CLHS: Issue AREF-1D Writeup

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## Issue AREF-1D Writeup

    Issue:        AREF-1D
    References:   Arrays (pp286-298)
    Category:     ENHANCEMENT
    Edit history: 22-Apr-87, Version 1 by Pitman
                  02-Jun-87, Version 2 by Pitman (ROW-MAJOR-AREF)
                  6-Jun-87, Versions 3, 4 by Masinter (editorial)
                  11-Jun-87, Version 5, to X3J13 (no changes)
                   6-Jul-87, Version 6, by Masinter
                  14-Nov-87, Version 7, by Masinter (update discussion)

    Problem Description:

    It's hard to write functions like Maclisp's LISTARRAY and FILLARRAY efficiently
    in Common Lisp because they take arguments of varying rank. Currently, you have
    to make a displaced array to work with temporarily and then throw away the
    displaced array when you're done. In many cases, this is bothersome because
    there is no a priori reason why they should have to cons at all.

    Proposal (AREF-1D:ROW-MAJOR-AREF):

    Introduce a new function ROW-MAJOR-AREF that allows one-dimensional access to
    the storage backing up a given array assuming the normal row-major storage
    layout.

    ROW-MAJOR-AREF is valid for use with SETF.

    row-major-aref array index              [Function]

    This accesses and returns the element of array specified by index when the
    elements of array are considered in row-major order. Array may be an array of
    any dimensionality. row-major-aref may be used with setf. For reference, the
    following sets of expressions are equivalent:

    (row-major-aref array index) ==
        (aref (make-array (array-total-size array)
                          :displaced-to array
                          :element-type (array-element-type array))
              index)

    and

    (aref array .. subscripts ..) ==
        (row-major-aref array (array-row-major-index array .. subscripts ..))

    Rationale:

    Common Lisp requires row-major storage layout of arrays and has a number of
    operators that allow users to exploit that order. ROW-MAJOR-AREF is a useful,
    simple addition.

    LISTARRAY and FILLARRAY, for example, could be trivially defined by loops that
    had the following form:

        (DOTIMES (I (ARRAY-TOTAL-SIZE ARRAY))
          ... (ROW-MAJOR-AREF ARRAY I) ...)

    Currently, the only really efficient way to write this would involve something
    like:

        (ECASE (ARRAY-RANK ARRAY1)
          ((0) (SETF (AREF ARRAY1) (AREF ARRAY2)))
          ((1) (DOTIMES (I (ARRAY-DIMENSION ARRAY 0))
    	     (SETF (AREF ARRAY1 I) (AREF ARRAY2 I))))
          ((2) (DOTIMES (I (ARRAY-DIMENSION ARRAY 0))
    	     (DOTIMES (I (ARRAY-DIMENSION ARRAY 1))
    	       (SETF (AREF ARRAY1 I J) (AREF ARRAY2 I J)))))
          ...some finite number of clauses...)

    Current Practice:

    Many implementations have this primitive under some other name for use
    internally. In Symbolics systems, for example, it is SYS:%1D-AREF.

    Adoption Cost:

    This change is fairly localized. In implementations that already use this
    primitive internally, it's little more than a matter of changing the name of or
    otherwise releasing the existing primitive. In some implementations, it might
    involve writing a small amount of code or compiler work to make ROW-MAJOR-AREF
    work efficiently.

    Benefits:

    This gives users efficient access to something to which they already have
    inefficient access.

    Conversion Cost:

    This is an upward-compatible change; the name ROW-MAJOR-AREF is unlikely to be
    used by any current program.

    Aesthetics:

    This allows certain programs to be written in a more aesthetic way.

    Discussion:

    This issue was conditionally passed at X3J13/June 1987, pending clarification of
    some details. Those clarifications have been made in this version.

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