local vim = vim
local uv = vim.loop
local log = require('vim.lsp.log')
local protocol = require('vim.lsp.protocol')
local validate, schedule, schedule_wrap = vim.validate, vim.schedule, vim.schedule_wrap

-- TODO replace with a better implementation.
--@private
--- Encodes to JSON.
---
--@param data (table) Data to encode
--@returns (string) Encoded object
local function json_encode(data)
  local status, result = pcall(vim.fn.json_encode, data)
  if status then
    return result
  else
    return nil, result
  end
end
--@private
--- Decodes from JSON.
---
--@param data (string) Data to decode
--@returns (table) Decoded JSON object
local function json_decode(data)
  local status, result = pcall(vim.fn.json_decode, data)
  if status then
    return result
  else
    return nil, result
  end
end

--@private
--- Checks whether a given path exists and is a directory.
--@param filename (string) path to check
--@returns (bool)
local function is_dir(filename)
  local stat = vim.loop.fs_stat(filename)
  return stat and stat.type == 'directory' or false
end

local NIL = vim.NIL
--@private
--- Returns its argument, but converts `vim.NIL` to Lua `nil`.
--@param v (any) Argument
--@returns (any)
local function convert_NIL(v)
  if v == NIL then return nil end
  return v
end

--@private
--- Merges current process env with the given env and returns the result as
--- a list of "k=v" strings.
---
--- <pre>
--- Example:
---
---  in:    { PRODUCTION="false", PATH="/usr/bin/", PORT=123, HOST="0.0.0.0", }
---  out:   { "PRODUCTION=false", "PATH=/usr/bin/", "PORT=123", "HOST=0.0.0.0", }
--- </pre>
--@param env (table) table of environment variable assignments
--@returns (table) list of `"k=v"` strings
local function env_merge(env)
  if env == nil then
    return env
  end
  -- Merge.
  env = vim.tbl_extend('force', vim.fn.environ(), env)
  local final_env = {}
  for k,v in pairs(env) do
    assert(type(k) == 'string', 'env must be a dict')
    table.insert(final_env, k..'='..tostring(v))
  end
  return final_env
end

--@private
--- Embeds the given string into a table and correctly computes `Content-Length`.
---
--@param encoded_message (string)
--@returns (table) table containing encoded message and `Content-Length` attribute
local function format_message_with_content_length(encoded_message)
  return table.concat {
    'Content-Length: '; tostring(#encoded_message); '\r\n\r\n';
    encoded_message;
  }
end

--@private
--- Parses an LSP Message's header
---
--@param header: The header to parse.
--@returns Parsed headers
local function parse_headers(header)
  if type(header) ~= 'string' then
    return nil
  end
  local headers = {}
  for line in vim.gsplit(header, '\r\n', true) do
    if line == '' then
      break
    end
    local key, value = line:match("^%s*(%S+)%s*:%s*(.+)%s*$")
    if key then
      key = key:lower():gsub('%-', '_')
      headers[key] = value
    else
      local _ = log.error() and log.error("invalid header line %q", line)
      error(string.format("invalid header line %q", line))
    end
  end
  headers.content_length = tonumber(headers.content_length)
      or error(string.format("Content-Length not found in headers. %q", header))
  return headers
end

-- This is the start of any possible header patterns. The gsub converts it to a
-- case insensitive pattern.
local header_start_pattern = ("content"):gsub("%w", function(c) return "["..c..c:upper().."]" end)

--@private
--- The actual workhorse.
local function request_parser_loop()
  local buffer = ''
  while true do
    -- A message can only be complete if it has a double CRLF and also the full
    -- payload, so first let's check for the CRLFs
    local start, finish = buffer:find('\r\n\r\n', 1, true)
    -- Start parsing the headers
    if start then
      -- This is a workaround for servers sending initial garbage before
      -- sending headers, such as if a bash script sends stdout. It assumes
      -- that we know all of the headers ahead of time. At this moment, the
      -- only valid headers start with "Content-*", so that's the thing we will
      -- be searching for.
      -- TODO(ashkan) I'd like to remove this, but it seems permanent :(
      local buffer_start = buffer:find(header_start_pattern)
      local headers = parse_headers(buffer:sub(buffer_start, start-1))
      buffer = buffer:sub(finish+1)
      local content_length = headers.content_length
      -- Keep waiting for data until we have enough.
      while #buffer < content_length do
        buffer = buffer..(coroutine.yield()
            or error("Expected more data for the body. The server may have died.")) -- TODO hmm.
      end
      local body = buffer:sub(1, content_length)
      buffer = buffer:sub(content_length + 1)
      -- Yield our data.
      buffer = buffer..(coroutine.yield(headers, body)
          or error("Expected more data for the body. The server may have died.")) -- TODO hmm.
    else
      -- Get more data since we don't have enough.
      buffer = buffer..(coroutine.yield()
          or error("Expected more data for the header. The server may have died.")) -- TODO hmm.
    end
  end
end

local client_errors = vim.tbl_add_reverse_lookup {
  INVALID_SERVER_MESSAGE       = 1;
  INVALID_SERVER_JSON          = 2;
  NO_RESULT_CALLBACK_FOUND     = 3;
  READ_ERROR                   = 4;
  NOTIFICATION_HANDLER_ERROR   = 5;
  SERVER_REQUEST_HANDLER_ERROR = 6;
  SERVER_RESULT_CALLBACK_ERROR = 7;
}

--- Constructs an error message from an LSP error object.
---
--@param err (table) The error object
--@returns (string) The formatted error message
local function format_rpc_error(err)
  validate {
    err = { err, 't' };
  }

  -- There is ErrorCodes in the LSP specification,
  -- but in ResponseError.code it is not used and the actual type is number.
  local code
  if protocol.ErrorCodes[err.code] then
    code = string.format("code_name = %s,", protocol.ErrorCodes[err.code])
  else
    code = string.format("code_name = unknown, code = %s,", err.code)
  end

  local message_parts = {"RPC[Error]", code}
  if err.message then
    table.insert(message_parts, "message =")
    table.insert(message_parts, string.format("%q", err.message))
  end
  if err.data then
    table.insert(message_parts, "data =")
    table.insert(message_parts, vim.inspect(err.data))
  end
  return table.concat(message_parts, ' ')
end

--- Creates an RPC response object/table.
---
--@param code RPC error code defined in `vim.lsp.protocol.ErrorCodes`
--@param message (optional) arbitrary message to send to server
--@param data (optional) arbitrary data to send to server
local function rpc_response_error(code, message, data)
  -- TODO should this error or just pick a sane error (like InternalError)?
  local code_name = assert(protocol.ErrorCodes[code], 'Invalid RPC error code')
  return setmetatable({
    code = code;
    message = message or code_name;
    data = data;
  }, {
    __tostring = format_rpc_error;
  })
end

local default_handlers = {}
--@private
--- Default handler for notifications sent to an LSP server.
---
--@param method (string) The invoked LSP method
--@param params (table): Parameters for the invoked LSP method
function default_handlers.notification(method, params)
  local _ = log.debug() and log.debug('notification', method, params)
end
--@private
--- Default handler for requests sent to an LSP server.
---
--@param method (string) The invoked LSP method
--@param params (table): Parameters for the invoked LSP method
--@returns `nil` and `vim.lsp.protocol.ErrorCodes.MethodNotFound`.
function default_handlers.server_request(method, params)
  local _ = log.debug() and log.debug('server_request', method, params)
  return nil, rpc_response_error(protocol.ErrorCodes.MethodNotFound)
end
--@private
--- Default handler for when a client exits.
---
--@param code (number): Exit code
--@param signal (number): Number describing the signal used to terminate (if
---any)
function default_handlers.on_exit(code, signal)
  local _ = log.info() and log.info("client_exit", { code = code, signal = signal })
end
--@private
--- Default handler for client errors.
---
--@param code (number): Error code
--@param err (any): Details about the error
---any)
function default_handlers.on_error(code, err)
  local _ = log.error() and log.error('client_error:', client_errors[code], err)
end

--- Starts an LSP server process and create an LSP RPC client object to
--- interact with it.
---
--@param cmd (string) Command to start the LSP server.
--@param cmd_args (table) List of additional string arguments to pass to {cmd}.
--@param handlers (table, optional) Handlers for LSP message types. Valid
---handler names are:
--- - `"notification"`
--- - `"server_request"`
--- - `"on_error"`
--- - `"on_exit"`
--@param extra_spawn_params (table, optional) Additional context for the LSP
--- server process. May contain:
--- - {cwd} (string) Working directory for the LSP server process
--- - {env} (table) Additional environment variables for LSP server process
--@returns Client RPC object.
---
--@returns Methods:
--- - `notify()` |vim.lsp.rpc.notify()|
--- - `request()` |vim.lsp.rpc.request()|
---
--@returns Members:
--- - {pid} (number) The LSP server's PID.
--- - {handle} A handle for low-level interaction with the LSP server process
---   |vim.loop|.
local function start(cmd, cmd_args, handlers, extra_spawn_params)
  local _ = log.info() and log.info("Starting RPC client", {cmd = cmd, args = cmd_args, extra = extra_spawn_params})
  validate {
    cmd = { cmd, 's' };
    cmd_args = { cmd_args, 't' };
    handlers = { handlers, 't', true };
  }

  if not (vim.fn.executable(cmd) == 1) then
    error(string.format("The given command %q is not executable.", cmd))
  end
  if handlers then
    local user_handlers = handlers
    handlers = {}
    for handle_name, default_handler in pairs(default_handlers) do
      local user_handler = user_handlers[handle_name]
      if user_handler then
        if type(user_handler) ~= 'function' then
          error(string.format("handler.%s must be a function", handle_name))
        end
        -- server_request is wrapped elsewhere.
        if not (handle_name == 'server_request'
          or handle_name == 'on_exit') -- TODO this blocks the loop exiting for some reason.
        then
          user_handler = schedule_wrap(user_handler)
        end
        handlers[handle_name] = user_handler
      else
        handlers[handle_name] = default_handler
      end
    end
  else
    handlers = default_handlers
  end

  local stdin = uv.new_pipe(false)
  local stdout = uv.new_pipe(false)
  local stderr = uv.new_pipe(false)

  local message_index = 0
  local message_callbacks = {}

  local handle, pid
  do
    --@private
    --- Callback for |vim.loop.spawn()| Closes all streams and runs the
    --- `on_exit` handler.
    --@param code (number) Exit code
    --@param signal (number) Signal that was used to terminate (if any)
    local function onexit(code, signal)
      stdin:close()
      stdout:close()
      stderr:close()
      handle:close()
      -- Make sure that message_callbacks can be gc'd.
      message_callbacks = nil
      handlers.on_exit(code, signal)
    end
    local spawn_params = {
      args = cmd_args;
      stdio = {stdin, stdout, stderr};
    }
    if extra_spawn_params then
      spawn_params.cwd = extra_spawn_params.cwd
      if spawn_params.cwd then
        assert(is_dir(spawn_params.cwd), "cwd must be a directory")
      end
      spawn_params.env = env_merge(extra_spawn_params.env)
    end
    handle, pid = uv.spawn(cmd, spawn_params, onexit)
  end

  --@private
  --- Encodes {payload} into a JSON-RPC message and sends it to the remote
  --- process.
  ---
  --@param payload (table) Converted into a JSON string, see |json_encode()|
  --@returns true if the payload could be scheduled, false if the main event-loop is in the process of closing.
  local function encode_and_send(payload)
    local _ = log.debug() and log.debug("rpc.send.payload", payload)
    if handle:is_closing() then return false end
    -- TODO(ashkan) remove this once we have a Lua json_encode
    schedule(function()
      local encoded = assert(json_encode(payload))
      stdin:write(format_message_with_content_length(encoded))
    end)
    return true
  end

  -- FIXME: DOC: Should be placed on the RPC client object returned by
  -- `start()`
  --
  --- Sends a notification to the LSP server.
  --@param method (string) The invoked LSP method
  --@param params (table): Parameters for the invoked LSP method
  --@returns (bool) `true` if notification could be sent, `false` if not
  local function notify(method, params)
    return encode_and_send {
      jsonrpc = "2.0";
      method = method;
      params = params;
    }
  end

  --@private
  --- sends an error object to the remote LSP process.
  local function send_response(request_id, err, result)
    return encode_and_send {
      id = request_id;
      jsonrpc = "2.0";
      error = err;
      result = result;
    }
  end

  -- FIXME: DOC: Should be placed on the RPC client object returned by
  -- `start()`
  --
  --- Sends a request to the LSP server and runs {callback} upon response.
  ---
  --@param method (string) The invoked LSP method
  --@param params (table) Parameters for the invoked LSP method
  --@param callback (function) Callback to invoke
  --@returns (bool, number) `(true, message_id)` if request could be sent, `false` if not
  local function request(method, params, callback)
    validate {
      callback = { callback, 'f' };
    }
    message_index = message_index + 1
    local message_id = message_index
    local result = encode_and_send {
      id = message_id;
      jsonrpc = "2.0";
      method = method;
      params = params;
    }
    if result then
      message_callbacks[message_id] = schedule_wrap(callback)
      return result, message_id
    else
      return false
    end
  end

  stderr:read_start(function(_err, chunk)
    if chunk then
      local _ = log.error() and log.error("rpc", cmd, "stderr", chunk)
    end
  end)

  --@private
  local function on_error(errkind, ...)
    assert(client_errors[errkind])
    -- TODO what to do if this fails?
    pcall(handlers.on_error, errkind, ...)
  end
  --@private
  local function pcall_handler(errkind, status, head, ...)
    if not status then
      on_error(errkind, head, ...)
      return status, head
    end
    return status, head, ...
  end
  --@private
  local function try_call(errkind, fn, ...)
    return pcall_handler(errkind, pcall(fn, ...))
  end

  -- TODO periodically check message_callbacks for old requests past a certain
  -- time and log them. This would require storing the timestamp. I could call
  -- them with an error then, perhaps.

  --@private
  local function handle_body(body)
    local decoded, err = json_decode(body)
    if not decoded then
      on_error(client_errors.INVALID_SERVER_JSON, err)
      return
    end
    local _ = log.debug() and log.debug("decoded", decoded)

    if type(decoded.method) == 'string' and decoded.id then
      -- Server Request
      decoded.params = convert_NIL(decoded.params)
      -- Schedule here so that the users functions don't trigger an error and
      -- we can still use the result.
      schedule(function()
        local status, result
        status, result, err = try_call(client_errors.SERVER_REQUEST_HANDLER_ERROR,
            handlers.server_request, decoded.method, decoded.params)
        local _ = log.debug() and log.debug("server_request: callback result", { status = status, result = result, err = err })
        if status then
          if not (result or err) then
            -- TODO this can be a problem if `null` is sent for result. needs vim.NIL
            error(string.format("method %q: either a result or an error must be sent to the server in response", decoded.method))
          end
          if err then
            assert(type(err) == 'table', "err must be a table. Use rpc_response_error to help format errors.")
            local code_name = assert(protocol.ErrorCodes[err.code], "Errors must use protocol.ErrorCodes. Use rpc_response_error to help format errors.")
            err.message = err.message or code_name
          end
        else
          -- On an exception, result will contain the error message.
          err = rpc_response_error(protocol.ErrorCodes.InternalError, result)
          result = nil
        end
        send_response(decoded.id, err, result)
      end)
    -- This works because we are expecting vim.NIL here
    elseif decoded.id and (decoded.result or decoded.error) then
      -- Server Result
      decoded.error = convert_NIL(decoded.error)
      decoded.result = convert_NIL(decoded.result)

      -- Do not surface RequestCancelled to users, it is RPC-internal.
      if decoded.error
        and decoded.error.code == protocol.ErrorCodes.RequestCancelled then
        local _ = log.debug() and log.debug("Received cancellation ack", decoded)
        local result_id = tonumber(decoded.id)
        -- Clear any callback since this is cancelled now.
        -- This is safe to do assuming that these conditions hold:
        -- - The server will not send a result callback after this cancellation.
        -- - If the server sent this cancellation ACK after sending the result, the user of this RPC
        -- client will ignore the result themselves.
        if result_id then
          message_callbacks[result_id] = nil
        end
        return
      end

      -- We sent a number, so we expect a number.
      local result_id = tonumber(decoded.id)
      local callback = message_callbacks[result_id]
      if callback then
        message_callbacks[result_id] = nil
        validate {
          callback = { callback, 'f' };
        }
        if decoded.error then
          decoded.error = setmetatable(decoded.error, {
            __tostring = format_rpc_error;
          })
        end
        try_call(client_errors.SERVER_RESULT_CALLBACK_ERROR,
            callback, decoded.error, decoded.result)
      else
        on_error(client_errors.NO_RESULT_CALLBACK_FOUND, decoded)
        local _ = log.error() and log.error("No callback found for server response id "..result_id)
      end
    elseif type(decoded.method) == 'string' then
      -- Notification
      decoded.params = convert_NIL(decoded.params)
      try_call(client_errors.NOTIFICATION_HANDLER_ERROR,
          handlers.notification, decoded.method, decoded.params)
    else
      -- Invalid server message
      on_error(client_errors.INVALID_SERVER_MESSAGE, decoded)
    end
  end
  -- TODO(ashkan) remove this once we have a Lua json_decode
  handle_body = schedule_wrap(handle_body)

  local request_parser = coroutine.wrap(request_parser_loop)
  request_parser()
  stdout:read_start(function(err, chunk)
    if err then
      -- TODO better handling. Can these be intermittent errors?
      on_error(client_errors.READ_ERROR, err)
      return
    end
    -- This should signal that we are done reading from the client.
    if not chunk then return end
    -- Flush anything in the parser by looping until we don't get a result
    -- anymore.
    while true do
      local headers, body = request_parser(chunk)
      -- If we successfully parsed, then handle the response.
      if headers then
        handle_body(body)
        -- Set chunk to empty so that we can call request_parser to get
        -- anything existing in the parser to flush.
        chunk = ''
      else
        break
      end
    end
  end)

  return {
    pid = pid;
    handle = handle;
    request = request;
    notify = notify
  }
end

return {
  start = start;
  rpc_response_error = rpc_response_error;
  format_rpc_error = format_rpc_error;
  client_errors = client_errors;
}
-- vim:sw=2 ts=2 et