feat: re-enable ALP-RD as top-level float compressor (#1725)

Fixes issue seen in #1723

In #1000, changes were made to remove ALP-RD as a top-level compressor
and instead to only use it for patches. However, it seems that it was
not getting selected anymore, whether that was due to the patching cost
overhead or something else. This was noticed by a user, and confirmed by
me in a Python shell.

<img width="946" alt="image"
src="https://github.com/user-attachments/assets/c42caed5-12e3-448b-aea6-3f33a7c97bfc"
/>

After this change, ALP-RD indeed does get selected again.

<img width="907" alt="image"
src="https://github.com/user-attachments/assets/bbb996fc-5223-43ed-9b4c-4b0262a417dc"
/>

vortex-sampling-compressor/src/compressors/alp.rs

@@ -56,6 +56,7 @@ impl EncodingCompressor for ALPCompressor {
             .excluding(self)
             .compress(&encoded, like.as_ref().and_then(|l| l.child(0)))?;
 
+        // Attempt to compress patches with ALP-RD encoding
         let compressed_patches = patches
             .map(|p| {
                 ctx.auxiliary("patches")
@@ -76,6 +77,7 @@ impl EncodingCompressor for ALPCompressor {
     fn used_encodings(&self) -> HashSet<EncodingRef> {
         HashSet::from([
             &ALPEncoding as EncodingRef,
+            // ALP-RD + BitPacking possibly used for patches
             &ALPRDEncoding,
             &BitPackedEncoding,
         ])

vortex-sampling-compressor/src/lib.rs

@@ -50,8 +50,11 @@ mod sampling_compressor;
 
 pub use sampling_compressor::*;
 
-pub const DEFAULT_COMPRESSORS: [CompressorRef; 15] = [
+use crate::compressors::alp_rd::ALPRDCompressor;
+
+pub const DEFAULT_COMPRESSORS: [CompressorRef; 16] = [
     &ALPCompressor as CompressorRef,
+    &ALPRDCompressor,
     &BITPACK_WITH_PATCHES,
     &DEFAULT_CHUNKED_COMPRESSOR,
     &ConstantCompressor,
@@ -72,8 +75,9 @@ pub const DEFAULT_COMPRESSORS: [CompressorRef; 15] = [
 ];
 
 #[cfg(not(target_arch = "wasm32"))]
-pub const ALL_COMPRESSORS: [CompressorRef; 18] = [
+pub const ALL_COMPRESSORS: [CompressorRef; 19] = [
     &ALPCompressor as CompressorRef,
+    &ALPRDCompressor,
     &BITPACK_WITH_PATCHES,
     &DEFAULT_CHUNKED_COMPRESSOR,
     &ConstantCompressor,
@@ -94,8 +98,9 @@ pub const ALL_COMPRESSORS: [CompressorRef; 18] = [
 ];
 
 #[cfg(target_arch = "wasm32")]
-pub const ALL_COMPRESSORS: [CompressorRef; 16] = [
+pub const ALL_COMPRESSORS: [CompressorRef; 17] = [
     &ALPCompressor as CompressorRef,
+    &ALPRDCompressor,
     &BITPACK_WITH_PATCHES,
     &DEFAULT_CHUNKED_COMPRESSOR,
     &ConstantCompressor,

vortex-sampling-compressor/src/sampling_compressor.rs

@@ -368,3 +368,28 @@ pub(crate) fn find_best_compression<'a>(
 
     Ok(best)
 }
+
+#[cfg(test)]
+mod tests {
+    use itertools::Itertools;
+    use vortex_alp::ALPRDEncoding;
+    use vortex_array::array::PrimitiveArray;
+    use vortex_array::encoding::Encoding;
+    use vortex_array::IntoArrayData;
+
+    use crate::SamplingCompressor;
+
+    #[test]
+    fn test_default() {
+        let values = (0..4096)
+            .map(|x| (x as f64) / 1234567890.0f64)
+            .collect_vec();
+        let array = PrimitiveArray::from(values).into_array();
+
+        let compressed = SamplingCompressor::default()
+            .compress(&array, None)
+            .unwrap()
+            .into_array();
+        assert_eq!(compressed.encoding().id(), ALPRDEncoding::ID);
+    }
+}