perf(prof): sample filtering and hashing

profiling/Cargo.toml

@@ -48,6 +48,7 @@ features = ["env-filter", "fmt", "smallvec", "std"]
 allocator-api2 = { version = "0.2", default-features = false, features = ["alloc"] }
 criterion = { version = "0.5.1" }
 datadog-php-profiling = { path = ".", features = ["test"] }
+proptest = { version = "1" }
 
 [target.'cfg(target_arch = "x86_64")'.dev-dependencies]
 criterion-perf-events = "0.4.0"

profiling/src/bitset.rs

@@ -0,0 +1,239 @@
+// Copyright 2025-Present Datadog, Inc. https://www.datadoghq.com/
+// SPDX-License-Identifier: Apache-2.0 OR BSD-3-Clause
+
+use std::hash::{Hash, Hasher};
+
+/// Very simple bitset which doesn't allocate, and doesn't change after it has
+/// been created.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+#[repr(transparent)]
+pub struct BitSet {
+    bits: u32,
+}
+
+impl Hash for BitSet {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        let bits = self.bits;
+        bits.count_ones().hash(state);
+        bits.hash(state);
+    }
+}
+
+impl BitSet {
+    pub const MAX: usize = u32::BITS as usize;
+
+    /// Creates a new bitset from the provided number.
+    pub const fn new(bits: u32) -> Self {
+        Self { bits }
+    }
+
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.bits.count_ones() as usize
+    }
+
+    #[inline]
+    pub fn is_empty(&self) -> bool {
+        self.bits == 0
+    }
+
+    #[inline]
+    pub fn contains(&self, bit: usize) -> bool {
+        if bit < 32 {
+            let mask = 1u32 << bit;
+            let masked = self.bits & mask;
+            masked != 0
+        } else {
+            false
+        }
+    }
+
+    pub fn iter(&self) -> BitSetIter {
+        BitSetIter::new(self)
+    }
+}
+
+impl FromIterator<usize> for BitSet {
+    /// Creates a new bitset from the iterator.
+    ///
+    /// # Panics
+    ///
+    /// Panics if an item is out of the range of the bitset e.g. [`u32::MAX`].
+    fn from_iter<I: IntoIterator<Item = usize>>(iter: I) -> BitSet {
+        let mut bits = 0;
+        let mut insert = |bit| {
+            // todo: add non-panic API
+            assert!(bit < BitSet::MAX);
+            bits |= 1u32 << bit;
+        };
+        for bit in iter {
+            insert(bit);
+        }
+        BitSet { bits }
+    }
+}
+
+pub struct BitSetIter {
+    bitset: u32,
+    offset: u32,
+    end: u32,
+}
+
+impl BitSetIter {
+    pub fn new(bitset: &BitSet) -> BitSetIter {
+        let bitset = bitset.bits;
+        let offset = 0;
+        let end = {
+            let num_bits = u32::BITS;
+            let leading_zeros = bitset.leading_zeros();
+            num_bits - leading_zeros
+        };
+        BitSetIter {
+            bitset,
+            offset,
+            end,
+        }
+    }
+}
+
+impl Iterator for BitSetIter {
+    type Item = usize;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        while self.offset != self.end {
+            let offset = self.offset;
+            self.offset += 1;
+            let mask = 1 << offset;
+            let masked = self.bitset & mask;
+            if masked != 0 {
+                return Some(offset as usize);
+            }
+        }
+        None
+    }
+}
+
+impl ExactSizeIterator for BitSetIter {
+    fn len(&self) -> usize {
+        if self.offset < self.end {
+            let shifted = self.bitset >> self.offset;
+            shifted.count_ones() as usize
+        } else {
+            0
+        }
+    }
+}
+
+impl IntoIterator for BitSet {
+    type Item = usize;
+    type IntoIter = BitSetIter;
+    fn into_iter(self) -> Self::IntoIter {
+        BitSetIter::new(&self)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use proptest::prelude::*;
+    use proptest::test_runner::{RngAlgorithm, TestRng};
+    use std::collections::HashSet;
+
+    #[test]
+    fn bitset_full() {
+        let bitset = BitSet::new(u32::MAX);
+        assert_eq!(bitset.len(), BitSet::MAX);
+        assert!(!bitset.is_empty());
+
+        for i in 0..BitSet::MAX {
+            assert!(bitset.contains(i));
+        }
+
+        for (offset, bit) in bitset.iter().enumerate() {
+            assert_eq!(bit, offset);
+        }
+
+        let mut iter = bitset.iter();
+        let mut len = BitSet::MAX;
+        assert_eq!(len, iter.len());
+
+        while let Some(_) = iter.next() {
+            len -= 1;
+            assert_eq!(len, iter.len());
+        }
+    }
+
+    #[test]
+    fn bitset_empty() {
+        let bitset = BitSet::new(0);
+        assert_eq!(0, bitset.len());
+        assert!(bitset.is_empty());
+        for i in 0..BitSet::MAX {
+            assert!(!bitset.contains(i));
+        }
+
+        let mut iter = bitset.iter();
+        let len = 0;
+        assert_eq!(len, iter.len());
+        assert_eq!(None, iter.next());
+    }
+
+    // There's nothing special about 27, just testing a single possible number.
+    #[test]
+    fn bitset_27() {
+        let bitset = BitSet::new(1 << 27);
+        assert_eq!(1, bitset.len());
+        assert!(!bitset.is_empty());
+        assert!(bitset.contains(27));
+
+        let mut iter = bitset.iter();
+        let len = 1;
+        assert_eq!(len, iter.len());
+        assert_eq!(Some(27), iter.next());
+    }
+
+    static IOTA: [usize; BitSet::MAX] = [
+        0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,
+        25, 26, 27, 28, 29, 30, 31,
+    ];
+
+    proptest! {
+        #[test]
+        fn bitset_acts_like_a_special_hashset(
+            oracle in proptest::sample::subsequence(&IOTA, 1..IOTA.len())
+                .prop_map(HashSet::<usize>::from_iter),
+        ) {
+            let bitset1 = BitSet::from_iter(oracle.iter().cloned());
+            prop_assert_eq!(bitset1.len(), oracle.len());
+
+            // Items in the oracle exist in the bitset.
+            for item in oracle.iter() {
+                prop_assert!(bitset1.contains(*item));
+            }
+
+            // Test the other way around to check the iterator implementation.
+            let mut i = 0;
+            for item in bitset1.iter() {
+                prop_assert!(oracle.contains(&item));
+                i += 1;
+            }
+            // Make sure the iterator ran as many times as we expected.
+            prop_assert_eq!(i, oracle.len(),
+                "BitSet's iterator didn't have the expected number of iterations"
+            );
+
+            // Like regular sets, insertion order doesn't matter in bitsets.
+            let mut shuffled = oracle.iter().copied().collect::<Vec<_>>();
+            let mut rng = TestRng::deterministic_rng(RngAlgorithm::ChaCha);
+            use rand::seq::SliceRandom;
+            shuffled.shuffle(&mut rng);
+            let bitset2 = BitSet::from_iter(shuffled.iter().cloned());
+
+            prop_assert_eq!(
+                bitset1, bitset2,
+                "Insertion order unexpectedly mattered, diff in binary: {:b} vs {:b}",
+                bitset1.bits, bitset2.bits
+            );
+        }
+    }
+}