#+TITLE: Terminal Capability Detection (v0.12.0)
#+DATE: 2026-05-11
#+AUTHOR: Amr Gharbeia / Hermes
#+STARTUP: content

* Overview

Currently, users must manually choose between ~modern-backend~ and
~simple-backend~ when initializing cl-tty. This module adds auto-detection:

1. Check if stdout is a real TTY (not piped/redirected)
2. Check the =COLORTERM= environment variable for truecolor support
3. Optionally query the terminal via DA1/DA3 escape sequences
4. Return the appropriate backend, cached for subsequent calls

Detection is best-effort: the COLORTERM env var is the most reliable single
signal. DA1 queries are asynchronous and many terminals don't respond.
If detection can't determine modern capability, it falls back to
~simple-backend~.

** Contract

- ~detect-backend~ → ~modern-backend~ or ~simple-backend~
  Auto-detect and return the appropriate backend. Results are cached
  in ~*detected-backend*~.

- ~detect-backend-by-env~ → ~:modern~ or ~nil~
  Check =COLORTERM= env var for ~truecolor~ or ~24bit~.

- ~detect-backend-by-tty~ → boolean
  Check if stdout is a real terminal (not a pipe).

- ~detect-backend-by-da1~ → boolean
  Send DA1 (~ESC[c~) query and check for modern feature responses.

- ~*detected-backend*~ — variable
  Cache for detection result. ~nil~ = not yet detected.

* Plan

See =docs/plans/2026-05-11-terminal-detection.md= for implementation tasks.

1. Create ~detection.lisp~ with all detection functions
2. Wire into ASDF
3. Update ~demo.lisp~ to use ~detect-backend~
4. Tangle, test, commit

* Tests

#+BEGIN_SRC lisp :tangle no
;; Tests are manually added to backend/tests.lisp
(def-test detection-returns-backend-instance ()
  (let ((be (cl-tty.backend:detect-backend)))
    (is-true (typep be 'cl-tty.backend:backend))))

(def-test detection-caches-result ()
  (let ((*detected-backend* nil))
    (cl-tty.backend:detect-backend)
    (is-true (not (null cl-tty.backend::*detected-backend*)))))
#+END_SRC

* Implementation

** Package

Detection functions are added to the existing ~cl-tty.backend~ package.
No new package definition needed.

** Environment probe

Check ~COLORTERM~ first — it's the simplest and most reliable signal.

#+BEGIN_SRC lisp :tangle ../backend/detection.lisp
(in-package :cl-tty.backend)

;;; ─── Detection cache ────────────────────────────────────────────────────────

(defvar *detected-backend* nil
  "Cached backend instance from detect-backend. Nil = not yet detected.")

;;; ─── Environment probe ──────────────────────────────────────────────────────

(defun detect-backend-by-env ()
  "Check COLORTERM environment variable for modern terminal support.
Returns :modern if COLORTERM contains 'truecolor' or '24bit', nil otherwise."
  (let ((colorterm (sb-ext:posix-getenv "COLORTERM")))
    (when (and colorterm
               (or (search "truecolor" colorterm :test #'char-equal)
                   (search "24bit" colorterm :test #'char-equal)))
      :modern)))
#+END_SRC

** TTY probe

Check if stdout is connected to a terminal (not a pipe or file).

#+BEGIN_SRC lisp :tangle ../backend/detection.lisp
;;; ─── TTY probe ──────────────────────────────────────────────────────────────

(defun detect-backend-by-tty ()
  "Check if stdout is a real terminal (not a pipe/redirect).
Returns T if stdout is interactive, nil otherwise."
  (interactive-stream-p *standard-output*))
#+END_SRC

** DA1 terminal query (best-effort)

Send a DA1 (Device Attributes) query and briefly listen for a response.
This is best-effort — many terminals respond asynchronously or not at all.

#+BEGIN_SRC lisp :tangle ../backend/detection.lisp
;;; ─── DA1 terminal query ─────────────────────────────────────────────────────

(defun query-terminal (query &optional (timeout 0.1))
  "Send QUERY string to terminal and return any response received within
TIMEOUT seconds. Returns the response string, or nil if no response."
  (write-string query *query-io*)
  (force-output *query-io*)
  (sleep timeout)
  (let ((response (make-array 0 :element-type 'character
                              :fill-pointer 0 :adjustable t)))
    (loop while (listen *query-io*)
          do (vector-push-extend (read-char-no-hang *query-io*) response))
    (when (plusp (length response))
      response)))

(defun detect-backend-by-da1 ()
  "Send DA1 (ESC[c) query and check for kitty terminal response code.
Returns T if terminal reports kitty compatibility codes."
  (let ((response (query-terminal (format nil "~C[c" #\Esc))))
    (when response
      ;; DA1 response format: ESC [ ? digits ; digits c
      ;; Kitty reports code 62 in the response
      (search "?62" response))))
#+END_SRC

** Orchestrator

Tie all probes together into ~detect-backend~.

#+BEGIN_SRC lisp :tangle ../backend/detection.lisp
;;; ─── Orchestrator ───────────────────────────────────────────────────────────

(defun detect-backend ()
  "Auto-detect the appropriate backend for the current terminal.
Returns a backend instance (modern-backend or simple-backend).
Result is cached in *detected-backend* for subsequent calls."
  (or *detected-backend*
      (setf *detected-backend*
            (if (and (detect-backend-by-tty)
                     (or (eql (detect-backend-by-env) :modern)
                         (detect-backend-by-da1)))
                (make-modern-backend)
                (make-simple-backend)))))
#+END_SRC