test(writer): cover PatchedEncodingEncoder narrow-width + error paths

Add tests exercising every integer PType through toLongs/fromLongs/PTypeIO.set,
the non-primitive and unsupported-ptype throw paths, and the no-bit-width-gain
notApplicable branch. Coverage on PatchedEncodingEncoder goes from 42 missed
lines to 1 (an unreachable defensive default in fromLongs).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dfa1

writer/src/test/java/io/github/dfa1/vortex/writer/encode/PatchedEncodingEncoderTest.java

@@ -2,12 +2,15 @@
 
 import io.github.dfa1.vortex.core.DType;
 import io.github.dfa1.vortex.core.PType;
+import io.github.dfa1.vortex.core.VortexException;
 import io.github.dfa1.vortex.encoding.DTypes;
 import io.github.dfa1.vortex.encoding.EncodingId;
 import io.github.dfa1.vortex.reader.ReadRegistry;
 import io.github.dfa1.vortex.reader.array.Array;
+import io.github.dfa1.vortex.reader.array.ByteArray;
 import io.github.dfa1.vortex.reader.array.IntArray;
 import io.github.dfa1.vortex.reader.array.LongArray;
+import io.github.dfa1.vortex.reader.array.ShortArray;
 import io.github.dfa1.vortex.reader.decode.ArrayNode;
 import io.github.dfa1.vortex.reader.decode.DecodeContext;
 import io.github.dfa1.vortex.reader.decode.PatchedEncodingDecoder;
@@ -16,13 +19,15 @@
 import org.junit.jupiter.api.Nested;
 import org.junit.jupiter.api.Test;
 import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.EnumSource;
 import org.junit.jupiter.params.provider.ValueSource;
 
 import java.lang.foreign.Arena;
 import java.lang.foreign.MemorySegment;
 import java.util.List;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
 
 class PatchedEncodingEncoderTest {
 
@@ -119,6 +124,47 @@ void encode_roundTrip_i32_multipleChunks() {
             }
         }
 
+        @ParameterizedTest
+        @EnumSource(value = PType.class, names = {"I8", "U8", "I16", "U16", "I32", "U32", "I64", "U64"})
+        void encode_roundTrip_allIntegerPTypes(PType ptype) {
+            // Given: 100 small values + 1 outlier — exercises toLongs/PTypeIO.set for every width
+            int n = 100;
+            DType dtype = dtypeOf(ptype);
+            Object data = buildData(ptype, n);
+
+            // When
+            EncodeResult result = SUT.encode(dtype, data, EncodeTestHelper.testCtx());
+            Array decoded = decode(result, dtype, n);
+
+            // Then
+            for (int i = 0; i < n; i++) {
+                assertThat(readLong(decoded, i, ptype)).as("ptype %s index %d", ptype, i)
+                        .isEqualTo(expectedAt(ptype, i, n));
+            }
+        }
+
+        @Test
+        void encode_throws_onUnsupportedPType() {
+            // Given: a primitive float dtype slips past the instanceof check but has no integer mapping
+            double[] data = {1.0, 2.0, 3.0};
+
+            // When / Then
+            assertThatThrownBy(() -> SUT.encode(DTypes.F64, data, EncodeTestHelper.testCtx()))
+                    .isInstanceOf(VortexException.class)
+                    .hasMessageContaining("unsupported ptype");
+        }
+
+        @Test
+        void encode_throws_onNonPrimitiveDtype() {
+            // Given
+            long[] data = {1L, 2L, 3L};
+
+            // When / Then: non-primitive dtype is a programmer error on the encode path
+            assertThatThrownBy(() -> SUT.encode(DTypes.UTF8, data, EncodeTestHelper.testCtx()))
+                    .isInstanceOf(VortexException.class)
+                    .hasMessageContaining("expected primitive");
+        }
+
         @ParameterizedTest
         @ValueSource(ints = {10, 100, 1023, 1024, 1025, 2048})
         void encode_roundTrip_i64_variousLengths(int n) {
@@ -212,5 +258,122 @@ void encodeCascade_notApplicable_whenEmpty() {
             // Then
             assertThat(step.applicable()).isFalse();
         }
+
+        @Test
+        void encodeCascade_notApplicable_whenNonPrimitiveDtype() {
+            // Given
+            long[] data = {1L, 2L, 3L};
+
+            // When
+            CascadeStep step = SUT.encodeCascade(DTypes.UTF8, data, EncodeTestHelper.testCtx());
+
+            // Then
+            assertThat(step.applicable()).isFalse();
+        }
+
+        @Test
+        void encodeCascade_notApplicable_whenNoBitWidthGain() {
+            // Given: every value needs the full 64 bits, so no target width beats the raw cost
+            long[] data = new long[100];
+            for (int i = 0; i < data.length; i++) {
+                data[i] = Long.MIN_VALUE | i; // bit 63 set → width 64 for all
+            }
+
+            // When
+            CascadeStep step = SUT.encodeCascade(DTypes.I64, data, EncodeTestHelper.testCtx());
+
+            // Then
+            assertThat(step.applicable()).isFalse();
+        }
+
+        @ParameterizedTest
+        @EnumSource(value = PType.class, names = {"I8", "U8", "I16", "U16", "I32", "U32"})
+        void encodeCascade_applicable_smallIntegerPTypes(PType ptype) {
+            // Given: 100 small values + 1 outlier — exercises fromLongs for every narrow width
+            int n = 100;
+            DType dtype = dtypeOf(ptype);
+            Object data = buildData(ptype, n);
+
+            // When
+            CascadeStep step = SUT.encodeCascade(dtype, data, EncodeTestHelper.testCtx());
+
+            // Then
+            assertThat(step.applicable()).isTrue();
+            assertThat(step.openChildren()).hasSize(4);
+            assertThat(step.openChildren().get(0).childDtype()).isEqualTo(dtype);
+            assertThat(step.openChildren().get(3).childDtype()).isEqualTo(dtype);
+        }
+    }
+
+    private static DType dtypeOf(PType ptype) {
+        return new DType.Primitive(ptype, false);
+    }
+
+    /// Builds an array of the given primitive type: small values everywhere except a
+    /// single high outlier at the midpoint, which forces a patch.
+    private static Object buildData(PType ptype, int n) {
+        return switch (ptype) {
+            case I8, U8 -> {
+                byte[] a = new byte[n];
+                for (int i = 0; i < n; i++) {
+                    a[i] = (byte) (i & 3);
+                }
+                a[n / 2] = (byte) (ptype == PType.U8 ? 0xC8 : 0x7F);
+                yield a;
+            }
+            case I16, U16 -> {
+                short[] a = new short[n];
+                for (int i = 0; i < n; i++) {
+                    a[i] = (short) (i & 3);
+                }
+                a[n / 2] = (short) (ptype == PType.U16 ? 0xFFFF : 0x7FFF);
+                yield a;
+            }
+            case I32, U32 -> {
+                int[] a = new int[n];
+                for (int i = 0; i < n; i++) {
+                    a[i] = i & 3;
+                }
+                a[n / 2] = ptype == PType.U32 ? 0xFFFF_FFFF : 0x7FFF_FFFF;
+                yield a;
+            }
+            case I64, U64 -> {
+                long[] a = new long[n];
+                for (int i = 0; i < n; i++) {
+                    a[i] = i & 3;
+                }
+                a[n / 2] = ptype == PType.U64 ? -1L : Long.MAX_VALUE;
+                yield a;
+            }
+            default -> throw new IllegalArgumentException("unsupported: " + ptype);
+        };
+    }
+
+    /// Expected raw element value for [#buildData], used to compare against the decoded array.
+    private static long expectedAt(PType ptype, int i, int n) {
+        if (i != n / 2) {
+            return i & 3;
+        }
+        return switch (ptype) {
+            case I8 -> 0x7F;
+            case U8 -> (byte) 0xC8;
+            case I16 -> 0x7FFF;
+            case U16 -> (short) 0xFFFF;
+            case I32 -> 0x7FFF_FFFF;
+            case U32 -> 0xFFFF_FFFF;
+            case I64 -> Long.MAX_VALUE;
+            case U64 -> -1L;
+            default -> throw new IllegalArgumentException("unsupported: " + ptype);
+        };
+    }
+
+    private static long readLong(Array array, int i, PType ptype) {
+        return switch (ptype) {
+            case I8, U8 -> ((ByteArray) array).getByte(i);
+            case I16, U16 -> ((ShortArray) array).getShort(i);
+            case I32, U32 -> ((IntArray) array).getInt(i);
+            case I64, U64 -> ((LongArray) array).getLong(i);
+            default -> throw new IllegalArgumentException("unsupported: " + ptype);
+        };
     }
 }