-- @@2023-07-07 release
-- LICENSED UNDER CC0

-- stylua: ignore start
--[[
js autogen:
var a ="АБВГДЕЁЖЗИЙКЛМНОПРСТУФХЦЧШЩЪЫЬЭЮЯ";
var q = a + a.toLowerCase();
var ret = q.split('').map(L=>`["${L}"]="${L.toLowerCase()}"`).join(", ")
console.log(ret)

second lcased half is not "strictly" required in this example though;
]]
local cheat_sheet_map_RU = {
  ["А"]="а", ["Б"]="б", ["В"]="в", ["Г"]="г", ["Д"]="д", ["Е"]="е", ["Ё"]="ё", ["Ж"]="ж", ["З"]="з", ["И"]="и", ["Й"]="й", ["К"]="к", ["Л"]="л", ["М"]="м", ["Н"]="н", ["О"]="о", ["П"]="п", ["Р"]="р", ["С"]="с", ["Т"]="т", ["У"]="у", ["Ф"]="ф", ["Х"]="х", ["Ц"]="ц", ["Ч"]="ч", ["Ш"]="ш", ["Щ"]="щ", ["Ъ"]="ъ", ["Ы"]="ы", ["Ь"]="ь", ["Э"]="э", ["Ю"]="ю", ["Я"]="я",
  ["а"]="а", ["б"]="б", ["в"]="в", ["г"]="г", ["д"]="д", ["е"]="е", ["ё"]="ё", ["ж"]="ж", ["з"]="з", ["и"]="и", ["й"]="й", ["к"]="к", ["л"]="л", ["м"]="м", ["н"]="н", ["о"]="о", ["п"]="п", ["р"]="р", ["с"]="с", ["т"]="т", ["у"]="у", ["ф"]="ф", ["х"]="х", ["ц"]="ц", ["ч"]="ч", ["ш"]="ш", ["щ"]="щ", ["ъ"]="ъ", ["ы"]="ы", ["ь"]="ь", ["э"]="э", ["ю"]="ю", ["я"]="я",
}
-- stylua: ignore end

local s_lower = string.lower
local s_sub = string.sub
local t_concat = table.concat

local function manual_lowercase(s)
  local ret = {}
  local ret_len = 0

  local idx = 1
  local len_s = #s
  local cheat
  while idx <= len_s do
    cheat = cheat_sheet_map_RU[s_sub(s, idx, idx + 1)]
    if cheat then
      ret_len = ret_len + 1
      ret[ret_len] = cheat
      idx = idx + 1 -- since we consumed 2 "bytes"
    else
      ret_len = ret_len + 1
      ret[ret_len] = s_lower(s_sub(s, idx, idx))
    end
    idx = idx + 1
  end
  return t_concat(ret, "")
end

print(manual_lowercase("ЫХАЛО walks into the bar"))
print(manual_lowercase("then 1 ПРЫНЦ Машет Ему Лапкой"))
print(manual_lowercase('and lastly, "THE PARTY" beginz!'))