return function()
	local Log = require(script.Parent.Parent.Parent.Log)

	local InstanceMap = require(script.Parent.Parent.InstanceMap)
	local PatchSet = require(script.Parent.Parent.PatchSet)

	local diff = require(script.Parent.diff)

	local function isEmpty(table)
		return next(table) == nil, "Table was not empty"
	end

	local function size(dict)
		local len = 0

		for _ in pairs(dict) do
			len = len + 1
		end

		return len
	end

	it("should generate an empty patch for empty instances", function()
		local knownInstances = InstanceMap.new()
		local virtualInstances = {
			ROOT = {
				ClassName = "Folder",
				Name = "Some Name",
				Properties = {},
				Children = {},
			},
		}

		local rootInstance = Instance.new("Folder")
		rootInstance.Name = "Some Name"
		knownInstances:insert("ROOT", rootInstance)

		local ok, patch = diff(knownInstances, virtualInstances, "ROOT")

		assert(ok, tostring(patch))

		assert(isEmpty(patch.removed))
		assert(isEmpty(patch.added))
		assert(isEmpty(patch.updated))
	end)

	it("should generate a patch with a changed name", function()
		local knownInstances = InstanceMap.new()
		local virtualInstances = {
			ROOT = {
				ClassName = "Folder",
				Name = "Some Name",
				Properties = {},
				Children = {},
			},
		}

		local rootInstance = Instance.new("Folder")
		knownInstances:insert("ROOT", rootInstance)

		local ok, patch = diff(knownInstances, virtualInstances, "ROOT")

		assert(ok, tostring(patch))

		assert(isEmpty(patch.removed))
		assert(isEmpty(patch.added))
		expect(#patch.updated).to.equal(1)

		local update = patch.updated[1]
		expect(update.id).to.equal("ROOT")
		expect(update.changedName).to.equal("Some Name")
		assert(isEmpty(update.changedProperties))
	end)

	it("should generate a patch with a changed property", function()
		local knownInstances = InstanceMap.new()
		local virtualInstances = {
			ROOT = {
				ClassName = "StringValue",
				Name = "Value",
				Properties = {
					Value = {
						String = "Hello, world!",
					},
				},
				Children = {},
			},
		}

		local rootInstance = Instance.new("StringValue")
		rootInstance.Value = "Initial Value"
		knownInstances:insert("ROOT", rootInstance)

		local ok, patch = diff(knownInstances, virtualInstances, "ROOT")

		assert(ok, tostring(patch))

		assert(isEmpty(patch.removed))
		assert(isEmpty(patch.added))
		expect(#patch.updated).to.equal(1)

		local update = patch.updated[1]
		expect(update.id).to.equal("ROOT")
		expect(update.changedName).to.equal(nil)
		expect(size(update.changedProperties)).to.equal(1)

		local patchProperty = update.changedProperties["Value"]
		expect(patchProperty).to.be.a("table")
		local ty, value = next(patchProperty)
		expect(ty).to.equal("String")
		expect(value).to.equal("Hello, world!")
	end)

	it("should generate an empty patch if no properties changed", function()
		local knownInstances = InstanceMap.new()
		local virtualInstances = {
			ROOT = {
				ClassName = "StringValue",
				Name = "Value",
				Properties = {
					Value = {
						String = "Hello, world!",
					},
				},
				Children = {},
			},
		}

		local rootInstance = Instance.new("StringValue")
		rootInstance.Value = "Hello, world!"
		knownInstances:insert("ROOT", rootInstance)

		local ok, patch = diff(knownInstances, virtualInstances, "ROOT")

		assert(ok, tostring(patch))
		assert(PatchSet.isEmpty(patch), "expected empty patch")
	end)

	it("should ignore unknown properties", function()
		local knownInstances = InstanceMap.new()
		local virtualInstances = {
			ROOT = {
				ClassName = "Folder",
				Name = "Folder",
				Properties = {
					FAKE_PROPERTY = {
						String = "Hello, world!",
					},
				},
				Children = {},
			},
		}

		local rootInstance = Instance.new("Folder")
		knownInstances:insert("ROOT", rootInstance)

		local ok, patch = diff(knownInstances, virtualInstances, "ROOT")

		assert(ok, tostring(patch))

		assert(isEmpty(patch.removed))
		assert(isEmpty(patch.added))
		assert(isEmpty(patch.updated))
	end)

	--[[
		Because rbx_dom_lua resolves non-canonical properties to their canonical
		variants, this test does not work as intended.

		Instead, heat_xml is diffed with Heat, the canonical property variant,
		and a patch trying to assign to heat_xml is generated. This is
		incorrect, but will require more invasive changes to fix later.
	]]
	itFIXME("should ignore unreadable properties", function()
		local knownInstances = InstanceMap.new()
		local virtualInstances = {
			ROOT = {
				ClassName = "Fire",
				Name = "Fire",
				Properties = {
					-- heat_xml is a serialization-only property that is not
					-- exposed to Lua.
					heat_xml = {
						Float32 = 5,
					},
				},
				Children = {},
			},
		}

		local rootInstance = Instance.new("Fire")
		knownInstances:insert("ROOT", rootInstance)

		local ok, patch = diff(knownInstances, virtualInstances, "ROOT")

		Log.warn("{:#?}", patch)

		assert(ok, tostring(patch))

		assert(isEmpty(patch.removed))
		assert(isEmpty(patch.added))
		assert(isEmpty(patch.updated))
	end)

	it("should generate a patch removing unknown children by default", function()
		local knownInstances = InstanceMap.new()
		local virtualInstances = {
			ROOT = {
				ClassName = "Folder",
				Name = "Folder",
				Properties = {},
				Children = {},
			},
		}

		local rootInstance = Instance.new("Folder")
		local unknownChild = Instance.new("Folder")
		unknownChild.Parent = rootInstance
		knownInstances:insert("ROOT", rootInstance)

		local ok, patch = diff(knownInstances, virtualInstances, "ROOT")

		assert(ok, tostring(patch))

		assert(isEmpty(patch.added))
		assert(isEmpty(patch.updated))
		expect(#patch.removed).to.equal(1)
		expect(patch.removed[1]).to.equal(unknownChild)
	end)

	it("should generate an empty patch if unknown children should be ignored", function()
		local knownInstances = InstanceMap.new()
		local virtualInstances = {
			ROOT = {
				ClassName = "Folder",
				Name = "Folder",
				Properties = {},
				Children = {},
				Metadata = {
					ignoreUnknownInstances = true,
				},
			},
		}

		local rootInstance = Instance.new("Folder")
		local unknownChild = Instance.new("Folder")
		unknownChild.Parent = rootInstance
		knownInstances:insert("ROOT", rootInstance)

		local ok, patch = diff(knownInstances, virtualInstances, "ROOT")

		assert(ok, tostring(patch))

		assert(isEmpty(patch.added))
		assert(isEmpty(patch.updated))
		assert(isEmpty(patch.removed))
	end)

	it("should generate a patch with an added child", function()
		local knownInstances = InstanceMap.new()
		local virtualInstances = {
			ROOT = {
				ClassName = "Folder",
				Name = "Folder",
				Properties = {},
				Children = {"CHILD"},
			},

			CHILD = {
				ClassName = "Folder",
				Name = "Child",
				Properties = {},
				Children = {},
			},
		}

		local rootInstance = Instance.new("Folder")
		knownInstances:insert("ROOT", rootInstance)

		local ok, patch = diff(knownInstances, virtualInstances, "ROOT")

		assert(ok, tostring(patch))

		assert(isEmpty(patch.updated))
		assert(isEmpty(patch.removed))
		expect(size(patch.added)).to.equal(1)
		expect(patch.added["CHILD"]).to.equal(virtualInstances["CHILD"])
	end)
end