make benchmark deterministic

Cargo.toml

@@ -17,7 +17,7 @@ thiserror = { package = "thiserror-core", version = "1.0.38", default-features =
 
 [dev-dependencies]
 criterion = "0.3"
-rand = "0.8.5"
+rand = {version = "0.8.5", features = ["small_rng"]}
 
 [features]
 default = ["std"]

benches/reversedbitreader_bench.rs

@@ -1,8 +1,8 @@
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
-use rand::Rng;
+use rand::{Rng, SeedableRng};
 use ruzstd::decoding::bit_reader_reverse::BitReaderReversed;
 
-fn fibonacci(br: &mut BitReaderReversed, accesses: &[u8]) -> u64 {
+fn do_all_accesses(br: &mut BitReaderReversed, accesses: &[u8]) -> u64 {
  let mut sum = 0;
  for x in accesses {
  sum += br.get_bits(*x).unwrap() as u64;
@@ -12,24 +12,26 @@ fn fibonacci(br: &mut BitReaderReversed, accesses: &[u8]) -> u64 {
 }
 
 fn criterion_benchmark(c: &mut Criterion) {
- let mut rng = rand::thread_rng();
+ const DATA_SIZE: usize = 1024 * 1024;
+
+ let mut rng = rand::rngs::SmallRng::seed_from_u64(0xDEADBEEF);
  let mut rand_vec = vec![];
- for _ in 0..100000 {
+ for _ in 0..DATA_SIZE {
  rand_vec.push(rng.gen());
  }
 
  let mut access_vec = vec![];
  let mut br = BitReaderReversed::new(&rand_vec);
  while br.bits_remaining() > 0 {
- let x = rng.gen_range(1..20);
+ let x = rng.gen_range(0..20);
  br.get_bits(x).unwrap();
  access_vec.push(x);
  }
 
- c.bench_function("fib 20", |b| {
+ c.bench_function("reversed bitreader", |b| {
  b.iter(|| {
  br.reset(&rand_vec);
- fibonacci(&mut br, &access_vec)
+ do_all_accesses(&mut br, &access_vec)
  })
  });
 }