fix: respect negative values on deserialization in REQ sketch

Author: proost

src/main/java/org/apache/datasketches/req/ReqSerDe.java

@@ -205,7 +205,7 @@ static final Compactor extractCompactor(final PositionalSegment posSeg, final bo
     final float[] arr = new float[count];
     posSeg.getFloatArray(arr, 0, count);
     float minItem = Float.MAX_VALUE;
-    float maxItem = Float.MIN_VALUE;
+    float maxItem = -Float.MAX_VALUE;
     for (int i = 0; i < count; i++) {
       minItem = min(minItem, arr[i]);
       maxItem = max(maxItem, arr[i]);

src/test/java/org/apache/datasketches/req/ReqCompactorTest.java

@@ -101,4 +101,47 @@ private static void checkSerDeImpl(final int k, final boolean hra) {
     assertEquals(fbuf2.getDelta(), delta);
     assertTrue(fbuf.isEqualTo(fbuf2));
   }
+
+  @Test
+  public void checkSerDeWithNegativeValues() {
+    checkSerDeNegativeImpl(12, false);
+    checkSerDeNegativeImpl(12, true);
+  }
+
+  private static void checkSerDeNegativeImpl(final int k, final boolean hra) {
+    final ReqCompactor c1 = new ReqCompactor((byte)0, hra, k, null);
+    final int nomCap = 2 * 3 * k;
+    final FloatBuffer fbuf = c1.getBuffer();
+
+    for (int i = 1; i <= nomCap; i++) {
+      fbuf.append(-i); //all negative values
+    }
+    final byte[] c1ser = c1.toByteArray();
+    final PositionalSegment posSeg = PositionalSegment.wrap(MemorySegment.ofArray(c1ser));
+    final Compactor compactor = ReqSerDe.extractCompactor(posSeg, fbuf.isSorted(), hra);
+    assertEquals(compactor.minItem, -nomCap);
+    assertEquals(compactor.maxItem, -1f);
+  }
+
+  @Test
+  public void checkSerDeWithMixedValues() {
+    checkSerDeMixedImpl(12, false);
+    checkSerDeMixedImpl(12, true);
+  }
+
+  private static void checkSerDeMixedImpl(final int k, final boolean hra) {
+    final ReqCompactor c1 = new ReqCompactor((byte)0, hra, k, null);
+    final int nomCap = 2 * 3 * k;
+    final int half = nomCap / 2;
+    final FloatBuffer fbuf = c1.getBuffer();
+
+    for (int i = 0; i < nomCap; i++) {
+      fbuf.append(i - half); // range: -half to half-1
+    }
+    final byte[] c1ser = c1.toByteArray();
+    final PositionalSegment posSeg = PositionalSegment.wrap(MemorySegment.ofArray(c1ser));
+    final Compactor compactor = ReqSerDe.extractCompactor(posSeg, fbuf.isSorted(), hra);
+    assertEquals(compactor.minItem, (float) -half);
+    assertEquals(compactor.maxItem, (float) (half - 1));
+  }
 }

src/test/java/org/apache/datasketches/req/ReqSketchCrossLanguageTest.java

@@ -51,6 +51,30 @@ public void generateBinariesForCompatibilityTesting() throws IOException {
     }
   }
 
+  @Test(groups = {GENERATE_JAVA_FILES})
+  public void generateNegativeBinariesForCompatibilityTesting() throws IOException {
+    final int[] nArr = {1, 10};
+    for (final int n: nArr) {
+      final ReqSketch sk = ReqSketch.builder().build();
+      for (int i = 1; i <= n; i++) {
+        sk.update(-i);
+      }
+      putBytesToJavaPath("req_float_negative_n" + n + "_java.sk", sk.toByteArray());
+    }
+  }
+
+  @Test(groups = {GENERATE_JAVA_FILES})
+  public void generateMixedBinariesForCompatibilityTesting() throws IOException {
+    final int[] nArr = {1, 10};
+    for (final int n: nArr) {
+      final ReqSketch sk = ReqSketch.builder().build();
+      for (int i = -n; i <= n; i++) {
+        sk.update(i);
+      }
+      putBytesToJavaPath("req_float_mixed_n" + n + "_java.sk", sk.toByteArray());
+    }
+  }
+
   @Test(groups = {CHECK_CPP_FILES})
   public void deserializeFromCpp() throws IOException {
     final int[] nArr = {0, 1, 10, 100, 1000, 10000, 100000, 1000000};