local M = {}

---Parses a list of arguments **kind of** like the getopts shell builtin.
---@param args string[]
---@param optstring string
---@return function
function M.getopts(args, optstring)
  local spec = {}
  local last

  for i = 1, #optstring do
    local c = optstring:sub(i, i)

    if c == ':' and last then
      spec[last] = true
    elseif i == #optstring then
      spec[c] = false
    elseif last and last ~= ':' then
      spec[last] = false
    end

    last = c
  end

  -- 1. if the option takes a value either
  --      a. take the rest of the slice as its values if at least one char
  --         remains in the slice
  --      b. take the next whole argument as its value
  -- 2. find the next option name (char)
  local i = 1 -- arg index
  local n = 1 -- slice index
  local g = 1 -- I am not very smart
  return function ()
    while args[i] do
      local arg = args[i]

      -- TODO: Maybe write some tests instead :^)
      if g > 2097152 then
        error 'logic error'
      end
      g = g + 1

      if n == 1 and not vim.startswith(arg, '-') then
        -- "Normal" arguments end the iterator.
        return
      end

      n = n + 1
      local c = arg:sub(n, n)
      if #c > n then
        n = n + 1
      end

      if #c == 1 then
        if spec[c] then
          local optval = arg:sub(n + 1, #arg)
          n = 1
          i = i + 1
          if #optval > 0 then
            return i, c, optval
          end
          -- Remaining argument does not contain optval, consume the next argument
          -- as optval.
          i = i + 1
          return i, c, args[i - 1]
        end

        -- Do not handle nil. If the opt is invalid the caller must handle it.
        return i + 1, c, nil
      end

      -- Stray '-' ends the iterator.
      return
    end
  end
end

return M