ColdFusion 128-bit unsigned int to IPv6

I've posted a function that will convert an IPv6 address to a 128-bit unsigned int value here: ColdFusion IPv6 to 128-bit unsigned int

I need a function that will go in the other direction now.

This function turned out to be more complicated and I'll explain the complications in the answer.

Here is the function that will convert a 128-bit unsigned int to an IPv6 address with the correct (concise) IPv6 formatting.

Explanations: Part of the problem with a function like this is that the number passed into the function (nUInt128) is not guaranteed to be a 128-bit unsigned int. It might be 8-bits (::1) or even weird stuff like a signed 136-bit number (ColdFusion/Java seems to prefer signed ints). Not having exactly a 128-bit number that gets converted to a Java Byte array with 16 values will cause java.net.Inet6Address.getAddress() to throw an error. My solution was to create a ColdFusion array with 16 zeros and back-fill it and then use that with java.net.Inet6Address.getAddress(). I was surprised that this worked as I have no idea how large the numbers are in that array. ColdFusion/Java somehow did some magic and turned the array into a Byte[]. The back-fill also strips off the numbers that are larger and fixes the 136-bit signed int problem.

<cffunction name="UInt128ToIPv6" returntype="string" output="no" access="public" hint="returns IPv6 address for uint128 number">
    <cfargument name="nUInt128" type="numeric" required="yes" hint="uint128 to convert to ipv6 address">

    <cfif arguments.nUInt128 EQ 0>
        <cfreturn "">
    </cfif>

    <cftry>
        <cfset local['javaMathBigInteger'] = CreateObject("java", "java.math.BigInteger").init(arguments.nUInt128)>
        <cfset local['JavaNetInet6Address'] = CreateObject("java", "java.net.Inet6Address")>
        <cfset local['arrBytes'] = local.javaMathBigInteger.toByteArray()>

        <!--- correct the array length if !=16 bytes --->
        <cfset local['arrFixedBytes'] = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]>
        <cfif arrayLen(local.arrBytes) NEQ 16>
            <cfset local['nFixedIndex'] = 16>
            <cfset local['nBytesIndex'] = arrayLen(local.arrBytes)>

            <cfloop condition="local.nFixedIndex NEQ 0 && local.nBytesIndex NEQ 0">
                <cfset local.arrFixedBytes[local.nFixedIndex] = local.arrBytes[local.nBytesIndex]>
                <cfset local.nFixedIndex-->
                <cfset local.nBytesIndex-->
            </cfloop>
        </cfif>
        <!--- /correct the array length if !=16 bytes --->

        <cfif arrayLen(local.arrBytes) NEQ 16>
            <cfset local['vcIPv6'] = local.JavaNetInet6Address.getByAddress(local.arrFixedBytes).getHostAddress()>
        <cfelse>
            <cfset local['vcIPv6'] = local.JavaNetInet6Address.getByAddress(local.javaMathBigInteger.toByteArray()).getHostAddress()>
        </cfif>
        <cfcatch type="any">
            <cfset local['vcIPv6'] = "">
        </cfcatch>
    </cftry>

    <cfreturn formatIPv6(vcIPv6 = local.vcIPv6)>
</cffunction>

Here is the formatIPv6() utility function that is called at the end of the previous function.

<cffunction name="formatIPv6" returntype="string" output="yes" access="public" hint="returns a compressed ipv6 address">
    <cfargument name="vcIPv6" type="string" required="yes" hint="IPv6 address">

    <!--- inside reReplace removes leading zeros, outside reReplace removes repeating ":" and "0:" --->
    <cfreturn reReplace(reReplace(LCase(arguments.vcIPv6), "(:|^)(0{0,3})([1-9a-f]*)", "\1\3", "all"), "(^|:)[0|:]+:", "::", "all")>
</cffunction>

If you have any suggestions/questions, please leave comments.