pr-description-873.md

Optimize get_check_in_history_page to Avoid Full Deserialization

Summary

Changed check-in history storage from a single Vec<CheckInHistoryEntry> blob under DataKey::CheckInHistory(vault_id) to individual entries stored under DataKey::CheckInEntry(vault_id, index) with a ring buffer approach for O(1) writes.

Changes

Storage Layout

• Added DataKey::CheckInEntry(u64, u32) — stores a single CheckInHistoryEntry
• Added DataKey::CheckInHistoryHead(u64) — ring buffer head pointer (only meaningful at 50 entries)
• Added DataKey::CheckInHistoryLen(u64) — total number of entries stored (capped at 50)
• Old DataKey::CheckInHistory(u64) key is deprecated but remains in the enum to avoid orphaned data

record_check_in_history (write path)

• No longer deserializes the full Vec
• Uses a ring buffer: entries are stored at indices 0–49, wrapping around when full
• O(1) write cost (single entry write + two metadata writes)

get_check_in_history (read path, backward compatible)

• Reimplemented to read individual entries in chronological order
• Still returns the full history when needed

get_check_in_history_page (new, paginated)

• Only deserializes the entries for the requested page
• Parameters: vault_id, page, page_size
• Returns empty vec if page is out of bounds
• Entries returned in chronological order (oldest first)

predict_expiry (optimized)

• No longer loads all 50 entries; only reads the first and last entry in the sample window (up to 10 entries)

Performance

• Write: O(1) entry storage instead of O(50) serialization of full Vec
• Read (paginated): Only deserializes page_size entries instead of all 50
• predict_expiry: Only reads 2 entries instead of all 50

Testing

• Added 5 new tests covering:
  • First page returns correct number of entries
  • Second page returns remaining entries
  • Out-of-bounds page returns empty
  • Full 50-entry ring buffer pagination
  • Entries are in chronological order

Closes #873