feat(HashingAlgorithm): provide public access via GetAlgorithm

doc/articles/multihash.md

@@ -52,5 +52,24 @@ data into a [CID](cid.md).
 
 ## Registry
 
-The [hashing registry](xref:Ipfs.Registry.HashingAlgorithm) contains the metadata on a hashing algorithm.  You can use
+The [hashing registry](xref:Ipfs.Registry.HashingAlgorithm) contains the metadata on hashing algorithms.  You can use
 [Register](xref:Ipfs.Registry.HashingAlgorithm.Register*) to add a new hashing algorithm.
+
+### Example
+
+Using an hashing algorithm.  Note that `ComputeHash` can take a byte array or a `Stream`.
+
+```csharp
+public void GetHasher()
+{
+    using (var hasher = HashingAlgorithm.GetAlgorithm("sha3-256"))
+    {
+        Assert.IsNotNull(hasher);
+        var input = new byte[] { 0xe9 };
+        var expected = "f0d04dd1e6cfc29a4460d521796852f25d9ef8d28b44ee91ff5b759d72c1e6d6".ToHexBuffer();
+
+        var actual = hasher.ComputeHash(input);
+        CollectionAssert.AreEqual(expected, actual);
+    }
+}
+```

src/Registry/HashingAlgorithm.cs

@@ -223,5 +223,30 @@ public static IEnumerable<HashingAlgorithm> All
             get { return Names.Values; }
         }
 
+        /// <summary>
+        ///   Gets the <see cref="HashAlgorithm"/> with the specified IPFS multi-hash name.
+        /// </summary>
+        /// <param name="name">
+        ///   The name of a hashing algorithm, see <see href="https://github.com/multiformats/multicodec/blob/master/table.csv"/>
+        ///   for IPFS defined names.
+        /// </param>
+        /// <returns>
+        ///   The hashing implementation associated with the <paramref name="name"/>.
+        /// </returns>
+        /// <exception cref="KeyNotFoundException">
+        ///   When <paramref name="name"/> is not registered.
+        /// </exception>
+        public static HashAlgorithm GetAlgorithm(string name)
+        {
+            try
+            {
+                return HashingAlgorithm.Names[name].Hasher();
+            }
+            catch (KeyNotFoundException)
+            {
+                throw new KeyNotFoundException($"Hash algorithm '{name}' is not registered.");
+            }
+        }
+
     }
 }

test/Registry/HashingAlgorithmTest.cs

@@ -11,6 +11,26 @@ namespace Ipfs.Registry
     [TestClass]
     public class HashingAlgorithmTest
     {
+        [TestMethod]
+        public void GetHasher()
+        {
+            using (var hasher = HashingAlgorithm.GetAlgorithm("sha3-256"))
+            {
+                Assert.IsNotNull(hasher);
+                var input = new byte[] { 0xe9 };
+                var expected = "f0d04dd1e6cfc29a4460d521796852f25d9ef8d28b44ee91ff5b759d72c1e6d6".ToHexBuffer();
+
+                var actual = hasher.ComputeHash(input);
+                CollectionAssert.AreEqual(expected, actual);
+            }
+        }
+
+        [TestMethod]
+        public void GetHasher_Unknown()
+        {
+            ExceptionAssert.Throws<KeyNotFoundException>(() => HashingAlgorithm.GetAlgorithm("unknown"));
+        }
+
         [TestMethod]
         public void HashingAlgorithm_Bad_Name()
         {