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Custom StreamReader with FSM



This post is about Finite State Machines, the Flying Spaghetti Monster is for next time.

For one of the EAI projects I'm working on I needed a specific StreamReader that adds field-wrappers (around the fields, obviously) of a char-based stream. For example:

field1-1,field1-2;field2-1,field2-2 => "field1-1","field1-2";"field2-1","field2-2"

My first approach was the currently popular test-driven one: a default passthru read method and a bunch of tests, that of-course all initially failed. I trusted my creativeness in order to randomly identify all different types of input-string combinations:

NoFieldSeperator
NoRecordSeperator
EmptyString
FieldWithSingleWrapCharInfix
FieldWithSingleWrapCharPostfix
etc...

Inside the reader I added different if-then-else clauses to manipulate the output, until a given test succeeded. Every newly added if-clause of course potentially broke 1 or more of the previous tests. One can imagine it took quite a lot of time and way too many iterations to get this very simple parser working!

After a while I was lucky enough to get all cases working: hurray! In fact, not really :-(
I didn't have complete confidence in what I had just built and how it was built. This wouldn't work for more complex scenarios. How could I ever be sure that I didn't forget some special case? I needed a more formal approach...
 
So I modeled the reader as a finite state machine, thereby defining the actions i.e. the desired output character(s) for every input character depending on the state. It's very easy, just start drawing on a piece of paper and think about the states and the output for every input. The reader can be in the 2 following states: 'inside a field' or 'outside a field'.


I have defined 'special' input tokens that require special handling (different action) or trigger a state transition:

 RD: record delimiter
FD: field delimiter
WS: whitespace
EoF: end of file
WC: wrap character
C: any other char

The output (or action) for a given input greatly depends upon the state the reader is in and if the input character triggers a transition. EoF, field- and record-delimiters trigger a state transition from insidefield to outsidefield.

I didn't put the actions on the diagram.  For every 1 char in the input, the reader can output 0, 1 or 2 chars. If the input contains the field-wrap character inside a field, it should be doubled. This requires the ability to cache a few characters.

field1,fiel"d2; => "field1","fiel""d2"

Whitespaces could be configured to be skipped if non-significant (trimming).
Also RD or FD when 'OutsideField' could be configured to generate empty fields.
For this implementation I choose the object-oriented state design pattern.

internal abstract class StreamState
        {
            public abstract int CharInput(WrappingFsmStreamReader context, int character);
            public abstract int WhitespaceInput(WrappingFsmStreamReader context);
            public abstract int FieldDelimiterInput(WrappingFsmStreamReader context);
            public abstract int RecordDelimiterInput(WrappingFsmStreamReader context);
            public abstract int WrapInput(WrappingFsmStreamReader context);
            public abstract int EoFInput(WrappingFsmStreamReader context);
        }

Here is a sample.

So next time you need a custom StreamReader it makes sense to model it as a FSM. Define and document the states and behavior, use this inside knowledge to write the unit tests.
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Posted On Monday, May 25, 2009 9:55 AM
Filed Under [ BizTalk - EAI - B2B ]

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