# davearonson's solution

## to Queen Attack in the Elixir Track

Published at Jul 13 2018 · 0 comments
Instructions
Test suite
Solution

#### Note:

This solution was written on an old version of Exercism. The tests below might not correspond to the solution code, and the exercise may have changed since this code was written.

Given the position of two queens on a chess board, indicate whether or not they are positioned so that they can attack each other.

In the game of chess, a queen can attack pieces which are on the same row, column, or diagonal.

A chessboard can be represented by an 8 by 8 array.

So if you're told the white queen is at (2, 3) and the black queen at (5, 6), then you'd know you've got a set-up like so:

``````_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ W _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ B _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
``````

You'd also be able to answer whether the queens can attack each other. In this case, that answer would be yes, they can, because both pieces share a diagonal.

## Running tests

Execute the tests with:

``````\$ elixir queen_attack_test.exs
``````

### Pending tests

In the test suites, all but the first test have been skipped.

Once you get a test passing, you can unskip the next one by commenting out the relevant `@tag :pending` with a `#` symbol.

For example:

``````# @tag :pending
test "shouting" do
assert Bob.hey("WATCH OUT!") == "Whoa, chill out!"
end
``````

Or, you can enable all the tests by commenting out the `ExUnit.configure` line in the test suite.

``````# ExUnit.configure exclude: :pending, trace: true
``````

For more detailed information about the Elixir track, please see the help page.

## Source

J Dalbey's Programming Practice problems http://users.csc.calpoly.edu/~jdalbey/103/Projects/ProgrammingPractice.html

## Submitting Incomplete Solutions

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

### queen_attack_test.exs

``````if !System.get_env("EXERCISM_TEST_EXAMPLES") do
end

ExUnit.start()
ExUnit.configure(exclude: :pending, trace: true)

defmodule QueenAttackTest do
use ExUnit.Case

# @tag :pending
test "default positions" do
queens = Queens.new()
assert queens.white == {0, 3}
assert queens.black == {7, 3}
end

@tag :pending
test "specific placement" do
queens = Queens.new({3, 7}, {6, 1})
assert queens.white == {3, 7}
assert queens.black == {6, 1}
end

@tag :pending
test "cannot occupy same space" do
assert_raise ArgumentError, fn ->
Queens.new({2, 4}, {2, 4})
end
end

@tag :pending
test "string representation" do
queens = Queens.new({2, 4}, {6, 6})

board =
String.trim("""
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ W _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ B _
_ _ _ _ _ _ _ _
""")

assert Queens.to_string(queens) == board
end

@tag :pending
test "another string representation" do
queens = Queens.new({7, 1}, {0, 0})

board =
String.trim("""
B _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ W _ _ _ _ _ _
""")

assert Queens.to_string(queens) == board
end

@tag :pending
test "yet another string representation" do
queens = Queens.new({4, 3}, {3, 4})

board =
String.trim("""
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ B _ _ _
_ _ _ W _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
""")

assert Queens.to_string(queens) == board
end

@tag :pending
test "queen placed on the bottom right corner" do
queens = Queens.new({4, 3}, {7, 7})

board =
String.trim("""
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ W _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ B
""")

assert Queens.to_string(queens) == board
end

@tag :pending
test "queen placed on the edge of the board" do
queens = Queens.new({4, 3}, {2, 7})

board =
String.trim("""
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ B
_ _ _ _ _ _ _ _
_ _ _ W _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
_ _ _ _ _ _ _ _
""")

assert Queens.to_string(queens) == board
end

@tag :pending
test "cannot attack" do
queens = Queens.new({2, 3}, {4, 7})
refute Queens.can_attack?(queens)
end

@tag :pending
test "can attack on same row" do
queens = Queens.new({2, 4}, {2, 7})
assert Queens.can_attack?(queens)
end

@tag :pending
test "can attack on same column" do
queens = Queens.new({5, 4}, {2, 4})
assert Queens.can_attack?(queens)
end

@tag :pending
test "can attack on diagonal" do
queens = Queens.new({1, 1}, {6, 6})
assert Queens.can_attack?(queens)
end

@tag :pending
test "can attack on other diagonal" do
queens = Queens.new({0, 6}, {1, 7})
assert Queens.can_attack?(queens)
end

@tag :pending
test "can attack on yet another diagonal" do
queens = Queens.new({4, 1}, {6, 3})
assert Queens.can_attack?(queens)
end

@tag :pending
test "can attack on a diagonal slanted the other way" do
queens = Queens.new({6, 1}, {1, 6})
assert Queens.can_attack?(queens)
end
end``````
``````# Thanks to exercism.io / github.com user veelenga for
# telling me about map_join and reminding me of "pinned" matching.

defmodule Queens do
@type t :: %Queens{ black: {integer, integer}, white: {integer, integer} }
defstruct black: nil, white: nil

@doc """
Creates a new set of Queens
"""
@spec new() :: Queens.t()
@spec new({integer, integer}, {integer, integer}) :: Queens.t()
def new(white \\ {0, 3}, black \\ {7, 3})
def new(white, white), do: raise ArgumentError
def new(white, black), do: %Queens{ black: black, white: white }

@doc """
Gives a string reprentation of the board with
white and black queen locations shown
"""
@spec to_string(Queens.t()) :: String.t()
def to_string(queens) do
(0..7) |> Enum.map_join("\n", &(row(&1, queens)))
end

def row(row, queens) do
(0..7) |> Enum.map_join(" ", &(cell(row, &1, queens)))
end

def cell(row, col, %{black: black_loc, white: white_loc}) do
case {row, col} do
^black_loc -> "B"
^white_loc -> "W"
_          -> "_"
end
end

@doc """
Checks if the queens can attack each other
"""
@spec can_attack?(Queens.t()) :: boolean
def can_attack?(queens) do
match(queens.white, queens.black)
end

def match({row , _   }, {row , _   }), do: true
def match({_   , col }, {_   , col }), do: true
def match({row1, col1}, {row2, col2}) do
row1 - col1 == row2 - col2 ||
(7 - row1) - col1 == (7 - row2) - col2
end
end``````