Microsoft.Scripting.Utils.MathUtils.GetRandBits C# (CSharp) Method

        public static BigInteger GetRandBits(this Random generator, int bits) {
            ContractUtils.Requires(bits > 0);

            // equivalent to (bits + 7) / 8 without possibility of overflow
            int count = bits % 8 == 0 ? bits / 8 : bits / 8 + 1;

            // Pad the end (most significant) with zeros if we align to the byte
            // to ensure that we end up with a positive value
            byte[] bytes = new byte[bits % 8 == 0 ? count + 1 : count];
            generator.NextBytes(bytes);
            if (bits % 8 == 0) {
                bytes[bytes.Length - 1] = 0;
            } else {
                bytes[bytes.Length - 1] = (byte)(bytes[bytes.Length - 1] & ((1 << (bits % 8)) - 1));
            }

            if (bits <= 32) {
                return (BigInteger)GetWord(bytes, 0, bits);
            } else if (bits <= 64) {
                ulong a = GetWord(bytes, 0, bits);
                ulong b = GetWord(bytes, 32, bits);
                return (BigInteger)(a | (b << 32));
            } else {
                count = (count + 3) / 4;
                uint[] data = new uint[count];
                for (int i = 0; i < count; i++) {
                    data[i] = GetWord(bytes, i * 32, bits);
                }
                return new BigInteger(1, data);
            }
        }