Nvim core source
================

Module-specific details are documented at the top of each module (`terminal.c`,
`screen.c`, ...).

See `:help development` for more guidelines.

Logs
----

Low-level log messages sink to `$NVIM_LOG_FILE`.

You can use `LOG_CALLSTACK();` anywhere in the source to log the current
stacktrace. To log in an alternate file, e.g. stderr, use
`LOG_CALLSTACK_TO_FILE(FILE*)`. (Currently Linux-only.)

UI events are logged at level 0 (`DEBUG_LOG_LEVEL`).

    rm -rf build/
    make CMAKE_EXTRA_FLAGS="-DMIN_LOG_LEVEL=0"

Filename conventions
--------------------

The source files use extensions to hint about their purpose.

- `*.c`, `*.generated.c` - full C files, with all includes, etc.
- `*.c.h` - parametrized C files, contain all necessary includes, but require
  defining macros before actually using. Example: `typval_encode.c.h`
- `*.h` - full headers, with all includes. Does *not* apply to `*.generated.h`.
- `*.h.generated.h` - exported functions’ declarations.
- `*.c.generated.h` - static functions’ declarations.

Nvim lifecycle
--------------

Following describes how Nvim processes input.

Consider a typical Vim-like editing session:

01. Vim dispays the welcome screen
02. User types: `:`
03. Vim enters command-line mode
04. User types: `edit README.txt<CR>`
05. Vim opens the file and returns to normal mode
06. User types: `G`
07. Vim navigates to the end of the file
09. User types: `5`
10. Vim enters count-pending mode
11. User types: `d`
12. Vim enters operator-pending mode
13. User types: `w`
14. Vim deletes 5 words
15. User types: `g`
16. Vim enters the "g command mode"
17. User types: `g`
18. Vim goes to the beginning of the file
19. User types: `i`
20. Vim enters insert mode
21. User types: `word<ESC>`
22. Vim inserts "word" at the beginning and returns to normal mode

Note that we split user actions into sequences of inputs that change the state
of the editor. While there's no documentation about a "g command mode" (step
16), internally it is implemented similarly to "operator-pending mode".

From this we can see that Vim has the behavior of an input-driven state machine
(more specifically, a pushdown automaton since it requires a stack for
transitioning back from states). Assuming each state has a callback responsible
for handling keys, this pseudocode represents the main program loop:

```py
def state_enter(state_callback, data):
  do
    key = readkey()                 # read a key from the user
  while state_callback(data, key)   # invoke the callback for the current state
```

That is, each state is entered by calling `state_enter` and passing a
state-specific callback and data. Here is a high-level pseudocode for a program
that implements something like the workflow described above:

```py
def main()
  state_enter(normal_state, {}):

def normal_state(data, key):
  if key == ':':
    state_enter(command_line_state, {})
  elif key == 'i':
    state_enter(insert_state, {})
  elif key == 'd':
    state_enter(delete_operator_state, {})
  elif key == 'g':
    state_enter(g_command_state, {})
  elif is_number(key):
    state_enter(get_operator_count_state, {'count': key})
  elif key == 'G'
    jump_to_eof()
  return true

def command_line_state(data, key):
  if key == '<cr>':
    if data['input']:
      execute_ex_command(data['input'])
    return false
  elif key == '<esc>'
    return false

  if not data['input']:
    data['input'] = ''

  data['input'] += key
  return true

def delete_operator_state(data, key):
  count = data['count'] or 1
  if key == 'w':
    delete_word(count)
  elif key == '$':
    delete_to_eol(count)
  return false  # return to normal mode

def g_command_state(data, key):
  if key == 'g':
    go_top()
  elif key == 'v':
    reselect()
  return false  # return to normal mode

def get_operator_count_state(data, key):
  if is_number(key):
    data['count'] += key
    return true
  unshift_key(key)  # return key to the input buffer
  state_enter(delete_operator_state, data)
  return false

def insert_state(data, key):
  if key == '<esc>':
    return false  # exit insert mode
  self_insert(key)
  return true
```

The above gives an idea of how Nvim is organized internally. Some states like
the `g_command_state` or `get_operator_count_state` do not have a dedicated
`state_enter` callback, but are implicitly embedded into other states (this
will change later as we continue the refactoring effort). To start reading the
actual code, here's the recommended order:

1. `state_enter()` function (state.c). This is the actual program loop,
   note that a `VimState` structure is used, which contains function pointers
   for the callback and state data.
2. `main()` function (main.c). After all startup, `normal_enter` is called
   at the end of function to enter normal mode.
3. `normal_enter()` function (normal.c) is a small wrapper for setting
   up the NormalState structure and calling `state_enter`.
4. `normal_check()` function (normal.c) is called before each iteration of
   normal mode.
5. `normal_execute()` function (normal.c) is called when a key is read in normal
   mode.

The basic structure described for normal mode in 3, 4 and 5 is used for other
modes managed by the `state_enter` loop:

- command-line mode: `command_line_{enter,check,execute}()`(`ex_getln.c`)
- insert mode: `insert_{enter,check,execute}()`(`edit.c`)
- terminal mode: `terminal_{enter,execute}()`(`terminal.c`)

Async event support
-------------------

One of the features Nvim added is the support for handling arbitrary
asynchronous events, which can include:

- RPC requests
- job control callbacks
- timers

Nvim implements this functionality by entering another event loop while
waiting for characters, so instead of:

```py
def state_enter(state_callback, data):
  do
    key = readkey()                 # read a key from the user
  while state_callback(data, key)   # invoke the callback for the current state
```

Nvim program loop is more like:

```py
def state_enter(state_callback, data):
  do
    event = read_next_event()       # read an event from the operating system
  while state_callback(data, event) # invoke the callback for the current state
```

where `event` is something the operating system delivers to us, including (but
not limited to) user input. The `read_next_event()` part is internally
implemented by libuv, the platform layer used by Nvim.

Since Nvim inherited its code from Vim, the states are not prepared to receive
"arbitrary events", so we use a special key to represent those (When a state
receives an "arbitrary event", it normally doesn't do anything other update the
screen).