Add nextletter function

spec/nextfile_spec.lua

@@ -18,6 +18,22 @@ describe("splitfilename", function()
 	end)
 end)
 
+describe("nextletter", function()
+	local nextfile = require "nextfile"
+
+	it("should return the next letter when the current letter is not 'z'", function()
+		local letter = nextfile.nextletter("q")
+
+		assert.are.equal("r", letter)
+	end)
+
+	it("should return 'za' when the current letter is 'z'", function()
+		local letter = nextfile.nextletter("z")
+
+		assert.are.equal("za", letter)
+	end)
+end)
+
 describe("nextfile", function()
 	local prevfile = require "prevfile"
 	local nextfile = require "nextfile"
@@ -44,10 +60,20 @@ describe("nextfile", function()
 	insulate("prevfile.prevfile returns a numeric filename", function()
 		prevfile.prevfile = function() return "/testdir/701.txt" end
 
-		--it("should return the result of incrementing the filename", function()
+		it("should return the result of incrementing the filename", function()
+			local file = nextfile.nextfile("/testdir")
+
+			assert.are.equal("/testdir/702.txt", file)
+		end)
+	end)
+
+	insulate("prevfile.prevfile returns a non-numeric filename", function()
+		prevfile.prevfile = function() return "/testdir/421b.txt" end
+
+		--it("should return the result of replacing the last character with the next letter of the alphabet", function()
 			--local file = nextfile.nextfile("/testdir")
 
-			--assert.are.equal("/testdir/702.txt", file)
+			--assert.are.equal("/testdir/421c.txt", file)
 		--end)
 	end)
 end)

src/nextfile.lua

@@ -7,16 +7,26 @@ function nextfile.splitfilename(path)
 	return dir, name, ext
 end
 
+function nextfile.nextletter(letter)
+	if letter == "z" then return "za" end
+
+	local ascii = letter:byte()
+	local nextascii = ascii + 1
+	return string.char(nextascii)
+end
+
 function nextfile.nextfile(directory)
 	local previous = prevfile.prevfile(directory)
 	if previous == nil then return nil end
 
 	local dir, name, ext = nextfile.splitfilename(previous)
-	print(dir)
-	print(name)
-	print(ext)
 
-	return ""
+	if tonumber(name) == nil then
+		local lastletter = name:sub(#name)
+		return dir .. name:gsub() .. ext
+	end
+	
+	return dir .. name + 1 .. ext
 end
 
 return nextfile