[GitHub] [incubator-daffodil] mbeckerle commented on a change in pull request #192: Fix issue where suspensions with mixed byteOrder caused incorrect bitOrder to be used

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mbeckerle commented on a change in pull request #192: Fix issue where suspensions with mixed byteOrder caused incorrect bitOrder to be used
URL: https://github.com/apache/incubator-daffodil/pull/192#discussion_r262228845
 
 

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 File path: daffodil-io/src/main/scala/org/apache/daffodil/io/DataOutputStreamImplMixin.scala
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 @@ -757,32 +757,46 @@ trait DataOutputStreamImplMixin extends DataStreamCommonState
    * Assumed to be called from inner loops, so should be fast as possible.
    */
   protected final def putLongUnchecked(signedLong: Long, bitLengthFrom1To64: Int, finfo: FormatInfo): Boolean = {
+    // The order we check bitOrder vs byteOrder here is actually very
+    // important, primarily due to how suspensions end up resolved and data
+    // output streams (DOS) collapsed. To explain why:
     //
-    // based on bit and byte order, dispatch to specific code
+    // When we create new a buffered DOS, no byteOrder is really associated
+    // with the DOS, because a DOS is just a series of bytes. To figure out
+    // what bytes to write to the DOS, we use the FormatInfo associated with an
+    // element, call the appropriate function below to convert that element to
+    // bytes, and then write those bytes to the DOS. This works as expected
+    // most of the time.
     //
+    // However, a buffered DOS maintains a fragment byte, and at some point we
+    // collapse that fragment byte into a direct DOS using this function. But
+    // we just said that there not really a byteOrder associated with a DOS,
+    // only with elements. So if we do not know the byteOrder of the fragment
+    // byte of a DOS, how do we know which function below to call when
+    // collapsing a fragment byte?
+    //
+    // Well, we do keep track of the bitOrder, since we need to know if we
 
 Review comment:
   Excellent comment. 

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