# 4d47's solution

## to Grains in the Erlang Track

Published at Jul 22 2018 · 0 comments
Instructions
Test suite
Solution

#### Note:

This exercise has changed since this solution was written.

Calculate the number of grains of wheat on a chessboard given that the number on each square doubles.

There once was a wise servant who saved the life of a prince. The king promised to pay whatever the servant could dream up. Knowing that the king loved chess, the servant told the king he would like to have grains of wheat. One grain on the first square of a chess board. Two grains on the next. Four on the third, and so on.

There are 64 squares on a chessboard.

Write code that shows:

• how many grains were on each square, and
• the total number of grains

## For bonus points

Did you get the tests passing and the code clean? If you want to, these are some additional things you could try:

• Optimize for speed.

Then please share your thoughts in a comment on the submission. Did this experiment make the code better? Worse? Did you learn anything from it?

## 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
``````

### Test versioning

Each problem defines a macro `TEST_VERSION` in the test file and verifies that the solution defines and exports a function `test_version` returning that same value.

``````-export([test_version/0]).

test_version() ->
1.
``````

The benefit of this is that reviewers can see against which test version an iteration was written if, for example, a previously posted solution does not solve the current problem or passes current tests.

## 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.

## Source

JavaRanch Cattle Drive, exercise 6 http://www.javaranch.com/grains.jsp

## Submitting Incomplete Solutions

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

### grains_tests.erl

``````-module(grains_tests).

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

-define(assertGrains(Grains, Square), ?assertMatch(Grains, grains:square(Square))).

square_1_test() ->
?assertGrains(1, 1).

square_2_test() ->
?assertGrains(2, 2).

square_3_test() ->
?assertGrains(4, 3).

square_4_test() ->
?assertGrains(8, 4).

square_16_test() ->
?assertGrains(32768, 16).

square_32_test() ->
?assertGrains(2147483648, 32).

square_64_test() ->
?assertGrains(9223372036854775808, 64).

total_grains_test() ->
?assertMatch(18446744073709551615, grains:total()).

version_test() ->
?assertMatch(1, grains:test_version()).``````
``````-module(grains).

-export([square/1, total/0, test_version/0]).

%
square(1) ->
1;
square(N) ->
2 * square(N - 1).

total() ->
lists:sum([square(X) || X <- lists:seq(1, 64)]).

test_version() ->
1.``````