tustvold commented on code in PR #2593: URL: https://github.com/apache/arrow-rs/pull/2593#discussion_r966359089
########## arrow/src/row/interner.rs: ########## @@ -0,0 +1,326 @@ +// Licensed to the Apache Software Foundation (ASF) under one +// or more contributor license agreements. See the NOTICE file +// distributed with this work for additional information +// regarding copyright ownership. The ASF licenses this file +// to you under the Apache License, Version 2.0 (the +// "License"); you may not use this file except in compliance +// with the License. You may obtain a copy of the License at +// +// http://www.apache.org/licenses/LICENSE-2.0 +// +// Unless required by applicable law or agreed to in writing, +// software distributed under the License is distributed on an +// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY +// KIND, either express or implied. See the License for the +// specific language governing permissions and limitations +// under the License. + +use hashbrown::hash_map::RawEntryMut; +use hashbrown::HashMap; +use std::cmp::Ordering; +use std::num::NonZeroU32; +use std::ops::Index; + +#[derive(Debug, Copy, Clone, PartialEq, Eq)] +pub struct Interned(NonZeroU32); + +/// A byte array interner that generates normalized keys that are sorted with respect +/// to the interned values, e.g. `inter(a) < intern(b) => a < b` +#[derive(Debug, Default)] +pub struct OrderPreservingInterner { + keys: InternBuffer, + values: InternBuffer, + bucket: Box<Bucket>, + + hasher: ahash::RandomState, + lookup: HashMap<Interned, (), ()>, +} + +impl OrderPreservingInterner { + /// Interns an iterator of values returning a list of [`Interned`] which can be + /// used with [`Self::normalized_key`] to retrieve the normalized keys with a + /// lifetime not tied to the mutable borrow passed to this method + pub fn intern<I, V>(&mut self, input: I) -> Vec<Option<Interned>> + where + I: IntoIterator<Item = Option<V>>, + V: AsRef<[u8]>, + { + let iter = input.into_iter(); + let capacity = iter.size_hint().0; + let mut out = Vec::with_capacity(capacity); + let mut to_intern: Vec<(usize, u64, V)> = Vec::with_capacity(capacity); + let mut to_intern_len = 0; + + for (idx, item) in iter.enumerate() { + let value: V = match item { + Some(value) => value, + None => { + out.push(None); + continue; + } + }; + + let v = value.as_ref(); + let hash = self.hasher.hash_one(v); + let entry = self + .lookup + .raw_entry_mut() + .from_hash(hash, |a| &self.values[*a] == v); + + match entry { + RawEntryMut::Occupied(o) => out.push(Some(*o.key())), + RawEntryMut::Vacant(_) => { + // Push placeholder + out.push(None); + to_intern_len += v.len(); + to_intern.push((idx, hash, value)); + } + }; + } + + to_intern.sort_unstable_by(|(_, _, a), (_, _, b)| a.as_ref().cmp(b.as_ref())); + + self.keys.offsets.reserve(to_intern.len()); + self.keys.values.reserve(to_intern.len()); // Approximation + self.values.offsets.reserve(to_intern.len()); + self.values.values.reserve(to_intern_len); + + for (idx, hash, value) in to_intern { + let val = value.as_ref(); + + let entry = self + .lookup + .raw_entry_mut() + .from_hash(hash, |a| &self.values[*a] == val); + + match entry { + RawEntryMut::Occupied(o) => { + out[idx] = Some(*o.key()); + } + RawEntryMut::Vacant(v) => { + let val = value.as_ref(); + self.bucket + .insert(&mut self.values, val, &mut self.keys.values); + self.keys.values.push(0); + let interned = self.keys.append(); + + let hasher = &mut self.hasher; + let values = &self.values; + v.insert_with_hasher(hash, interned, (), |key| { + hasher.hash_one(&values[*key]) + }); + out[idx] = Some(interned); + } + } + } + + out + } + + /// Returns a null-terminated byte array that can be compared against other normalized_key + /// returned by this instance, to establish ordering of the interned values + pub fn normalized_key(&self, key: Interned) -> &[u8] { + &self.keys[key] + } +} + +/// A buffer of `[u8]` indexed by `[Interned]` +#[derive(Debug)] +struct InternBuffer { + values: Vec<u8>, + offsets: Vec<usize>, +} + +impl Default for InternBuffer { + fn default() -> Self { + Self { + values: Default::default(), + offsets: vec![0], + } + } +} + +impl InternBuffer { + /// Insert `data` returning the corresponding [`Interned`] + fn insert(&mut self, data: &[u8]) -> Interned { + self.values.extend_from_slice(data); + self.append() + } + + /// Appends the next value based on data written to `self.values` + /// returning the corresponding [`Interned`] + fn append(&mut self) -> Interned { + let idx: u32 = self.offsets.len().try_into().unwrap(); + let key = Interned(NonZeroU32::new(idx).unwrap()); + self.offsets.push(self.values.len()); + key + } +} + +impl Index<Interned> for InternBuffer { + type Output = [u8]; + + fn index(&self, key: Interned) -> &Self::Output { + let index = key.0.get() as usize; + let end = self.offsets[index]; + let start = self.offsets[index - 1]; + unsafe { self.values.get_unchecked(start..end) } + } +} + +/// A slot corresponds to a single byte-value in the generated normalized key +/// +/// It may contain a value, if not the first slot, and may contain a child [`Bucket`] representing +/// the next byte in the generated normalized key +#[derive(Debug, Default, Clone)] +struct Slot { + value: Option<Interned>, + /// Child values smaller than `self.value` if any + child: Option<Box<Bucket>>, +} + +/// Each bucket corresponds to a single byte in the normalized key +#[derive(Debug, Clone)] +struct Bucket { + slots: Box<[Slot]>, +} + +impl Default for Bucket { + fn default() -> Self { + let slots = (0..255).map(|_| Slot::default()).collect::<Vec<_>>().into(); + Self { slots } + } +} + +impl Bucket { + /// Perform a skewed binary search to find the first slot that is empty or less + /// + /// Returns `Ok(idx)` if an exact match is found, otherwise returns `Err(idx)` + /// containing the slot index to insert at + fn insert_pos(&self, values_buf: &InternBuffer, data: &[u8]) -> Result<usize, usize> { + let mut size = self.slots.len() - 1; + let mut left = 0; + let mut right = size; + while left < right { + // Skew binary search to leave gaps of at most 3 elements + let mid = left + (size / 2).min(3); + + let slot = &self.slots[mid]; + let val = match slot.value { + Some(val) => val, + None => return Err(mid), + }; + + let cmp = values_buf[val].cmp(data); + if cmp == Ordering::Less { + left = mid + 1; + } else if cmp == Ordering::Greater { + right = mid; + } else { + return Ok(mid); + } + + size = right - left; + } + Err(left) + } + + /// Insert `data` into this bucket or one of its children, appending the + /// normalized key to `out` as it is constructed + /// + /// # Panics + /// + /// Panics if the value already exists + fn insert(&mut self, values_buf: &mut InternBuffer, data: &[u8], out: &mut Vec<u8>) { + match self.insert_pos(values_buf, data) { + Ok(_) => unreachable!("value already exists"), + Err(idx) => { + let slot = &mut self.slots[idx]; + // Must always spill final slot so can always create a value less than + if idx != 254 && slot.value.is_none() { + out.push(idx as u8 + 2); + slot.value = Some(values_buf.insert(data)) + } else { + out.push(idx as u8 + 1); + slot.child + .get_or_insert_with(Default::default) + .insert(values_buf, data, out); + } + } + } + } +} + +#[cfg(test)] +mod tests { + use super::*; + use rand::prelude::*; + + // Clippy isn't smart enough to understand dropping mutability + #[allow(clippy::needless_collect)] + fn test_intern_values(values: &[u64]) { + let mut interner = OrderPreservingInterner::default(); + + // Intern a single value at a time to check ordering + let interned: Vec<_> = values + .iter() + .flat_map(|v| interner.intern([Some(&v.to_be_bytes())])) + .map(Option::unwrap) + .collect(); + + let interned: Vec<_> = interned + .into_iter() + .map(|x| interner.normalized_key(x)) + .collect(); + + for (i, a) in interned.iter().enumerate() { + for (j, b) in interned.iter().enumerate() { + let interned_cmp = a.cmp(b); + let values_cmp = values[i].cmp(&values[j]); + assert_eq!( + interned_cmp, values_cmp, + "({:?} vs {:?}) vs ({} vs {})", + a, b, values[i], values[j] + ) + } + } + } + + #[test] + #[cfg_attr(miri, ignore)] + fn test_interner() { Review Comment: The test is too heavy for miri, it makes it OOM -- This is an automated message from the Apache Git Service. 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