jecsand838 commented on code in PR #8274:
URL: https://github.com/apache/arrow-rs/pull/8274#discussion_r2322665974


##########
arrow-avro/src/writer/encoder.rs:
##########
@@ -275,3 +384,434 @@ impl F64Encoder<'_> {
             .map_err(|e| ArrowError::IoError(format!("write f64: {e}"), e))
     }
 }
+
+struct Utf8GenericEncoder<'a, O: OffsetSizeTrait>(&'a GenericStringArray<O>);
+
+impl<'a, O: OffsetSizeTrait> Utf8GenericEncoder<'a, O> {
+    #[inline]
+    fn encode<W: Write + ?Sized>(&mut self, idx: usize, out: &mut W) -> 
Result<(), ArrowError> {
+        write_len_prefixed(out, self.0.value(idx).as_bytes())
+    }
+}
+
+type Utf8Encoder<'a> = Utf8GenericEncoder<'a, i32>;
+type Utf8LargeEncoder<'a> = Utf8GenericEncoder<'a, i64>;
+
+/// Unified field encoder:
+/// - Holds the inner `Encoder` (by value)
+/// - Tracks the column/site null buffer and whether any nulls exist
+/// - Carries per-site Avro `Nullability` and precomputed union branch (fast 
path)
+pub struct FieldEncoder<'a> {
+    encoder: Encoder<'a>,
+    nulls: Option<NullBuffer>,
+    has_nulls: bool,
+    nullability: Option<Nullability>,
+    /// Precomputed constant branch byte if the site is nullable but contains 
no nulls
+    pre: Option<u8>,
+}
+
+impl<'a> FieldEncoder<'a> {
+    fn make_encoder(
+        array: &'a dyn Array,
+        field: &Field,
+        plan: PlanRef<'_>,
+    ) -> Result<Self, ArrowError> {
+        let nulls = array.nulls().cloned();
+        let has_nulls = array.null_count() > 0;
+        let encoder = match plan {
+            FieldPlan::Struct { encoders } => {
+                let arr = array
+                    .as_any()
+                    .downcast_ref::<StructArray>()
+                    .ok_or_else(|| ArrowError::SchemaError("Expected 
StructArray".into()))?;
+                Encoder::Struct(Box::new(StructEncoder::try_new(arr, 
encoders)?))
+            }
+            FieldPlan::List {
+                items_nullability,
+                item_plan,
+            } => match array.data_type() {
+                DataType::List(_) => {
+                    let arr = array
+                        .as_any()
+                        .downcast_ref::<ListArray>()
+                        .ok_or_else(|| ArrowError::SchemaError("Expected 
ListArray".into()))?;
+                    Encoder::List(Box::new(ListEncoder32::try_new(
+                        arr,
+                        *items_nullability,
+                        item_plan.as_ref(),
+                    )?))
+                }
+                DataType::LargeList(_) => {
+                    let arr = array
+                        .as_any()
+                        .downcast_ref::<LargeListArray>()
+                        .ok_or_else(|| ArrowError::SchemaError("Expected 
LargeListArray".into()))?;
+                    Encoder::LargeList(Box::new(ListEncoder64::try_new(
+                        arr,
+                        *items_nullability,
+                        item_plan.as_ref(),
+                    )?))
+                }
+                other => {
+                    return Err(ArrowError::SchemaError(format!(
+                        "Avro array site requires Arrow List/LargeList, found: 
{other:?}"
+                    )))
+                }
+            },
+            FieldPlan::Scalar => match array.data_type() {
+                DataType::Boolean => 
Encoder::Boolean(BooleanEncoder(array.as_boolean())),
+                DataType::Utf8 => {
+                    
Encoder::Utf8(Utf8GenericEncoder::<i32>(array.as_string::<i32>()))
+                }
+                DataType::LargeUtf8 => {
+                    
Encoder::Utf8Large(Utf8GenericEncoder::<i64>(array.as_string::<i64>()))
+                }
+                DataType::Int32 => 
Encoder::Int(IntEncoder(array.as_primitive::<Int32Type>())),
+                DataType::Int64 => 
Encoder::Long(LongEncoder(array.as_primitive::<Int64Type>())),
+                DataType::Float32 => {
+                    
Encoder::Float32(F32Encoder(array.as_primitive::<Float32Type>()))
+                }
+                DataType::Float64 => {
+                    
Encoder::Float64(F64Encoder(array.as_primitive::<Float64Type>()))
+                }
+                DataType::Binary => 
Encoder::Binary(BinaryEncoder(array.as_binary::<i32>())),
+                DataType::LargeBinary => {
+                    
Encoder::LargeBinary(BinaryEncoder(array.as_binary::<i64>()))
+                }
+                DataType::Timestamp(TimeUnit::Microsecond, _) => 
Encoder::Timestamp(LongEncoder(
+                    array.as_primitive::<TimestampMicrosecondType>(),
+                )),
+                other => {
+                    return Err(ArrowError::NotYetImplemented(format!(
+                        "Avro scalar type not yet supported: {other:?}"
+                    )));
+                }
+            },
+            other => {
+                return Err(ArrowError::NotYetImplemented(
+                    "Avro writer: {other:?} not yet supported".into(),
+                ));
+            }
+        };
+        Ok(Self {
+            encoder,
+            nulls,
+            has_nulls,
+            nullability: None,
+            pre: None,
+        })
+    }
+
+    #[inline]
+    fn has_nulls(&self) -> bool {
+        self.has_nulls
+    }
+
+    #[inline]
+    fn is_null(&self, idx: usize) -> bool {
+        self.nulls
+            .as_ref()
+            .is_some_and(|null_buffer| null_buffer.is_null(idx))
+    }
+
+    #[inline]
+    fn with_effective_nullability(mut self, order: Option<Nullability>) -> 
Self {
+        self.nullability = order;
+        self.pre = self.precomputed_union_value_branch(order);
+        self
+    }
+
+    #[inline]
+    fn precomputed_union_value_branch(&self, order: Option<Nullability>) -> 
Option<u8> {
+        match (order, self.has_nulls()) {
+            (Some(Nullability::NullFirst), false) => Some(0x02), // value 
branch index 1
+            (Some(Nullability::NullSecond), false) => Some(0x00), // value 
branch index 0
+            _ => None,
+        }
+    }
+
+    #[inline]
+    fn encode_inner<W: Write + ?Sized>(
+        &mut self,
+        idx: usize,
+        out: &mut W,
+    ) -> Result<(), ArrowError> {
+        self.encoder.encode(idx, out)
+    }
+
+    #[inline]
+    fn encode<W: Write + ?Sized>(&mut self, idx: usize, out: &mut W) -> 
Result<(), ArrowError> {
+        if let Some(b) = self.pre {
+            return out
+                .write_all(&[b])
+                .map_err(|e| ArrowError::IoError(format!("write union value 
branch: {e}"), e))
+                .and_then(|_| self.encode_inner(idx, out));
+        }
+        if let Some(order) = self.nullability {
+            let is_null = self.is_null(idx);
+            write_optional_index(out, is_null, order)?;
+            if is_null {
+                return Ok(());
+            }
+        }
+        self.encode_inner(idx, out)

Review Comment:
   @scovich I think you're really onto something here. I'll dive into this 
further to see how this can be simplified and re-thought out.



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