Change representation of lesson configs from classNo to lesson indices

[zehata]

Implementation

• The lesson index of a lesson is its index in the timetable in the API response.
• Lesson configs are now saved as mapping of lesson types to arrays of lesson indices. Example:

{
    CS1010S: {
        Lecture: [53],
        Tutorial: [19],
        Recitation: [32],
    },
}

• TA configs have been removed. TA modules are now saved as an array of module codes. ["CS1010S"]
• TA lessons will use the same lesson config as non-TA modules. TA lessons can now be added and removed individually. Example:

{
    CS1010S: {
        Lecture: [53],
        Tutorial: [19],
        Recitation: [32, 33],
    },
}

• When a TA module is changed back to non-TA, it will use the closest matching valid non-TA lesson config.
• Current format classNo lesson configs and TA lesson configs will be migrated to the new lessonGroup format
• Serialization format has been changed

Change	Before	After
(..., ...) denotes arrays of lesson indices	?CG1111A=LAB:03	?CG1111A=LAB:(5,6)
; separates different lesson types	?CS1010S=LEC:1,TUT:1	?CS1010S=LEC:(53);TUT:(41)
TA modules serialized similar to hidden	?CG1111A=LAB:03&ta=CG1111A(LAB:03)	?CG1111A=LAB:(5,6)&ta=CG1111A

• Current share link serialization format will be deserialized into the new format
• Optimiser has been updated to share links in the new lessonGroup format.

Resolves

All modules can now be set as TA modules.
Resolves #3929 and #3930 , from which this implementation came from

Before	After

Issue with deserialization of TA module lesson configs with multiple lessons in a module, from #4214 (comment).

Before	After

All lessons of a single classNo now show up, and are highlighted when one of them is hovered over. Provided that the module is not set as a TA mod. Resolves #4168.

Before	After

TA module lessons can now be added and removed individually.
User request from: https://t.me/NUSMods/12508 (No issue opened, at least I can't find one)

Before	After

Known issues

Array Index Stability

Using the index of an array comes with the inherent issue of stability. Furthermore, it may fail silently, since lessons of the same lesson type are typically placed next to each other in the timetable list in the API response. So the module lesson config may remain valid even after the module timetable had been updated.

An alternative is to serialize additional info as proposed in #3930 . It will make share links very long, because of the serialization of weeks information.

Personally I think the lessons provided by the faculties are stable enough, and they will most likely not add an additional class in the middle of the semester, so I think it will be better idea to create UI to inform users that the module info has changed if it changes, which can be done simply by hashing the module info during scraping.

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
nusmods-export	Ready	Preview	Comment	Sep 12, 2025 9:24pm
nusmods-website	Ready	Preview	Comment	Sep 12, 2025 9:24pm

[vercel]

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[codecov]

Codecov Report

❌ Patch coverage is 86.77130% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 54.38%. Comparing base (988c6fd) to head (e0d9ba3).
⚠️ Report is 142 commits behind head on master.

Files with missing lines	Patch %	Lines
website/src/views/timetable/TimetableContent.tsx	42.46%	42 Missing ⚠️
...site/src/views/timetable/TimetableModulesTable.tsx	14.28%	6 Missing ⚠️
website/src/views/tetris/board.ts	0.00%	3 Missing ⚠️
website/src/reducers/timetables.ts	94.11%	2 Missing ⚠️
website/src/utils/timetables.ts	99.06%	2 Missing ⚠️
website/src/actions/export.ts	0.00%	1 Missing ⚠️
...c/views/components/module-info/LessonTimetable.tsx	66.66%	1 Missing ⚠️
website/src/views/settings/previewTimetable.ts	0.00%	1 Missing ⚠️
website/src/views/timetable/TimetableContainer.tsx	97.67%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4225      +/-   ##
==========================================
- Coverage   54.52%   54.38%   -0.14%     
==========================================
  Files         274      289      +15     
  Lines        6076     6862     +786     
  Branches     1455     1675     +220     
==========================================
+ Hits         3313     3732     +419     
- Misses       2763     3130     +367     

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[zehata]

Changes from #4226 is only a bugfix. The delta from this PR will apply to the fixed master too.