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Helot's solution

to Allergies in the Erlang Track

Published at Nov 07 2018 · 0 comments
Test suite


This exercise has changed since this solution was written.

Given a person's allergy score, determine whether or not they're allergic to a given item, and their full list of allergies.

An allergy test produces a single numeric score which contains the information about all the allergies the person has (that they were tested for).

The list of items (and their value) that were tested are:

  • eggs (1)
  • peanuts (2)
  • shellfish (4)
  • strawberries (8)
  • tomatoes (16)
  • chocolate (32)
  • pollen (64)
  • cats (128)

So if Tom is allergic to peanuts and chocolate, he gets a score of 34.

Now, given just that score of 34, your program should be able to say:

  • Whether Tom is allergic to any one of those allergens listed above.
  • All the allergens Tom is allergic to.

Note: a given score may include allergens not listed above (i.e. allergens that score 256, 512, 1024, etc.). Your program should ignore those components of the score. For example, if the allergy score is 257, your program should only report the eggs (1) allergy.

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.

To make tests pass, add the following to your solution:


test_version() ->

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.


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Submitting Incomplete Solutions

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no_allergies_at_all_test() ->
  ?assertEqual([], allergies:allergies(0)).

allergic_to_just_eggs_test() ->
  ?assertEqual(['eggs'], allergies:allergies(1)).

allergic_to_just_peanuts_test() ->
  ?assertEqual(['peanuts'], allergies:allergies(2)).

allergic_to_just_strawberries_test() ->
  ?assertEqual(['strawberries'], allergies:allergies(8)).

allergic_to_eggs_and_peanuts_test() ->
  ?assertEqual(['eggs', 'peanuts'], allergies:allergies(3)).

allergic_to_more_than_eggs_but_not_peanuts_test() ->
  ?assertEqual(['eggs', 'shellfish'], allergies:allergies(5)).

allergic_to_lots_of_stuff_test() ->
     ['strawberries', 'tomatoes', 'chocolate', 'pollen', 'cats'], allergies:allergies(248)).

allergic_to_everything_test() ->
     ['eggs', 'peanuts', 'shellfish', 'strawberries', 'tomatoes', 'chocolate', 'pollen', 'cats'],

no_allergies_means_not_allergic_test() ->
  ?assertNot(allergies:is_allergic_to('peanuts', 0)),
  ?assertNot(allergies:is_allergic_to('cats', 0)),
  ?assertNot(allergies:is_allergic_to('strawberries', 0)).

is_allergic_to_eggs_test() ->
  ?assert(allergies:is_allergic_to('eggs', 1)).

allergic_to_eggs_and_other_stuff_test() ->
  ?assert(allergies:is_allergic_to('eggs', 5)).

ignore_non_allergen_score_parts_test() ->
     ['eggs', 'shellfish', 'strawberries', 'tomatoes', 'chocolate', 'pollen', 'cats'],

version_test() ->
  ?assertMatch(1, allergies:test_version()).


-type allergen() :: eggs | peanuts | shellfish | strawberries | tomatoes | chocolate | pollen | cats.

%% API

-spec allergies(non_neg_integer()) -> [allergen()].
allergies(Score) ->
      Eggs:1>> = <<Score:8>>,

    Flags = lists:zip(
              [Eggs, Peanuts, Shellfish, Strawberries, Tomatoes, Chocolate, Pollen, Cats],
              [eggs, peanuts, shellfish, strawberries, tomatoes, chocolate, pollen, cats]),
    [Allergen || {Flag, Allergen} <- Flags, Flag == 1].

-spec is_allergic_to(allergen(), non_neg_integer()) -> boolean().
is_allergic_to(Allergen, Score) ->
    lists:member(Allergen, allergies(Score)).

-spec test_version() -> integer().
test_version() ->

%% Internal

Community comments

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Helot's Reflection

This is very nearly the same problem as "secret handshake". As such, I've reused much of the structure of that solution here. In both cases, I found binary pattern matching the best way to extract single bits, though it does cause some clunkiness in mapping the bits to the expected values.

This solution is also the first time I found an exercise where the tests required an exported function that wasn't provided in the initial skeleton. Easy enough to add, but uncertain if that's an oversight in the test construction.