Get compress performance to match paper algorithm 4

.gitignore

@@ -1,9 +1,2 @@
 /target
 .idea/
-
-
-# Added by cargo
-#
-# already existing elements were commented out
-
-#/target

Cargo.toml

@@ -1,6 +1,10 @@
 [package]
 name = "fsst-rs"
 version = "0.0.1"
+description = "Pure-Rust implementation of Fast Static Symbol Tables algorithm for string compression"
+authors = ["SpiralDB Developers <[email protected]>"]
+license = "Apache-2.0"
+repository = "https://github.com/spiraldb/fsst"
 edition = "2021"
 
 [lints.rust]
@@ -22,7 +26,16 @@ use_debug = { level = "deny" }
 criterion = "0.5"
 lz4 = "1"
 
+[[example]]
+name = "round_trip"
+bench = false
+test = false
+
 [[bench]]
 name = "compress"
 harness = false
-bench = true
+
+[[test]]
+name = "correctness"
+test = true
+bench = false

benches/compress.rs

@@ -4,13 +4,14 @@
 //!
 //! Also contains LZ4 baseline.
 #![allow(missing_docs)]
+use core::str;
 use std::io::{Cursor, Read, Write};
 
 use criterion::{black_box, criterion_group, criterion_main, Criterion};
 use lz4::liblz4::BlockChecksum;
 use lz4::{BlockSize, ContentChecksum};
 
-use fsst_rs::{train, Code};
+use fsst_rs::{train, ESCAPE_CODE};
 
 const CORPUS: &str = include_str!("dracula.txt");
 const TEST: &str = "I found my smattering of German very useful here";
@@ -26,17 +27,17 @@ fn bench_fsst(c: &mut Criterion) {
     let plaintext = TEST.as_bytes();
 
     let compressed = table.compress(plaintext);
-    let escape_count = compressed
-        .iter()
-        .filter(|b| **b == Code::ESCAPE_CODE)
-        .count();
+    let escape_count = compressed.iter().filter(|b| **b == ESCAPE_CODE).count();
     let ratio = (plaintext.len() as f64) / (compressed.len() as f64);
     println!(
         "Escapes = {escape_count}/{}, compression_ratio = {ratio}",
         compressed.len()
     );
 
-    assert_eq!(table.decompress(&compressed), TEST.as_bytes());
+    let decompressed = table.decompress(&compressed);
+    let decompressed = str::from_utf8(&decompressed).unwrap();
+    println!("DECODED: {}", decompressed);
+    assert_eq!(decompressed, TEST);
 
     group.bench_function("compress-single", |b| {
         b.iter(|| black_box(table.compress(black_box(plaintext))));
@@ -50,29 +51,6 @@ fn bench_fsst(c: &mut Criterion) {
 fn bench_lz4(c: &mut Criterion) {
     let mut group = c.benchmark_group("lz4");
 
-    // {
-    //     let compressed = Vec::with_capacity(10_000);
-    //     let mut encoder = lz4::EncoderBuilder::new()
-    //         .block_size(BlockSize::Max64KB)
-    //         .build(compressed)
-    //         .unwrap();
-    //
-    //     encoder.write_all(TEST.as_bytes()).unwrap();
-    //     let (compressed, result) = encoder.finish();
-    //     result.unwrap();
-    //
-    //     let ratio = (TEST.as_bytes().len() as f64) / (compressed.len() as f64);
-    //     println!("LZ4 compress_ratio = {ratio}");
-    //
-    //     // ensure decodes cleanly
-    //     let cursor = Cursor::new(compressed);
-    //     let mut decoder = lz4::Decoder::new(cursor).unwrap();
-    //     let mut output = String::new();
-    //
-    //     decoder.read_to_string(&mut output).unwrap();
-    //     assert_eq!(output.as_str(), TEST);
-    // }
-
     group.bench_function("compress-single", |b| {
         let mut compressed = Vec::with_capacity(100_000_000);
         let mut encoder = lz4::EncoderBuilder::new()

examples/round_trip.rs

@@ -0,0 +1,19 @@
+//! Simple example where we show round-tripping a string through the static symbol table.
+
+use core::str;
+
+fn main() {
+    // Train on a sample.
+    let sample = "the quick brown fox jumped over the lazy dog";
+    let trained = fsst_rs::train(sample.as_bytes());
+    let compressed = trained.compress(sample.as_bytes());
+    println!("compressed: {} => {}", sample.len(), compressed.len());
+    // decompress now
+    let decode = trained.decompress(&compressed);
+    let output = str::from_utf8(&decode).unwrap();
+    println!(
+        "decoded to the original: len={} text='{}'",
+        decode.len(),
+        output
+    );
+}

rust-toolchain.toml

@@ -1,5 +1,4 @@
 [toolchain]
-channel = "nightly-2024-06-19"
+channel = "stable"
 components = ["rust-src", "rustfmt", "clippy"]
 profile = "minimal"
-

src/builder.rs

@@ -7,7 +7,8 @@
 use std::cmp::Ordering;
 use std::collections::BinaryHeap;
 
-use crate::{Code, Symbol, SymbolTable};
+use crate::find_longest::FindLongestSymbol;
+use crate::{Symbol, SymbolTable, MAX_CODE};
 
 #[derive(Debug, Clone)]
 struct Counter {
@@ -21,29 +22,29 @@ struct Counter {
 impl Counter {
     fn new() -> Self {
         Self {
-            counts1: vec![0; Code::CODE_MAX as usize],
-            counts2: vec![vec![0; Code::CODE_MAX as usize]; Code::CODE_MAX as usize],
+            counts1: vec![0; MAX_CODE as usize],
+            counts2: vec![vec![0; MAX_CODE as usize]; MAX_CODE as usize],
         }
     }
 
     #[inline]
-    fn record_count1(&mut self, code1: Code) {
-        self.counts1[code1.0 as usize] += 1;
+    fn record_count1(&mut self, code1: u16) {
+        self.counts1[code1 as usize] += 1;
     }
 
     #[inline]
-    fn record_count2(&mut self, code1: Code, code2: Code) {
-        self.counts2[code1.0 as usize][code2.0 as usize] += 1;
+    fn record_count2(&mut self, code1: u16, code2: u16) {
+        self.counts2[code1 as usize][code2 as usize] += 1;
     }
 
     #[inline]
-    fn count1(&self, code: Code) -> usize {
-        self.counts1[code.0 as usize]
+    fn count1(&self, code: u16) -> usize {
+        self.counts1[code as usize]
     }
 
     #[inline]
-    fn count2(&self, code1: Code, code2: Code) -> usize {
-        self.counts2[code1.0 as usize][code2.0 as usize]
+    fn count2(&self, code1: u16, code2: u16) -> usize {
+        self.counts2[code1 as usize][code2 as usize]
     }
 }
 
@@ -65,6 +66,9 @@ pub fn train(corpus: impl AsRef<[u8]>) -> SymbolTable {
     let mut table = SymbolTable::default();
     // TODO(aduffy): handle truncating/sampling if corpus > requires sample size.
     let sample = corpus.as_ref();
+    if sample.is_empty() {
+        return table;
+    }
     for _generation in 0..MAX_GENERATIONS {
         let counter = table.compress_count(sample);
         table = table.optimize(counter);
@@ -81,13 +85,13 @@ impl SymbolTable {
         let len = sample.len();
         let mut prev_code = self.find_longest_symbol(sample);
         counter.record_count1(prev_code);
-        let mut pos = self.symbols[prev_code.0 as usize].len();
+        let mut pos = self.symbols[prev_code as usize].len();
 
         while pos < len {
             let code = self.find_longest_symbol(&sample[pos..len]);
             counter.record_count1(code);
             counter.record_count2(prev_code, code);
-            pos += self.symbols[code.0 as usize].len();
+            pos += self.symbols[code as usize].len();
             prev_code = code;
         }
 
@@ -100,17 +104,15 @@ impl SymbolTable {
         let mut res = SymbolTable::default();
         let mut pqueue = BinaryHeap::new();
         for code1 in 0..511 {
-            let code1 = Code::from_u16(code1);
-            let symbol1 = self.symbols[code1.0 as usize];
+            let symbol1 = self.symbols[code1 as usize];
             let gain = counters.count1(code1) * symbol1.len();
             pqueue.push(Candidate {
                 symbol: symbol1,
                 gain,
             });
 
             for code2 in 0..511 {
-                let code2 = Code::from_u16(code2);
-                let symbol2 = &self.symbols[code2.0 as usize];
+                let symbol2 = &self.symbols[code2 as usize];
                 // If either symbol is zero-length, or if merging would yield a symbol of
                 // length greater than 8, skip.
                 if symbol1.len() + symbol2.len() >= 8 || symbol1.is_empty() || symbol2.is_empty() {
@@ -133,10 +135,13 @@ impl SymbolTable {
         }
 
         // Pop the 255 best symbols.
-        pqueue
-            .iter()
-            .take(255)
-            .for_each(|candidate| res.insert(candidate.symbol));
+        let mut n_symbols = 0;
+        while !pqueue.is_empty() && n_symbols < 255 {
+            let candidate = pqueue.pop().unwrap();
+            if res.insert(candidate.symbol) {
+                n_symbols += 1;
+            }
+        }
 
         res
     }
@@ -181,7 +186,7 @@ impl Ord for Candidate {
 
 #[cfg(test)]
 mod test {
-    use crate::{train, Code};
+    use crate::{train, ESCAPE_CODE};
 
     #[test]
     fn test_builder() {
@@ -193,26 +198,26 @@ mod test {
         let compressed = table.compress(text.as_bytes());
 
         // Ensure that the compressed string has no escape bytes
-        assert!(compressed.iter().all(|b| *b != Code::ESCAPE_CODE));
+        assert!(compressed.iter().all(|b| *b != ESCAPE_CODE));
 
-        // Ensure that we can compress a string with no values seen at training time.
+        // Ensure that we can compress a string with no values seen at training time, with escape bytes
         let compressed = table.compress("xyz123".as_bytes());
         assert_eq!(
             compressed,
             vec![
-                Code::ESCAPE_CODE,
+                ESCAPE_CODE,
                 b'x',
-                Code::ESCAPE_CODE,
+                ESCAPE_CODE,
                 b'y',
-                Code::ESCAPE_CODE,
+                ESCAPE_CODE,
                 b'z',
-                Code::ESCAPE_CODE,
+                ESCAPE_CODE,
                 b'1',
-                Code::ESCAPE_CODE,
+                ESCAPE_CODE,
                 b'2',
-                Code::ESCAPE_CODE,
+                ESCAPE_CODE,
                 b'3',
             ]
-        )
+        );
     }
 }

src/find_longest/mod.rs

@@ -0,0 +1,5 @@
+mod naive;
+
+pub trait FindLongestSymbol {
+    fn find_longest_symbol(&self, text: &[u8]) -> u16;
+}

src/find_longest/naive.rs

@@ -0,0 +1,28 @@
+use crate::find_longest::FindLongestSymbol;
+use crate::SymbolTable;
+
+// Find the code that maps to a symbol with longest-match to a piece of text.
+//
+// This is the naive algorithm that just scans the whole table and is very slow.
+
+impl FindLongestSymbol for SymbolTable {
+    // NOTE(aduffy): if you don't disable inlining, this function won't show up in profiles.
+    #[inline(never)]
+    fn find_longest_symbol(&self, text: &[u8]) -> u16 {
+        debug_assert!(!text.is_empty(), "text must not be empty");
+
+        // Find the code that best maps to the provided text table here.
+        // Start with the code corresponding to the escape of the first character in the text
+        let mut best_code = text[0] as u16;
+        let mut best_overlap = 1;
+        for code in 256..511 {
+            let symbol = &self.symbols[code as usize];
+            if symbol.is_prefix(text) && symbol.len() > best_overlap {
+                best_code = code;
+                best_overlap = symbol.len();
+            }
+        }
+
+        best_code
+    }
+}