Crosspoint/docs/contributing/koreader-synchronization.md

KOReader Synchronization Architecture

This document explains the intent and internal structure of the KOReader synchronization code in CrossPoint.

Scope:

• Synchronization logic that maps between CrossPoint reading position and KOReader sync payloads.
• Module boundaries and responsibilities.
• Matching rules, fallback strategy, and expected behavior.

For XPath-specific details and examples, see koreader-sync-xpath-mapping.md.

Goals

The synchronization layer is designed to:

• Be robust on constrained devices (ESP32-C3 memory constraints).
• Be deterministic and debuggable when mapping positions.
• Keep transport/client logic separated from parsing/mapping logic.
• Prefer precise anchors when available, but degrade gracefully.

Data Model Mismatch

CrossPoint stores position as chapter/page-centric state. KOReader sync payload stores position as XPath-like anchor plus percentage.

Because layout engines differ, page equality cannot be guaranteed across devices. The synchronization strategy therefore combines:

• Structural anchor mapping (XPath).
• Percent-based fallback.
• Paragraph LUT refinement when available.

Module Responsibilities

Client / orchestration

• lib/KOReaderSync/KOReaderSyncClient.cpp

  • HTTP calls and payload exchange.

• lib/KOReaderSync/ProgressMapper.cpp

  • High-level mapping from app state to KOReader payload and back.
  • Chooses XPath path or percentage fallback.

XPath indexing facade

• lib/KOReaderSync/ChapterXPathIndexer.h
• lib/KOReaderSync/ChapterXPathIndexer.cpp
  • Public API consumed by ProgressMapper.
  • Thin facade over forward/reverse mapper internals.
  • Utility extraction helpers (DocFragment index, paragraph index).

Forward mapping engine

• lib/KOReaderSync/ChapterXPathForwardMapper.cpp
  • Maps intra-spine progress to XPath.
  • Emits /text()[N].M for body-level text-node locations.

Reverse mapping engine

• lib/KOReaderSync/ChapterXPathReverseMapper.cpp
  • Maps XPath to intra-spine progress.
  • Supports exact and tolerant matching tiers.
  • Handles /text()[N].M codepoint offsets.

Shared parser/state/utilities

• lib/KOReaderSync/ChapterXPathIndexerInternal.cpp

• lib/KOReaderSync/ChapterXPathIndexerInternal.h

  • UTF-8 helpers, XPath normalization, parse runner, and chapter text-byte counting.

• lib/KOReaderSync/ChapterXPathIndexerState.h

  • Shared stack model and generic Expat callback adapters.
  • Common parser code pattern used by both forward/reverse engines.

Core Logic

Forward (CrossPoint -> KOReader)

1. Decompress one spine XHTML to a temporary file.
2. Count total visible text bytes.
3. Cache that total per spine (cache-path + spine index + href) so repeated mappings for the same chapter can skip the expensive counting pass.
4. Convert intra-spine progress to target visible-byte offset.
5. Stream parse and stop at target.
6. Emit anchor:
   • element XPath, or
   • /text()[N].M when in body-level text-node context.

Reverse (KOReader -> CrossPoint)

1. Decompress one spine XHTML to a temporary file.
2. Stream parse chapter while evaluating candidate matches.
3. Resolve best tier in this order:
   • exact
   • exact-no-index
   • ancestor
   • ancestor-no-index
4. Convert resolved byte offset to intra-spine progress.

For text-node anchors /text()[N].M:

• N is treated as 1-based text node index.
• M is treated as 0-based codepoint offset.

Fallback Strategy

When XPath mapping fails or is ambiguous:

• Fall back to percentage-driven chapter/page estimation.
• Use paragraph LUT refinement where available.

This guarantees user progress continuity even for malformed or sparse content.

Constraints and Non-Goals

• No full DOM materialization for entire books.
• Parse only one spine item on demand.
• Keep memory usage bounded and transient.
• Do not attempt pixel-perfect page parity with KOReader.