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Re: About shared substrings

From: Tom Lord
Subject: Re: About shared substrings
Date: Fri, 16 Jan 2004 14:00:27 -0800 (PST)

    > From: Roland Orre <address@hidden>

    > I discussed this issue with Mikael Djurfeldt today and then he came up
    > with the following solution:

    > SCM substring_table;

    > SCM scm_make_shared_substring (SCM parent, SCM start, SCM end)
    > {
    >   SCM substring;
    >   char *mem;
    >   int c_start, c_end;
    >   SCM_VALIDATE_SUBSTRING_SPEC_COPY (1, parent, mem,
    >                                     2, start, c_start,
    >                                     3, end, c_end);
    >   substring = scm_cell (SCM_MAKE_STRING_TAG (c_end - c_start),
    >                         (scm_t_bits) (mem + c_start));
    >   scm_hash_set_x (substring_table, substring, parent);
    >   return substring;
    > }

    > where the following is put in the main:
    > substring_table
    > = scm_permanent_object (scm_make_weak_key_hash_table (SCM_UNDEFINED));

    > This is almost magical :) It works perfectly well and I don't need to
    > bother about any explicit deallocation. This is also the first time I
    > really understand the purpose of these weak hash tables. For weak hash
    > tables the hash entry will be garbage collected first when the key is
    > seen as garbage. With this scheme we still have the same essential
    > functionality from my perspective about shared substrings but we do
    > no longer need an explicit tag for shared substrings.

How can this actually work?

It ensures that SUBSTRING, while live, protects PARENT.  I see that
and it's _roughly_ what's wanted.

But SUBSTRING is tagged as a string, no?  When that key (the
substring) is collected -- won't that lead to a bogus free of the
substring data?

(I'm looking at the 1.6.4 GC implementation.  Apologies if things have
changed in some significant way in 1.7)


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