Merge pull request #208 from sgeisler/hex_bytes

Use more performant hex_bytes function

Cargo.toml

@@ -1,4 +1,3 @@
-
 [package]
 name = "bitcoin"
 version = "0.15.1"
@@ -18,6 +17,7 @@ path = "src/lib.rs"
 [features]
 fuzztarget = ["secp256k1/fuzztarget"]
 serde-decimal = ["serde", "strason"]
+unstable = []
 
 [dependencies]
 bitcoin-bech32 = "0.8.0"

src/util/iter.rs

@@ -1,80 +0,0 @@
-// Rust Bitcoin Library
-// Written in 2014 by
-//     Andrew Poelstra <apoelstra@wpsoftware.net>
-//
-// To the extent possible under law, the author(s) have dedicated all
-// copyright and related and neighboring rights to this software to
-// the public domain worldwide. This software is distributed without
-// any warranty.
-//
-// You should have received a copy of the CC0 Public Domain Dedication
-// along with this software.
-// If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
-//
-
-//! Iterator adaptors
-//!
-//! Iterator adaptors needed by Bitcoin but not provided by the Rust
-//! standard library.
-
-/// An iterator that returns pairs of elements
-pub struct Pair<I>
-    where I: Iterator
-{
-    iter: I,
-    last_elem: Option<I::Item>
-}
-
-impl<I: Iterator> Iterator for Pair<I> {
-    type Item = (I::Item, I::Item);
-
-    #[inline]
-    fn next(&mut self) -> Option<(I::Item, I::Item)> {
-        let elem1 = self.iter.next();
-        if elem1.is_none() {
-            None
-        } else {
-            let elem2 = self.iter.next();
-            if elem2.is_none() {
-                self.last_elem = elem1;
-                None
-            } else {
-                Some((elem1.unwrap(), elem2.unwrap()))
-            }
-        }
-    }
-
-    #[inline]
-    fn size_hint(&self) -> (usize, Option<usize>) {
-        match self.iter.size_hint() {
-            (n, None) => (n/2, None),
-            (n, Some(m)) => (n/2, Some(m/2))
-        }
-    }
-}
-
-impl<I: Iterator> Pair<I> {
-    /// Returns the last element of the iterator if there were an odd
-    /// number of elements remaining before it was Pair-ified.
-    #[inline]
-    pub fn remainder(self) -> Option<I::Item> {
-        self.last_elem
-    }
-}
-
-/// Returns an iterator that returns elements of the original iterator 2 at a time
-pub trait Pairable : Sized + Iterator {
-    /// Returns an iterator that returns elements of the original iterator 2 at a time
-    fn pair(self) -> Pair<Self>;
-}
-
-impl<I: Iterator> Pairable for I {
-    /// Creates an iterator that yields pairs of elements from the underlying
-    /// iterator, yielding `None` when there are fewer than two elements to
-    /// return.
-    #[inline]
-    fn pair(self) -> Pair<I> {
-        Pair {iter: self, last_elem: None }
-    }
-}
-

src/util/misc.rs

@@ -17,29 +17,59 @@
 //! Various utility functions
 
 use blockdata::opcodes;
-use util::iter::Pairable;
 use consensus::encode;
 
+/// Helper function to convert hex nibble characters to their respective value
+#[inline]
+fn hex_val(c: u8) -> Result<u8, encode::Error> {
+    let res = match c {
+        b'0' ... b'9' => c - '0' as u8,
+        b'a' ... b'f' => c - 'a' as u8 + 10,
+        b'A' ... b'F' => c - 'A' as u8 + 10,
+        _ => return Err(encode::Error::UnexpectedHexDigit(c as char)),
+    };
+    Ok(res)
+}
+
 /// Convert a hexadecimal-encoded string to its corresponding bytes
-pub fn hex_bytes(s: &str) -> Result<Vec<u8>, encode::Error> {
-    let mut v = vec![];
-    let mut iter = s.chars().pair();
-    // Do the parsing
-    iter.by_ref().fold(Ok(()), |e, (f, s)| 
-        if e.is_err() { e }
-        else {
-            match (f.to_digit(16), s.to_digit(16)) {
-                (None, _) => Err(encode::Error::UnexpectedHexDigit(f)),
-                (_, None) => Err(encode::Error::UnexpectedHexDigit(s)),
-                (Some(f), Some(s)) => { v.push((f * 0x10 + s) as u8); Ok(()) }
+pub fn hex_bytes(data: &str) -> Result<Vec<u8>, encode::Error> {
+    // This code is optimized to be as fast as possible without using unsafe or platform specific
+    // features. If you want to refactor it please make sure you don't introduce performance
+    // regressions (run the benchmark with `cargo bench --features unstable`).
+
+    // If the hex string has an uneven length fail early
+    if data.len() % 2 != 0 {
+        return Err(encode::Error::ParseFailed("hexstring of odd length"));
+    }
+
+    // Preallocate the uninitialized memory for the byte array
+    let mut res = Vec::with_capacity(data.len() / 2);
+
+    let mut hex_it = data.bytes();
+    loop {
+        // Get most significant nibble of current byte or end iteration
+        let msn = match hex_it.next() {
+            None => break,
+            Some(x) => x,
+        };
+
+        // Get least significant nibble of current byte
+        let lsn = match hex_it.next() {
+            None => unreachable!("len % 2 == 0"),
+            Some(x) => x,
+        };
+
+        // Convert bytes representing characters to their represented value and combine lsn and msn.
+        // The and_then and map are crucial for performance, in comparision to using ? and then
+        // using the results of that for the calculation it's nearly twice as fast. Using bit
+        // shifting and or instead of multiply and add on the other hand doesn't show a significant
+        // increase in performance.
+        match hex_val(msn).and_then(|msn_val| hex_val(lsn).map(|lsn_val| msn_val * 16 + lsn_val)) {
+            Ok(x) => res.push(x),
+            Err(e) => return Err(e),
         }
     }
-    )?;
-    // Check that there was no remainder
-    match iter.remainder() {
-        Some(_) => Err(encode::Error::ParseFailed("hexstring of odd length")),
-        None => Ok(v)
-    }
+    Ok(res)
 }
 
 /// Search for `needle` in the vector `haystack` and remove every
@@ -77,6 +107,55 @@ pub fn script_find_and_remove(haystack: &mut Vec<u8>, needle: &[u8]) -> usize {
     n_deleted
 }
 
+#[cfg(all(test, feature="unstable"))]
+mod benches {
+    use rand::{Rng, thread_rng};
+    use super::hex_bytes;
+    use test::Bencher;
+
+    fn join<I: Iterator<Item=IT>, IT: AsRef<str>>(iter: I, expected_len: usize) -> String {
+        let mut res = String::with_capacity(expected_len);
+        for s in iter {
+            res.push_str(s.as_ref());
+        }
+        res
+    }
+
+    fn bench_from_hex(b: &mut Bencher, data_size: usize) {
+        let data_bytes = thread_rng()
+            .gen_iter()
+            .take(data_size)
+            .collect::<Vec<u8>>();
+        let data = join(data_bytes.iter().map(|x| format!("{:02x}", x)), data_size * 2);
+
+        assert_eq!(hex_bytes(&data).unwrap(), data_bytes);
+
+        b.iter(move || {
+            hex_bytes(&data).unwrap()
+        })
+    }
+
+    #[bench]
+    fn from_hex_16_bytes(b: &mut Bencher) {
+        bench_from_hex(b, 16);
+    }
+
+    #[bench]
+    fn from_hex_64_bytes(b: &mut Bencher) {
+        bench_from_hex(b, 64);
+    }
+
+    #[bench]
+    fn from_hex_256_bytes(b: &mut Bencher) {
+        bench_from_hex(b, 256);
+    }
+
+    #[bench]
+    fn from_hex_4m_bytes(b: &mut Bencher) {
+        bench_from_hex(b, 1024 * 1024 * 4);
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::script_find_and_remove;

src/util/mod.rs

@@ -24,7 +24,6 @@ pub mod bip143;
 pub mod contracthash;
 pub mod decimal;
 pub mod hash;
-pub mod iter;
 pub mod misc;
 pub mod uint;