fix(ArrowBuilder): correct null bit checking logic and add tests

.gitignore

@@ -23,7 +23,7 @@
 DerivedData/
 Packages/
 xcuserdata/
-
+.vscode/
 # Docker
 /.docker/
 

Sources/Arrow/ArrowBufferBuilder.swift

@@ -42,7 +42,7 @@ public class BaseBufferBuilder {
     }
 
     public func isNull(_ index: UInt) -> Bool {
-        return self.nulls.length == 0 || BitUtility.isSet(index + self.offset, buffer: self.nulls)
+        return !BitUtility.isSet(index + self.offset, buffer: self.nulls)
     }
 
     func resizeLength(_ data: ArrowBuffer, len: UInt = 0) -> UInt {
@@ -429,10 +429,6 @@ public class ListBufferBuilder: BaseBufferBuilder, ArrowBufferBuilder {
         }
     }
 
-    public override func isNull(_ index: UInt) -> Bool {
-        return !BitUtility.isSet(index + self.offset, buffer: self.nulls)
-    }
-
     public func resize(_ length: UInt) {
         if length > self.offsets.length {
             let resizeLength = resizeLength(self.offsets)

Tests/ArrowTests/ArrayBuilderTest.swift

@@ -82,4 +82,98 @@ final class ArrayBuilderTests: XCTestCase {
         XCTAssertThrowsError(try ArrowArrayBuilders.loadBuilder(Int?.self))
         XCTAssertThrowsError(try ArrowArrayBuilders.loadBuilder(UInt?.self))
     }
+
+    func testFixedBufferBuilderIsNull() throws {
+        let builder: FixedBufferBuilder<Int32> = try FixedBufferBuilder()
+        builder.append(42)
+        builder.append(nil)
+        builder.append(7)
+
+        XCTAssertFalse(builder.isNull(0), "Value 42 should not be null")
+        XCTAssertTrue(builder.isNull(1), "Nil value should be null")
+        XCTAssertFalse(builder.isNull(2), "Value 7 should not be null")
+        XCTAssertEqual(builder.nullCount, 1)
+    }
+
+    func testBoolBufferBuilderIsNull() throws {
+        let builder = try BoolBufferBuilder()
+        builder.append(true)
+        builder.append(nil)
+        builder.append(false)
+
+        XCTAssertFalse(builder.isNull(0), "Value true should not be null")
+        XCTAssertTrue(builder.isNull(1), "Nil value should be null")
+        XCTAssertFalse(builder.isNull(2), "Value false should not be null")
+        XCTAssertEqual(builder.nullCount, 1)
+    }
+
+    func testVariableBufferBuilderIsNull() throws {
+        let builder = try VariableBufferBuilder<String>()
+        builder.append("hello")
+        builder.append(nil)
+        builder.append("world")
+
+        XCTAssertFalse(builder.isNull(0), "String 'hello' should not be null")
+        XCTAssertTrue(builder.isNull(1), "Nil value should be null")
+        XCTAssertFalse(builder.isNull(2), "String 'world' should not be null")
+        XCTAssertEqual(builder.nullCount, 1)
+    }
+
+    func testFixedBufferBuilderIsNullMultiple() throws {
+        let builder: FixedBufferBuilder<Double> = try FixedBufferBuilder()
+        builder.append(1.5)
+        builder.append(nil)
+        builder.append(2.5)
+        builder.append(nil)
+        builder.append(3.5)
+
+        XCTAssertFalse(builder.isNull(0))
+        XCTAssertTrue(builder.isNull(1))
+        XCTAssertFalse(builder.isNull(2))
+        XCTAssertTrue(builder.isNull(3))
+        XCTAssertFalse(builder.isNull(4))
+        XCTAssertEqual(builder.nullCount, 2)
+    }
+
+    func testBoolBufferBuilderIsNullMultiple() throws {
+        let builder = try BoolBufferBuilder()
+        builder.append(true)
+        builder.append(true)
+        builder.append(nil)
+        builder.append(false)
+        builder.append(nil)
+        builder.append(false)
+
+        XCTAssertFalse(builder.isNull(0))
+        XCTAssertFalse(builder.isNull(1))
+        XCTAssertTrue(builder.isNull(2))
+        XCTAssertFalse(builder.isNull(3))
+        XCTAssertTrue(builder.isNull(4))
+        XCTAssertFalse(builder.isNull(5))
+        XCTAssertEqual(builder.nullCount, 2)
+    }
+
+    func testFixedBufferBuilderIsNullAllNulls() throws {
+        let builder: FixedBufferBuilder<Int8> = try FixedBufferBuilder()
+        builder.append(nil)
+        builder.append(nil)
+        builder.append(nil)
+
+        XCTAssertTrue(builder.isNull(0))
+        XCTAssertTrue(builder.isNull(1))
+        XCTAssertTrue(builder.isNull(2))
+        XCTAssertEqual(builder.nullCount, 3)
+    }
+
+    func testFixedBufferBuilderIsNullNoNulls() throws {
+        let builder: FixedBufferBuilder<UInt64> = try FixedBufferBuilder()
+        builder.append(100)
+        builder.append(200)
+        builder.append(300)
+
+        XCTAssertFalse(builder.isNull(0))
+        XCTAssertFalse(builder.isNull(1))
+        XCTAssertFalse(builder.isNull(2))
+        XCTAssertEqual(builder.nullCount, 0)
+    }
 }