Add prevfile function

spec/prevfile_spec.lua

@@ -39,7 +39,7 @@ describe("ls", function()
 
 			prevfile.ls(test_directory)
 
-			assert.stub(lfs.dir).was_called_with(test_directory)
+			assert.spy(lfs.dir).was_called_with(test_directory)
 		end)
 	end)
 
@@ -73,3 +73,36 @@ describe("ls", function()
 		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)

src/prevfile.lua

@@ -27,4 +27,10 @@ function prevfile.ls(directory)
 	return files
 end
 
+function prevfile.prevfile(directory)
+	local files = prevfile.ls(directory)
+	if files == nil then return nil end
+	return files[#files]
+end
+
 return prevfile