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Source code

fun countDigitsIn(v: Long): Int {
    val guess = ((64 - v.countLeadingZeroBits()) * 10) ushr 5
    return guess + (if (v > DigitCountToLargestValue[guess]) 1 else 0)
}

fun countDigitsIn_32(v: Long): Int {
    val guess = ((64 - v.countLeadingZeroBits()) * 10) ushr 5
    return guess + (if (v > DigitCountToLargestValue32[guess.and(0x1f)]) 1 else 0)
}

private val DigitCountToLargestValue = longArrayOf(
    -1, // Every value has more than 0 digits.
    9L, // For 1 digit (index 1), the largest value is 9.
    99L,
    999L,
    9999L,
    99999L,
    999999L,
    9999999L,
    99999999L,
    999999999L,
    9999999999L,
    99999999999L,
    999999999999L,
    9999999999999L,
    99999999999999L,
    999999999999999L,
    9999999999999999L,
    99999999999999999L,
    999999999999999999L, // For 18 digits (index 18), the largest value is 999999999999999999.
    Long.MAX_VALUE, // For 19 digits (index 19), the largest value is MAX_VALUE.
)

val DigitCountToLargestValue32 = LongArray(32) {
    if (it in DigitCountToLargestValue.indices) {
        DigitCountToLargestValue[it]
    } else {
        -1
    }
}