Rubberduck VBA Parser, Episode VI: Return of the Abstraction

VBA comment syntax is fun... and VBA line continuation makes it even more fun.

Picture a VBA module like this:

Rem this is an old-style comment.
' this is a more standard comment

Rem this _
    is _
    a _
    multiline _
    comment

Private Sub Foo() ' this _
                    is _
                    also _
                    a _
                    multiline _
                    comment _
                    _
...don't do this at home.

End Sub

'@TestMethod
Private Sub Bar()
    ' todo: call Foo
End Sub

(no wonder syntax highlighting is getting confused!)

If you don't know what Rubberduck is: Rubberduck is a COM add-in for the VBE / VBA's IDE that I'm building with ...@RubberDuck. I have a branch where I've burned the whole parser namespace and replaced it with ANTLR-generated code.

The only problem is that the .g4 VB6 grammar file I'm using to generate the parser, does not support comments. So I ended up [re-]inserting an abstraction layer between ANTLR's IParseTree and the rest of Rubberduck.. albeit very differently this time.

I added two methods to the IRubberduckParser interface:

/// 
/// Parses all code modules in specified project.
/// 
/// Returns an IParseTree for each code module in the project; the qualified module name being the key.
IEnumerable Parse(VBProject vbProject);

IEnumerable ParseComments(VBComponent vbComponent);

The VbModuleParseResult class encapsulates a module's IParseTree and its CommentNodes:

```
public class VbModuleParseResult
{
public VbModuleParseResult(QualifiedModuleName qualifiedName, IParseTree parseTree, IEnumerable comments)
{
_qualifiedName = qualifiedName;
_parseTree = parseTree;
_comments = comments;
}

private readonly QualifiedModuleName _qualifiedName;
public QualifiedModuleName QualifiedName { get { return _qualifiedName; } }

private IParseTree _parseTree;
public IParseTree ParseTree { get { return _parseTree; } }

Let's tackle this piece of code, shall we?

public IEnumerable ParseComments(VBComponent component)
{
    var code = component.CodeModule.Code();
    var qualifiedName = new QualifiedModuleName(component.Collection.Parent.Name, component.Name);

    var commentBuilder = new StringBuilder();
    var continuing = false;

    var startLine = 0;
    var startColumn = 0;

    for (var i = 0; i < code.Length; i++)
    {
        var line = code[i];                
        var index = 0;

        if (continuing || line.HasComment(out index))
        {
            startLine = continuing ? startLine : i;
            startColumn = continuing ? startColumn : index;

            var commentLength = line.Length - index;

            continuing = line.EndsWith("_");
            if (!continuing)
            {
                commentBuilder.Append(line.Substring(index, commentLength).TrimStart());
                var selection = new Selection(startLine + 1, startColumn + 1, i + 1, line.Length);

                var result = new CommentNode(commentBuilder.ToString(), new QualifiedSelection(qualifiedName, selection));
                commentBuilder.Clear();

                yield return result;
            }
            else
            {
                // ignore line continuations in comment text:
                commentBuilder.Append(line.Substring(index, commentLength).TrimStart()); 
            }
        }
    }
}

Alright, first thing I see is some duplication:

continuing = line.EndsWith("_");
if (!continuing)
{
    commentBuilder.Append(line.Substring(index, commentLength).TrimStart());
    var selection = new Selection(startLine + 1, startColumn + 1, i + 1, line.Length);

    var result = new CommentNode(commentBuilder.ToString(), new QualifiedSelection(qualifiedName, selection));
    commentBuilder.Clear();

    yield return result;
}
else
{
    // ignore line continuations in comment text:
    commentBuilder.Append(line.Substring(index, commentLength).TrimStart()); 
}

commentBuilder.Append(line.Substring(index, commentLength).TrimStart()); is duplicated.

So lets remove it.

continuing = line.EndsWith("_");
commentBuilder.Append(line.Substring(index, commentLength).TrimStart());
if (!continuing)
{
    var selection = new Selection(startLine + 1, startColumn + 1, i + 1, line.Length);

    var result = new CommentNode(commentBuilder.ToString(), new QualifiedSelection(qualifiedName, selection));
    commentBuilder.Clear();

    yield return result;
}

Additionally, there's some duplication here...

if (continuing || line.HasComment(out index))
{
    startLine = continuing ? startLine : i;
    startColumn = continuing ? startColumn : index;

    var commentLength = line.Length - index;

    continuing = line.EndsWith("_");
    commentBuilder.Append(line.Substring(index, commentLength).TrimStart());
    if (!continuing)
    {
        var selection = new Selection(startLine + 1, startColumn + 1, i + 1, line.Length);

        var result = new CommentNode(commentBuilder.ToString(), new QualifiedSelection(qualifiedName, selection));
        commentBuilder.Clear();

        yield return result;
    }
}

continuing is checked three times between changes. That's a bit of a waste.

Maybe we can fix it?

if (continuing || line.HasComment(out index))
{
    if(!continuing){
        startLine = i;
        startColumn = index;
    }
    var commentLength = line.Length - index;

    continuing = line.EndsWith("_");
    commentBuilder.Append(line.Substring(index, commentLength).TrimStart());
    if (!continuing)
    {
        var selection = new Selection(startLine + 1, startColumn + 1, i + 1, line.Length);

        var result = new CommentNode(commentBuilder.ToString(), new QualifiedSelection(qualifiedName, selection));
        commentBuilder.Clear();

        yield return result;
    }
}

Hmm, we're still checking it twice... and if (a || b) { if(!a) structures are messy. I wonder if there's something we can do about that?

Invert it, maybe? (if A OR B is true then NOT A implies B)

if (continuing || line.HasComment(out index))
{
    if(line.HasComment(out index)){
        startLine = i;
        startColumn = index;
    }

Hmm...

Here's a batshit crazy idea.

Put a for loop inside the current for loop to repeat the bits you need.

```
for (var i = 0; i < code.Length; i++)
{
var line = code[i];
var index = 0;

if (line.HasComment(out index))
{
startLine = i;
startColumn = index;

//multiline comment forloop...
for (; i < code.Length; i++)
{
line = code[i];
var commentLength = line.Length - index;
commentBuilder.Append(line.Substring(index, commentLength).TrimStart());
if(!line.EndsWith("_"))
{
break;
}
}
var selection = new S

StackExchange Code Review Q#79532, answer score: 7