Recursively generate checksums of all videos in a directory on Windows

checksums.lua

--Run an OS command, returning its output rather than its exit status.
local function run(cmd,mode)
	local prog = io.popen(cmd)
	local output
	if mode == 1 then --String of full output
		output = prog:read("*all"):sub(1,-2)
		prog:close()
	elseif mode == 2 then --Table of each line
		output = {}
		for line in prog:lines() do
			table.insert(output,line)
		end
		prog:close()
	elseif mode == 3 then --Iterator over lines of output
		output = prog:lines()
	else
		error("Unknown mode")
	end
	return output
end

--Decide where the root of this is going to happen
local toplevel
local arg = select(1,...)
if arg == nil then
	toplevel = "C:\\Users\\Andrew\\Media\\"
else
	toplevel = run("cd /D "..arg.." && cd",1)..'\\'
end

--Determine where the home folder is, so it can be abbreviated to ~
local USERPROFILE = run("echo %USERPROFILE%",1)

function main(dir)
	--Figure out where we are
	dir = dir or toplevel
	dir = dir:gsub("\\\\",'\\')
	local pwd = dir:gsub(USERPROFILE,'~')
	print(pwd)
	
	--Read existing SHA256SUMS file to build a dictionary mapping filenames to hashes
	local checksums = {}
	local sums = io.open(dir.."SHA256SUMS",'r')
	if sums ~= nil then
		for line in sums:lines() do
			local checksum = line:sub(1,64)
			local filename = line:sub(66)
			checksums[filename] = checksum
		end
		sums:close()
	end
	
	--List directory contents to build a (sorted) list of files to be checksummed
	local filenames = {}
	for file in run([[dir /B /A:-D "]]..dir..[["]],3) do
		if file:find("%.mkv$") or file:find("%.mp4$") then
			table.insert(filenames,file)
		end
	end
	table.sort(filenames)
	
	--Calculate checksums for any new files
	local newFiles = false
	for _,file in ipairs(filenames) do
		if not checksums[file] then
			io.write("Generating checksum for new file "..dir..file.."...")
			out = run([[sha256sum.exe "]]..dir..file..[["]],1)
			checksums[file] = out:sub(2,65)
			print("done.")
			newFiles = true
		end
	end
	
	--If there were any new files, rebuild SHA256SUMS using the dictionary and the sorted list
	if newFiles then
		io.write("Rewriting "..dir.."SHA256SUMS...")
		os.execute([[del /A "]]..dir..[[SHA256SUMS" 2>nul]])
		local sums,err = io.open(dir.."SHA256SUMS",'w')
		for _,file in ipairs(filenames) do
			local sum = checksums[file]
			sums:write(sum..' '..file..'\n')
		end
		sums:close()
		os.execute([[attrib +h "]]..dir..[[SHA256SUMS"]])
		print("done.")
	end
	
	--Perform the above steps for all subdirectories of the current directory
	for subdir in run([[dir /B /A:D "]]..dir..[["]],3) do
		main(dir..subdir..'\\')
	end
end

--do the thing
main()