[Variant] test: add variant object tests with different sizes (apache#7896)

# Which issue does this PR close?

We generally require a GitHub issue to be filed for all bug fixes and
enhancements and this helps us generate change logs for our releases.
You can link an issue to this PR using the GitHub syntax.

- Closes apache#7821 .

# Rationale for this change

- [x] VariantObject with between 2^8 and 2^16 elements (
field_id_size_minus_1 = 1, 2 byte field ids)

- [x] VariantObject with between 2^16 and 2^24 elements (
field_id_size_minus_1 = 2, 3 byte field ids)

- [x] VariantObject with between 2^24 and 2^32 elements (
field_id_size_minus_1 = 3, 4 byte field ids)
	- Inserting 2^24 + 1 elements takes too long.

- [x] VariantObject with total child data length between 2^8 and 2^16
elements ( field_offset_size_minus_1 = 1, 2 byte field offsets)

- [x] VariantObject with total child data length between 2^16 and 2^24
elements ( field_offset_size_minus_1 = 2, 3 byte field offsets)

- [x] VariantObject with total child data length between 2^24 and 2^32
elements ( field_offset_size_minus_1 = 3, 4 byte field offsets)

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

Author: odysa

parquet-variant/src/variant/object.rs

@@ -389,13 +389,15 @@ impl<'m, 'v> VariantObject<'m, 'v> {
 
 #[cfg(test)]
 mod tests {
+    use crate::VariantBuilder;
+
     use super::*;
 
     #[test]
     fn test_variant_object_simple() {
         // Create metadata with field names: "age", "name", "active" (sorted)
         // Header: version=1, sorted=1, offset_size=1 (offset_size_minus_one=0)
-        // So header byte = 00_0_1_0001 = 0x10
+        // So header byte = 00_0_1_0001 = 0x11
         let metadata_bytes = vec![
             0b0001_0001,
             3, // dictionary size
@@ -607,4 +609,113 @@ mod tests {
             ArrowError::InvalidArgumentError(ref msg) if msg.contains("Tried to extract byte(s) ..16 from 15-byte buffer")
         ));
     }
+
+    fn test_variant_object_with_count(count: i32, expected_field_id_size: OffsetSizeBytes) {
+        let field_names: Vec<_> = (0..count).map(|val| val.to_string()).collect();
+        let mut builder =
+            VariantBuilder::new().with_field_names(field_names.iter().map(|s| s.as_str()));
+
+        let mut obj = builder.new_object();
+
+        for i in 0..count {
+            obj.insert(&field_names[i as usize], i);
+        }
+
+        obj.finish().unwrap();
+        let (metadata, value) = builder.finish();
+        let variant = Variant::new(&metadata, &value);
+
+        if let Variant::Object(obj) = variant {
+            assert_eq!(obj.len(), count as usize);
+
+            assert_eq!(obj.get(&field_names[0]).unwrap(), Variant::Int32(0));
+            assert_eq!(
+                obj.get(&field_names[(count - 1) as usize]).unwrap(),
+                Variant::Int32(count - 1)
+            );
+            assert_eq!(
+                obj.header.field_id_size, expected_field_id_size,
+                "Expected {}-byte field IDs, got {}-byte field IDs",
+                expected_field_id_size as usize, obj.header.field_id_size as usize
+            );
+        } else {
+            panic!("Expected object variant");
+        }
+    }
+
+    #[test]
+    fn test_variant_object_257_elements() {
+        test_variant_object_with_count((1 << 8) + 1, OffsetSizeBytes::Two); // 2^8 + 1, expected 2-byte field IDs
+    }
+
+    #[test]
+    fn test_variant_object_65537_elements() {
+        test_variant_object_with_count((1 << 16) + 1, OffsetSizeBytes::Three);
+        // 2^16 + 1, expected 3-byte field IDs
+    }
+
+    /* Can't run this test now as it takes 45x longer than other tests
+    #[test]
+    fn test_variant_object_16777217_elements() {
+        test_variant_object_with_count((1 << 24) + 1, OffsetSizeBytes::Four);
+        // 2^24 + 1, expected 4-byte field IDs
+    }
+     */
+
+    #[test]
+    fn test_variant_object_small_sizes_255_elements() {
+        test_variant_object_with_count(255, OffsetSizeBytes::One);
+    }
+
+    fn test_variant_object_with_large_data(
+        data_size_per_field: usize,
+        expected_field_offset_size: OffsetSizeBytes,
+    ) {
+        let num_fields = 20;
+        let mut builder = VariantBuilder::new();
+        let mut obj = builder.new_object();
+
+        let str_val = "a".repeat(data_size_per_field);
+
+        for val in 0..num_fields {
+            let key = format!("id_{val}");
+            obj.insert(&key, str_val.as_str());
+        }
+
+        obj.finish().unwrap();
+        let (metadata, value) = builder.finish();
+        let variant = Variant::new(&metadata, &value);
+
+        if let Variant::Object(obj) = variant {
+            assert_eq!(obj.len(), num_fields);
+            assert_eq!(
+                obj.header.field_offset_size, expected_field_offset_size,
+                "Expected {}-byte field offsets, got {}-byte field offsets",
+                expected_field_offset_size as usize, obj.header.field_offset_size as usize
+            );
+        } else {
+            panic!("Expected object variant");
+        }
+    }
+
+    #[test]
+    fn test_variant_object_child_data_0_byte_offsets_minus_one() {
+        test_variant_object_with_large_data(10, OffsetSizeBytes::One);
+    }
+
+    #[test]
+    fn test_variant_object_256_bytes_child_data_3_byte_offsets() {
+        test_variant_object_with_large_data(256 + 1, OffsetSizeBytes::Two); // 2^8 - 2^16 elements
+    }
+
+    #[test]
+    fn test_variant_object_16777216_bytes_child_data_4_byte_offsets() {
+        test_variant_object_with_large_data(65536 + 1, OffsetSizeBytes::Three); // 2^16 - 2^24 elements
+    }
+
+    #[test]
+    fn test_variant_object_65535_bytes_child_data_2_byte_offsets() {
+        test_variant_object_with_large_data(16777216 + 1, OffsetSizeBytes::Four);
+        // 2^24
+    }
 }