feat: support previous special term when academic year changes

[jloh02]

Closes #4109

[vercel]

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2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
nusmods-export	Ignored	Preview	Jun 21, 2026 3:27pm
nusmods-website	Ignored	Preview	Jun 21, 2026 3:27pm

[codecov]

Codecov Report

❌ Patch coverage is 79.51807% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.97%. Comparing base (988c6fd) to head (9dd1d49).
⚠️ Report is 241 commits behind head on master.

Files with missing lines	Patch %	Lines
website/src/actions/moduleBank.ts	60.86%	9 Missing ⚠️
website/src/utils/specialTerm.ts	88.13%	7 Missing ⚠️
website/src/views/modules/ModulePageContent.tsx	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4402      +/-   ##
==========================================
+ Coverage   54.52%   56.97%   +2.44%     
==========================================
  Files         274      317      +43     
  Lines        6076     7128    +1052     
  Branches     1455     1724     +269     
==========================================
+ Hits         3313     4061     +748     
- Misses       2763     3067     +304     

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[greptile-apps]

Greptile Summary

This PR introduces automatic detection of the previous-AY Special Term I/II overlap window, so that when the academic year config rotates to the new AY before ST I/II complete, the website and scraper transparently source sem 3/4 data from the previous year without manual intervention.

• A new shared nusmods-academic-calendar package exposes calendar-aware helpers (isPreviousAySpecialTermActive, getEffectiveSpecialTermAcadYear, shouldUsePreviousAyForSemester, etc.) consumed by both the scraper and website.
• The scraper's DataPipeline detects the overlap window at runtime and fetches sem 3/4 data from the previous AY, writing venue files under both AYs; combineModules gains a preserveModuleInfoSemesters option that keeps current-AY module metadata while merging previous-AY timetable/exam data.
• The website's fetchModule action is enriched with an async archive-fetch step that calls mergePreviousAySpecialTermData to replace unreliable new-AY ModReg R0 special-term data with authoritative archive data; the ST2 external-exam-link logic is updated to use the new helpers.

Confidence Score: 5/5

The change is safe to merge. The new code paths are well-tested, have safe fallbacks (archive fetch errors silently degrade to current-AY data), and are gated on an opt-in overlap window that does nothing outside the ST I/II period.

The overlap detection and merging logic is thoroughly tested in both the new shared package and the website utils. Scraper behaviour is exercised by the updated CollateModules tests. Error paths (archive fetch failure, missing calendar entries) all fall back gracefully. The only non-blocking concern is that moduleDataCheck logs spurious warnings for every module with previous-AY ST data, but this does not affect correctness.

scrapers/nus-v2/src/tasks/CollateModules.ts — the warn-before-guard ordering produces noisy logs during the overlap period

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant W as "Website (fetchModule)"
    participant S as "Redux Store"
    participant API as "NUSMods API"
    participant ARC as "Archive API"

    W->>API: "GET /module/{code} (current AY)"
    API-->>W: "Module (may lack ST1/ST2 timetable)"
    W->>S: "dispatch SUCCESS_KEY(FETCH_MODULE) via requestAction"
    Note over W,S: "Components render with current-AY data"

    W->>W: "enrichModuleWithPreviousAySpecialTerm()"
    alt "During overlap window (prevAY ST active)"
        W->>S: "check moduleArchive cache"
        alt "Not cached"
            W->>ARC: "GET /module/{code}?year=prevAY"
            ARC-->>W: "archiveModule"
        end
        W->>W: "mergePreviousAySpecialTermData()"
        alt "Archive data wins"
            W->>S: "dispatch SUCCESS_KEY(FETCH_MODULE) with merged module"
            Note over W,S: "Components re-render with archive ST1/ST2 data"
        end
    end

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%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant W as "Website (fetchModule)"
    participant S as "Redux Store"
    participant API as "NUSMods API"
    participant ARC as "Archive API"

    W->>API: "GET /module/{code} (current AY)"
    API-->>W: "Module (may lack ST1/ST2 timetable)"
    W->>S: "dispatch SUCCESS_KEY(FETCH_MODULE) via requestAction"
    Note over W,S: "Components render with current-AY data"

    W->>W: "enrichModuleWithPreviousAySpecialTerm()"
    alt "During overlap window (prevAY ST active)"
        W->>S: "check moduleArchive cache"
        alt "Not cached"
            W->>ARC: "GET /module/{code}?year=prevAY"
            ARC-->>W: "archiveModule"
        end
        W->>W: "mergePreviousAySpecialTermData()"
        alt "Archive data wins"
            W->>S: "dispatch SUCCESS_KEY(FETCH_MODULE) with merged module"
            Note over W,S: "Components re-render with archive ST1/ST2 data"
        end
    end

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_{Reviews (6): Last reviewed commit: "chore: add unit tests for boundary value..." | Re-trigger Greptile}

[ravern]

@greptileai review

[greptile-apps]

Want your agent to iterate on Greptile's feedback? Try greploops.

[leslieyip02]

Mostly LGTM, just some minor nits.