Add isValid method to Multibase class

README.md

@@ -17,6 +17,7 @@ Simply clone this repo.
 byte[] data = ...
 String encoded = Multibase.encode(Multibase.Base.Base58BTC, data);
 byte[] decoded = Multibase.decode(encoded);
+boolean isValid = Multibase.isValid(encoded);
 ```
 
 ## Dependency

src/main/java/io/ipfs/multibase/Multibase.java

@@ -1,6 +1,7 @@
 package io.ipfs.multibase;
 
 import java.util.Map;
+import java.util.Optional;
 import java.util.TreeMap;
 
 import io.ipfs.multibase.binary.Base32;
@@ -45,14 +46,21 @@ public enum Base {
                 lookup.put(b.prefix, b);
         }
 
-        public static Base lookup(String data) {
+        private static Optional<Base> lookupOptional(String data) {
+            if (data == null || data.isEmpty())
+                return Optional.empty();
             String p = Character.toString(data.codePointAt(0));
             Base base = lookup.get(p);
             if (base != null)
-                return base;
+                return Optional.of(base);
             if (data.startsWith(Base256Emoji.prefix))
-                return Base256Emoji;
-            throw new IllegalArgumentException("Unknown Multibase type: " + p);
+                return Optional.of(Base256Emoji);
+            return Optional.empty();
+        }
+
+        public static Base lookup(String data) {
+            return lookupOptional(data)
+                    .orElseThrow(() -> new IllegalArgumentException("Unknown Multibase type: " + data));
         }
     }
 
@@ -154,4 +162,22 @@ private static String safeSubstringFromIndexOne(String data) {
         int charIndex = data.offsetByCodePoints(0, 1);
         return data.substring(charIndex);
     }
+
+    /**
+     * Check if the given data has a valid multibase prefix.
+     *
+     * <p>
+     * Please note that "having a valid prefix" is NOT the same as "being an
+     * entirely valid multibase string"; even if <tt>true</tt>, it's still entirely
+     * possible for {@link #decode(String)} to throw an
+     * <tt>IllegalArgumentException</tt>, if prefix is valid, but the following
+     * data is not.
+     *
+     * @param data Multibase string to check.
+     * @return true if the data has a valid multibase prefix, false otherwise; but
+     *         see above.
+     */
+    public static boolean hasValidPrefix(String data) {
+        return Base.lookupOptional(data).isPresent();
+    }
 }

src/test/java/io/ipfs/multibase/MultibaseBadInputsTest.java

@@ -1,5 +1,6 @@
 package io.ipfs.multibase;
 
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertThrows;
 
 import java.util.Arrays;
@@ -29,4 +30,17 @@ public void badInputTest(String input) {
         });
     }
 
+    public static Collection<String> invalidPrefix() {
+        return Arrays.asList(
+                "2", // '2' is not a valid encoding marker
+                "", // Empty string is not a valid multibase
+                "🫕🚀" // This not a valid Emoji Multibase (note how it's inverted)
+        );
+    }
+
+    @MethodSource("invalidPrefix")
+    @ParameterizedTest(name = "{index}: \"{0}\"")
+    public void invalidPrefix(String input) {
+        assertFalse(Multibase.hasValidPrefix(input));
+    }
 }

src/test/java/io/ipfs/multibase/MultibaseTest.java

@@ -8,6 +8,7 @@
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertArrayEquals;
+import static org.junit.jupiter.api.Assertions.assertTrue;
 
 public class MultibaseTest {
 
@@ -67,6 +68,12 @@ public void testDecode(Multibase.Base base, byte[] raw, String encoded) {
         assertArrayEquals(raw, output, String.format("Expected %s, but got %s", bytesToHex(raw), bytesToHex(output)));
     }
 
+    @MethodSource("data")
+    @ParameterizedTest(name = "{index}: {0}, {2}")
+    public void testHasValidPrefix(Multibase.Base base, byte[] raw, String encoded) {
+        assertTrue(Multibase.hasValidPrefix(encoded));
+    }
+
     //Copied from https://stackoverflow.com/a/140861
     private static byte[] hexToBytes(String s) {
         int len = s.length();