fix(optimiser): Add default slots to unallocated lessons

[thejus03]

Context

• Fixes Optimiser partial timetable does not show unallocated lessons #4145

Implementation

• Added default slots for unassigned lessons

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[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.64%. Comparing base (988c6fd) to head (81fe1a5).
⚠️ Report is 154 commits behind head on master.

Files with missing lines	Patch %	Lines
.../optimiser/OptimiserContainer/OptimiserContent.tsx	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4264      +/-   ##
==========================================
+ Coverage   54.52%   56.64%   +2.11%     
==========================================
  Files         274      297      +23     
  Lines        6076     6935     +859     
  Branches     1455     1674     +219     
==========================================
+ Hits         3313     3928     +615     
- Misses       2763     3007     +244     

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Project	Deployment	Review	Updated (UTC)
nusmods-export	Ready	Preview, Comment	Dec 15, 2025 1:19am
nusmods-website	Ready	Preview, Comment	Dec 15, 2025 1:19am

[leslieyip02]

The code is LGTM, but I'm not convinced that assigning a default slot is necessarily better than doing nothing. I feel that omitting unassigned modules makes it more explicit to the user that they need to tinker a bit with those modules (and/or adjust their preferences).

However, that's just my opinion so maybe a better way is to add an opt-in for this? So some sort of checkbox for "Assign default lessons if needed".

Warning	Result Timetable

What do you think?

[thejus03]

Sure, I think adding an opt-in would make sense for some people. However, the reason why I would think most would want to have the default slot is because, let’s say, from the above example, CS2100 Lecture was unassigned. Now, if no default slots are added, he needs to go and remove the mod and re-add CS2100. Doing this, however, resets the other slots chosen for the other CS2100 lessons by the optimiser, like tutorial, lab, etc.

So I thought adding a default slot was better because now users can just modify the slot for CS2100 Lec without having the need to re-add and lose the other lessons chosen by the optimiser.

Hope that makes sense. I am okay with anything. Maybe what we can do is put the default state for the opt-in as 'assign default slot' ?

[jloh02]

alternatively, what if we do a flow where we show the warning, then have a button to auto-assign to default slots. i reckon this would not require a new Vercel function call, and allows for both behaviours. if we feel this is too convoluted, i'm more in favor of a warning due to the above reasons @leslieyip02 mentioned