-- Simulate a raycast by one tick.
local function SimulateCast(cast: ActiveCast, delta: number, expectingShortCall: boolean)
	assert(cast.StateInfo.UpdateConnection ~= nil, ERR_OBJECT_DISPOSED)
	PrintDebug("Casting for frame.")
	local latestTrajectory = cast.StateInfo.Trajectories[#cast.StateInfo.Trajectories]

	local origin = latestTrajectory.Origin
	local totalDelta = cast.StateInfo.TotalRuntime - latestTrajectory.StartTime
	local initialVelocity = latestTrajectory.InitialVelocity
	local acceleration = latestTrajectory.Acceleration

	local lastPoint = GetPositionAtTime(totalDelta, origin, initialVelocity, acceleration)
	local lastVelocity = GetVelocityAtTime(totalDelta, initialVelocity, acceleration)
	local lastDelta = cast.StateInfo.TotalRuntime - latestTrajectory.StartTime

	cast.StateInfo.TotalRuntime += delta

	-- Recalculate this.
	totalDelta = cast.StateInfo.TotalRuntime - latestTrajectory.StartTime

	local currentTarget = GetPositionAtTime(totalDelta, origin, initialVelocity, acceleration)
	local segmentVelocity = GetVelocityAtTime(totalDelta, initialVelocity, acceleration) 
	local totalDisplacement = currentTarget - lastPoint -- This is the displacement from where the ray was on the last from to where the ray is now.

	local rayDir = totalDisplacement.Unit * segmentVelocity.Magnitude * delta
	local targetWorldRoot = cast.RayInfo.WorldRoot
	local resultOfCast = targetWorldRoot:Raycast(lastPoint, rayDir, cast.RayInfo.Parameters)

	local point = currentTarget
	local part: Instance? = nil
	local material = Enum.Material.Air
	local normal = Vector3.new()

	if (resultOfCast ~= nil) then
		point = resultOfCast.Position
		part = resultOfCast.Instance
		material = resultOfCast.Material
		normal = resultOfCast.Normal
	end

	local rayDisplacement = (point - lastPoint).Magnitude
	-- For clarity -- totalDisplacement is how far the ray would have traveled if it hit nothing,
	-- and rayDisplacement is how far the ray really traveled (which will be identical to totalDisplacement if it did indeed hit nothing)

	SendLengthChanged(cast, lastPoint, rayDir.Unit, rayDisplacement, segmentVelocity, cast.RayInfo.CosmeticBulletObject)
	cast.StateInfo.DistanceCovered += rayDisplacement

	local rayVisualization: ConeHandleAdornment? = nil
	if (delta > 0) then
		rayVisualization = DbgVisualizeSegment(CFrame.new(lastPoint, lastPoint + rayDir), rayDisplacement)
	end


	-- HIT DETECTED. Handle all that garbage, and also handle behaviors 1 and 2 (default behavior, go high res when hit) if applicable.
	-- CAST BEHAVIOR 2 IS HANDLED IN THE CODE THAT CALLS THIS FUNCTION.

	if part and part ~= cast.RayInfo.CosmeticBulletObject then
		local start = tick()
		PrintDebug("Hit something, testing now.")

		-- SANITY CHECK: Don't allow the user to yield or run otherwise extensive code that takes longer than one frame/heartbeat to execute.
		if (cast.RayInfo.CanPierceCallback ~= nil) then
			if expectingShortCall == false then
				if (cast.StateInfo.IsActivelySimulatingPierce) then
					cast:Terminate()
					error("ERROR: The latest call to CanPierceCallback took too long to complete! This cast is going to suffer desyncs which WILL cause unexpected behavior and errors. Please fix your performance problems, or remove statements that yield (e.g. wait() calls)")
					-- Use error. This should absolutely abort the cast.
				end
			end
			-- expectingShortCall is used to determine if we are doing a forced resolution increase, in which case this will be called several times in a single frame, which throws this error.
			cast.StateInfo.IsActivelySimulatingPierce = true
		end
		------------------------------

		if cast.RayInfo.CanPierceCallback == nil or (cast.RayInfo.CanPierceCallback ~= nil and cast.RayInfo.CanPierceCallback(cast, resultOfCast, segmentVelocity, cast.RayInfo.CosmeticBulletObject) == false) then
			PrintDebug("Piercing function is nil or it returned FALSE to not pierce this hit.")
			cast.StateInfo.IsActivelySimulatingPierce = false

			if (cast.StateInfo.HighFidelityBehavior == 2 and latestTrajectory.Acceleration ~= Vector3.new() and cast.StateInfo.HighFidelitySegmentSize ~= 0) then
				cast.StateInfo.CancelHighResCast = false -- Reset this here.

				if cast.StateInfo.IsActivelyResimulating then
					cast:Terminate()
					error("Cascading cast lag encountered! The caster attempted to perform a high fidelity cast before the previous one completed, resulting in exponential cast lag. Consider increasing HighFidelitySegmentSize.")
				end


				cast.StateInfo.IsActivelyResimulating = true

				-- This is a physics based cast and it needs to be recalculated.
				PrintDebug("Hit was registered, but recalculation is on for physics based casts. Recalculating to verify a real hit...")

				-- Split this ray segment into smaller segments of a given size.
				-- In 99% of cases, it won't divide evently (e.g. I have a distance of 1.25 and I want to divide into 0.1 -- that won't work)
				-- To fix this, the segments need to be stretched slightly to fill the space (rather than having a single shorter segment at the end)

				local numSegmentsDecimal = rayDisplacement / cast.StateInfo.HighFidelitySegmentSize -- say rayDisplacement is 5.1, segment size is 0.5 -- 10.2 segments
				local numSegmentsReal = math.floor(numSegmentsDecimal) -- 10 segments + 0.2 extra segments
				local realSegmentLength = rayDisplacement / numSegmentsReal -- this spits out 0.51, which isn't exact to the defined 0.5, but it's close

				local timeIncrement = delta / numSegmentsReal
				for segmentIndex = 1, numSegmentsReal do
					if cast.StateInfo.CancelHighResCast then
						cast.StateInfo.CancelHighResCast = false
						break
					end

					local subPosition = GetPositionAtTime(lastDelta + (timeIncrement * segmentIndex), origin, initialVelocity, acceleration)
					local subVelocity = GetVelocityAtTime(lastDelta + (timeIncrement * segmentIndex), initialVelocity, acceleration) 
					local subRayDir = subVelocity * delta
					local subResult = targetWorldRoot:Raycast(subPosition, subRayDir, cast.RayInfo.Parameters)

					local subDisplacement = (subPosition - (subPosition + subVelocity)).Magnitude

					if (subResult ~= nil) then
						local subDisplacement = (subPosition - subResult.Position).Magnitude
						local dbgSeg = DbgVisualizeSegment(CFrame.new(subPosition, subPosition + subVelocity), subDisplacement)
						if (dbgSeg ~= nil) then dbgSeg.Color3 = Color3.new(0.286275, 0.329412, 0.247059) end
						if cast.RayInfo.CanPierceCallback == nil or (cast.RayInfo.CanPierceCallback ~= nil and cast.RayInfo.CanPierceCallback(cast, subResult, subVelocity, cast.RayInfo.CosmeticBulletObject) == false) then
							cast.StateInfo.IsActivelyResimulating = false

							SendRayHit(cast, subResult, subVelocity, cast.RayInfo.CosmeticBulletObject)
							cast:Terminate()
							local vis = DbgVisualizeHit(CFrame.new(point), false)
							if (vis ~= nil) then vis.Color3 = Color3.new(0.0588235, 0.87451, 1) end
							return
						else
							SendRayPierced(cast, subResult, subVelocity, cast.RayInfo.CosmeticBulletObject)
							local vis = DbgVisualizeHit(CFrame.new(point), true)
							if (vis ~= nil) then vis.Color3 = Color3.new(1, 0.113725, 0.588235) end
							if (dbgSeg ~= nil) then dbgSeg.Color3 = Color3.new(0.305882, 0.243137, 0.329412) end
						end
					else
						local dbgSeg = DbgVisualizeSegment(CFrame.new(subPosition, subPosition + subVelocity), subDisplacement)
						if (dbgSeg ~= nil) then dbgSeg.Color3 = Color3.new(0.286275, 0.329412, 0.247059) end
					end
				end
				cast.StateInfo.IsActivelyResimulating = false
			elseif (cast.StateInfo.HighFidelityBehavior ~= 1 and cast.StateInfo.HighFidelityBehavior ~= 3) then
				cast:Terminate()
				error("Invalid value " .. (cast.StateInfo.HighFidelityBehavior) .. " for HighFidelityBehavior.")
			else
				PrintDebug("Hit was successful. Terminating.")
				SendRayHit(cast, resultOfCast, segmentVelocity, cast.RayInfo.CosmeticBulletObject)
				cast:Terminate()
				DbgVisualizeHit(CFrame.new(point), false)
				return
			end
		else
			PrintDebug("Piercing function returned TRUE to pierce this part.")
			if rayVisualization ~= nil then
				rayVisualization.Color3 = Color3.new(0.4, 0.05, 0.05)
			end
			DbgVisualizeHit(CFrame.new(point), true)

			local params = cast.RayInfo.Parameters
			local alteredParts = {}
			local currentPierceTestCount = 0
			local originalFilter = params.FilterDescendantsInstances
			local brokeFromSolidObject = false
			while true do
				if resultOfCast.Instance:IsA("Terrain") then
					if material == Enum.Material.Water then
						cast:Terminate()
						error("Do not add Water as a piercable material. If you need to pierce water, set cast.RayInfo.Parameters.IgnoreWater = true instead", 0)
					end
					warn("WARNING: The pierce callback for this cast returned TRUE on Terrain! This can cause severely adverse effects.")
				end

				if params.FilterType == Enum.RaycastFilterType.Exclude then
					local filter = params.FilterDescendantsInstances
					table.insert(filter, resultOfCast.Instance)
					table.insert(alteredParts, resultOfCast.Instance)
					params.FilterDescendantsInstances = filter
				else
					local filter = params.FilterDescendantsInstances
					table.removeObject(filter, resultOfCast.Instance)
					table.insert(alteredParts, resultOfCast.Instance)
					params.FilterDescendantsInstances = filter
				end

				SendRayPierced(cast, resultOfCast, segmentVelocity, cast.RayInfo.CosmeticBulletObject)

				resultOfCast = targetWorldRoot:Raycast(lastPoint, rayDir, params)

				if resultOfCast == nil then
					break
				end

				if currentPierceTestCount >= MAX_PIERCE_TEST_COUNT then
					warn("WARNING: Exceeded maximum pierce test budget for a single ray segment (attempted to test the same segment " .. MAX_PIERCE_TEST_COUNT .. " times!)")
					break
				end
				currentPierceTestCount = currentPierceTestCount + 1;

				if cast.RayInfo.CanPierceCallback(cast, resultOfCast, segmentVelocity, cast.RayInfo.CosmeticBulletObject) == false then
					brokeFromSolidObject = true
					break
				end
			end

			cast.RayInfo.Parameters.FilterDescendantsInstances = originalFilter
			cast.StateInfo.IsActivelySimulatingPierce = false

			if brokeFromSolidObject then
				PrintDebug("Broke because the ray hit something solid (" .. tostring(resultOfCast.Instance) .. ") while testing for a pierce. Terminating the cast.")
				SendRayHit(cast, resultOfCast, segmentVelocity, cast.RayInfo.CosmeticBulletObject)
				cast:Terminate()
				DbgVisualizeHit(CFrame.new(resultOfCast.Position), false)
				return
			end
		end
	end

	if (cast.StateInfo.DistanceCovered >= cast.RayInfo.MaxDistance) then
		cast:Terminate()
		DbgVisualizeHit(CFrame.new(currentTarget), false)
	end
end