scovich commented on code in PR #8349:
URL: https://github.com/apache/arrow-rs/pull/8349#discussion_r2370419272


##########
arrow-avro/src/reader/record.rs:
##########
@@ -1518,19 +1104,340 @@ impl Decoder {
                     .map_err(|e| ArrowError::ParseError(e.to_string()))?;
                 Arc::new(vals)
             }
-            Self::Union(fields, type_ids, offsets, encodings, _, None) => {
-                flush_union!(fields, type_ids, offsets, encodings)
-            }
-            Self::Union(fields, type_ids, offsets, encodings, _, 
Some(union_resolution)) => {
-                match &mut union_resolution.kind {
-                    UnionResolvedKind::Both { .. } | 
UnionResolvedKind::FromSingle { .. } => {
-                        flush_union!(fields, type_ids, offsets, encodings)
-                    }
-                    UnionResolvedKind::ToSingle { target } => 
target.flush(nulls)?,
+            Self::Union(u) => u.flush(nulls)?,
+        })
+    }
+}
+
+#[derive(Debug)]
+struct DispatchLut {
+    to_reader: Box<[i16]>,
+    promotion: Box<[Promotion]>,
+}
+
+impl DispatchLut {
+    fn from_writer_to_reader(promotion_map: &[Option<(usize, Promotion)>]) -> 
Self {
+        let mut to_reader = Vec::with_capacity(promotion_map.len());
+        let mut promotion = Vec::with_capacity(promotion_map.len());
+        for map in promotion_map {
+            match *map {
+                Some((idx, promo)) => {
+                    debug_assert!(idx <= i16::MAX as usize);
+                    to_reader.push(idx as i16);

Review Comment:
   Just double checking -- `UnionFields` forbids negative type ids, so we 
actually _can_ use them as sentinel values?



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