Specification (tests!) for not yet implemented Lua HistArray

histArray_spec.lua

local HistArray = require "histArray"

-- The HistArray module provides an array that automatically purges
-- old values. It lets you write code like:
--
-- y[n] = c1*x[n] + c2*x[n-1] + c3*x[n-2]
--                - c4*y[n-1] - c5*y[n-2]
--
-- If x and y are HistArrays created with history size 2, all samples from
-- x[n-3] and y[n-3] and below will be purged (accessing them is an error)
-- so that the no longer needed values do not waste any memory.
--
-- FAQ:    How does a HistArray know when to purge old values?
-- Answer: For all n, when y[n] is assigned to, y[n - (y.histSize+1)]
--         and below are purged.  This allows most recurrence relations
--         (like the above) to be defined in a natural manner.

describe("a history array", function()
    it("is initialized with a history size", function()
        local y = HistArray.new(2)
        assert.are.equal(2, y.histSize)
    end)

    it("holds as many values as its history size, plus one", function()
        local y = HistArray.new(2)
        y[1] = 0.1
        y[2] = 0.2
        y[3] = 0.3
        y[4] = 0.4
        assert.are.equal(0.4, y[4])  -- current value
        assert.are.equal(0.3, y[3])  -- current minus 1
        assert.are.equal(0.2, y[2])  -- current minus 2
        assert.has_error(function() local _ = y[1] end)  -- too old!
    end)
end)