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# ryanplusplus's solution

## to Circular Buffer in the Lua Track

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

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 β 2 & 3 β 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 β A & B β 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3 & 4 have been replaced by A & B making 5 now the oldest data in the buffer.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 the tests

To run the tests, run the command `busted` from within the exercise directory.

## Further information

For more detailed information about the Lua track, including how to get help if you're having trouble, please visit the exercism.io Lua language page.

## Submitting Incomplete Solutions

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

### circular-buffer_spec.lua

``````local CircularBuffer = require('circular-buffer')

describe('circular-buffer', function()
it('reading an empty buffer throws a BufferEmptyException', function()
local buffer = CircularBuffer:new(1)
assert.has_error((function() buffer:read() end), 'buffer is empty')
end)

it('write and read back one item', function()
local buffer = CircularBuffer:new(1)
buffer:write('1')
assert.has_error((function() buffer:read() end), 'buffer is empty')
end)

it('write and read back multiple items', function()
local buffer = CircularBuffer:new(2)
buffer:write('1')
buffer:write('2')
assert.has_error((function() buffer:read() end), 'buffer is empty')
end)

it('clearing a buffer', function()
local buffer = CircularBuffer:new(2)
buffer:write('1')
buffer:write('2')
buffer:clear()
assert.has_error((function() buffer:read() end), 'buffer is empty')
buffer:write('3')
buffer:write('4')
end)

it('alternate write and read', function()
local buffer = CircularBuffer:new(2)
buffer:write('1')
buffer:write('2')
end)

it('reads back oldest item', function()
local buffer = CircularBuffer:new(3)
buffer:write('1')
buffer:write('2')
buffer:write('3')
end)

it('writes of nil do not occupy buffer', function()
local buffer = CircularBuffer:new(3)
buffer:write(nil)
for i = 1, 3 do
buffer:write(tostring(i))
end
end)

it('writing to a full buffer throws a BufferFullException', function()
local buffer = CircularBuffer:new(2)
buffer:write('1')
buffer:write('2')
assert.has_error((function() buffer:write('A') end), 'buffer is full')
end)

it('forced writes over write oldest item in a full buffer', function()
local buffer = CircularBuffer:new(2)
buffer:write('1')
buffer:write('2')
buffer:forceWrite('A')
assert.has_error((function() buffer:read() end), 'buffer is empty')
end)

it('forced writes act like normal writes in a non-full buffer', function()
local buffer = CircularBuffer:new(2)
buffer:write('1')
buffer:forceWrite('2')
assert.has_error((function() buffer:read() end), 'buffer is empty')
end)

it('alternate force write and read into full buffer', function()
local buffer = CircularBuffer:new(5)
for i = 1, 3 do
buffer:write(tostring(i))
end
buffer:write('4')
for i = 5, 8 do
buffer:write(tostring(i))
end
buffer:forceWrite('A')
buffer:forceWrite('B')
assert.has_error((function() buffer:read() end), 'buffer is empty')
end)
end)``````
``````local CircularBuffer = {}
CircularBuffer.__index = CircularBuffer

if self.head == self.tail then error('buffer is empty') end
self.tail = self.tail + 1
return self.items[self.tail - 1]
end

function CircularBuffer:write(item)
if item == nil then return end
if (self.head - self.tail) == self.capacity then error('buffer is full') end
end

function CircularBuffer:forceWrite(item)
if item == nil then return end
if (self.head - self.tail) == self.capacity then self.tail = self.tail + 1 end
self:write(item)
end

function CircularBuffer:clear()
self.items = {}
self.tail = 1
end

return {
new = function(_, capacity)
local self = setmetatable({ capacity = capacity }, CircularBuffer)
self:clear()
return self
end
}``````