case-study 2026-10-09 windowsnativeopensource

Windows Calculator %: open-source contrast for a familiar quirk

Percent on a calculator is a classic “brochure vs binary” puzzle: after addition, 10% often means “10% of the first operand,” while after multiply it may mean “divide by 100.” Microsoft publishes Calculator sources, so claims can be cross-checked.

Source meta

31075 GitHub stars (snapshot)

Language: C#

License: MIT

Updated: 2026-10-02

GitHub source ↗

rea.run rating

4/5 — Open-source arbitration lab

Quality. Claims can be checked against Microsoft’s public sources.

Evidence. Official Calculator repo is the arbiter.

Limits. rea.run summary — not a Microsoft-affiliated guide.

For. Native/Windows teaching pairs.

The question. Reproduce the operator-dependent percent rule with citations (command IDs, branches, constants such as 100).

Agent loop. Ask REA for the percent handler Evidence, then open the matching files in the official repository. Prefer named symbols from public source when aligning decompilation.

Teaching payoff. Students learn to distrust a single numeric demo until the branch is named and matched to tests in the open tree.

Guardrails. This is interoperability / education on an open project — not advice to tamper with licensing or OS components you are not authorized to modify.

Takeaways

Operator context changes percent semantics.
Open-source trees are the arbitration layer.
Cite branches, do not meme screenshots alone.