(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)))

;;; ─── 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*))

;;; ─── 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))))

;;; ─── 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)))))