Alex D Herbert created RNG-54:
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Summary: StringSampler
Key: RNG-54
URL: https://issues.apache.org/jira/browse/RNG-54
Project: Commons RNG
Issue Type: Improvement
Components: sampling
Affects Versions: 1.1
Reporter: Alex D Herbert
There is currently no equivalent for the function
{{org.apache.commons.math3.random.RandomDataGenerator.nextHexString(int)}}.
Here is the original version adapted to use the {{UniformRandomProvider:}}
{code:java}
public String nextHexStringOriginal(UniformRandomProvider ran, int len) {
// Initialize output buffer
StringBuilder outBuffer = new StringBuilder();
// Get int(len/2)+1 random bytes
byte[] randomBytes = new byte[(len / 2) + 1];
ran.nextBytes(randomBytes);
// Convert each byte to 2 hex digits
for (int i = 0; i < randomBytes.length; i++) {
Integer c = Integer.valueOf(randomBytes[i]);
/*
* Add 128 to byte value to make interval 0-255 before doing hex
conversion.
* This guarantees <= 2 hex digits from toHexString() toHexString would
* otherwise add 2^32 to negative arguments.
*/
String hex = Integer.toHexString(c.intValue() + 128);
// Make sure we add 2 hex digits for each byte
if (hex.length() == 1) {
hex = "0" + hex;
}
outBuffer.append(hex);
}
return outBuffer.toString().substring(0, len);
}
{code}
Note: I removed the length check to make the speed test (see below) fair.
This makes use of {{StringBuider}} and is not very efficient. I have created a
version based on the Hex encoding within
{{org.apache.commons.codec.digest.DigestUtils}} and
{{org.apache.commons.codec.binary.Hex}}. This uses a direct look-up of the hex
character using the index from successive 4 bits of a byte array to form an
index from 0-15.
Here's the function without details of how the {{byte[]}} is correctly sized:
{code:java}
private static final char[] DIGITS_LOWER = {
'0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 'a', 'b', 'c', 'd', 'e',
'f' };
private static String nextHexString(UniformRandomProvider rng, byte[] bytes,
int length) {
rng.nextBytes(bytes);
// Use the upper and lower 4 bits of each byte as an
// index in the range 0-15 for each hex character.
final char[] out = new char[length];
// Run the loop without checking index j by producing characters
// up to the size below the desired length.
final int loopLimit = length / 2;
int i = 0, j = 0;
while (i < loopLimit) {
final byte b = bytes[i];
// 0x0F == 0x01 | 0x02 | 0x04 | 0x08
out[j++] = DIGITS_LOWER[(b >>> 4) & 0x0F];
out[j++] = DIGITS_LOWER[b & 0x0F];
i++;
}
// The final character
if (j < length)
out[j++] = DIGITS_LOWER[(bytes[i] >>> 4) & 0x0F];
return new String(out);
}
{code}
I've compared this to the original function and a modified one below that
computes the exact same strings:
{code:java}
public String nextHexStringModified(UniformRandomProvider ran, int len) {
// Initialize output buffer
StringBuilder outBuffer = new StringBuilder();
// byte[] randomBytes = new byte[(len/2) + 1]; // ORIGINAL
byte[] randomBytes = new byte[(len + 1) / 2];
ran.nextBytes(randomBytes);
// Convert each byte to 2 hex digits
for (int i = 0; i < randomBytes.length; i++) {
// ORIGINAL
// Integer c = Integer.valueOf(randomBytes[i]);
// String hex = Integer.toHexString(c.intValue() + 128);
String hex = Integer.toHexString(randomBytes[i] & 0xff);
// Make sure we add 2 hex digits for each byte
if (hex.length() == 1) {
outBuffer.append('0');
}
outBuffer.append(hex);
}
return outBuffer.toString().substring(0, len);
}
{code}
The timings are:
||Name||Time||Relative||
|StringSampler|316103|0.073|
|nextHexStringModified|3708104|0.853|
|nextHexStringOriginal|4348063|1.000|
This is not using JMH but the results show the method performs better.
The full {{StringSampler}} class supports a radix of 2, 8, and 16 for binary,
octal and hex strings.
JUnit tests show: the sampler computes the same values as
{{nextHexStringModified(int);}} edges cases are handled with exceptions; and
the output strings are uniform for each of the supported character sets (using
a Chi Squared test).
Can I create a PR for a {{org.apache.commons.rng.sampling.StringSampler}}?
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