Add a test to EncoderStringWriter

Making sure that
#247 is not actually
a bug.

src/write/encoder_string_writer.rs

@@ -44,11 +44,6 @@ use std::io;
 /// assert_eq!("base64: YXNkZg==", &buf);
 /// ```
 ///
-/// # Panics
-///
-/// Calling `write()` (or related methods) or `finish()` after `finish()` has completed without
-/// error is invalid and will panic.
-///
 /// # Performance
 ///
 /// Because it has to validate that the base64 is UTF-8, it is about 80% as fast as writing plain
@@ -144,6 +139,7 @@ mod tests {
         engine::Engine, tests::random_engine, write::encoder_string_writer::EncoderStringWriter,
     };
     use rand::Rng;
+    use std::cmp;
     use std::io::Write;
 
     #[test]
@@ -158,9 +154,8 @@ mod tests {
             orig_data.clear();
             normal_encoded.clear();
 
-            for _ in 0..size {
-                orig_data.push(rng.gen());
-            }
+            orig_data.resize(size, 0);
+            rng.fill(&mut orig_data[..]);
 
             let engine = random_engine(&mut rng);
             engine.encode_string(&orig_data, &mut normal_encoded);
@@ -172,6 +167,40 @@ mod tests {
 
             let stream_encoded = stream_encoder.into_inner();
 
+            assert_eq!(normal_encoded, stream_encoded);
+        }
+    }
+    #[test]
+    fn incremental_writes() {
+        let mut rng = rand::thread_rng();
+        let mut orig_data = Vec::<u8>::new();
+        let mut normal_encoded = String::new();
+
+        let size = 5_000;
+
+        for _ in 0..size {
+            orig_data.clear();
+            normal_encoded.clear();
+
+            orig_data.resize(size, 0);
+            rng.fill(&mut orig_data[..]);
+
+            let engine = random_engine(&mut rng);
+            engine.encode_string(&orig_data, &mut normal_encoded);
+
+            let mut stream_encoder = EncoderStringWriter::new(&engine);
+            // write small nibbles of data
+            let mut offset = 0;
+            while offset < size {
+                let nibble_size = cmp::min(rng.gen_range(0..=64), size - offset);
+                let len = stream_encoder
+                    .write(&orig_data[offset..offset + nibble_size])
+                    .unwrap();
+                offset += len;
+            }
+
+            let stream_encoded = stream_encoder.into_inner();
+
             assert_eq!(normal_encoded, stream_encoded);
         }
     }