Crosspoint/docs/contributing/architecture.md

Architecture Overview

CrossPoint is firmware for the Xteink X4 (unaffiliated with Xteink), built with PlatformIO targeting the ESP32-C3 microcontroller.

At a high level, it is firmware that uses an activity-driven application architecture loop with persistent settings/state, SD-card-first caching, and a rendering pipeline optimized for e-ink constraints.

System at a glance

graph TD
    A[Hardware: ESP32-C3 + SD + E-ink + Buttons] --> B[open-x4-sdk]
    B --> C[lib/hal wrappers]
    C --> D[src/main.cpp runtime loop]
    D --> E[Activities layer]
    D --> F[State and settings]
    E --> G[Reader flows]
    E --> H[Home/Library/Settings flows]
    E --> I[Network/Web server flows]
    G --> J[lib/Epub parsing + layout + hyphenation]
    J --> K[SD cache in .crosspoint]
    E --> L[GfxRenderer]
    L --> M[E-ink display buffer]

Runtime lifecycle

Primary entry point is src/main.cpp.

flowchart TD
    A[Boot] --> B[Init GPIO and optional serial]
    B --> C[Init SD storage]
    C --> D[Load settings and app state]
    D --> E[Init display and fonts]
    E --> F{Resume reader?}
    F -->|No| G[Enter Home activity]
    F -->|Yes| H[Enter Reader activity]
    G --> I[Main loop]
    H --> I
    I --> J[Poll input and run current activity]
    J --> K{Sleep condition met?}
    K -->|No| I
    K -->|Yes| L[Persist state and enter deep sleep]

In each loop iteration, the firmware updates input, runs the active activity, handles auto-sleep/power behavior, and applies a short delay policy to balance responsiveness and power.

Activity model

Activities are screen-level controllers deriving from src/activities/Activity.h. Some flows use src/activities/ActivityWithSubactivity.h to host nested activities.

• onEnter() and onExit() manage setup/teardown
• loop() handles per-frame behavior
• skipLoopDelay() and preventAutoSleep() are used by long-running flows (for example web server mode)

Top-level activity groups:

• src/activities/home/: home and library navigation
• src/activities/reader/: EPUB/XTC/TXT reading flows
• src/activities/settings/: settings menus and configuration
• src/activities/network/: Wi-Fi selection, AP/STA mode, file transfer server
• src/activities/boot_sleep/: boot and sleep transitions

Reader and content pipeline

Reader orchestration starts in src/activities/reader/ReaderActivity.h and dispatches to format-specific readers. EPUB processing is implemented in lib/Epub/.

flowchart LR
    A[Select book] --> B[ReaderActivity]
    B --> C{Format}
    C -->|EPUB| D[lib/Epub/Epub]
    C -->|XTC| E[lib/Xtc reader]
    C -->|TXT| F[lib/Txt reader]
    D --> G[Parse OPF/TOC and collect CSS refs]
    G --> H[Build/load book.bin and css_rules.cache]
    H --> I[Layout pages/sections]
    I --> J[Write section cache]
    J --> K[Render current page via GfxRenderer]

Why caching matters:

• RAM is limited on ESP32-C3, so expensive parsed/layout data is persisted to SD
• repeat opens/page navigation can reuse cached data instead of full reparsing

Reader internals call graph

This diagram zooms into the EPUB path to show the main control and data flow from activity entry to on-screen draw.

flowchart TD
    A[ReaderActivity onEnter] --> B{File type}
    B -->|EPUB| C[Create Epub object]
    B -->|XTC/TXT| Z[Use format-specific reader]

    C --> D[Epub load]
    D --> E[Locate container and OPF]
    E --> F[Build or load BookMetadataCache]
    F --> G[Load TOC and spine]
    G --> H[Load CSS cache or parse manifest/base-dir CSS]

    H --> I[EpubReaderActivity]
    I --> J{Section cache exists for current settings?}
    J -->|Yes| K[Read section bin from SD cache]
    J -->|No| L[Parse chapter HTML and layout text]
    L --> M[Apply typography settings and hyphenation]
    M --> N[Write section cache bin]

    K --> O[Build page model]
    N --> O
    O --> P[GfxRenderer draw calls]
    P --> Q[HAL display framebuffer update]
    Q --> R[E-ink refresh policy]

    S[SETTINGS singleton] -. influences .-> J
    S -. influences .-> M
    T[APP_STATE singleton] -. persists .-> U[Reading progress and resume context]
    U -. used by .-> I

Notes:

• CSS files are collected from the OPF manifest and, when needed, discovered by streaming ZIP paths under the OPF content base directory; the firmware avoids preloading the full ZIP central directory for large books.
• "section cache exists" depends on cache-busting parameters such as font, viewport size, paragraph alignment, hyphenation, embedded CSS, image rendering, and Focus Reading settings
• rendering favors reusing precomputed layout data to keep page turns responsive on constrained hardware
• progress/session state is persisted so the reader can reopen at the last position after reboot/sleep

State and persistence

Two singletons are central:

• src/CrossPointSettings.h (SETTINGS): user preferences and behavior flags
• src/CrossPointState.h (APP_STATE): runtime/session state such as current book and sleep context

Typical persisted areas on SD:

/.crosspoint/
  epub_<hash>/
    book.bin
    css_rules.cache
    progress.bin
    cover.bmp
    sections/*.bin
    img_* cache files
  settings.json
  state.json

sections/*.bin contains rendered pages plus anchor, paragraph, and list-item lookup tables used for TOC/footnote jumps and KOReader sync refinement. For binary cache formats, see docs/file-formats.md.

Networking architecture

Network file transfer is controlled by src/activities/network/CrossPointWebServerActivity.h and served by src/network/CrossPointWebServer.h.

Modes:

• STA: join existing Wi-Fi network
• AP: create hotspot
• Calibre Wireless: STA flow specialized for Calibre plugin uploads

Server behavior:

• HTTP server on port 80
• WebSocket upload server on port 81
• WebDAV handler on the HTTP server
• UDP discovery listener for upload clients
• file operations backed by SD storage
• browser APIs for file management, settings, fonts, OPDS servers, and saved Wi-Fi networks
• activity requests faster loop responsiveness while server is running

Endpoint reference: docs/webserver-endpoints.md.

Build-time generated assets

Some sources are generated and should not be edited manually.

• scripts/build_html.py generates src/network/html/*.generated.h from HTML files
• scripts/gen_i18n.py generates lib/I18n/I18nKeys.h, I18nStrings.h, and I18nStrings.cpp
• scripts/generate_hyphenation_trie.py generates hyphenation headers under lib/Epub/Epub/hyphenation/generated/

When editing related source assets, regenerate via normal build steps/scripts.

Key directories

• src/: app orchestration, settings/state, and activity implementations
• src/network/: web server and OTA/update networking
• src/components/: theming and shared UI components
• lib/hal/: hardware abstraction wrappers around open-x4-sdk
• lib/Epub/: EPUB parser, layout, CSS handling, and hyphenation
• lib/: supporting libraries (fonts, text, filesystem helpers, etc.)
• open-x4-sdk/: hardware SDK submodule (display, input, storage, battery)
• docs/: user and technical documentation

Embedded constraints that shape design

• constrained RAM drives SD-first caching and careful allocations
• e-ink refresh cost drives render/update batching choices
• main loop responsiveness matters for input, power handling, and watchdog safety
• background/network flows must cooperate with sleep and loop timing logic

Scope guardrails

Before implementing larger ideas, check:

• SCOPE.md
• GOVERNANCE.md