VarInt: fix long values accepted as an int value

`-1` and `Integer.MIN_VALUE` (and all values inbetween) are in fact
very large VarInts that can only be expressed as a long. So their
test data have been moved from `VarIntTest.integerTestVectors` to
`longTestVectors`.

`intValue()` is changed to guard against such cases, and a helper
`fitsInt()` is introduced.

JavaDocs are added to `intValue()` and `longValue()` to clarify the
expected value ranges.

core/src/main/java/org/bitcoinj/base/VarInt.java

@@ -107,12 +107,36 @@ public class VarInt {
         originallyEncodedSize = copy.originallyEncodedSize;
     }
 
+    /**
+     * Gets the value as a long. For values greater than {@link Long#MAX_VALUE} the returned long
+     * will be negative. It is still to be interpreted as an unsigned value.
+     *
+     * @return value as a long
+     */
     public long longValue() {
         return value;
     }
 
-    public int intValue() {
+    /**
-        return Math.toIntExact(value);
+     * Determine if the value would fit an int, i.e. it is in the range of {@code 0} to {@link Integer#MAX_VALUE}.
+     * If this is true, it's safe to call {@link #intValue()}.
+     *
+     * @return true if the value fits an int, false otherwise
+     */
+    public boolean fitsInt() {
+        return value >= 0 && value <= Integer.MAX_VALUE;
+    }
+
+    /**
+     * Gets the value as an unsigned int in the range of {@code 0} to {@link Integer#MAX_VALUE}.
+     *
+     * @return value as an unsigned int
+     * @throws ArithmeticException if the value doesn't fit an int
+     */
+    public int intValue() throws ArithmeticException {
+        check(fitsInt(), () ->
+                new ArithmeticException("value too large for an int: " + Long.toUnsignedString(value)));
+        return (int) value;
     }
 
     /**

core/src/test/java/org/bitcoinj/base/VarIntTest.java

@@ -26,6 +26,7 @@ import java.nio.ByteBuffer;
 
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
 
 @RunWith(JUnitParamsRunner.class)
 public class VarIntTest {
@@ -34,6 +35,7 @@ public class VarIntTest {
     @Parameters(method = "integerTestVectors")
     public void testIntCreation(int value, int size) {
         VarInt a = VarInt.of(value);
+        assertTrue(a.fitsInt());
         assertEquals(value, a.intValue());
         assertEquals(size, a.getSizeInBytes());
         assertEquals(size, a.getOriginalSizeInBytes());
@@ -45,6 +47,7 @@ public class VarIntTest {
     @Parameters(method = "longTestVectors")
     public void testIntGetErr(int value, int size) {
         VarInt a = VarInt.of(value);
+        assertFalse(a.fitsInt());
         a.intValue();
     }
 
@@ -52,6 +55,7 @@ public class VarIntTest {
     @Parameters(method = "longTestVectors")
     public void testIntGetErr2(int value, int size) {
         VarInt a = VarInt.of(value);
+        assertFalse(a.fitsInt());
         VarInt.ofBytes(a.serialize(), 0).intValue();
     }
 
@@ -89,10 +93,7 @@ public class VarIntTest {
                 new Object[]{ 0x7FFF, 3},
                 new Object[]{ 0x8000, 3},
                 new Object[]{ 0x10000, 5},
-                new Object[]{ Integer.MIN_VALUE, 9},
                 new Object[]{ Integer.MAX_VALUE, 5},
-                // -1 shouldn't normally be passed, but at least stay consistent (bug regression test)
-                new Object[]{ -1, 9}
         };
     }
 
@@ -105,9 +106,9 @@ public class VarIntTest {
                 new Object[]{ 0xAABBCCDDL, 5},
                 new Object[]{ 0xFFFFFFFFL, 5},
                 new Object[]{ 0xCAFEBABEDEADBEEFL, 9},
+                new Object[]{ Integer.MIN_VALUE, 9},
                 new Object[]{ Long.MIN_VALUE, 9},
                 new Object[]{ Long.MAX_VALUE, 9},
-                // -1 shouldn't normally be passed, but at least stay consistent (bug regression test)
                 new Object[]{ -1L, 9}
         };
     }