--- a/src/goodjava/io/IoUtils.java	Sat May 16 12:33:41 2020 +0300
+++ b/src/goodjava/io/IoUtils.java	Sat May 16 17:56:02 2020 -0600
@@ -57,6 +57,16 @@
 		in.close();
 	}
 
+	public static long checksum(InputStream in) throws IOException {
+		long cs = 0;
+		int c;
+		while( (c=in.read()) != -1 ) {
+			cs = 31 * cs + c;
+		}
+		in.close();
+		return cs;
+	}
+
 
 	static {
 		// undo restrictions of modern scum
@@ -71,4 +81,4 @@
 		return (SSLServerSocketFactory)SSLServerSocketFactory.getDefault();
 	}
 
-}
\ No newline at end of file
+}