Rekhon

// XIRR is normally NPV(r) = Σ cf · (1+r)^(−d/365), solved by Newton-Raphson from a guess.
// Both halves are hostile to exactness: the fractional exponent has no exact decimal form,
// so an implementation reaches for Double; and Newton's answer depends on the starting guess.
//
// Substituting x = (1+r)^(1/365) and multiplying through by x^dMax clears every division:
//     F(x) = Σ cf_i · x^(dMax − d_i)
// — a polynomial with whole-number exponents, so BigDecimal.pow is all it needs.
private fun bisect(
    signAtLow: Int,
    values: List<Long>,
    exponents: List<Int>,
): BigDecimal {
    var low = X_LOW
    var high = X_HIGH
    // Fixed iteration count over a literal bracket, with no early exit: no guess to depend
    // on and no tolerance branch to drift, so the same flows give the same basis points on
    // every machine and every build. Slower than Newton, and it does not care.
    repeat(ITERATIONS) {
        val mid = low.add(high).divide(TWO, CONTEXT)
        if (polynomialAt(mid, values, exponents).signum() == signAtLow) {
            low = mid
        } else {
            high = mid
        }
    }
    return low.add(high).divide(TWO, CONTEXT)
}