		false ->
			case LastVal == last_of_first(Matrix) of
				true ->
					{Values, NewMatrix} = last_values(Matrix),
					follow2(NewMatrix, Acc ++ Values);
				false ->
					case LastVal == last_of_last(Matrix) of
						true ->
							[Values | Tail] = lists:reverse(Matrix),
							follow2(lists:reverse(Tail), Acc ++ lists:reverse(Values));
						false ->
							case LastVal == first_of_last(Matrix) of
								true ->
									{Values, NewMatrix} = first_values(Matrix),
									follow2(NewMatrix, Acc ++ lists:reverse(Values));
								false ->
									exit({{error, invalid_spiral_matrix}, erlang:get_stacktrace()})
							end
					end
			end
	end.

first_of_first([ [ Value | _ ] | _]) -> Value.

last_of_first([Row | _Matrix]) ->
	[Value | _ ] = lists:reverse(Row),
	Value.

last_of_last(Matrix) ->
	[LastRow | _] = lists:reverse(Matrix),
	[Value | _] = lists:reverse(LastRow),
	Value.

first_of_last(Matrix) ->
	[[Value | _] | _] = lists:reverse(Matrix),
	Value.

last_values(Matrix) ->
	last_values(Matrix, [], []).

last_values([], Values, Acc) -> {lists:reverse(Values), lists:reverse(Acc)};
last_values([Row | Matrix], Values, Acc) ->
	[Value | RowTail] = lists:reverse(Row),
	last_values(Matrix, [Value | Values], [lists:reverse(RowTail) | Acc]).

first_values(Matrix) ->
	first_values(Matrix, [], []).

first_values([], Values, Acc) -> {lists:reverse(Values), lists:reverse(Acc)};
first_values([[Value | RowTail] | Matrix], Values, Acc) ->
	first_values(Matrix, [Value | Values], [RowTail | Acc]).