nextfile/spec/prevfile_spec.lua

function get_directory_iterator_from_table(table)
	return function(directory) 
		local index = 0
		local filenames = table

		return function()
			index = index + 1
			return filenames[index]
		end
	end
end

describe("ls", function()
	local lfs = require "lfs"
	local prevfile = require "prevfile"

	insulate("if directory does not exist", function()
		lfs.dir = function(directory) error("No such file or directory") end

		it("should return nil", function()
			local files = prevfile.ls("/testdir")

			assert.equals(nil, files)
		end)
	end)

	insulate("if directory is empty", function()
		lfs.dir = get_directory_iterator_from_table({})

		it("should return empty table", function()
			local files = prevfile.ls("/testdir")

			assert.are.same({}, files)
		end)

		it("should call lfs.dir with target directory", function()
			spy.on(lfs, "dir")
			local test_directory = "/testdir"

			prevfile.ls(test_directory)

			assert.spy(lfs.dir).was_called_with(test_directory)
		end)
	end)

	insulate("if directory contains files", function()
		lfs.dir = get_directory_iterator_from_table({"a.txt", "b.txt", "z.txt", "c.txt"})

		it("should return the files in lexographical order", function()
			local files = prevfile.ls("/testdir")

			assert.are.same({"a.txt", "b.txt", "c.txt", "z.txt"}, files)
		end)
	end)

	insulate("if directory contains numerically-named files", function()
		lfs.dir = get_directory_iterator_from_table({"/testdir/709.txt", "/testdir/71.txt", "/testdir/710.txt"})

		it("should return the files in numerical order", function()
			local files = prevfile.ls("/testdir")

			assert.are.same({"/testdir/71.txt", "/testdir/709.txt", "/testdir/710.txt"}, files)
		end)
	end)

	insulate("if directory contains hidden files", function()
		lfs.dir = get_directory_iterator_from_table({"a.txt", ".b.txt.swp", "c.txt"})

		it("should exclude them from the listing", function()
			local files = prevfile.ls("/testdir")

			assert.are.same({"a.txt", "c.txt"}, files)
		end)
	end)
end)

describe("prevfile", function()
	local prevfile = require "prevfile"

	insulate("if ls returns nil", function()
		prevfile.ls = function() return nil end

		it("should return nil", function()
			local file = prevfile.prevfile()
		
			assert.are.same(nil, file)
		end)
	end)

	insulate("if ls returns files", function()
		prevfile.ls = function() return {"a.txt", "b.txt", "c.txt", "z.txt"} end

		it("should call prevfile.ls to get files", function()
			spy.on(prevfile, "ls")
			test_directory = "/testdir"

			prevfile.prevfile(test_directory)

			assert.spy(prevfile.ls).was_called_with(test_directory)
		end)

		it("should return the last file in the list", function()
			local file = prevfile.prevfile("/testdir")

			assert.are.same("z.txt", file)
		end)
	end)
end)