A Reaper action to open UST files

Searching through the documentation (with Firefox's C-f search):

• reaper.GetSelectedMediaItem(project, index) returns the selected media item.

  • project is 0 for the active project — a lot of functions take this argument for some reason.
  • index is the 0-based index to choose which one to return if there are multiple selected items. Despite this being Lua, Reaper's own indices are still 0-based; thankfully the documentation does acknowledge this.
• reaper.GetActiveTake(item) returns the active take of item.
• reaper.GetMediaItemTake_Source(take) returns the source of take; a source has other properties (like length, type, sample rate…), so we don't have a file name yet.
• Finally, reaper.GetMediaSourceFileName(source) returns the file name of source.

The naming conventions seem a little inconsistent. Ideally a more cohesive integration with Lua might model these objects as Lua tables, so instead of reaper.GetMediaItemTake_Source(take) one could write take.source instead, but this is fine.

Every time we save the file in Reaper's builtin editor, the entire file is evaluated, and global variables are shown on the right pane. So I find it useful to put code in a main function and not declare local until I don't need the value to be shown anymore.

function main()
    selected_item = reaper.GetSelectedMediaItem(0, 0)
    active_take = reaper.GetActiveTake(selected_item)
    source = reaper.GetMediaItemTake_Source(active_take)
    source_path = reaper.GetMediaSourceFileName(source)
end

Now that we have the path in the source_path variable, we try to find a UST that exists.

What I want to do would be something like this in Emacs Lisp:

(let ((source-path
       "/kisaragi-common/cloud/Projects/1.done/20210625 ハルノ寂寞 - 稲葉曇[utau cover music]/lead.wav"))
  (-first
   #'f-exists?
   (list
    ;; root/abc.wav -> root/abc.ust
    (concat (f-no-ext (f-filename source-path)) ".ust")
    ;; root/abc.wav -> root/ust/abc.ust
    (f-join (f-dirname source-path)
            "ust"
            (concat (f-no-ext (f-filename source-path)) ".ust"))
    ;; root/wav/abc.wav -> root/ust/abc.ust
    (f-join (f-dirname source-path)
            ".."
            "ust"
            (concat (f-no-ext (f-filename source-path)) ".ust")))))

;; -> "/kisaragi-common/cloud/Projects/1.done/20210625 ハルノ寂寞 - 稲葉曇[utau cover music]/ust/lead.ust"

Unfortunately I wasn't able to find builtin functions in Lua that manipulated paths like this. In the end I had to copy some code from github:moteus/lua-path, as I didn't find a proven way to import modules from LuaRocks.

Extracting some functions and adopting them so that they don't depend on other lua-path modules:

-- From
-- https://github.com/moteus/lua-path/blob/5a32c705/lua/path.lua#L147
function splitext(P)
    local s1, s2 = string.match(P, "(.-[^\\/.])(%.[^\\/.]*)$")
    if s1 then
        return s1, s2
    end
    return P, ""
end

-- From
-- https://github.com/moteus/lua-path/blob/5a32c705/lua/path.lua#L153
function splitpath(P)
    return string.match(P, "^(.-)[\\/]?([^\\/]*)$")
end

-- Modified from
-- https://github.com/moteus/lua-path/blob/5a32c705/lua/path.lua#L134
--
-- No, I have zero idea what select('#') or #var means
if reaper.GetOS():match("^Win") then
    PATH_SEPARATOR = "\\"
else
    PATH_SEPARATOR = "/"
end
function joinpaths(...)
    local t, n = {...}, select("#", ...)
    local r = t[1]
    for i = 2, #t do
        r = r .. PATH_SEPARATOR .. t[i]
    end
    return r
end

These are enough to write down our three candidates:

source_dir, filename = splitpath(source_path)
base = splitext(filename)

candidates = {}
-- Intention: root/abc.wav -> root/abc.ust
candidates[1] = joinpaths(source_dir, base .. ".ust")
-- Intention: root/abc.wav -> root/ust/abc.ust
candidates[2] = joinpaths(source_dir, "ust", base .. ".ust")
-- Intention: root/wav/abc.wav -> root/ust/abc.ust
candidates[3] = joinpaths(source_dir, "..", "ust", base .. ".ust")

Checking for file existence can be done in Reaper with reaper.file_exists(path).

Next, a simple question: how do I iterate through a list of items and return the first one that matches a condition in Lua?

In short, like this:

-- Assuming `candidates` is already prepared beforehand

local ret
for _idx, path in next, candidates do
    if reaper.file_exists(path) then
        ret = path
        break
    end
end

• _idx is just me carrying Emacs Lisp's naming convention for throwaway variables over into Lua
• for … in … is the generic for statement. It calls the first argument (next here) on the second and third arguments (candidates and nil here). In this case, it iterates through each element of candidates in an unspecified order (this is done by next).

Assuming we found the UST file (and saved it into ret), we want to open it with UTAU.

We can delegate figuring out where the UTAU executable is to the system via xdg-open (and equivalent commands on Windows and macOS).

In Lua, the function to run a system command is os.execute. Unfortunately it doesn't seem to have an option to skip the shell, but for what I'm doing it's good enough.

Additionally, Reaper provides a function that returns the operating system name, so we don't have to do this ourselves.

-- Be sure to not name it lowercase `os` because that will overwrite
-- the builtin os table.
OS = reaper.GetOS()
if OS:match("^Win") then
    command = "start"
-- Lua's string matching patterns don't have the or operator ("|").
elseif OS:match("^macOS") or OS:match("^OSX") then
    command = "open"
elseif OS:match("^Other") then
    command = "xdg-open"
end
if ret then
    os.execute(command .. " " .. '"' .. ret .. '"')
end

Granted, I never actually tested this outside of Linux, but I like to at least make an effort to write cross-platform code.