# Circular Buffer

A circular buffer, cyclic buffer or ring buffer is a data structure that
uses a single, fixed-size buffer as if it were connected end-to-end.

A circular buffer first starts empty and of some predefined length. For
example, this is a 7-element buffer:

    [ ][ ][ ][ ][ ][ ][ ]

Assume that a 1 is written into the middle of the buffer (exact starting
location does not matter in a circular buffer):

    [ ][ ][ ][1][ ][ ][ ]

Then assume that two more elements are added — 2 & 3 — which get
appended after the 1:

    [ ][ ][ ][1][2][3][ ]

If two elements are then removed from the buffer, the oldest values
inside the buffer are removed. The two elements removed, in this case,
are 1 & 2, leaving the buffer with just a 3:

    [ ][ ][ ][ ][ ][3][ ]

If the buffer has 7 elements then it is completely full:

    [6][7][8][9][3][4][5]

When the buffer is full an error will be raised, alerting the client
that further writes are blocked until a slot becomes free.

When the buffer is full, the client can opt to overwrite the oldest
data with a forced write. In this case, two more elements — A & B —
are added and they overwrite the 3 & 4:

    [6][7][8][9][A][B][5]

3 & 4 have been replaced by A & B making 5 now the oldest data in the
buffer. Finally, if two elements are removed then what would be
returned is 5 & 6 yielding the buffer:

    [ ][7][8][9][A][B][ ]

Because there is space available, if the client again uses overwrite
to store C & D then the space where 5 & 6 were stored previously will
be used not the location of 7 & 8. 7 is still the oldest element and
the buffer is once again full.

    [D][7][8][9][A][B][C]

## 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](http://exercism.io/languages/erlang/installation) on how to do that.

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

```bash
$ 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:

```erlang
-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](http://exercism.io/languages/erlang) on the Exercism site.
This covers the basic information on setting up the development
environment expected by the exercises.

## Source

Wikipedia [http://en.wikipedia.org/wiki/Circular_buffer](http://en.wikipedia.org/wiki/Circular_buffer)

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