Add standard java.math.BigDecimal to BigDecimal converter

[vdshb]

It would be nice to have function like:

fun java.math.BigDecimal.toKotlinBigDecimal(): com.ionspin.kotlin.bignum.decimal.BigDecimal

near https://github.com/ionspin/kotlin-multiplatform-bignum/blob/main/bignum/src/jvmMain/kotlin/com/ionspin/kotlin/bignum/decimal/DecimalUtility.kt

[ionspin]

Thanks for the recommendation, a contribution on this would be welcome!

[vdshb]

My draft implementation:

import com.ionspin.kotlin.bignum.decimal.BigDecimal
import com.ionspin.kotlin.bignum.integer.BigInteger
import kotlin.math.absoluteValue

fun java.math.BigDecimal.toKotlinBigDecimal(): BigDecimal {
    if (this == java.math.BigDecimal.ZERO) {
        return BigDecimal.ZERO
    }
    return if (scale() > 0) {
        BigDecimal.fromBigIntegerWithExponent(
            this.unscaledValue().toKotlinBigInteger(),
            (this.precision() - this.scale() - 1).toLong()
        )
    } else {
        BigDecimal.fromBigIntegerWithExponent(
            this.unscaledValue().toKotlinBigInteger(),
            (this.precision() - this.scale().absoluteValue - 1).toLong()
        )
    }
}

fun java.math.BigInteger.toKotlinBigInteger(): BigInteger {
    return BigInteger.parseString(this.toString(10), 10)
}

@ionspin I can see someone is already working on it, so I probably shouldn't be involved?

[ionspin]

@vdshb that pull request was closed, so you can go ahead if you'd like. I won't be able to look at the draft in depth today as I have to leave right now, but just by glancing it looks OK.