[PR] fix boundary and performance issues in MathUtil conversio ns [poi-xmlbeans]

pjfanning opened a new pull request, #94:
URL: https://github.com/apache/poi-xmlbeans/pull/94

   Three issues in `MathUtil`, each covered by a new test.
   
   **`safeFloatToInt` accepted `2147483648f`.** Widening `Integer.MAX_VALUE` to `float` rounds it up to `2^31`, so `f > Integer.MAX_VALUE` was false for exactly `2^31` and the `(int)` cast silently saturated to `2147483647` — the wrong answer from a method whose job is to reject out-of-range input. Comparing as `double` fixes it. The lower bound was already correct, since `-2^31` is exactly representable, and `safeDoubleToInt` was never affected.
   
   **`toBigInteger` guarded a large negative scale but not a large positive one.** A value like `1E-10000000` has `integerDigits == -9999999` and `scale == +10000000`, so neither existing condition tripped and `BigDecimal.toBigInteger()` was reached. That calls `setScale(0)`, which computes `10^scale`: measured at just over 4 seconds to return `0` from a 12-character input, and at larger exponents it throws `ArithmeticException: BigInteger would overflow supported range` instead of the documented `IllegalArgumentException`. The string passes `parseAsBigDecimal`'s length limit easily, and the path is reachable from parsed content via `JavaDecimalHolder` / `XmlObjectBase.getBigIntegerValue()`. Any such value truncates to zero, so return `BigInteger.ZERO` directly — which also makes every s
 ub-1 conversion cheaper.
   
   **`toBigInteger` had no explicit null check**, unlike every other method here. It already threw `NullPointerException` from `stripTrailingZeros()`, so this is consistency rather than a behaviour change; `toLong` and `toInt` inherit it.
   
   Also added a comment on the existing `scale() < -DEFAULT_MAX_NUMBER_CHARS` condition, which reads as redundant but is not: for `1E+2147483647` the `precision() - scale()` subtraction overflows to `Integer.MIN_VALUE` and only the scale check catches it. And corrected a few copy-pasted javadoc `@return` types (`parseAsDouble` documented `float`).
   
   Ran the `impl.util` and `impl.values` tests locally — all pass. Leaving the full suite to CI.
   
   🤖 Generated with [Claude Code](https://claude.com/claude-code)


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