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selector-lookup.scm
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selector-lookup.scm
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;;; FILE: "selector-lookup.scm"
;;; IMPLEMENTS: Selector->Method lookup strategy
;;; AUTHOR: Ken Dickey
;;; DATE: 10 December 2017
;; Strategy for selector lookup
;;
;; In place of a Class or hash-table, each St object has a slot with
;; a vector of closures which are its methods.
;;
;; Selectors are assigned an id when first used to refer to a method.
;; The id is used to find the appropriate method in the method vector.
;; Holes in the vector are kept track of in slot zero.
;;
;; No hashes. No caches. Holes elided.
;;
;; E.g. (#f ==> unused)
;; Logical Actual ..(hole-index num-adjacent-holes)..
;; 0 | #f | | --> | ((5 . 1) (9 . 2) (15 . 0))
;; 1 | m1 | | m1 |
;; 2 | m2 | | m2 |
;; 3 | m3 | | m3 |
;; 4 | m4 | | m4 |
;; 5 | #f | | m7 |
;; 6 | #f | | m8 |
;; 7 | m7 | | m12 |
;; 8 | m8 | | m13 |
;; 9 | #f | | m14 |
;;10 | #f | | m16 |
;;11 | #f |
;;12 | m12 |
;;13 | m13 |
;;14 | m14 |
;;15 | #f |
;;16 | m16 |
;;
;; Tuning: Could use method Object>>doesNotUnderstand: in unused
;; slots, only condensing when the number of unused 'holes'
;; exceeds a given threshold.
;;
;; Alt: Check against Larceny's optimized case dispatch.
;; Alt: Large # of gaps -> binary search
(define (make-selector-table) (vector nil))
(define assign-id-to-selector ;; return the selector id
(let ( (counter 0) )
(lambda (sym)
(cond
((not (symbol? sym))
doesNotUnderstand:
)
((selector->id sym)) ;; Answer the index
(else
(set! counter (+ 1 counter))
(set-selector-index! sym counter)
counter)
) ) ) )
(define (selector->id sym)
(getprop sym '%%method-index%%))
(define (set-selector-index! sym idx)
(putprop sym '%%method-index%% idx))
(define (selector+table->method sym vec)
(let ( (idx (selector->id sym)) )
(if (not idx)
doesNotUnderstand:
(let loop ( (hole-count 0)
(holes-alist (vector-ref vec 0))
)
(cond
((null? holes-alist)
(let ( (index (- idx hole-count)) )
(if (< index (vector-length vec))
(vector-ref vec index)
doesNotUnderstand:)
))
(else
(let ( (next-hole (caar holes-alist))
(num-adjacent (cdar holes-alist))
)
(cond
((< idx next-hole)
(vector-ref vec (- idx hole-count))
)
((<= idx (+ next-hole num-adjacent))
doesNotUnderstand: ;; not in table
)
(else (loop
(+ hole-count 1 num-adjacent)
(cdr holes-alist)))))))
) ) ) )
;;;=================Quick Checks===============;;;
;; (define doesNotUnderstand: 'doesNotUnderstand: )
;; (map assign-id-to-selector
;; '(m1 m2 m3 m4 m5 m6 m7 m8 m9 m10 m11 m12 m13 m14 m15 m16 m17))
;; (define mvec (vector '((5 . 1) (9 . 2) (15 . 0))
;; 'm1 'm2 'm3 'm4 'm7 'm8 'm12 'm13 'm14 'm16))
;; (map (lambda (s) (selector+table->method s mvec))
;; '(m1 m2 m3 m4 m5 m6 m7 m8 m9 m10 m11 m12 m13 m14 m15 m16 m17))
;;;; Expect:
;;'(m1
;; m2
;; m3
;; m4
;; doesNotUnderstand:
;; doesNotUnderstand:
;; m7
;; m8
;; doesNotUnderstand:
;; doesNotUnderstand:
;; doesNotUnderstand:
;; m12
;; m13
;; m14
;; doesNotUnderstand:
;; m16
;; doesNotUnderstand:)
;;; FIXME: Add & Remove Methods
(define (add-selector+method-to-mvec selector method mvec)
;; (unless (and (symbol? selector)
;; (procedure? method)
;; (vector? mvec))
;; (error @@FIXME: checks@@)
(let ( (idx (assign-id-to-selector selector))
(mvec-len (vector-length mvec))
)
(let loop ( (hole-count 0)
(holes-alist (vector-ref mvec 0)) ;; FIXME: checks
)
(cond
((null? holes-alist) ;; go ahead and add
(let ( (index (- idx hole-count)) )
(if (< index mvec-len)
;; replace
(vector-set! mvec index method)
;; add at end
(let ( (result-vec
(vector-add-at-end method))
)
(cond
((= index mvec-len)
result-vec ;; no change to holes-alist
)
(else
(vector-set! result-vec
0
(append ;; add to end of alist
(vector-ref mvec 0)
(list
(cons
;; new hole
mvec-len
;; num adjacent holes
(- index mvec-len 1)
))))
result-vec)
)))
) )
(else
(let ( (next-hole (caar holes-alist))
(num-adjacent (cdar holes-alist))
)
(cond
((< index next-hole)
(vector-ref vec (- idx hole-count))
)
((<= idx (+ next-hole num-adjacent))
doesNotUnderstand: ;; not in table
)
(else (loop
(+ hole-count 1 num-adjacent)
(cdr holes-alist)))))))
) ) ) )
;; --- E O F --- ;;