---@brief
---
--- https://github.com/golang/tools/tree/master/gopls
---
--- Google's lsp server for golang.

--- @class go_dir_custom_args
---
--- @field envvar_id string
---
--- @field custom_subdir string?

local mod_cache = nil
local std_lib = nil

---@param custom_args go_dir_custom_args
---@param on_complete fun(dir: string | nil)
local function identify_go_dir(custom_args, on_complete)
  local cmd = { "go", "env", custom_args.envvar_id }
  vim.system(cmd, { text = true }, function(output)
    local res = vim.trim(output.stdout or "")
    if output.code == 0 and res ~= "" then
      if custom_args.custom_subdir and custom_args.custom_subdir ~= "" then
        res = res .. custom_args.custom_subdir
      end
      on_complete(res)
    else
      vim.schedule(function()
        vim.notify(
          ("[gopls] identify " .. custom_args.envvar_id .. " dir cmd failed with code %d: %s\n%s"):format(
            output.code,
            vim.inspect(cmd),
            output.stderr
          )
        )
      end)
      on_complete(nil)
    end
  end)
end

---@return string?
local function get_std_lib_dir()
  if std_lib and std_lib ~= "" then
    return std_lib
  end

  identify_go_dir({ envvar_id = "GOROOT", custom_subdir = "/src" }, function(dir)
    if dir then
      std_lib = dir
    end
  end)
  return std_lib
end

---@return string?
local function get_mod_cache_dir()
  if mod_cache and mod_cache ~= "" then
    return mod_cache
  end

  identify_go_dir({ envvar_id = "GOMODCACHE" }, function(dir)
    if dir then
      mod_cache = dir
    end
  end)
  return mod_cache
end

---@param fname string
---@return string?
local function get_root_dir(fname)
  if mod_cache and fname:sub(1, #mod_cache) == mod_cache then
    local clients = vim.lsp.get_clients({ name = "gopls" })
    if #clients > 0 then
      return clients[#clients].config.root_dir
    end
  end
  if std_lib and fname:sub(1, #std_lib) == std_lib then
    local clients = vim.lsp.get_clients({ name = "gopls" })
    if #clients > 0 then
      return clients[#clients].config.root_dir
    end
  end
  return vim.fs.root(fname, "go.work") or vim.fs.root(fname, "go.mod") or vim.fs.root(fname, ".git")
end

---@type vim.lsp.Config
return {
  cmd = { "gopls" },
  filetypes = { "go", "gomod", "gowork", "gotmpl" },
  root_dir = function(bufnr, on_dir)
    local fname = vim.api.nvim_buf_get_name(bufnr)
    get_mod_cache_dir()
    get_std_lib_dir()
    -- see: https://github.com/neovim/nvim-lspconfig/issues/804
    on_dir(get_root_dir(fname))
  end,
}