#+TITLE: cl-tui Architecture
#+STARTUP: content
#+FILETAGS: :project:cl-tui:architecture:

* Architecture

cl-tui is a layered framework. Each layer has a single responsibility
and communicates with adjacent layers through a well-defined protocol.

** Layer Diagram

#+BEGIN_SRC
  Application Code (user's CL project)
  ┌───────────────────────────────────────────────┐
  │  Component Tree                               │
  │  (user constructs via macros: vbox, hbox,     │
  │   text, box, select, markdown, etc.)          │
  └──────────────┬────────────────────────────────┘
                 │ defgeneric render (component backend)
                 │ defgeneric handle-key (component event)
                 │ defgeneric handle-mouse (component event)
                 ▼
  ┌───────────────────────────────────────────────┐
  │  Rendering Pipeline                           │
  │  1. Layout pass (constraint solve)            │
  │  2. Dirty walk (only changed branches)        │
  │  3. Render commands (component → cmds)        │
  │  4. Framebuffer diff (changed cells only)     │
  └──────────────┬────────────────────────────────┘
                 │ Render commands:
                 │   (:box x y w h style)
                 │   (:text x y str fg bg attrs)
                 │   (:rect x y w h ch)
                 ▼
  ┌───────────────────────────────────────────────┐
  │  Backend Protocol                             │
  │  ┌─────────────┐  ┌─────────────────────────┐ │
  │  │ modern      │  │ simple                  │ │
  │  │ truecolor   │  │ ASCII borders           │ │
  │  │ rounded     │  │ no color                │ │
  │  │ OSC 8 links │  │ universal compatibility │ │
  │  │ DECICM sync │  │ SSH-safe                │ │
  │  │ kitty proto │  │ pipe-safe               │ │
  │  └─────────────┘  └─────────────────────────┘ │
  └───────────────────────────────────────────────┘
#+END_SRC

** The Backend Protocol

The backend protocol is the central abstraction. Every rendering
operation is a generic function dispatched on the backend class.

*** Backend Classes

- =modern-backend= — raw escape sequences, truecolor, modern features
- =simple-backend= — ASCII art, no color, universal compatibility
- =backend= — abstract base (both inherit from this)

Backend selection happens once at startup, via terminal capability
detection. The same component tree renders correctly on both.

*** Backend Generic Functions

#+BEGIN_SRC
  ;; ── Lifecycle ──
  (initialize-backend backend)        → setup terminal, enable features
  (shutdown-backend backend)          → restore terminal, cleanup
  (suspend-backend backend)           → temporary suspend (SIGTSTP)
  (resume-backend backend)            → re-initialize after resume

  ;; ── Output ──
  (backend-size backend)              → (values columns lines)
  (backend-write backend string)      → raw output to terminal
  (begin-sync backend)                → start synchronized update
  (end-sync backend)                  → flush synchronized update
  (backend-clear backend)             → clear entire screen

  ;; ── Drawing primitives ──
  (draw-rect backend x y w h ch style)           → fill rectangle
  (draw-text backend x y str fg bg attrs)        → render text at position
  (draw-border backend x y w h style attrs)      → draw border rectangle
  (draw-ellipsis backend x y w)                  → truncated text marker
  (draw-link backend x y str url fg bg attrs)    → OSC 8 hyperlink

  ;; ── Cursor ──
  (cursor-move backend x y)            → position cursor
  (cursor-hide backend)                → hide cursor
  (cursor-show backend)                → show cursor
  (cursor-style backend :bar|:block|:underline &optional blink)

  ;; ── Input ──
  (read-event backend)                 → (values event-type event-data)
  (enable-mouse backend)               → enable SGR mouse reporting
  (enable-bracketed-paste backend)     → enable paste detection
  (set-keyboard-mode backend :kitty|:default)

  ;; ── Capability queries ──
  (capable-p backend :truecolor|:osc8|:kitty-keyboard|:sync|:sixel|:mouse)
#+END_SRC

*** Style structure

All drawing functions accept a =style= plist that is resolved through
the theme engine before reaching the backend:

#+BEGIN_SRC
  (:fg :error                     ; semantic role from theme
   :bg :background-panel          ; resolved to hex by theme
   :bold t
   :italic nil
   :underline nil
   :blink nil
   :reverse nil
   :dim nil
   :hyperlink-url nil)            ; OSC 8 URL if set
#+END_SRC

The backend receives resolved hex colors, not semantic roles. Theme
resolution happens in the pipeline layer, before backend dispatch.

*** Backend Selection

At startup:

#+BEGIN_SRC
  1. Check if stdout is a TTY (if not → simple-backend)
  2. Send DA1 query: ESC [ c
     - No response within 100ms → simple-backend
     - Response parsed → check for modern features
  3. Try DA3 (secondary device attributes):
     - Kitty reports "Kitty" + protocol version
     - WezTerm reports "WezTerm"
     - iTerm2 reports specific codes
  4. Query DECRPM for DECICM sync:
     - ESC [?2026$p
     - Response ESC [?2026;1$y = supported
  5. If sync + truecolor + kitty keyboard → modern-backend
     Otherwise → simple-backend
#+END_SRC

** Layout Engine

The layout engine is pure Common Lisp — no Yoga FFI, no C dependencies.

*** Constraint Model

A terminal has ~200x80 cells. The constraint solver only needs to
handle one-dimensional layout in two passes:

1. **Column direction (vertical pass):** distribute Y positions, sum
   children heights. Width is inherited from parent (minus padding).
2. **Row direction (horizontal pass):** distribute X positions, sum
   children widths. Height is inherited from parent.

Flex properties:
- =:grow= — proportional distribution of remaining space
- =:shrink= — proportional reduction when content overflows
- =:basis= — initial size before grow/shrink
- =:wrap= — overflow moves to next row/column
- =:gap= — spacing between children

Position properties:
- =:relative= — normal flow (default)
- =:absolute= — positioned relative to parent's content box
- =:top=, =:right=, =:bottom=, =:left= — offset for absolute

This is a subset of CSS Flexbox. Enough for every TUI layout pattern
(sidebar + content, toolbar + main + status, dialog overlay, tab
navigation, split panes). ~200 lines.

*** Layout Node

#+BEGIN_SRC
  (defclass layout-node ()
    ;; Computed by solver
    (x y width height             ; computed position + size
     children                     ; list of child layout-nodes
     parent                       ; parent layout-node (or nil for root)
     ;; Style input
     direction                    ; :row | :column | :row-reverse | :column-reverse
     wrap                         ; :nowrap | :wrap | :wrap-reverse
     grow shrink basis            ; flex sizing
     align-self align-items       ; cross-axis alignment
     justify-content              ; main-axis distribution
     padding margin border        ; box model
     gap                          ; spacing between children
     position-type                ; :relative | :absolute
     position-offset))            ; top/left for absolute
#+END_SRC

*** Composable API

#+BEGIN_SRC
  (vbox (:gap 1 :padding 1)
    (header "Title")
    (hbox (:grow 1)
      (sidebar (:width 30) ...)
      (content ...)))
#+END_SRC

Macros expand to layout-node construction + child wiring.

** Component Tree

Components are CLOS objects. Every component has a =layout-node=
slot that drives positioning. Components define =render= methods.

*** Base Component Class

#+BEGIN_SRC
  (defclass component ()
    (layout-node         ; layout-node for this component
     parent              ; parent component (or nil for root)
     children            ; list of child components
     dirty               ; t/nil — needs re-render
     theme               ; theme reference
     visible))           ; t/nil
#+END_SRC

*** Generic Functions

- =(render component backend)= — returns list of render commands
- =(handle-key component event)= — returns t if consumed
- =(handle-mouse component event)= — returns t if consumed
- =(measure component max-width max-height)= — returns (values w h)
- =(children component)= — returns list of child components
- =(find-focused component)= — returns the focused child (or nil)

*** Rendering Pipeline

#+BEGIN_SRC
  1. (propagate-dirty root)     → mark ancestors dirty
  2. (compute-layout root w h)  → pure CL constraint solve
  3. (collect-commands root)     → walk dirty branches, call render
  4. (diff-framebuffer prev curr) → emit only changed cells
  5. (begin-sync backend)       → DECICM start
  6. (flush-commands backend)   → write escape sequences
  7. (end-sync backend)         → DECICM end
  8. (clear-dirty root)         → mark all clean
#+END_SRC

** Input Handling

Input goes through a layered keybinding system:

1. Terminal emits escape sequences → parser converts to events
2. Events dispatched through layers: =:global= → =:local= → =:focused=
3. Focused component's =handle-key= fires first
4. Unconsumed events bubble to =:local= keymap, then =:global=
5. Modal layers (dialog) intercept before global

Mouse events follow the same path, with hit-testing routing to the
deepest component containing the click coordinates.

** Theme Engine

Semantic tokens → hex colors → backend color pairs. No code references
hex values directly. =:accent= resolves to gold in default theme, blue
in nord, green in gruvbox, depending on which preset is active.

Presets define both =:dark= and =:light= variants. Auto-detection
reads terminal background color at startup.

** File Structure

#+BEGIN_SRC
  cl-tui/
  ├── cl-tui.asd
  ├── cl-tui-tests.asd
  ├── README.org
  ├── LICENSE
  ├── docs/
  │   ├── ROADMAP.org
  │   └── ARCHITECTURE.org          ← this file
  ├── src/
  │   ├── package.lisp
  │   ├── backend/
  │   │   ├── protocol.lisp
  │   │   ├── detection.lisp
  │   │   ├── simple.lisp
  │   │   └── modern.lisp
  │   ├── layout/
  │   │   ├── nodes.lisp
  │   │   ├── solver.lisp
  │   │   └── api.lisp
  │   ├── components/
  │   │   ├── base.lisp
  │   │   ├── box.lisp
  │   │   └── text.lisp
  │   ├── rendering/
  │   │   ├── pipeline.lisp
  │   │   ├── dirty.lisp
  │   │   └── diff.lisp
  │   └── theme/
  │       ├── tokens.lisp
  │       └── presets.lisp
  └── tests/
      ├── package.lisp
      ├── backend.lisp
      ├── layout.lisp
      └── components.lisp
#+END_SRC

** Dependency Graph

  backend/     (no deps)
  layout/      (no deps — pure math)
  theme/       (backend for color resolution)
  components/  (layout, theme, rendering)
  rendering/   (layout, components, backend, theme)
  input/       (backend for raw events)

Init order:
  1. Backend — detect, initialize
  2. Theme — load default preset
  3. Layout — construct component tree
  4. Render — layout → commands → flush
  5. Input — event loop (blocks on read-event)