perf: Improve performance of md5 (#19568)

## Which issue does this PR close?

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- Part of #19569

## Rationale for this change

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I ran microbenchmarks comparing DataFusion with DuckDB for string
functions (see apache/datafusion-benchmarks#26)
and noticed that DF was very slow for `md5`.

This PR improves performance:

| Benchmark                  | Before | After  | Speedup     |
|----------------------------|--------|--------|-------------|
| md5_array (1024 strings)   | 206 µs | 100 µs | 2.1x faster |
| md5_scalar (single string) | 337 ns | 221 ns | 1.5x faster |

## What changes are included in this PR?

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Avoid using `write!` with a format string and use a more efficient
approach

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Author: andygrove

datafusion/functions/src/crypto/basic.rs

@@ -33,7 +33,7 @@ use datafusion_common::{
 use datafusion_expr::ColumnarValue;
 use md5::Md5;
 use sha2::{Sha224, Sha256, Sha384, Sha512};
-use std::fmt::{self, Write};
+use std::fmt;
 use std::str::FromStr;
 use std::sync::Arc;
 
@@ -157,14 +157,18 @@ pub fn md5(args: &[ColumnarValue]) -> Result<ColumnarValue> {
     })
 }
 
-/// this function exists so that we do not need to pull in the crate hex. it is only used by md5
-/// function below
+/// Hex encoding lookup table for fast byte-to-hex conversion
+const HEX_CHARS_LOWER: &[u8; 16] = b"0123456789abcdef";
+
+/// Fast hex encoding using a lookup table instead of format strings.
+/// This is significantly faster than using `write!("{:02x}")` for each byte.
 #[inline]
 fn hex_encode<T: AsRef<[u8]>>(data: T) -> String {
-    let mut s = String::with_capacity(data.as_ref().len() * 2);
-    for b in data.as_ref() {
-        // Writing to a string never errors, so we can unwrap here.
-        write!(&mut s, "{b:02x}").unwrap();
+    let bytes = data.as_ref();
+    let mut s = String::with_capacity(bytes.len() * 2);
+    for &b in bytes {
+        s.push(HEX_CHARS_LOWER[(b >> 4) as usize] as char);
+        s.push(HEX_CHARS_LOWER[(b & 0x0f) as usize] as char);
     }
     s
 }