Rewrite IntMath.mod in terms of Math.floorMod.

RELNOTES=n/a
PiperOrigin-RevId: 755542679

Author: eamonnmcmanus

guava-tests/test/com/google/common/math/IntMathTest.java

@@ -25,6 +25,7 @@
 import static com.google.common.math.MathTesting.POSITIVE_INTEGER_CANDIDATES;
 import static com.google.common.math.ReflectionFreeAssertThrows.assertThrows;
 import static com.google.common.math.TestPlatform.intsCanGoOutOfRange;
+import static com.google.common.truth.Truth.assertWithMessage;
 import static java.lang.Math.min;
 import static java.math.BigInteger.valueOf;
 import static java.math.RoundingMode.FLOOR;
@@ -373,15 +374,17 @@ public void testDivByZeroAlwaysFails() {
   public void testMod() {
     for (int x : ALL_INTEGER_CANDIDATES) {
       for (int m : POSITIVE_INTEGER_CANDIDATES) {
-        assertEquals(valueOf(x).mod(valueOf(m)).intValue(), IntMath.mod(x, m));
+        assertWithMessage("%s mod %s", x, m)
+            .that(IntMath.mod(x, m))
+            .isEqualTo(valueOf(x).mod(valueOf(m)).intValue());
       }
     }
   }
 
-  public void testModNegativeModulusFails() {
+  public void testModNegativeModulus() {
     for (int x : POSITIVE_INTEGER_CANDIDATES) {
       for (int m : NEGATIVE_INTEGER_CANDIDATES) {
-        assertThrows(ArithmeticException.class, () -> IntMath.mod(x, m));
+        assertWithMessage("%s mod %s", x, m).that(IntMath.mod(x, m)).isEqualTo(Math.floorMod(x, m));
       }
     }
   }

guava-tests/test/com/google/common/math/LongMathTest.java

@@ -422,10 +422,10 @@ public void testIntMod() {
   }
 
   @GwtIncompatible // TODO
-  public void testIntModNegativeModulusFails() {
+  public void testIntModNegativeModulus() {
     for (long x : ALL_LONG_CANDIDATES) {
       for (int m : NEGATIVE_INTEGER_CANDIDATES) {
-        assertThrows(ArithmeticException.class, () -> LongMath.mod(x, m));
+        assertEquals(Math.floorMod(x, m), LongMath.mod(x, m));
       }
     }
   }
@@ -448,10 +448,10 @@ public void testMod() {
   }
 
   @GwtIncompatible // TODO
-  public void testModNegativeModulusFails() {
+  public void testModNegativeModulus() {
     for (long x : ALL_LONG_CANDIDATES) {
       for (long m : NEGATIVE_LONG_CANDIDATES) {
-        assertThrows(ArithmeticException.class, () -> LongMath.mod(x, m));
+        assertEquals(Math.floorMod(x, m), LongMath.mod(x, m));
       }
     }
   }

guava/src/com/google/common/math/IntMath.java

@@ -385,8 +385,7 @@ public static int mod(int x, int m) {
     if (m <= 0) {
       throw new ArithmeticException("Modulus " + m + " must be > 0");
     }
-    int result = x % m;
-    return (result >= 0) ? result : result + m;
+    return Math.floorMod(x, m);
   }
 
   /**

guava/src/com/google/common/math/LongMath.java

@@ -470,11 +470,7 @@ public static int mod(long x, int m) {
    */
   @GwtIncompatible // TODO
   public static long mod(long x, long m) {
-    if (m <= 0) {
-      throw new ArithmeticException("Modulus must be positive");
-    }
-    long result = x % m;
-    return (result >= 0) ? result : result + m;
+    return Math.floorMod(x, m);
   }
 
   /**