First attempt at writing the installer

index.json

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+[
+  {
+    "src": "src/startup.lua",
+    "dest": "/startup.lua",
+    "hash": "07219cd9561b41ce1f39209958076c471b17855679c968b42767b0122423c782"
+  }
+]

install.lua

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+local repoRoot = "https://git.colon-three.com/kodi/snowier/raw/branch/main/"
+
+local URLs = {
+  fileIndex = repoRoot .. "index.json",
+  -- Implementation of sha256 pulled from here
+  sha2 = "https://raw.githubusercontent.com/Egor-Skriptunoff/pure_lua_SHA/refs/heads/master/sha2.lua"
+}
+
+local log = {}
+
+--- @param value string
+function log.info(value)
+  print("[I] " .. value)
+end
+
+--- @param value string
+function log.error(value)
+    printError("[E] " .. value)
+end
+
+local function main()
+  log.info("Starting installer")
+
+  if not fs.exists("sha2.lua") then
+    log.info("Downloading sha2.lua")
+    local sharequest = http.get(URLs.sha2)
+    local shafile = fs.open("sha2.lua", "w")
+    shafile.write(sharequest.readAll())
+    shafile.close()
+    sharequest.close()
+  end
+  local hash = require("sha2")
+
+  log.info("Downloading index file")
+  local indexrequest = http.get(URLs.fileIndex)
+  local index = textutils.unserializeJson(request.readAll())
+  indexrequest.close()
+
+  if (index == nil) then
+    log.error("Failed to deserialize index file.")
+    return
+  end
+
+  log.info("Checking for updates")
+  local toDownload = {}
+  for k, v in ipairs(index) do
+    -- A file should be redownloaded if it either:
+    -- - Doesn't exist locally
+    -- - Exists locally but has a different hash
+    if not fs.exists(v.dest) then
+      toDownload[#toDownload+1] = v
+    else
+      local destfile = fs.open(v.dest, "r")
+      local filehash = hash.sha256(destfile.readAll())
+      destfile.close()
+      if filehash ~= v.sha256 then
+        toDownload[#toDownload+1] = v
+      end
+    end
+  end
+
+  log.info(#toDownload .. " files to download")
+  for k, v in ipairs(toDownload) do
+    if not fs.exists("/tmp") then
+      fs.makeDir("/tmp")
+    end
+    log.info("Downloading '" .. v.dest .. "'")
+    local filerequest = http.get(repoRoot .. v.src)
+    local tempfile = fs.open("/tmp/" .. v.hash .. ".lua", "w")
+    tempfile.write(filerequest.readAll())
+    filerequest.close()
+    tempfile.close()
+    -- Check the validity of the hash, this acts both as a checksum
+    -- and a way to raise an error if the index is incorrect
+    tempfile = fs.open("/tmp/" .. v.hash .. ".lua", "r")
+    local filehash = hash.sha256(tempfile.readAll())
+    tempfile.close()
+    if filehash ~= v.hash then
+      log.error("File " .. v.dest .. " (from " .. v.src .. ") has a mismatched hash.")
+      log.error("Installation aborted.")
+      return
+    end
+  end
+
+  log.info("Installing")
+  for k, v in ipairs(toDownload) do
+    fs.move("/tmp/" .. v.hash .. ".lua", v.dest)
+  end
+
+  log.info("Installation complete!")
+end
+
+main()

src/startup.lua

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+print("Hello, World!")

updateindex.nu

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+#!/usr/bin/env nix-shell
+#!nix-shell -p jq nushell -i nu
+
+open index.json
+| each {|entry| 
+  {src: $entry.src, dest: $entry.dest, 
+  hash: (cat $entry.src | hash sha256)}
+}
+| collect
+| save -f index.json