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## Issue ADJUST-ARRAY-DISPLACEMENT Writeup

    Issue:        ADJUST-ARRAY-DISPLACEMENT
    Reference:    ADJUST-ARRAY (CLtL p.297)
    Category:     Clarification
    Edit history: Version 1 by Fahlman, 18-Apr-87 (from Steele's list)
                  Version 2 by Masinter
                  Version 3 by Masinter, 5-Jun-87 (respond to comments)
                  Version 4 by Masinter, 23-Nov-87

    Problem Description:

    The interaction of ADJUST-ARRAY and displaced arrays is insufficiently specified
    in the case where the array being adjusted is displaced.

    Proposal: ADJUST-ARRAY-DISPLAYCEMENT:RULES

    Interaction of adjusting and displacement:

    Suppose we are adjusting array A, which is perhaps displaced to B before the
    ADJUST-ARRAY call and perhaps to C after the call.

    (1) A is not displaced before or after: The dimensions of A are altered, and the
    contents rearranged as appropriate.  Additional elements of A are taken from the
    :INITIAL-ELEMENT.  The use of :INITIAL-CONTENTS causes all old contents to be
    discarded.

    (2) A is not displaced before, but is displaced to C after.  As specified in
    CLtL, none of the original contents of A appears in A afterwards; A now contains
    the contents of C, without any rearrangement of C.

    (3) A is displaced to B before the call, and is displaced to C after the call.
    (B and C may be the same.) As in case (2), the contents of B do not appear in A
    afterward (unless such contents also happen to be in C).  If
    :DISPLACED-INDEX-OFFSET is not specified in the ADJUST-ARRAY call, it defaults
    to zero; the old offset (into B) is not retained.

    (4) A is displaced to B before the call, but not displaced afterward.  A gets a
    new "data region", and contents of B are copied into it as appropriate to
    maintain the existing old contents; additional elements of A are taken from the
    :INITIAL-ELEMENT.  However, the use of :INITIAL-CONTENTS causes all old contents
    to be discarded.

    If array X is displaced to array Y, and array Y is displaced to array Z, and
    array Y is altered by ADJUST-ARRAY, array X must now refer to the adjusted
    contents of Y.  This means that an implementation may not collapse the chain to
    make X refer to Z directly and forget that the chain of reference passes through
    array Y.  (Cacheing techniques are of course permitted, as long as they preserve
    the semantics specified here and in CLtL.)

    If X is displaced to Y, it is an error to adjust Y in such a way that it no
    longer has enough elements to satisfy X.  This error may be signalled at the
    time of the adjustment, but this is not required.

    Note: Omitting the :DISPLACED-TO argument to ADJUST-ARRAY is equivalent to
    specifying :DISPLACED-TO NIL; in either case, the array is not displaced after
    the call and case (1) or (4) hold.

    Rationale:

    This interaction must be clarified.  This set of rules was proposed some time
    ago, as a result of discussions on the Common Lisp mailing list, and this model
    has been adopted by many Common Lisp implementations.

    Current Practice:

    Many implementations currently follow the model proposed here, although they
    differ in some detail. For example, Symbolics Common Lisp behaves as indicated
    except for case (4); in that case, it never copies the contents of B, and all
    elements are taken from the :INITIAL-ELEMENT.

    Cost to implementors:

    Some existing implementations may have to be changed, but adopting any other
    model would be worse.  Public-domain code implementing this model is available
    from CMU.

    Cost to users:

    This is a relatively uncommon situation, which is the reason it didn't occur to
    the original language designers to specify how it works.  Any user code that
    cares about this issue probably already follows the proposed model.

    Benefits:

    Clarification of a situation that is currently not addressed by the standard.

    Discussion:

    The cleanup committee supports this clarification.

    Some consideration was given to adding DISPLACED-ARRAY-P or ARRAY-DISPLACED-TO
    and ARRAY-DISPLACED-INDEX-OFFSET which would allow access to information as to
    whether an array was or was not displaced. However, these are not part of the
    current proposal.

    A similar issue arises with ADJUST-ARRAY and fill pointers, and will be the
    subject of a separate issue.

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