[Ironruby-core] Code Review: MutableString5

Peter Bacon Darwin bacondarwin at googlemail.com
Sat May 10 05:27:28 EDT 2008

This is a big old diff to search through.  I couldn't work out a way of
easily patching it onto my source at home due to the folder differences.

I really like this hybrid idea and it looks like it will work well.  I have
one question with regards to encodings and KCODE.

I appreciate that String is changing between Ruby 1.8 and 1.9.  It appears
that this MutableString implementation is leaning more toward the 1.9
implementation (i.e. holding on to an Encoding within the String itself).


1.8 does hold the encoding and as I understand it the implicit encoding of
the bytes held in a String is driven off KCODE.  Is that correct?  If so you
have a number of scenarios which I think could cause problems with
MutableString holding on to its own Encoding, which stem from times when
KCODE is changed at runtime.  I'll try to describe a concrete example and
you can tell me where I am going wrong...


Assume that KCODE is set to UTF8.  If you create a String from an array of
bytes in Ruby, the bytes are just stored as-is.  You can do stuff which is
encoding dependent and UTF8 is assumed.

If you now change KCODE to say EUC, then the bytes in the String are
unchanged but now encoding dependent operations will possibly produce
different results on the same string since they interpret the bytes

The worry I have with MutableString, is that if you create a string from
bytes but then do an operation that requires it to be converted to a CLR
string internally.  What happens when you change KCODE?  You can't simply
change the Encoding value of the MutableString, since if you then access the
bytes you will not get the same bytes back as were originally put in.  I
suppose, on changing KCODE, you could go through all the strings in memory,
which have been converted from binary to CLR strings, and convert them (i.e.
back to bytes via the old encoding and then to CLR strings via the new
encoding).  What would be the optimal solution in this case?


Again, I am not talking from a position of deep knowledge here so I may be
missing something really obvious.  But I thought it was worth asking the







From: ironruby-core-bounces at rubyforge.org
[mailto:ironruby-core-bounces at rubyforge.org] On Behalf Of Tomas Matousek
Sent: Friday,09 May 09, 2008 19:08
To: IronRuby External Code Reviewers
Cc: ironruby-core at rubyforge.org
Subject: [Ironruby-core] Code Review: MutableString5


tfpt review /shelveset:MutableString5;REDMOND\tomat


A new implementation for Ruby MutableString and Ruby regular expression

This is just the first pass, w/o optimizations and w/o encodings (Default
system encoding is used for all strings). 

Many improvements and adjustments will come in future, some hacks will be


Basic architecture:

MutableString holds on Content and Encoding. Content is an abstract class
that has three subclasses:

1)      StringContent

-          Holds on an instance of System.String - an immutable .NET string.
This is the default representation for strings coming from CLR methods and
for Ruby string literals. 

-          A textual write operation on the mutable string that has this
content representation will cause implicit conversion of the representation
to StringBuilderContent. 

-          A binary read/write operation triggers a transition to
BinaryContent using the Encoding stored on the owning MutableString.


2)      StringBuilderContent

-          Holds on an instance of System.Text.StringBuilder - a mutable
Unicode string. 

-          A binary read/write operation transforms the content to
BinaryContent representation.

-          StringBuilder is not optimal for some operations (requires
unnecessary copying), we may consider to replace it with resizable char[].


3)      BinaryContent

-          A textual read/write operation transforms the content to
StringBuilderContent representation.

-          List<byte> is currently used, but it doesn't fit many operations
very well. We should replace it by resizable byte[].


The content representation is changed based upon operations that are
performed on the mutable string. There is currently no limit on number of
content type switches, so if one alternates binary and textual operations
the conversion will take place for each one of them. Although this shouldn't
be a common case we may consider to add some counters and keep the
representation binary/textual based upon their values. 


The design assumes that the nature of operations implemented by library
methods is of two kinds: textual and binary. And that data that are once
treated as text are not usually treated as raw binary data later. Any text
in the IronRuby runtime is represented as a sequence of 16bit Unicode
characters (standard .NET representation). Each binary data treated as text
is converted to this representation, regardless of the encoding used for
storage representation in the file. The encoding is remembered in the
MutableString instance and the original representation could be always
recreated. Not all Unicode characters fit into 16 bits, therefore some
exotic ones are represented by multiple characters (surrogates). If there is
such a character in the string, some operations (e.g. indexing) might not be
precise anymore - the n-th item in the char[] isn't the n-th Unicode
character in the string (there might be escape characters). We believe this
impreciseness is not a real world issue and is worth performance gain and
implementation simplicity. 



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