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# Helot's solution

## to Connect in the Erlang Track

Published at Dec 15 2018 · 0 comments
Instructions
Test suite
Solution

#### Note:

This exercise has changed since this solution was written.

Compute the result for a game of Hex / Polygon.

The abstract boardgame known as Hex / Polygon / CON-TAC-TIX is quite simple in rules, though complex in practice. Two players place stones on a rhombus with hexagonal fields. The player to connect his/her stones to the opposite side first wins. The four sides of the rhombus are divided between the two players (i.e. one player gets assigned a side and the side directly opposite it and the other player gets assigned the two other sides).

Your goal is to build a program that given a simple representation of a board computes the winner (or lack thereof). Note that all games need not be "fair". (For example, players may have mismatched piece counts.)

The boards look like this (with spaces added for readability, which won't be in the representation passed to your code):

``````. O . X .
. X X O .
O O O X .
. X O X O
X O O O X
``````

"Player `O`" plays from top to bottom, "Player `X`" plays from left to right. In the above example `O` has made a connection from left to right but nobody has won since `O` didn't connect top and bottom.

## Running tests

In order to run the tests, issue the following command from the exercise directory:

For running the tests provided, `rebar3` is used as it is the official build and dependency management tool for erlang now. Please refer to the tracks installation instructions on how to do that.

In order to run the tests, you can issue the following command from the exercise directory.

``````\$ rebar3 eunit
``````

## Questions?

For detailed information about the Erlang track, please refer to the help page on the Exercism site. This covers the basic information on setting up the development environment expected by the exercises.

## Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.

### connect_tests.erl

``````%% based on canonical data version 1.1.0
%% https://raw.githubusercontent.com/exercism/problem-specifications/master/exercises/connect/canonical-data.json

-module(connect_tests).

-include_lib("erl_exercism/include/exercism.hrl").
-include_lib("eunit/include/eunit.hrl").

an_empty_board_has_no_winner_test() ->
Input=[
". . . . .",
" . . . . .",
"  . . . . .",
"   . . . . .",
"    . . . . ."
],
Expected=undefined,
?assertMatch(Expected, connect:winner(Input)).

x_can_win_on_a_1x1_board_test() ->
Input=[
"X"
],
Expected=x,
?assertMatch(Expected, connect:winner(Input)).

o_can_win_on_a_1x1_board_test() ->
Input=[
"O"
],
Expected=o,
?assertMatch(Expected, connect:winner(Input)).

only_edges_does_not_make_a_winner_test() ->
Input=[
"O O O X",
" X . . X",
"  X . . X",
"   X O O O"
],
Expected=undefined,
?assertMatch(Expected, connect:winner(Input)).

illegal_diagonal_does_not_make_a_winner_test() ->
Input=[
"X O . .",
" O X X X",
"  O X O .",
"   . O X .",
"    X X O O"
],
Expected=undefined,
?assertMatch(Expected, connect:winner(Input)).

Input=[
"X . . .",
" . X O .",
"  O . X O",
"   . O . X",
"    . . O ."
],
Expected=undefined,
?assertMatch(Expected, connect:winner(Input)).

x_wins_crossing_from_left_to_right_test() ->
Input=[
". O . .",
" O X X X",
"  O X O .",
"   X X O X",
"    . O X ."
],
Expected=x,
?assertMatch(Expected, connect:winner(Input)).

o_wins_crossing_from_top_to_bottom_test() ->
Input=[
". O . .",
" O X X X",
"  O O O .",
"   X X O X",
"    . O X ."
],
Expected=o,
?assertMatch(Expected, connect:winner(Input)).

x_wins_using_a_convoluted_path_test() ->
Input=[
". X X . .",
" X . X . X",
"  . X . X .",
"   . X X . .",
"    O O O O O"
],
Expected=x,
?assertMatch(Expected, connect:winner(Input)).

x_wins_using_a_spiral_path_test() ->
Input=[
"O X X X X X X X X",
" O X O O O O O O O",
"  O X O X X X X X O",
"   O X O X O O O X O",
"    O X O X X X O X O",
"     O X O O O X O X O",
"      O X X X X X O X O",
"       O O O O O O O X O",
"        X X X X X X X X O"
],
Expected=x,
?assertMatch(Expected, connect:winner(Input)).``````
``````-module(connect).

-include_lib("stdlib/include/assert.hrl").

-define(GOAL_START_X, start_x).
-define(GOAL_END_X, end_x).
-define(GOAL_START_O, start_o).
-define(GOAL_END_O, end_o).

-export([winner/1]).

-type board() :: [\$X | \$O | \$. | \$ , ...].

%% API

-spec winner([board()]) -> x | o | undefined.
winner(Board) ->
Parsed = parse_board(Board),
case {is_winner(Parsed, x), is_winner(Parsed, o)} of
{true, true} ->
% This should never be possible unless there's a bug in
% the library.
throw(impossible_board);
{true, false} ->
x;
{false, true} ->
o;
{false, false} ->
undefined
end.

%% Internal

parse(\$X) -> x;
parse(\$O) -> o;
parse(\$.) -> undefined;
parse(\$ ) -> space.

parse_board([_ | _] = Board) ->
Parsed = [[parse(C) || C <- Row, parse(C) /= space] || Row <- Board],
Width = length(hd(Parsed)),
ok = ?assert(lists:all(fun (Row) -> Width == length(Row) end, Parsed),
"All rows should be of identical length."),
Parsed.

graph_board(G, Board) ->
ok.

dimensions([Row | _] = Board) ->
Width = length(Row),
Height = length(Board),
{Width, Height}.

Dimensions = dimensions(Board),
ok = add_vertices(G, Board, {0, 0}, Dimensions),
G.

_ = digraph:add_edge(G, V1, V2, Label),
_ = digraph:add_edge(G, V2, V1, Label),
ok.

add_w_edge(G, V, {X, Y}) when X > 0 ->
add_undirected_edge(G, V, {X - 1, Y}, west);
ok.

add_nw_edge(G, V, {X, Y}) when Y > 0 ->
add_undirected_edge(G, V, {X, Y - 1}, northwest);
ok.

add_ne_edge(G, V, {X, Y}, {Width, _Height}) when Y > 0, X < Width - 1 ->
add_undirected_edge(G, V, {X + 1, Y - 1}, northeast);
ok.

ok = add_ne_edge(G, V, Coordinates, Dimensions).

add_vertices(_G, [], {_X, _Y}, _Dimensions) ->
ok;
add_vertices(G, [[] | Board], {_X, Y}, Dimensions) ->
add_vertices(G, Board, {0, Y + 1}, Dimensions);
add_vertices(G, [[Space | Row] | Board], {X, Y}, Dimensions) ->
V = digraph:add_vertex(G, {X, Y}, Space),
ok = add_edges(G, V, {X, Y}, Dimensions),
add_vertices(G, [Row | Board], {X + 1, Y}, Dimensions).

ok = lists:foreach(fun ({X, Y}) -> digraph:add_edge(G, StartX, {X, Y}) end,
[{0, Y} || Y <- lists:seq(0, Height)]),
ok = lists:foreach(fun ({X, Y}) -> digraph:add_edge(G, StartO, {X, Y}) end,
[{X, 0} || X <- lists:seq(0, Width)]),
ok = lists:foreach(fun ({X, Y}) -> digraph:add_edge(G, {X, Y}, EndX) end,
[{Width - 1, Y} || Y <- lists:seq(0, Height)]),
ok = lists:foreach(fun ({X, Y}) -> digraph:add_edge(G, {X, Y}, EndO) end,
[{X, Height - 1} || X <- lists:seq(0, Width)]).

filter_board(G, Keep) ->
true = digraph:del_vertices(G, [V ||
Vertex <- digraph:vertices(G),
{V, Label} <- [digraph:vertex(G, Vertex)],
Label /= Keep]),
ok.

is_winner(Parsed, x) ->
is_winner(Parsed, x, ?GOAL_START_X, ?GOAL_END_X);
is_winner(Parsed, o) ->
is_winner(Parsed, o, ?GOAL_START_O, ?GOAL_END_O).

is_winner(Parsed, Side, Start, End) ->
G = digraph:new([private]),
try
% It'd have been nice if there was a way to copy a board, so I
% could re-use this base state for X and O.
ok = graph_board(G, Parsed),
% It turns out the hardest part of this for me to get right
% was the edge contstruction. As such, it made sense to me to
% construct the board the same way every time, then filter to
% only the interesting vertices instead of trying to only
% build edges for the currently interesting side.
ok = filter_board(G, Side),
% Creating a custom search function that knew how to respect
% the vertex labels to reject invalid paths would allow for
% easily reusing the same graph. That's probably what I'd do
% in a version of this that tried to implement more of the
% games rules.
Path = digraph:get_path(G, Start, End),
% io:format("Vertices: ~w~n", [digraph:vertices(G)]),
% io:format("Edges: ~w~n", [digraph:edges(G)]),
% io:format("Path: ~w~n", [Path]),
% case Path of
%     false ->
%         ok;
%     Path when is_list(Path) ->
%         io:format("Neighbours: ~w~n", [[digraph:out_neighbours(G, V) || V <- Path]])
% end,
Path /= false
after
true = digraph:delete(G)
end.``````