Sri Lanka A/L Subject Combination Validator

Every state-university Sri Lankan undergraduate intake routes through one of six Advanced Level streams defined by the University Grants Commission. Each stream is a rule: a small set of mandatory subjects, a basket of permitted optional subjects, and (sometimes) a disallowed-pair list. The validator above models each rule declaratively and checks your three picks against all six in turn.

1. Normalise. Each subject you pick is matched to its Department of Examinations code from the canonical A/L subject catalogue. A combination must contain exactly three distinct recognised subjects.
2. Walk the streams. The validator visits each rule in priority order (Bio → Phys → Eng Tech → Bio-systems Tech → Commerce → Arts) and asks:
   • Are all mandatory subjects in your combination?
   • Does the basket pick-N constraint hold?
   • Is any pair on the disallowed-pair list present?
   • For Eng Tech and BST: is the third subject inside the open basket (pure sciences and Combined Maths are excluded)?
3. Verdict.If exactly one stream matches, that's the stream. If more than one matches, both are surfaced (a rare borderline case). If none match, the rule that failed for the closest-fit stream becomes the verdict reason.
4. Suggest swaps. For invalid combinations, the validator runs a one-subject diff: for each of your three subjects it tries every catalogue subject as a replacement and reports up to three single-swap edits that would turn the combination valid.

The same evaluation is exposed twice in the data module — once as the main validateCombination function that returns every matching stream, and once as a strict classifyByPriorityWalk cross-check that returns the first match. Both must agree on every combination that yields a unique stream; ten regression cases assert this in the source.

Two rules trip up more candidates than any other, so they are worth spelling out. The first is the disallowed-pair rule: Biology and Combined Mathematics cannot sit in the same combination. A student who offers both is effectively splitting effort across two different streams, and the UGC stream-definition circular removes that combination from both the Biological Science and Physical Science baskets. The second is the third-subject open basket for the two technology streams. Engineering Technology and Bio-systems Technology each fix two compulsory subjects and then require the third to come from a wide list — ICT, Geography, Economics, Accounting, Business Studies, a language or an aesthetic subject — while explicitly barring Physics, Chemistry, Biology and Combined Mathematics. The validator models this as an allow-list rather than a basket count, which is why an otherwise sensible pick like Engineering Technology + Science for Technology + Biology is rejected.

A medium check runs alongside the stream logic but never blocks a verdict. If you flag a subject in a medium the Department of Examinations does not offer it in — say Home Economics in English — the tool raises a warning rather than failing the combination, because the stream basket itself is medium-agnostic. The one hard medium rule the UGC enforces is that the same subject offered in two mediums (for example History of Sri Lanka in Sinhala and in Tamil) counts once, so it can never fill two of your three slots.

Stream eligibility is only the first gate. Passing it tells you which faculties are open to you, not whether your marks clear the bar for a specific course. That second step depends on your Z-score, your district and the year's cut-offs — work through it with the Sri Lanka A/L Z-Score Calculator and the University Cutoff Finder once your combination is confirmed valid here.

• University Grants Commission of Sri Lanka — Admissions to Undergraduate Courses Handbook 2024/2025
• Department of Examinations Sri Lanka — G.C.E. (A/L) syllabus and subject-code catalogue
• Ministry of Education Sri Lanka — A/L syllabus revision circulars (Engineering Technology, Bio-systems Technology)
• University Grants Commission Sri Lanka — official site

Stream rules and subject baskets on this page were last cross-checked against the cited UGC and DoE sources on 2026-05-16. They are reviewed every April at the start of the new SL admission cycle, and again after every gazetted Ministry of Education circular revision. Coverage is state-university (UGC) admission only; private and foreign-equivalency rules are out of scope.