Optimize fsst_compress_offsets_overflow_i32 by 2X locally

encodings/fsst/src/tests.rs

@@ -1,9 +1,7 @@
 // SPDX-License-Identifier: Apache-2.0
 // SPDX-FileCopyrightText: Copyright the Vortex contributors
 
-use rand::SeedableRng;
-use rand::rngs::StdRng;
-use rand::seq::IndexedRandom;
+use fsst::CompressorBuilder;
 use vortex_array::ArrayRef;
 use vortex_array::IntoArray;
 use vortex_array::LEGACY_SESSION;
@@ -118,12 +116,17 @@ fn test_fsst_array_ops() {
 /// [`VarBinBuilder<i32>`] for the FSST output and panicked in
 /// [`VarBinBuilder::append_value`] once cumulative compressed bytes crossed the boundary.
 ///
-/// The input is built with [`VarBinBuilder<i64>`] so the input itself does not panic, which
-/// confirms the overflow is on the FSST output side. After the fix the test must succeed
-/// with the row count preserved.
+/// The compressor is built with an empty symbol table, so every input byte misses the table
+/// and is escape-coded at exactly two output bytes per input byte. This makes the output
+/// size deterministic and crosses the i32 boundary with roughly half the input bytes that
+/// an incompressible-input construction would need. The input is built with
+/// [`VarBinBuilder<i64>`] so the input itself does not panic, which confirms the overflow
+/// is on the FSST output side. After the fix the test must succeed with the row count
+/// preserved.
 ///
-/// Allocates ~2.5 GiB for the input and ~2.5 GiB for the FSST output (~5 GiB total), so it
-/// is gated to CI runs and skipped when `VORTEX_SKIP_SLOW_TESTS` is set. To run it locally:
+/// Allocates ~1.1 GiB for the input and ~2.1 GiB for the FSST output (~3.2 GiB total), so
+/// it is gated to CI runs and skipped when `VORTEX_SKIP_SLOW_TESTS` is set. To run it
+/// locally:
 ///
 /// ```text
 /// CI=1 cargo test --release -p vortex-fsst fsst_compress_offsets
@@ -133,38 +136,28 @@ fn test_fsst_array_ops() {
 #[test_with::env(CI)]
 #[test_with::no_env(VORTEX_SKIP_SLOW_TESTS)]
 fn fsst_compress_offsets_overflow_i32() {
-    // High-entropy ASCII strings sliced from a random pool. FSST is a symbol-table
-    // compressor; pseudo-random data with no recurring byte sequences resists compression,
-    // so the compressed output stays close to input size and crosses the i32 boundary.
     const STRING_LEN: usize = 64 * 1024;
-    const TOTAL_BYTES: usize = (1usize << 31) + (512 << 20); // ~2.5 GiB
+    // Escape coding doubles every byte, so ~1.06 GiB of input compresses to ~2.13 GiB,
+    // comfortably past i32::MAX.
+    const TOTAL_BYTES: usize = (1usize << 30) + (64 << 20);
     const N: usize = TOTAL_BYTES / STRING_LEN;
-    const POOL_LEN: usize = 64 * 1024 * 1024;
-
-    // Printable ASCII alphabet so the result is valid UTF-8.
-    const ALPHABET: &[u8; 95] =
-        b" !\"#$%&'()*+,-./0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\\]^_`abcdefghijklmnopqrstuvwxyz{|}~";
-
-    let mut rng = StdRng::seed_from_u64(0xC0DE_C011_B711);
-    let pool: Vec<u8> = (0..POOL_LEN)
-        .map(|_| *ALPHABET.choose(&mut rng).unwrap())
-        .collect();
 
     println!("building large VarBinArray");
+    let string = vec![b'a'; STRING_LEN];
     let mut builder = VarBinBuilder::<i64>::with_capacity(N);
-    for i in 0..N {
-        let off = i.wrapping_mul(31337) % (POOL_LEN - STRING_LEN);
-        builder.append_value(&pool[off..off + STRING_LEN]);
+    for _ in 0..N {
+        builder.append_value(&string);
     }
     let array = builder.finish(DType::Utf8(Nullability::NonNullable));
 
-    println!("training FSST compressor");
-    let compressor = fsst_train_compressor(&array);
+    let compressor = CompressorBuilder::default().build();
     let len = array.len();
     let dtype = array.dtype().clone();
     let mut ctx = LEGACY_SESSION.create_execution_ctx();
 
     println!("compressing to FSST");
     let compressed = fsst_compress(array, len, &dtype, &compressor, &mut ctx);
     assert_eq!(compressed.len(), len);
+    // Prove the regression condition was exercised: compressed bytes crossed i32::MAX.
+    assert!(compressed.codes_bytes().len() > i32::MAX as usize);
 }