在Golang中将大小可变的[] byte转换为int64

I have a method that transposes slice of int64 ([]int64) to a int64 but I haven't found a way to do it.

package main

import "fmt"
import "bytes"
import "encoding/binary"

func main() {
    var mySlice = []byte{0, 0, 0, 0, 0, 0, 0, 0, 0, 23}
    data := binary.BigEndian.Uint64(mySlice)
    fmt.Println(data)

    var ret int64
    buf := bytes.NewBuffer(mySlice)
    binary.Read(buf, binary.BigEndian, ret)

    fmt.Println(ret)
}

My method initializes []byte from a given size (say, make([]byte, 20)) and my method operates on a given bit and dimension size and interleaves it (bit operations):

So, say a []byte{0 0 0 0 0 0 0 0 0 23} gives 23 and a [more trailing zeroes..., 125, more tailing zeroes...] is 500

I guess I'm looking for something more like java's BigInteger class that takes in []byte (and signum) in BigEndian.

The method I'm trying to port (from Java) would be something like this:

BigInteger toIndex(long... transposedIndex) {
        byte[] b = new byte[length];
        int bIndex = length - 1;
        long mask = 1L << (bits - 1);
        for (int i = 0; i < bits; i++) {
            for (int j = 0; j < transposedIndex.length; j++) {
                if ((transposedIndex[j] & mask) != 0) {
                    b[length - 1 - bIndex / 8] |= 1 << (bIndex % 8);
                }
                bIndex--;
            }
            mask >>= 1;
        }
        // b is expected to be BigEndian
        return new BigInteger(1, b);
    }

and what I have in Golang is this:

func (s *TestStruct) untranspose(x []int64) (b int64) {
    t := make([]byte, s.length)
    bIndex := s.length - 1
    mask := int64(1 << (s.bits - 1))

    for i := 0; i < int(s.bits); i++ {
        for j := 0; j < len(x); j++ {
            if (x[j] & mask) != 0 {
                t[s.length - 1 - bIndex / 8] |= 1 << (bIndex % 8)
            }

            bIndex--
        }
        mask >>= 1
    }

    return int64(binary.BigEndian.Uint64(t))
}

which doesn't seem to be correct. []byte could be longer than 8-bit, say, [0 0 0 0 0 0 0 0 2 170]