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(in-package :alexandria-2)
(defun dim-in-bounds-p (dimensions &rest subscripts)
"Mirrors cl:array-in-bounds-p, but takes dimensions (list of integers) as its
first argument instead of an array.
(array-in-bounds-p arr ...) == (dim-in-bounds-p (array-dimensions arr) ...)"
(and (= (length dimensions) (length subscripts))
(every (lambda (i d) (and (integerp i) (< -1 i d)))
subscripts dimensions)))
(defun row-major-index (dimensions &rest subscripts)
"Mirrors cl:array-row-major-index, but takes dimensions (list of integers)
as its first argument instead of an array.
Signals an error if lengths of dimensions and subscripts are not equal
(array-row-major-index arr ...) == (row-major-index (array-dimensions arr) ...)"
(unless (apply #'dim-in-bounds-p dimensions subscripts)
(error (format nil "Indices ~a invalid for dimensions ~a" subscripts dimensions)))
(loop with word-idx = 0
with dimprod = 1
for dim-size in (reverse dimensions)
for dim-idx in (reverse subscripts)
do
(incf word-idx (* dim-idx dimprod))
(setf dimprod (* dimprod dim-size))
finally (return word-idx)))
(defun rmajor-to-indices (dimensions index)
"The inverse function to row-major-index. Given a set of dimensions and a
row-major index, produce the list of indices <subscripts> such that
(row-major-index dimensions sucscripts) = index"
(when (null dimensions) (error "Dimensions must be non-null"))
(let ((size (reduce #'* dimensions)))
(unless (< -1 index size)
(error (format nil "Row-major index ~a invalid for array of total size ~a" index size))))
(labels ((rec (dimensions index word-sizes acc)
(if (null (cdr dimensions))
(reverse (cons index acc))
(multiple-value-bind (idx remainder) (floor index (car word-sizes))
(rec (cdr dimensions) remainder (cdr word-sizes) (cons idx acc))))))
(rec dimensions index
(cdr (reduce (lambda (x y) (cons (* x (car y)) y)) dimensions
:initial-value '(1) :from-end t))
nil)))
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