refactor(rust): Add groupby partitioning and parallel groupby finishing to new-streaming engine

crates/polars-arrow/src/bitmap/builder.rs

@@ -0,0 +1,100 @@
+use crate::bitmap::{Bitmap, MutableBitmap};
+use crate::storage::SharedStorage;
+
+/// Used to build bitmaps bool-by-bool in sequential order.
+#[derive(Default, Clone)]
+pub struct BitmapBuilder {
+    buf: u64,
+    len: usize,
+    cap: usize,
+    set_bits: usize,
+    bytes: Vec<u8>,
+}
+
+impl BitmapBuilder {
+    pub fn new() -> Self {
+        Self::default()
+    }
+
+    pub fn len(&self) -> usize {
+        self.len
+    }
+
+    pub fn capacity(&self) -> usize {
+        self.cap
+    }
+
+    pub fn with_capacity(bits: usize) -> Self {
+        let bytes = Vec::with_capacity(bits.div_ceil(64) * 8);
+        let words_available = bytes.capacity() / 8;
+        Self {
+            buf: 0,
+            len: 0,
+            cap: words_available * 64,
+            set_bits: 0,
+            bytes,
+        }
+    }
+
+    #[inline(always)]
+    pub fn reserve(&mut self, additional: usize) {
+        if self.len + additional > self.cap {
+            self.reserve_slow(additional)
+        }
+    }
+
+    #[cold]
+    #[inline(never)]
+    fn reserve_slow(&mut self, additional: usize) {
+        let bytes_needed = (self.len + additional).div_ceil(64) * 8;
+        self.bytes.reserve(bytes_needed - self.bytes.capacity());
+        let words_available = self.bytes.capacity() / 8;
+        self.cap = words_available * 64;
+    }
+
+    #[inline(always)]
+    pub fn push(&mut self, x: bool) {
+        self.reserve(1);
+        unsafe { self.push_unchecked(x) }
+    }
+
+    /// # Safety
+    /// self.len() < self.capacity() must hold.
+    #[inline(always)]
+    pub unsafe fn push_unchecked(&mut self, x: bool) {
+        debug_assert!(self.len < self.cap);
+        self.buf |= (x as u64) << (self.len % 64);
+        self.len += 1;
+        if self.len % 64 == 0 {
+            let p = self.bytes.as_mut_ptr().add(self.bytes.len()).cast::<u64>();
+            p.write_unaligned(self.buf.to_le());
+            self.bytes.set_len(self.bytes.len() + 8);
+            self.set_bits += self.buf.count_ones() as usize;
+            self.buf = 0;
+        }
+    }
+
+    /// # Safety
+    /// May only be called once at the end.
+    unsafe fn finish(&mut self) {
+        if self.len % 64 != 0 {
+            self.bytes.extend_from_slice(&self.buf.to_le_bytes());
+            self.set_bits += self.buf.count_ones() as usize;
+        }
+    }
+
+    pub fn into_mut(mut self) -> MutableBitmap {
+        unsafe {
+            self.finish();
+            MutableBitmap::from_vec(self.bytes, self.len)
+        }
+    }
+
+    pub fn freeze(mut self) -> Bitmap {
+        unsafe {
+            self.finish();
+            let storage = SharedStorage::from_vec(self.bytes);
+            Bitmap::from_inner_unchecked(storage, 0, self.len, Some(self.len - self.set_bits))
+        }
+    }
+}

crates/polars-arrow/src/bitmap/mod.rs

@@ -19,3 +19,6 @@ pub use assign_ops::*;
 pub mod utils;
 
 pub mod bitmask;
+
+mod builder;
+pub use builder::*;

crates/polars-core/src/hashing/vector_hasher.rs

@@ -1,4 +1,5 @@
 use arrow::bitmap::utils::get_bit_unchecked;
+use polars_utils::hashing::folded_multiply;
 use polars_utils::total_ord::{ToTotalOrd, TotalHash};
 use rayon::prelude::*;
 use xxhash_rust::xxh3::xxh3_64_with_seed;
@@ -30,11 +31,6 @@ pub trait VecHash {
     }
 }
 
-pub(crate) const fn folded_multiply(s: u64, by: u64) -> u64 {
-    let result = (s as u128).wrapping_mul(by as u128);
-    ((result & 0xffff_ffff_ffff_ffff) as u64) ^ ((result >> 64) as u64)
-}
-
 pub(crate) fn get_null_hash_value(random_state: &PlRandomState) -> u64 {
     // we just start with a large prime number and hash that twice
     // to get a constant hash value for null/None

crates/polars-expr/Cargo.toml

@@ -24,6 +24,7 @@ polars-plan = { workspace = true }
 polars-row = { workspace = true }
 polars-time = { workspace = true, optional = true }
 polars-utils = { workspace = true }
+rand = { workspace = true }
 rayon = { workspace = true }
 
 [features]

crates/polars-expr/src/groups/mod.rs

@@ -23,21 +23,20 @@ pub trait Grouper: Any + Send {
     /// the ith group of other now has group index group_idxs[i] in self.
     fn combine(&mut self, other: &dyn Grouper, group_idxs: &mut Vec<IdxSize>);
 
-    /// Partitions this Grouper into the given partitions.
+    /// Partitions this Grouper into the given number of partitions.
     ///
-    /// Updates partition_idxs and group_idxs such that the ith group of self
-    /// has group index group_idxs[i] in partition partition_idxs[i].
+    /// Updates partition_idxs such that the ith group of self moves to partition
+    /// partition_idxs[i].
     ///
     /// It is guaranteed that two equal keys in two independent partition_into
     /// calls map to the same partition index if the seed and the number of
     /// partitions is equal.
-    fn partition_into(
+    fn partition(
         &self,
         seed: u64,
-        partitions: &mut [Box<dyn Grouper>],
+        num_partitions: usize,
         partition_idxs: &mut Vec<IdxSize>,
-        group_idxs: &mut Vec<IdxSize>,
-    );
+    ) -> Vec<Box<dyn Grouper>>;
 
     /// Returns the keys in this Grouper in group order, that is the key for
     /// group i is returned in row i.

crates/polars-expr/src/groups/row_encoded.rs

@@ -4,8 +4,10 @@ use hashbrown::hash_table::{Entry, HashTable};
 use polars_core::chunked_array::ops::row_encode::_get_rows_encoded_unordered;
 use polars_row::EncodingField;
 use polars_utils::aliases::PlRandomState;
+use polars_utils::hashing::{folded_multiply, hash_to_partition};
 use polars_utils::itertools::Itertools;
 use polars_utils::vec::PushUnchecked;
+use rand::Rng;
 
 use super::*;
 
@@ -27,24 +29,31 @@ pub struct RowEncodedHashGrouper {
     key_schema: Arc<Schema>,
     table: HashTable<Group>,
     key_data: Vec<u8>,
+
+    // Used for computing canonical hashes.
     random_state: PlRandomState,
+
+    // Internal random seed used to keep hash iteration order decorrelated.
+    // We simply store a random odd number and multiply the canonical hash by it.
+    seed: u64,
 }
 
 impl RowEncodedHashGrouper {
     pub fn new(key_schema: Arc<Schema>, random_state: PlRandomState) -> Self {
         Self {
             key_schema,
             random_state,
+            seed: rand::random::<u64>() | 1,
             ..Default::default()
         }
     }
 
     fn insert_key(&mut self, hash: u64, key: &[u8]) -> IdxSize {
         let num_groups = self.table.len();
         let entry = self.table.entry(
-            hash,
+            hash.wrapping_mul(self.seed),
             |g| unsafe { hash == g.key_hash && key == g.key(&self.key_data) },
-            |g| g.key_hash,
+            |g| g.key_hash.wrapping_mul(self.seed),
         );
 
         match entry {
@@ -64,6 +73,23 @@ impl RowEncodedHashGrouper {
         }
     }
 
+    /// Insert a key, without checking that it is unique.
+    fn insert_key_unique(&mut self, hash: u64, key: &[u8]) -> IdxSize {
+        let group_idx = self.table.len().try_into().unwrap();
+        let group = Group {
+            key_hash: hash,
+            key_offset: self.key_data.len(),
+            key_length: key.len().try_into().unwrap(),
+            group_idx,
+        };
+        self.key_data.extend(key);
+        self.table
+            .insert_unique(hash.wrapping_mul(self.seed), group, |g| {
+                g.key_hash.wrapping_mul(self.seed)
+            });
+        group_idx
+    }
+
     fn finalize_keys(&self, mut key_rows: Vec<&[u8]>) -> DataFrame {
         let key_dtypes = self
             .key_schema
@@ -125,7 +151,9 @@ impl Grouper for RowEncodedHashGrouper {
     fn combine(&mut self, other: &dyn Grouper, group_idxs: &mut Vec<IdxSize>) {
         let other = other.as_any().downcast_ref::<Self>().unwrap();
 
-        self.table.reserve(other.table.len(), |g| g.key_hash); // TODO: cardinality estimation.
+        // TODO: cardinality estimation.
+        self.table
+            .reserve(other.table.len(), |g| g.key_hash.wrapping_mul(self.seed));
 
         unsafe {
             group_idxs.clear();
@@ -167,14 +195,54 @@ impl Grouper for RowEncodedHashGrouper {
         )
     }
 
-    fn partition_into(
+    fn partition(
         &self,
-        _seed: u64,
-        _partitions: &mut [Box<dyn Grouper>],
-        _partition_idxs: &mut Vec<IdxSize>,
-        _group_idxs: &mut Vec<IdxSize>,
-    ) {
-        unimplemented!()
+        seed: u64,
+        num_partitions: usize,
+        partition_idxs: &mut Vec<IdxSize>,
+    ) -> Vec<Box<dyn Grouper>> {
+        assert!(num_partitions > 0);
+
+        // Two-pass algorithm to prevent reallocations.
+        let mut partition_size = vec![(0, 0); num_partitions]; // (keys, bytes)
+        unsafe {
+            for group in self.table.iter() {
+                let ph = folded_multiply(group.key_hash, seed | 1);
+                let p_idx = hash_to_partition(ph, num_partitions);
+                let (p_keys, p_bytes) = partition_size.get_unchecked_mut(p_idx as usize);
+                *p_keys += 1;
+                *p_bytes += group.key_length as usize;
+            }
+        }
+
+        let mut rng = rand::thread_rng();
+        let mut partitions = partition_size
+            .into_iter()
+            .map(|(keys, bytes)| Self {
+                key_schema: self.key_schema.clone(),
+                table: HashTable::with_capacity(keys),
+                key_data: Vec::with_capacity(bytes),
+                random_state: self.random_state.clone(),
+                seed: rng.gen::<u64>() | 1,
+            })
+            .collect_vec();
+
+        unsafe {
+            partition_idxs.clear();
+            partition_idxs.reserve(self.table.len());
+            let partition_idxs_out = partition_idxs.spare_capacity_mut();
+            for group in self.table.iter() {
+                let ph = folded_multiply(group.key_hash, seed | 1);
+                let p_idx = hash_to_partition(ph, num_partitions);
+                let p = partitions.get_unchecked_mut(p_idx);
+                p.insert_key_unique(group.key_hash, group.key(&self.key_data));
+                *partition_idxs_out.get_unchecked_mut(group.group_idx as usize) =
+                    MaybeUninit::new(p_idx as IdxSize);
+            }
+            partition_idxs.set_len(self.table.len());
+        }
+
+        partitions.into_iter().map(|p| Box::new(p) as _).collect()
     }
 
     fn as_any(&self) -> &dyn Any {

crates/polars-expr/src/reduce/len.rs

@@ -1,6 +1,7 @@
 use polars_core::error::constants::LENGTH_LIMIT_MSG;
 
 use super::*;
+use crate::reduce::partition::partition_vec;
 
 #[derive(Default)]
 pub struct LenReduce {
@@ -61,6 +62,17 @@ impl GroupedReduction for LenReduce {
         Ok(ca.into_series())
     }
 
+    unsafe fn partition(
+        self: Box<Self>,
+        partition_sizes: &[IdxSize],
+        partition_idxs: &[IdxSize],
+    ) -> Vec<Box<dyn GroupedReduction>> {
+        partition_vec(self.groups, partition_sizes, partition_idxs)
+            .into_iter()
+            .map(|groups| Box::new(Self { groups }) as _)
+            .collect()
+    }
+
     fn as_any(&self) -> &dyn Any {
         self
     }

crates/polars-expr/src/reduce/min_max.rs

@@ -11,6 +11,7 @@ use polars_utils::float::IsFloat;
 use polars_utils::min_max::MinMax;
 
 use super::*;
+use crate::reduce::partition::partition_mask;
 
 pub fn new_min_reduction(dtype: DataType, propagate_nans: bool) -> Box<dyn GroupedReduction> {
     use DataType::*;
@@ -344,6 +345,25 @@ impl GroupedReduction for BoolMinGroupedReduction {
         Ok(())
     }
 
+    unsafe fn partition(
+        self: Box<Self>,
+        partition_sizes: &[IdxSize],
+        partition_idxs: &[IdxSize],
+    ) -> Vec<Box<dyn GroupedReduction>> {
+        let p_values = partition_mask(&self.values.freeze(), partition_sizes, partition_idxs);
+        let p_mask = partition_mask(&self.mask.freeze(), partition_sizes, partition_idxs);
+        p_values
+            .into_iter()
+            .zip(p_mask)
+            .map(|(values, mask)| {
+                Box::new(Self {
+                    values: values.into_mut(),
+                    mask: mask.into_mut(),
+                }) as _
+            })
+            .collect()
+    }
+
     fn finalize(&mut self) -> PolarsResult<Series> {
         let v = core::mem::take(&mut self.values);
         let m = core::mem::take(&mut self.mask);
@@ -450,6 +470,25 @@ impl GroupedReduction for BoolMaxGroupedReduction {
         })
     }
 
+    unsafe fn partition(
+        self: Box<Self>,
+        partition_sizes: &[IdxSize],
+        partition_idxs: &[IdxSize],
+    ) -> Vec<Box<dyn GroupedReduction>> {
+        let p_values = partition_mask(&self.values.freeze(), partition_sizes, partition_idxs);
+        let p_mask = partition_mask(&self.mask.freeze(), partition_sizes, partition_idxs);
+        p_values
+            .into_iter()
+            .zip(p_mask)
+            .map(|(values, mask)| {
+                Box::new(Self {
+                    values: values.into_mut(),
+                    mask: mask.into_mut(),
+                }) as _
+            })
+            .collect()
+    }
+
     fn as_any(&self) -> &dyn Any {
         self
     }