[YunJ-Chang]

Problem and behavior

The task calculator floors // but used JavaScript's truncating %, so (a // b) * b + a % b did not give back a when the operands had opposite signs. For example, -7 % 3 showed -1 while -7 // 3 showed -3.

This takes approach 1, accepted in #602: % now returns the remainder of floored division, so its result takes the sign of the divisor. For integers this matches Python. -7 % 3 is 2 and 7 % -3 is -2. // does not change.

Notes

The divisor is added only when the signs differ, because ((a % b) + b) % b would turn 0.1 % 1 into 0.10000000000000009. An exact multiple returns 0 rather than -0, because atan2 reads the sign. A non-finite divisor keeps the plain remainder, so -1 % 10^400 stays -1 where Python gives inf; this differs from Python on purpose, so a finite expression keeps working. Non-negative operands keep their results, and a zero divisor is still refused. docs/task-mode.md now states the behavior for negative operands and infinite divisors.

The identity is promised for integers while the arithmetic stays exact. // rounds the quotient before flooring it, so 1 // 0.1 is 10 where Python gives 9, and that case already failed before this change. This change adds cases of its own: (-9.3 // 3.1) * 3.1 + (-9.3 % 3.1) was -9.3 and is now -6.2, and (5.4 // -1.8) * -1.8 + (5.4 % -1.8) was 5.4 and is now 3.6. % now follows Python there, while -9.3 // 3.1 stays -3 where Python gives -4. Deriving // from the remainder, as Python does, would fix this, but it changes 1 // 0.1 from 10 to 9, which #602 ruled out, so // is unchanged here.

Testing

tests/browser/calculator.test.js adds negative operands, the identity across sign combinations, non-negative results, zero divisors, the atan2 case, and infinite divisors. The tests for negative operands, negative zero, and infinite divisors fail without the fix. ./scripts/test.sh passes; cargo-audit, cargo-deny, actionlint, desktop checks, and one venv-only Python test were skipped locally.

Close #602

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Summary by cubic

Aligns the calculator's % with its floored //: % now returns the remainder of floored division, so the result takes the divisor's sign and (a // b) * b + a % b equals a for integers. -7 % 3 is now 2 instead of -1, matching Python, while // and non-negative operands are unchanged.

Notes

• The identity is guaranteed for integers only; // rounds the quotient before flooring, so 1 // 0.1 stays 10 and fractional operands can drift.
• An infinite divisor leaves the dividend unchanged, so -1 % 10^400 is -1 instead of Python's -inf.
• Docs and browser tests updated to cover sign combinations, zero divisors, infinite divisors, and unchanged positive results.

^{Written for commit 5be0cfa. Summary will update on new commits.}

[YunJ-Chang]

Updated at the current head, rebased onto main: % returns 0 for an exact multiple, a non-finite divisor keeps the plain remainder, and the doc states both limits on the identity.

One decision is yours: this change makes (-9.3 // 3.1) * 3.1 + (-9.3 % 3.1) give -6.2 instead of -9.3, because // keeps its rounding. Fixing that means deriving // from the remainder, which changes 1 // 0.1 from 10 to 9. Should I keep // as it is here, or make that change in this pull request or a follow-up?

[jserv]

A tiny negative dividend with a positive divisor makes remainder + divisor round to exactly divisor: -1e-20 % 3 is 3, and 1e-20 % -3 is -3. That breaks the range promise that the result lies strictly between 0 and the divisor, and (-1e-20 // 3) * 3 + (-1e-20 % 3) is 0 rather than -1e-20. Python gives 3.0 here too, so matching it is defensible, but the doc comment and docs/task-mode.md say the result takes the divisor's sign without mentioning this edge. State it next to the fractional-operand caveat, or return 0 when the sum equals divisor if you prefer to keep the range strict.

[jserv]

Nothing pins the rounding edge in flooredRemainder. Add ["-1e-20 % 3", 3] and ["1e-20 % -3", -3] here, or to a separate test, so whichever behavior you pick (document it or clamp to 0) is locked in and a later "fix" cannot change it unnoticed.

[jserv]

Rebase the current branch onto the upstream default branch and resolve any conflicts. Run /codetrial-review in AI agents (use $codetrial-review in Codex) and resolve all identified issues. Rework the series into minimal, logically coherent commits, squash closely related commits, and ensure all commit messages follow the project’s rules.