From bbc8f04db2f1916de2385620c10c5293bacaf102 Mon Sep 17 00:00:00 2001
From: Sebastian Rabenhorst <sebastian.rabenhorst@shopify.com>
Date: Tue, 5 May 2026 10:12:18 -0400
Subject: [PATCH] Pre-size dedup HashTable in
 GenericByteDictionaryBuilder::with_capacity
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`with_capacity` left the internal dedup `HashTable` at default capacity
(0), forcing 0 → 4 → 8 → 16 → … resize+rehash cycles on the first inserts
even when callers passed a non-zero `value_capacity`. `new()` already sizes
the dedup table from the keys-builder capacity; `with_capacity` should do
the same from `value_capacity`.

Adds a regression test asserting `dedup.capacity() >= value_capacity`
after construction.

Closes #9907.

Co-authored-by: AI <noreply@anthropic.com>
---
 .../generic_bytes_dictionary_builder.rs       | 21 ++++++++++++++++++-
 1 file changed, 20 insertions(+), 1 deletion(-)

diff --git a/arrow-array/src/builder/generic_bytes_dictionary_builder.rs b/arrow-array/src/builder/generic_bytes_dictionary_builder.rs
index 956014a72b7d..4ce451aec81e 100644
--- a/arrow-array/src/builder/generic_bytes_dictionary_builder.rs
+++ b/arrow-array/src/builder/generic_bytes_dictionary_builder.rs
@@ -84,7 +84,7 @@ where
     ) -> Self {
         Self {
             state: Default::default(),
-            dedup: Default::default(),
+            dedup: HashTable::with_capacity(value_capacity),
             keys_builder: PrimitiveBuilder::with_capacity(keys_capacity),
             values_builder: GenericByteBuilder::<T>::with_capacity(value_capacity, data_capacity),
         }
@@ -647,6 +647,25 @@ mod tests {
         test_bytes_dictionary_builder::<GenericBinaryType<i32>>(vec![b"abc", b"def"]);
     }
 
+    #[test]
+    fn test_with_capacity_presizes_dedup() {
+        // `with_capacity` must size the dedup `HashTable` from `value_capacity`,
+        // otherwise the first inserts force a chain of resize+rehash cycles.
+        let value_capacity = 128;
+        let builder =
+            GenericByteDictionaryBuilder::<Int32Type, GenericStringType<i32>>::with_capacity(
+                256,
+                value_capacity,
+                value_capacity * 32,
+            );
+        assert!(
+            builder.dedup.capacity() >= value_capacity,
+            "dedup HashTable not pre-sized: got capacity {}, expected >= {}",
+            builder.dedup.capacity(),
+            value_capacity,
+        );
+    }
+
     fn test_bytes_dictionary_builder_finish_cloned<T>(values: Vec<&T::Native>)
     where
         T: ByteArrayType,