fix(InputNumber): fix decimal rounding on spin

[selfplusplus2]

Fixes #8382

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[melloware]

GitHub Copilot Chat Assistant

Quick summary

• The new implementation is simpler than the old (it auto-computes precision from the operands), but it has a few pitfalls:
  • It fails for numbers printed in exponential notation (e.g. 1e-7) because toString().split('.') yields no decimal part.
  • It doesn't guard against string inputs, NaN, Infinity.
  • Math.pow(10, decimals) can exceed safe integers for large decimal counts (and large scale factors can overflow Number.MAX_SAFE_INTEGER).
  • Rounding via Math.round((base + increment) * precision) can still pick up floating errors before scaling; it's safer to scale each operand or the sum before rounding.
  • For financial/critical precision you should use a decimal library (decimal.js / Big.js / BigNumber).

Problems I found in current diff

• Exponential notation: (0.0000001).toString() === "1e-7" -> baseDec becomes 0, but you actually need 7 decimals.
• Input types: if base or increment are strings (common in UI code) you need to coerce to Number and handle invalid values.
• Very large decimals: Math.pow(10, n) with n > ~15 can cause factor > 2^53 and integer math becomes unsafe.
• Edge cases (NaN, Infinity) are not handled.

Suggested robust implementation

• Count decimals correctly including exponential notation.
• Cap decimals to a safe limit (e.g. 15) to avoid exceeding Number.MAX_SAFE_INTEGER.
• Use scaled integer addition and rounding: Math.round(base * factor + increment * factor) / factor
• Return NaN when inputs are not finite numbers, or fallback gracefully.

Example implementation (replace your function with this)
function countDecimals(value) {
if (!isFinite(value)) return 0;
const s = String(value);
// handle exponential notation
if (s.indexOf('e') >= 0) {
const [mantissa, expStr] = s.split('e');
const exp = parseInt(expStr, 10);
const dec = (mantissa.split('.')[1] || '').length;
return Math.max(0, dec - exp);
}
return (s.split('.')[1] || '').length;
}

const addWithPrecision = (base, increment) => {
base = Number(base);
increment = Number(increment);
if (!isFinite(base) || !isFinite(increment)) return NaN;

const baseDec = countDecimals(base);
const incDec = countDecimals(increment);

// cap decimals to avoid factor > Number.MAX_SAFE_INTEGER
const decimals = Math.min(Math.max(baseDec, incDec), 15);
const factor = Math.pow(10, decimals);

// If scaling would overflow, fall back to simple addition with an approximate rounding
const maxSafe = Number.MAX_SAFE_INTEGER;
if (Math.abs(base) * factor > maxSafe || Math.abs(increment) * factor > maxSafe) {
const sum = base + increment;
// round to a reasonable number of digits (15)
const fallbackFactor = Math.pow(10, 15);
return Math.round(sum * fallbackFactor) / fallbackFactor;
}

return Math.round(base * factor + increment * factor) / factor;
};

Behavior examples you can use to test

• addWithPrecision(0.1, 0.2) -> 0.3
• addWithPrecision(1e-7, 1e-7) -> 2e-7
• addWithPrecision('1.234', 0.001) -> 1.235
• addWithPrecision(NaN, 1) -> NaN
• addWithPrecision(Infinity, 1) -> NaN

Recommendations

• Add unit tests covering:
  • small decimals (0.1 + 0.2)
  • exponential inputs (1e-7)
  • string inputs
  • very large/safe boundary cases
• If this function is used for money or other critical decimal arithmetic, switch to a decimal/BigNumber library (decimal.js, big.js, bignumber.js) to avoid all floating-point pitfalls.
• Keep the decimal cap (15) documented so future maintainers understand the safety tradeoff.

If you want, I can:

• open a PR with this improved implementation and tests, or
• adapt the implementation to use a specific decimal library if you prefer. Which would you like?

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[selfplusplus2]

I have amended to factor in exponential numbers and added some tests

[melloware]

please run npm run format

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[selfplusplus2]

Formatting updated

[melloware]

ok npm run lint next please

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[selfplusplus2]

Done

[melloware]

weird now its saying the format is wrong again?

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[selfplusplus2]

It doesn't like my editor white space settings 🤣🤦. I've re-run format and lint.