test: mutation-harden math unit coverage

[thedavidmeister]

Module hardened

src/math/index.ts — selected as the weakest qualifying module from vitest run --coverage over the in-scope modules (excluding order/, router/, gas/, the core solve loop, index, and config). It had the lowest branch coverage (73.33%), driven entirely by two exported functions that had zero test coverage: toNumber and isBigNumberish. Every mutant planted in them survived the existing suite.

This PR is tests-only — no source line changed (git diff touches only src/math/index.test.ts).

Mutation matrix

Each mutation was applied to the exact source line, the math suite re-run, then the line restored.

Pre-existing coverage — already killed by existing tests (audited, left as-is)

Behavior	Mutation	Result
scaleTo18 down-branch	value / 10^(d-18) → *	KILLED
scaleTo18 up-branch	value * 10^(18-d) → /	KILLED
scaleTo18 operand	repeat(d-18) → repeat(18-d)	KILLED
scaleFrom18 up-branch	* → /	KILLED
scaleFrom18 down-branch	/ → *	KILLED
calculatePrice18 zero guard	amountIn === 0n → !== 0n	KILLED
calculatePrice18 numerator	out18 * ONE18 → out18 / ONE18	KILLED
calculatePrice18 denominator	/ amountIn18 → * amountIn18	KILLED
calculatePrice18 out decimals	decimalsOut → decimalsIn	KILLED
calculatePrice18 in decimals	decimalsIn → decimalsOut	KILLED

New coverage — mutants that SURVIVED before, now KILLED by added tests

Behavior	Mutation	Before	After
toNumber scaling	formatUnits(v, 18) → 17	SURVIVED	KILLED
toNumber scaling	formatUnits(v, 18) → 19	SURVIVED	KILLED
isBigNumberish int check	value % 1 === 0 → !== 0	SURVIVED	KILLED
isBigNumberish number branch	drop branch (→ false)	SURVIVED	KILLED
isBigNumberish string branch	drop branch (→ false)	SURVIVED	KILLED
isBigNumberish regex sign	/^-?[0-9]+$/ → drop -?	SURVIVED	KILLED
isBigNumberish regex dot	/^-?[0-9]+$/ → allow .	SURVIVED	KILLED
isBigNumberish isHex branch	drop isHex(value)	SURVIVED	KILLED
isBigNumberish bigint branch	drop typeof === "bigint"	SURVIVED	KILLED
isBigNumberish isBytes branch	drop isBytes(value)	SURVIVED	KILLED
isBigNumberish null guard	value != null → == null	SURVIVED	KILLED

Gaps checklist

• toNumber — fully covered (4 discriminating values pinning the 18-decimal scale).
• isBigNumberish — every OR-branch independently pinned by a discriminating case.
• scaleTo18 / scaleFrom18 / calculatePrice18 — pre-existing tests audited, all mutants killed.
• scaleTo18 / scaleFrom18 boundary > 18 → >= 18: equivalent mutant (at exactly 18 decimals both branches compute value * 1 == value / 1 == value); no test can distinguish — left uncovered intentionally.

Verification

• tsc -p tsconfig.check.json clean.
• eslint clean on the changed test file.
• Full vitest unit suite green: 847 tests passed (was 841; +6 new tests), 59 files.
• e2e fork test jobs are a pre-existing environmental red (RPC/secrets), out of scope and not touched.

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 1b5ff05b-ce27-4a51-a3c1-9c2a78f44ce9

📥 Commits

Reviewing files that changed from the base of the PR and between 4928590 and 5d9c122.

📒 Files selected for processing (1)

• src/math/index.test.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-06-15-solver-misc-coverage

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