feat(prereq): model program-type-gated prerequisites

[TheMythologist]

What & why

Prerequisites that branch by program type — e.g. (PROGRAM_TYPES IF_IN Graduate Degree Coursework THEN X) OR (PROGRAM_TYPES IF_IN Undergraduate Degree THEN Y) — previously failed to parse and produced no prerequisite tree at all. The grammar only allowed program_types THEN compound at the top level, so any program-type conditional that was parenthesised, combined with AND/OR, or gated on a disjunction of program types raised a "no viable alternative" error. This silently dropped the tree for ~65 modules (ESE5608, MA5202, IT5003, EE5113, the honours *HM modules, and others).

This PR parses those prerequisites and carries the program type as a first-class node in the tree ({ programType, then }), mirroring the existing cohort modelling. It is display-only: the planner has no program-type input to evaluate against, so these gates are shown in the tree but are never treated as unmet requirements.

Scraper

• Grammar: program_types_conditional now takes a program_types_gate — '(' program_types_gate ')' | program_types (OR program_types)* — so the gate may be a single program type, a disjunction of them, or that disjunction parenthesised. compound gained program_types_conditional, and overall was simplified to compound EOF. The parser was regenerated with pnpm antlr4ts.
• Visitor: builds { programType, then }; a gate that is a disjunction of IF_IN program types is collapsed into one condition over the union of types (since IF_IN X OR IF_IN Y is just IF_IN X, Y); bare program types are dropped; and the old "too many program types" bail-out is removed. The result is then normalised — redundant same-type repeats are flattened (the greedy THEN makes pt THEN A OR pt THEN B parse as pt THEN (A OR (pt THEN B)), and flattening recovers the symmetric A OR B), and the single outermost wrapper is dropped only when it is the lone "Undergraduate Degree" gate (the ubiquitous assumed audience). Any gate naming more is a real restriction and is kept.
• flattenTree walks only the gated requirement.

Website

• Mirrored the ProgramTypeRule / ProgramTypeCondition types and the new PrereqTree variant.
• Planner: checkPrerequisite skips program-type nodes (never unmet) and conflictToText handles them defensively.
• ModuleTree renders a program-type gate as "For " (joining a disjunction's types with " or "). A disjunction whose branches are all program-type gates is a per-degree split rather than a choice, so it drops the misleading "one of" label and collapses the now-empty junction so the connector line runs straight through.

Behaviour by case

• A normal module's lone Undergraduate Degree THEN … wrapper is dropped, so every existing tree is unchanged.
• Same-type branches (the honours modules) flatten to a plain A OR B.
• Differing-type branches are modelled and labelled — e.g. IT5003 shows IT5001 under both "For Graduate Degree Coursework" and "For CPE (Certificate)", and MA5202 shows three branches.
• An OR-of-program-types gate is collapsed and labelled with the union of its types — EE5113 (Undergraduate OR Graduate Coursework) shows "For Undergraduate Degree or Graduate Degree Coursework → EE5934", and EE6438 / BL5232D show "For Graduate Degree Coursework or Graduate Degree Research".
• A graduate/CPE-only wrapper is kept and labelled, so ESE5608 — whose undergraduate branch is entirely PROGRAMS/SPECIAL constraints the tree cannot represent — shows "For Graduate Degree Coursework → ESE5003" instead of a bare, unexplained ESE5003.

Testing

• Scraper parseString: added cases for differing-type modelling, same-type flattening, the ESE5608 degenerate shape, parenthesised and unparenthesised OR-of-program-types gates, and multi-type gates; updated the two previously-null "cannot parse" tests. Full scraper suite passes (151).
• Website: added planner cases (program-type nodes are never unmet, plus conflictToText) and a ModuleTree snapshot for the per-degree split. Planner + ModuleTree suites pass.
• Typecheck and lint clean on touched files.

Before:

with error ["no viable alternative at input '(PROGRAM_TYPESIF_INGraduate Degree CourseworkTHEN'"]

After:

with no errors

[vercel]

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2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
nusmods-export	Ignored	Preview	Jun 28, 2026 4:58am
nusmods-website	Ignored	Preview	Jun 28, 2026 4:58am

[codecov]

Codecov Report

❌ Patch coverage is 95.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.58%. Comparing base (988c6fd) to head (b3c05d0).
⚠️ Report is 247 commits behind head on master.

Files with missing lines	Patch %	Lines
website/src/utils/planner.ts	93.75%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4437      +/-   ##
==========================================
+ Coverage   54.52%   57.58%   +3.05%     
==========================================
  Files         274      317      +43     
  Lines        6076     7228    +1152     
  Branches     1455     1774     +319     
==========================================
+ Hits         3313     4162     +849     
- Misses       2763     3066     +303     

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[greptile-apps]

Greptile Summary

This PR adds first-class parsing and display support for program-type-gated prerequisites (e.g. PROGRAM_TYPES IF_IN Graduate Degree Coursework THEN …), which previously caused parser failures and silently dropped ~65 module prerequisite trees.

• Grammar & scraper: program_types_conditional is lifted to compound level with a new program_types_gate rule, allowing parenthesised and OR-of-program-types gates. The visitor builds { programType, then } nodes, collapses same-rule disjunctions, normalises redundant same-type nesting, and drops the lone "Undergraduate Degree" outer wrapper as an assumed default.
• Planner: resolveProgramTypes evaluates gates against the student's ScheduleType before walkTree runs — applicable gates are replaced by their consequence and non-applicable ones are nulled out (preventing vacuous OR satisfaction). The nOf count is clamped to the surviving pool to avoid an unsatisfiable node after pruning.
• ModuleTree: program-type gates render as "For <type>" labels; a disjunction whose every branch is a program-type gate omits the misleading "one of" label and collapses the connector line via the new .passthrough CSS class.

Confidence Score: 5/5

The change is safe to merge — it parses and displays previously-failing prerequisite trees without altering any existing tree output, and the planner evaluation is display-only for program-type gates.

All tree variants in both the scraper and the website planner are handled and tested exhaustively. The normalization, flattening, and outer-wrapper-drop logic each have matching test cases. The nOf count-clamping and defensive walkTree fallback address the concerns raised in earlier review threads. No existing behaviour changes — the lone Undergraduate Degree wrapper is dropped exactly as before, and modules without program-type nodes are untouched.

No files require special attention. The generated parser file (NusModsParser.ts) is large but is produced mechanically by ANTLR4 from the updated grammar.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[Raw prerequisite string] --> B[ANTLR4 parser NusModsParser]
    B --> C{Parse errors?}
    C -- Yes --> D[Return null]
    C -- No --> E[ReqTreeVisitor]
    E --> F{Visitor errors?}
    F -- Yes --> D
    F -- No --> G[Raw PrereqTree with programType nodes]
    G --> H[normalizeProgramTypes: Flatten same-type gates]
    H --> I[dropOuterProgramType: Drop lone Undergraduate wrapper]
    I --> J[Stored PrereqTree]
    J --> K[resolveProgramTypes with student scheduleType]
    K --> L{Gate applicable?}
    L -- Yes, replace with then --> M[Resolved tree, no programType nodes]
    L -- No, null, filter out --> M
    M --> N[walkTree / checkPrerequisite]
    N --> O[Unfulfilled prereqs array]
    J --> P[ModuleTree renderer]
    P --> Q{Node type}
    Q -- programType --> R[For X label]
    Q -- or of programType --> S[passthrough label]
    Q -- other --> T[Standard label]

Loading

%%{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"}}}%%
flowchart TD
    A[Raw prerequisite string] --> B[ANTLR4 parser NusModsParser]
    B --> C{Parse errors?}
    C -- Yes --> D[Return null]
    C -- No --> E[ReqTreeVisitor]
    E --> F{Visitor errors?}
    F -- Yes --> D
    F -- No --> G[Raw PrereqTree with programType nodes]
    G --> H[normalizeProgramTypes: Flatten same-type gates]
    H --> I[dropOuterProgramType: Drop lone Undergraduate wrapper]
    I --> J[Stored PrereqTree]
    J --> K[resolveProgramTypes with student scheduleType]
    K --> L{Gate applicable?}
    L -- Yes, replace with then --> M[Resolved tree, no programType nodes]
    L -- No, null, filter out --> M
    M --> N[walkTree / checkPrerequisite]
    N --> O[Unfulfilled prereqs array]
    J --> P[ModuleTree renderer]
    P --> Q{Node type}
    Q -- programType --> R[For X label]
    Q -- or of programType --> S[passthrough label]
    Q -- other --> T[Standard label]

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