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|
//! Swizzle reads a `narinfo.parquet` file, usually produced by `narinfo2parquet`.
//!
//! It swizzles the reference list, ie it converts the references from absolute,
//! global identifiers (store path hashes) to indices into the `store_path_hash`
//! column (ie, row numbers), so that we can later walk the reference graph
//! efficiently.
//!
//! Path hashes are represented as non-null, 20-byte `Binary` values.
//! The indices are represented as 32-bit unsigned integers, with in-band nulls
//! represented by [INDEX_NULL] (the all-1 bit pattern), to permit swizzling
//! partial datasets.
//!
//! In essence, it converts from names to pointers, so that `weave` can simply
//! chase pointers to trace the live set. This replaces an `O(log(n))` lookup
//! with `O(1)` indexing, and produces a much denser representation that actually
//! fits in memory.
//!
//! The in-memory representation is at least 80% smaller, and the indices compress
//! well in Parquet due to both temporal locality of reference and the power law
//! distribution of reference "popularity".
//!
//! Only two columns are read from `narinfo.parquet`:
//!
//! * `store_path_hash :: PathHash`
//! * `references :: List[PathHash]`
//!
//! Output is written to `narinfo-references.parquet` in the form of a single
//! `List[u32]` column, `reference_idxs`.
//!
//! This file is inherently bound to the corresponding `narinfo.parquet`,
//! since it essentially contains pointers into this file.
use anyhow::Result;
use hashbrown::HashTable;
use polars::{
lazy::dsl::{col, SpecialEq},
prelude::*,
};
use weave::{as_fixed_binary, hash64, leak, load_ph_array, DONE, INDEX_NULL};
fn main() -> Result<()> {
let ph_array: &'static [[u8; 20]] = leak(load_ph_array()?);
// TODO(edef): re-parallelise this
// We originally parallelised on chunks, but ph_array is only a single chunk, due to how Parquet loading works.
// TODO(edef): outline the 64-bit hash prefix? it's an indirection, but it saves ~2G of memory
eprint!("… build index\r");
let ph_map: &'static HashTable<(u64, u32)> = {
let mut ph_map = HashTable::with_capacity(ph_array.len());
for (offset, item) in ph_array.iter().enumerate() {
let offset = offset as u32;
let hash = hash64(item);
ph_map.insert_unique(hash, (hash, offset), |&(hash, _)| hash);
}
&*Box::leak(Box::new(ph_map))
};
eprintln!("{DONE}");
let ph_to_idx = |key: &[u8; 20]| -> u32 {
let hash = hash64(key);
ph_map
.find(hash, |&(candidate_hash, candidate_index)| {
candidate_hash == hash && &ph_array[candidate_index as usize] == key
})
.map(|&(_, index)| index)
.unwrap_or(INDEX_NULL)
};
eprint!("… swizzle references\r");
LazyFrame::scan_parquet("narinfo.parquet", ScanArgsParquet::default())?
.with_column(
col("references")
.map(
move |series: Series| -> PolarsResult<Option<Series>> {
Ok(Some(
series
.list()?
.apply_to_inner(&|series: Series| -> PolarsResult<Series> {
let series = series.binary()?;
let mut out: Vec<u32> = Vec::with_capacity(series.len());
out.extend(as_fixed_binary(series).flatten().map(ph_to_idx));
Ok(Series::from_vec("reference_idxs", out))
})?
.into_series(),
))
},
SpecialEq::from_type(DataType::List(DataType::UInt32.into())),
)
.alias("reference_idxs"),
)
.select([col("reference_idxs")])
.with_streaming(true)
.sink_parquet(
"narinfo-references.parquet".into(),
ParquetWriteOptions::default(),
)?;
eprintln!("{DONE}");
Ok(())
}
|
