Implement deterministic referral code generation
epic-membership-recruitment-core-services-task-002 — Implement the deterministic, URL-safe code generation algorithm in ReferralCodeService. The algorithm must produce one unique code per peer mentor per organisation, be reproducible given the same mentor and org identifiers, use a character set safe for deep-link URLs (alphanumeric, no ambiguous characters), and be short enough for display (8–12 chars). Include collision detection via the repository before persisting.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 1 - 540 tasks
Can start after Tier 0 completes
Implementation Notes
Recommended algorithm: HMAC-SHA256(key=orgSecret, message=mentorId+orgId), take first 8 bytes of digest, encode with base32-crockford alphabet (removes ambiguous chars automatically, URL-safe). This gives determinism, unguessability, and a clean character set in one step. orgSecret should be fetched from the repository (stored per organisation in Supabase, never in the app binary). For collision handling: append a 1-char numeric suffix (code + '2', '3', ...) and re-hash or simply append — document the strategy.
Store the generated code via ReferralCodeRepository immediately after generation so subsequent calls hit the early-exit path. Use Dart's crypto package (dart:convert + crypto) which is already available in Flutter projects.
Testing Requirements
Unit tests: assert same output for same inputs across 100 iterations. Assert character set validity with regex. Assert length bounds. Assert collision detection calls existsByCode() and retries.
Assert idempotency: second call to generateCode with same mentor returns early from repository lookup without calling save() again. Property-based test (if fast_check or equivalent available in Dart): generate codes for 1000 random mentor/org UUID pairs and assert no two produce the same code (birthday-paradox check for the chosen code length and alphabet).
Confirmed registration events originate from the membership system (Dynamics portal for HLF), which may call back asynchronously with significant delay. If the attribution service only accepts synchronous confirmation at registration time, late callbacks will fail to match the originating referral code, resulting in under-counted conversions.
Mitigation & Contingency
Mitigation: Design the attribution confirmation path as a webhook endpoint (Supabase Edge Function) that accepts a referral_code + new_member_id pair at any time after click. The service matches by code string, not by session. Persist pending_signup events immediately at onboarding screen submission so there is always a record to upgrade to 'confirmed' when the webhook fires.
Contingency: If the membership system cannot reliably call the webhook, implement a polling reconciliation job (Supabase pg_cron, daily) that queries the membership system for recently registered members and back-fills any unmatched attribution records.
If confirmRegistration() is called more than once for the same new member (e.g., idempotency retry from the webhook), duplicate milestone events could be emitted, causing the badge system to award badges multiple times.
Mitigation & Contingency
Mitigation: Use a UNIQUE constraint on (referral_code_id, new_member_id) in the referral_events table for confirmed events. The confirmRegistration() method uses upsert semantics; milestone evaluation reads the confirmed count from the aggregation query rather than counting individual calls.
Contingency: If duplicate awards occur in production, the badge system should support idempotent award checks (query existing badges before awarding). Add a deduplication guard in BadgeCriteriaIntegration as a secondary defence.
Stakeholder review may expand attribution requirements mid-epic to include click-through tracking per channel (WhatsApp vs SMS vs email), which is not currently in scope but was mentioned in user story discussions. This would require schema changes in the foundation epic and delay delivery.
Mitigation & Contingency
Mitigation: Capture per-channel data in the device_metadata JSONB field from day one as an unstructured field (share_channel: 'whatsapp'). This preserves data without requiring a schema column, allowing structured querying to be added later without migrations.
Contingency: If channel-level analytics become a hard requirement during this epic, timebox the change to adding a nullable channel column to referral_events and a corresponding filter parameter on the aggregation query, deferring dashboard UI to a separate task.