Efficient way to convert a hex representation of a number to a decimal value

Question

I have a byte[5] array which represents a decimal number when printed as a hex string. Two digits can be stored in one byte, the hex characters are not used. E.g.

[0x11,0x45,0x34,0x31,0x21] -> 1145343121.

Is there a more efficient way in Java (some bitshifting magic maybe) to do the conversion to a decimal number other than

long result = Long.parseLong(byteToHexString(bytes[]));?

An efficient conversion vice versa would also be interesting...

You're sacrificing a lot of CPU cycle for very little gain in memory. — awm, Nov 20 '11 at 22:16

score 3 · Answer 1 · edited Nov 20 '11 at 22:24

3

((a[0]>>>4)*1000000000L + (a[0]&15)*100000000L +
 (a[1]>>>4)*  10000000L + (a[1]&15)*  1000000L +
 (a[2]>>>4)*    100000L + (a[2]&15)*    10000L +
 (a[3]>>>4)*      1000L + (a[3]&15)*      100L +
 (a[4]>>>4)*        10L + (a[4]&15))

edited Nov 20 '11 at 22:24

x4u

13,877
6
48
58

answered Nov 20 '11 at 22:15

Marcelo Cantos

181,030
38
327
365

Sorry, way unable to get this running correctly. Assigned the result to a long variable. Wrong? – user462982 Nov 20 '11 at 23:33

score 2 · Accepted Answer · answered Nov 20 '11 at 23:04

2

Here you go, trick is to nibble a nibble at a time !

    byte[] buf = { 0x11, 0x45, 0x34, 0x31, 0x21 };
    long result = 0;
    for (int i = 0; i < buf.length; i++) {
        result = result * 100 + (buf[i] >> 4 & 0XF) * 10 + (buf[i] & 0XF);
    }
    System.out.println(result);

Output

1145343121

answered Nov 20 '11 at 23:04

Prashant Bhate

10,907
7
47
82

Yeah, thanks, works very well and looks short and sweet. Did a small test: Outperforms the trivial method by a factor of 6x. – user462982 Nov 20 '11 at 23:30

score 1 · Answer 3 · answered Nov 21 '11 at 02:00

After Knuth, Art of Computer Programming, Vol II Seminumerical Algorithms, answer to exercise 4.4(19):

public long binaryValue(long bcd)
{
    long    x = bcd;
    x -= ((x & 0xf0f0f0f0f0f0f0f0L) >> 4)*(0x10-10);
    x -= ((x & 0xff00ff00ff00ff00L) >> 8)*(0x100-100);
    x -= ((x & 0xffff0000ffff0000L) >> 16)*(0x10000-10000);
    x -= ((x & 0xffffffff00000000L) >> 32)*(0x100000000L-100000000);
    return x;
}

Efficient way to convert a hex representation of a number to a decimal value

3 Answers3