Overleaf/services/track-changes/app/coffee/UpdateCompressor.coffee

strInject = (s1, pos, s2) -> s1[...pos] + s2 + s1[pos..]
strRemove = (s1, pos, length) -> s1[...pos] + s1[(pos + length)..]

module.exports = UpdateCompressor =
	# Updates come from the doc updater in format
	# {
	# 	op:   [ { ... op1 ... }, { ... op2 ... } ]
	# 	meta: { ts: ..., user_id: ... }
	# }
	# but it's easier to work with on op per update, so convert these updates to
	# our compressed format
	# [{
	# 	op: op1
	# 	meta: { start_ts: ... , end_ts: ..., user_id: ... }
	# }, {
	# 	op: op2
	# 	meta: { start_ts: ... , end_ts: ..., user_id: ... }
	# }]
	convertRawUpdatesToCompressedFormat: (updates) ->
		normalizedUpdates = []
		for update in updates
			for op in update.op
				normalizedUpdates.push
					op: op
					meta:
						start_ts: update.meta.start_ts or update.meta.ts
						end_ts:   update.meta.end_ts   or update.meta.ts
						user_id:  update.meta.user_id
					v: update.v
		return normalizedUpdates

	compressRawUpdates: (lastPreviousUpdate, rawUpdates) ->
		updates = UpdateCompressor.convertRawUpdatesToCompressedFormat(rawUpdates)
		if lastPreviousUpdate?
			updates.unshift(lastPreviousUpdate)
		return UpdateCompressor.compressUpdates(updates)

	compressUpdates: (updates) ->
		return [] if updates.length == 0

		compressedUpdates = [updates.shift()]
		for update in updates
			lastCompressedUpdate = compressedUpdates.pop()
			if lastCompressedUpdate?
				compressedUpdates = compressedUpdates.concat UpdateCompressor._concatTwoUpdates lastCompressedUpdate, update
			else
				compressedUpdates.push update

		return compressedUpdates

	MAX_TIME_BETWEEN_UPDATES: oneMinute = 60 * 1000

	_concatTwoUpdates: (firstUpdate, secondUpdate) ->
		firstUpdate =
			op: firstUpdate.op
			meta:
				user_id:  firstUpdate.meta.user_id or null
				start_ts: firstUpdate.meta.start_ts or firstUpdate.meta.ts
				end_ts:   firstUpdate.meta.end_ts   or firstUpdate.meta.ts
			v: firstUpdate.v
		secondUpdate =
			op: secondUpdate.op
			meta:
				user_id:  secondUpdate.meta.user_id or null
				start_ts: secondUpdate.meta.start_ts or secondUpdate.meta.ts
				end_ts:   secondUpdate.meta.end_ts   or secondUpdate.meta.ts
			v: secondUpdate.v

		if firstUpdate.meta.user_id != secondUpdate.meta.user_id
			return [firstUpdate, secondUpdate]

		if secondUpdate.meta.start_ts - firstUpdate.meta.end_ts > UpdateCompressor.MAX_TIME_BETWEEN_UPDATES
			return [firstUpdate, secondUpdate]

		firstOp = firstUpdate.op
		secondOp = secondUpdate.op
		# Two inserts
		if firstOp.i? and secondOp.i? and firstOp.p <= secondOp.p <= (firstOp.p + firstOp.i.length)
			return [
				meta:
					start_ts: firstUpdate.meta.start_ts
					end_ts:   secondUpdate.meta.end_ts
					user_id:  firstUpdate.meta.user_id
				op:
					p: firstOp.p
					i: strInject(firstOp.i, secondOp.p - firstOp.p, secondOp.i)
				v: secondUpdate.v
			]
		# Two deletes
		else if firstOp.d? and secondOp.d? and secondOp.p <= firstOp.p <= (secondOp.p + secondOp.d.length)
			return [
				meta:
					start_ts: firstUpdate.meta.start_ts
					end_ts:   secondUpdate.meta.end_ts
					user_id:  firstUpdate.meta.user_id
				op:
					p: secondOp.p
					d: strInject(secondOp.d, firstOp.p - secondOp.p, firstOp.d)
				v: secondUpdate.v
			]
		# An insert and then a delete
		else if firstOp.i? and secondOp.d? and firstOp.p <= secondOp.p <= (firstOp.p + firstOp.i.length)
			offset = secondOp.p - firstOp.p
			insertedText = firstOp.i.slice(offset, offset + secondOp.d.length)
			if insertedText == secondOp.d
				insert = strRemove(firstOp.i, offset, secondOp.d.length)
				return [
					meta:
						start_ts: firstUpdate.meta.start_ts
						end_ts:   secondUpdate.meta.end_ts
						user_id:  firstUpdate.meta.user_id
					op:
						p: firstOp.p
						i: insert
					v: secondUpdate.v
				]
			else
				# This shouldn't be possible!
				return [firstUpdate, secondUpdate]

		else
			return [firstUpdate, secondUpdate]