Interpolation function for Roblox NumberSequence objects

interpolateSequence.lua

local errors = {
	incompatibleSequences = "Passed origin and goal NumberSequences do not have the same number of keypoints. Origin: $d Goal: $d",
}

-- double iter accepts two tables and the previous index and returns a new index and two values
local function doubleIter (tables, iterState)
	iterState = iterState + 1
	local valueOne = tables[1][iterState]
	local valueTwo = tables[2][iterState]
	
	if valueOne and valueTwo then
		return iterState, valueOne, valueTwo
	end
end

-- iipairs takes two tables and returns an iterator that iterates over both at the same time
local function iipairs (a,b)
  return doubleIter, {a,b}, 0
end

-- lerp takes two values and a percent and returns a new value that percent inbetween origin and goal
local function lerp(origin,goal,alpha)
	return origin + ((goal-origin) * alpha)
end

-- lerpNumberSequence
-- this function takes two number sequences and an alpha value and
-- returns a new number sequence that is blended between the two.
-- both passed sequences must contain the same number of keyframes.
local function lerpNumberSequence(originSequence, goalSequence, alpha)
	local originKeypoints = originSequence.Keypoints
	local goalKeypoints = goalSequence.Keypoints
	
	local originSize = #originKeypoints
	local goalSize = #goalKeypoints
	
	-- confirm keyframe count is identical
	assert(originSize == goalSize,
		errors.incompatibleSequences:format(originSize, goalSize)
	)
	
	local interpolatedKeyframes = {}
	
	for 
		index,
		originKeypoint,
		goalKeypoint
	in iipairs(originKeypoints, goalKeypoints) do
		local lerpedTime = lerp(originKeypoint.Time, goalKeypoint.Time, alpha)
		local lerpedValue = lerp(originKeypoint.Value, goalKeypoint.Value, alpha)
		local lerpedEnvelope = lerp(originKeypoint.Envelope, goalKeypoint.Envelope, alpha)
		interpolatedKeyframes[index] = NumberSequenceKeypoint.new(lerpedTime,lerpedValue,lerpedEnvelope)
	end
	
	return NumberSequence.new(interpolatedKeyframes)
end

return lerpNumberSequence