fix(usdc): decode integer-compressed float arrays

fixtures/integer_compressed_floats.usda

@@ -0,0 +1,13 @@
+#usda 1.0
+
+# Regression fixture for USDC's `i` (integer-compressed float array) encoding.
+# Generated with OpenUSD `usdcat -o integer_compressed_floats.usdc`.
+def Scope "IntegerCompressedFloats"
+{
+    float[] weights = [
+        1.0, 0.0, 1.0, 0.0,
+        0.0, 1.0, 0.0, 1.0,
+        1.0, 1.0, 0.0, 0.0,
+        0.0, 0.0, 1.0, 1.0,
+    ]
+}

src/usdc/mod.rs

@@ -772,6 +772,24 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn test_read_integer_compressed_float_array() -> Result<()> {
+        // OpenUSD serialises an all-integral float array with the `i` code:
+        // its LZ4 payload still needs Usd_IntegerCompression decoding.
+        let data = read_file("fixtures/integer_compressed_floats.usdc")?;
+        let weights = data
+            .get_field(&sdf::path("/IntegerCompressedFloats.weights")?, "default")?
+            .into_owned()
+            .try_as_float_vec()
+            .unwrap();
+
+        assert_eq!(
+            weights,
+            vec![1.0, 0.0, 1.0, 0.0, 0.0, 1.0, 0.0, 1.0, 1.0, 1.0, 0.0, 0.0, 0.0, 0.0, 1.0, 1.0,]
+        );
+        Ok(())
+    }
+
     #[test]
     fn test_read_halfs() -> Result<()> {
         let data = read_file("fixtures/floats.usdc")?;

src/usdc/reader.rs

@@ -662,9 +662,13 @@ impl<R: io::Read + io::Seek> CrateFile<R> {
             let code = self.reader.read_pod::<u8>()?;
 
             match code {
-                // Compressed integers
+                // Compressed integers. Pixar's `_ReadCompressedInts` runs the
+                // LZ4 block through `Usd_IntegerCompression` decoding after
+                // decompression, so the payload must be integer-decoded
+                // (`read_encoded_ints`), not reinterpreted as raw `i32`s
+                // straight out of LZ4.
                 b'i' => {
-                    let ints: Vec<i32> = self.read_compressed(count)?;
+                    let ints: Vec<i32> = self.read_encoded_ints(count)?;
                     ints.into_iter().map(|i| cast(i).unwrap()).collect()
                 }
                 // Lookup table and indexes
@@ -1546,6 +1550,30 @@ mod tests {
     use super::*;
     use std::fs;
 
+    #[test]
+    fn integer_compressed_float_fixture_uses_i_encoding() -> Result<()> {
+        let mut file = CrateFile::open(fs::File::open("fixtures/integer_compressed_floats.usdc")?)?;
+        let value = file
+            .fields
+            .iter()
+            .find_map(|field| {
+                let ty = field.value_rep.ty().ok()?;
+                (file.tokens[field.token_index] == "default"
+                    && ty == Type::Float
+                    && field.value_rep.is_array()
+                    && field.value_rep.is_compressed())
+                .then_some(field.value_rep)
+            })
+            .expect("fixture must contain one compressed float default value");
+
+        file.set_position(value.payload())?;
+        let (count, compressed) = file.unpack_array_len(value, ArrayKind::Floats)?;
+        assert_eq!(count, 16);
+        assert!(compressed);
+        assert_eq!(file.reader.read_pod::<u8>()?, b'i');
+        Ok(())
+    }
+
     #[test]
     fn test_read_crate_struct() {
         let path = "./vendor/usd-wg-assets/full_assets/ElephantWithMonochord/SoC-ElephantWithMonochord.usdc";