--- Line-delimited JSON-RPC client over hephd's unix socket (tech-spec §6).
---
--- The daemon speaks one JSON object per line: a request `{id, method, params}`
--- gets exactly one response line `{id, result}` xor `{id, error}`. We talk to
--- it over a libuv pipe and expose a **blocking** `call()` by pumping the event
--- loop with `vim.wait` until the matching id returns — synchronous ergonomics
--- over an async transport, which is what every surface call and the e2e tests
--- want.
---
--- A `Session` is one connection; the module keeps a default singleton for the
--- plugin and lets tests open isolated sessions (`new_session`) so an assertion
--- never shares state with the buffer under test.

local uv = vim.uv or vim.loop

local Session = {}
Session.__index = Session

--- Create an unconnected session bound to `socket_path` (lazy connect).
function Session.new(socket_path)
  return setmetatable({
    socket_path = socket_path,
    pipe = nil,
    buf = "", -- partial-line accumulator
    pending = {}, -- [id] = { done, result, err }
    next_id = 0,
    connected = false,
  }, Session)
end

--- Drain complete `\n`-terminated lines out of the read buffer. Runs in the
--- libuv fast-event context: string/table ops only, never `vim.api`/`vim.fn`.
function Session:_on_bytes(chunk)
  self.buf = self.buf .. chunk
  while true do
    local nl = self.buf:find("\n", 1, true)
    if not nl then
      break
    end
    local line = self.buf:sub(1, nl - 1)
    self.buf = self.buf:sub(nl + 1)
    if #line > 0 then
      self:_dispatch(line)
    end
  end
end

--- Match one response line to its pending call by id. A line with no id is a
--- server notification (tech-spec §6, slice 11d) — ignored for now.
function Session:_dispatch(line)
  -- `luanil` decodes JSON null to Lua nil (not the vim.NIL sentinel), so a
  -- `null` result / nullable field reads as a plain absent value.
  local ok, msg = pcall(vim.json.decode, line, { luanil = { object = true, array = true } })
  if not ok or type(msg) ~= "table" or msg.id == nil then
    return
  end
  local slot = self.pending[msg.id]
  if not slot then
    return
  end
  if msg.error ~= nil then
    slot.err = string.format("rpc error %s: %s", tostring(msg.error.code), tostring(msg.error.message))
  else
    slot.result = msg.result
  end
  slot.done = true
end

--- Fail every outstanding call so blocked `vim.wait`s unblock immediately
--- rather than each waiting out its full timeout. Safe to call from the read
--- callback (fast-event context): only touches tables and `vim.schedule`.
function Session:_fail_all(reason)
  self.connected = false
  for _, slot in pairs(self.pending) do
    if not slot.done then
      slot.err = reason
      slot.done = true
    end
  end
  local pipe = self.pipe
  self.pipe = nil
  if pipe then
    vim.schedule(function()
      pcall(function()
        pipe:close()
      end)
    end)
  end
end

--- Connect (idempotent). Blocks until the connect callback fires.
function Session:_ensure()
  if self.connected then
    return
  end
  assert(self.socket_path, "heph: no socket configured (call require('heph').setup{ socket = ... })")
  local pipe = uv.new_pipe(false)
  local done, cerr = false, nil
  pipe:connect(self.socket_path, function(e)
    cerr = e
    done = true
  end)
  if not vim.wait(5000, function()
    return done
  end, 10) then
    pcall(function()
      pipe:close()
    end)
    error("heph: timed out connecting to hephd at " .. self.socket_path)
  end
  if cerr then
    pcall(function()
      pipe:close()
    end)
    error("heph: cannot connect to hephd at " .. self.socket_path .. ": " .. cerr)
  end
  self.pipe = pipe
  self.buf = ""
  self.connected = true
  pipe:read_start(function(rerr, chunk)
    if rerr then
      self:_fail_all("connection error: " .. rerr)
    elseif chunk == nil then
      self:_fail_all("hephd closed the connection")
    else
      self:_on_bytes(chunk)
    end
  end)
end

--- Call `method` with `params`, blocking until the response. Raises a Lua error
--- on an rpc error or timeout. `opts.timeout` defaults to 5000ms.
function Session:call(method, params, opts)
  opts = opts or {}
  self:_ensure()
  self.next_id = self.next_id + 1
  local id = self.next_id
  local slot = { done = false }
  self.pending[id] = slot

  -- Empty params must serialize as `{}`, not `[]` (the daemon parses an object).
  if params == nil or (type(params) == "table" and vim.tbl_isempty(params)) then
    params = vim.empty_dict()
  end
  local line = vim.json.encode({ id = id, method = method, params = params }) .. "\n"
  self.pipe:write(line)

  local ok = vim.wait(opts.timeout or 5000, function()
    return slot.done
  end, 5)
  self.pending[id] = nil
  if not ok then
    error("heph: rpc timeout calling " .. method)
  end
  if slot.err then
    error("heph: " .. slot.err)
  end
  return slot.result
end

--- Close the connection, failing any in-flight calls.
function Session:close()
  self:_fail_all("connection closed")
end

local M = { Session = Session }

--- (Re)bind the default singleton session to `socket_path`.
function M.setup(socket_path)
  if M._default then
    M._default:close()
  end
  M._default = Session.new(socket_path)
  return M._default
end

--- The default singleton session (created unconnected if absent).
function M.session()
  if not M._default then
    M._default = Session.new(nil)
  end
  return M._default
end

--- Register a hook that (re)ensures the daemon — called once to self-heal a
--- dropped connection before a single retry. `nil` disables self-heal (used when
--- autostart is off, so a connect-only setup fails loudly instead of respawning).
function M.set_respawn(fn)
  M._respawn = fn
end

local function is_connection_error(msg)
  msg = tostring(msg)
  return msg:find("connect", 1, true) ~= nil
    or msg:find("connection", 1, true) ~= nil
    or msg:find("timeout", 1, true) ~= nil
end

--- Blocking call on the default session. If the call fails because the
--- connection is dead and a respawn hook is set, ensure the daemon and retry
--- once (the prior owner releases the DB lock on exit, so a respawn can claim it).
function M.call(method, params, opts)
  local ok, result = pcall(M.session().call, M.session(), method, params, opts)
  if ok then
    return result
  end
  if M._respawn and is_connection_error(result) then
    pcall(M._respawn)
    M.session():close() -- drop the dead connection so the retry reconnects
    return M.session():call(method, params, opts)
  end
  error(result)
end

--- An isolated session for a socket — used by tests for independent assertions.
function M.new_session(socket_path)
  return Session.new(socket_path)
end

--- Close the default session.
function M.close()
  if M._default then
    M._default:close()
  end
end

return M