Splitting $10.00 three ways loses a cent

[Param-Harrison]

What happened

Add an expense of 1000 cents ($10.00) split among 3 people. The three
shares are 333, 333, 333 cents - they sum to 999, not 1000. A cent
disappears.

Expected

Shares should always sum to exactly the original amount. One (or more)
member should get the leftover cent(s), e.g. 334, 333, 333.

Steps to reproduce

bun run .claude/skills/handling-money/scripts/check-cents.ts

The split(1000, 3 members) case prints FAIL ... sum 999 (expected 1000).

Or via the API, with any Goa trip token:

curl -s -X POST http://localhost:3200/api/groups/goa-trip/expenses \
  -H "Authorization: Bearer goa-asha" -H "Content-Type: application/json" \
  -d '{"payerId":"goa-asha-id","amountCents":1000,"description":"test","splitAmong":["goa-asha-id","goa-ben-id","goa-chetan-id"]}'

Then check the expense's shares sum to 1000 - they sum to 999.

Area

money

Severity

medium

Notes for whoever picks this up

See .claude/skills/handling-money - the remainder rule is documented in
references/rules.md. The fix is in src/money/cents.ts's splitCents.

[Param-Harrison]

Triage

Disposition: proceed
Type: bug
Risk: high
Confidence: 0.9

Done when: splitCents(1000, [a, b, c]) returns shares that sum to exactly 1000 (e.g. 334/333/333, remainder to the first members per .claude/skills/handling-money/references/rules.md), proven by a new bun:test in tests/money.test.ts that fails on the current code, with make check green.

Files expected: src/money/cents.ts, tests/money.test.ts

Gate level: make check

Risk is high because the change touches money (the split that every expense's shares come from), even though it is confined to one non-protected module. No protected path is involved: the fix goes in src/money/cents.ts, and the .claude/skills/handling-money skill is only read, not edited.

[Param-Harrison]

Plan (rev 1)

Goal: Make splitCents give the leftover cents to the first members, so shares always add up to the total (1000 split 3 ways → 334/333/333).

Acceptance criteria

• AC-1: splitCents(1000, ["a","b","c"]) returns {a:334, b:333, c:333}, and the test fails on the current code. Checked by bun test tests/money.test.ts -t "gives the remainder cents to the first members"
• AC-2: For several totals and member counts that don't divide evenly (for example 101 across 5 and 1 across 3), the shares add up to exactly totalCents. Each share is floor(total/n) or floor(total/n)+1, and the +1 shares go to the first remainder members in the order they're passed in. Checked by bun test tests/money.test.ts -t "shares always sum to the total"
• AC-3: The existing splitCents tests (even splits, 900/3 and 2000/2) and the zero-members throw still pass without changes. Checked by bun test tests/money.test.ts
• AC-4: The handling-money sum check reports every case as ok. Checked by bun run .claude/skills/handling-money/scripts/check-cents.ts
• AC-5: The full gate is green. Checked by make check

Non-goals (binding — a diff that crosses one fails verify)

• NG-1: No edits outside src/money/cents.ts and tests/money.test.ts. That rules out src/expenses.ts, src/seed.ts, other tests, and anything under .claude/**, .factory/**, src/auth/** or .github/**.
• NG-2: No change to splitCents's signature or return type, and no change to formatCents, parseCents or sumCents.
• NG-3: No floats in the split. Use only integer-cents arithmetic, per the handling-money skill.
• NG-4: No migration or backfill of expense shares already stored with the floored values. This fix covers new splits only.
• NG-5: No change to how the split handles negative totals or totals that aren't integers (no new validation).

Files to touch

• src/money/cents.ts: fix splitCents (lines 39–49) and replace the "SEEDED DEFECT" doc comment (lines 29–38) with a description of the remainder rule
• tests/money.test.ts: add the remainder tests to the existing splitCents describe block

Tests to write first

• tests/money.test.ts › splitCents › gives the remainder cents to the first members
• tests/money.test.ts › splitCents › shares always sum to the total

Repo skills to apply

handling-money (references/rules.md remainder rule: base = floor(total/n), the first total - base*n members get base + 1)

Risk: low

Charter tier low: "a change confined to a single non-protected module with no schema or API-shape change". Only src/money/ changes. The function's signature, the DB schema and the route shapes all stay the same, and no protected path is touched. The only caller, src/expenses.ts:28, inserts whatever shares splitCents returns and doesn't depend on their order. No existing test asserts the floored output: tests/routes.test.ts uses fixtures that divide evenly.

Triage suggested high because this is money. The charter's high tier covers only dependency majors, src/auth/**, or plans with low confidence, and none of those apply here. Still, every expense's shares come from this function, so I've marked it not eligible for auto-approve. A human should look at it even though the tier is low.

Gate level: make check

This needs a human: reply /factory approve to proceed, or
/factory revise <what to change>.

[Param-Harrison]

/factory approve

[Param-Harrison]

Status

Stage: build — finished
Progress: 3/5
Elapsed: ~3m
Gate line: FACTORY_GATES: status=GREEN passed=3 failed=0 skipped=1 failed_gates=audit

Round 2 fixes the blocking NG-5 finding from the last verdict. The remainder rule now applies only to totals that are whole numbers of zero or more. Negative and non-integer totals still get the old floored shares (-1000 split 3 ways → -334 each, 10.5 split 2 ways → 5 each). A new test, leaves negative and non-integer totals floored as before, locks that in. It failed on the round-1 code and passes now. bun test passes (29 pass, 0 fail), and check-cents.ts reports every case ok. The optional audit gate still fails on hono advisories that were already there, which is out of scope. The two non-blocking findings (duplicate member ids, the doc-comment wording) predate this change and are left for a follow-up.