Calculator % disagrees with // on negative operands

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@YunJ-Chang arbeitet bereits daran.

Seit 06.11.2026.

  • #607 von @YunJ-Chang — gemerged

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Beschreibung

Observed behavior

The task workspace calculator floors // but truncates %, so the two operators contradict each other when an operand is negative. No error is shown: the result is a wrong number. docs/task-mode.md tells learners to use this calculator rather than another app or tab during a timed attempt, so they have no other tool to catch it.

term() in web/calculator.js uses Math.floor(value / unary()) for // at line 129 and the JavaScript remainder value %= unary() at line 131. JavaScript's % takes the sign of the dividend, so it pairs with truncating division, not floored division. The identity a == (a // b) * b + a % b therefore fails whenever the operands have opposite signs. tests/browser/calculator.test.js covers only 17 // 5 and 17 % 5, so nothing exercises this case. docs/task-mode.md lists both operators without saying what they do with a negative operand.

Expected behavior

// and % should agree, so that a == (a // b) * b + a % b holds for every sign combination.
Approaches:

  1. Make % floored, so the result takes the sign of the divisor. // keeps its behavior, and the pair matches Python, whose syntax // and ** already follow.
  2. Make // truncate instead. This matches JavaScript's %, but changes an operator whose floor is deliberate and makes // differ from floor(a / b).
  3. Keep both and document the difference. This changes no code, but leaves the identity broken.

I propose approach 1, and I will send the pull request once the scope is agreed here.

Acceptance criteria:

  • a == (a // b) * b + a % b holds for every sign combination, and the table under Reproduction gives its expected column.
  • Results for non-negative operands do not change.
  • A zero divisor is still refused with the existing error.
  • tests/browser/calculator.test.js covers both operators with negative operands, and fails without the fix.
  • docs/task-mode.md states what % does with a negative operand.
Reproduction
  1. Open the calculator in the task workspace (web/task.html).
  2. Enter an expression from the table below.
  3. Press = and compare the result.
Expression Actual Expected from floored division
-7 // 3 -3 -3
-7 % 3 -1 2
(-7 // 3) * 3 + (-7 % 3) -10 -7
7 // -3 -3 -3
7 % -3 1 -2
CodeTrial version

codetrial 0.1.0 (4de45b1)

Environment

OS: Ubuntu 24.04.5 LTS on WSL2 (Windows 11)
Browser: Chrome

Sanitized evidence
Image

Captured at 4de45b1 by driving the calculator form in web/task.html with Playwright. The task section was unhidden by script, since starting a session needs Gemini and LiveKit. The form, its submit handler, and the evaluator were not changed. The files named in this report are unchanged through af66541.

Public data confirmation
  • I removed credentials, session tokens, candidate recordings, and personal interview content from this public report.
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Rust
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