[SPARK-48596][SQL] Perf improvement for calculating hex string for long

[yaooqinn]

What changes were proposed in this pull request?

This pull request optimizes the Hex.hex(num: Long) method by removing leading zeros, thus eliminating the need to copy the array to remove them afterward.

Why are the changes needed?

• Unit tests added
• Did a benchmark locally (30~50% speedup)

Hex Long Tests:                           Best Time(ms)   Avg Time(ms)   Stdev(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------------------------------
Legacy                                             1062           1094          16          9.4         106.2       1.0X
New                                                 739            807          26         13.5          73.9       1.4X

object HexBenchmark extends BenchmarkBase {
  override def runBenchmarkSuite(mainArgs: Array[String]): Unit = {
    val N = 10_000_000
    runBenchmark("Hex") {
      val benchmark = new Benchmark("Hex Long Tests", N, 10, output = output)
      val range = 1 to 12
      benchmark.addCase("Legacy") { _ =>
        (1 to N).foreach(x => range.foreach(y => hexLegacy(x - y)))
      }

      benchmark.addCase("New") { _ =>
        (1 to N).foreach(x => range.foreach(y => Hex.hex(x - y)))
      }
      benchmark.run()
    }
  }

  def hexLegacy(num: Long): UTF8String = {
    // Extract the hex digits of num into value[] from right to left
    val value = new Array[Byte](16)
    var numBuf = num
    var len = 0
    do {
      len += 1
      // Hex.hexDigits need to be seen here
      value(value.length - len) = Hex.hexDigits((numBuf & 0xF).toInt)
      numBuf >>>= 4
    } while (numBuf != 0)
    UTF8String.fromBytes(java.util.Arrays.copyOfRange(value, value.length - len, value.length))
  }
}

Does this PR introduce any user-facing change?

no

How was this patch tested?

Was this patch authored or co-authored using generative AI tooling?

no

[beliefer]

Could you add comments to explain the arithmetic expression ?

[yaooqinn]

I don't see the necessity for commenting on such a pretty common expression

[beliefer]

Good catch!

[beliefer]

value -> bytes

[yaooqinn]

bytes might be good but the naming is currently consistent with another variant.

[yaooqinn]

cc @cloud-fan @dongjoon-hyun @LuciferYang thanks

[LuciferYang]

+1, LGTM

[yaooqinn]

Merged to master. Thank you @LuciferYang and also for the offline suggestion