;;;
;;;  data.heap - Heaps
;;;
;;;   Copyright (c) 2014-2024  Shiro Kawai  <shiro@acm.org>
;;;
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;;;
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(define-module data.heap
  (use gauche.sequence)
  (use gauche.uvector)
  (use scheme.list)
  (export <binary-heap>
          make-binary-heap build-binary-heap
          binary-heap-comparator binary-heap-key-procedure
          binary-heap-capacity
          binary-heap-empty? binary-heap-num-entries
          binary-heap-copy binary-heap-check binary-heap-clear!
          binary-heap-push!
          binary-heap-find-min binary-heap-find-max
          binary-heap-pop-min! binary-heap-pop-max!
          binary-heap-swap-min! binary-heap-swap-max!
          binary-heap-find binary-heap-remove! binary-heap-delete!
          ))
(select-module data.heap)

;; we use sparse-vector by default; we just make it autoload
;; so that the tests won't depend on data.sparse.
(autoload data.sparse make-sparse-vector <sparse-vector-base>
          sparse-vector-num-entries sparse-vector-exists?
          sparse-vector-copy sparse-vector-clear!)

;;;
;;; Binary Heap
;;;

;; We use min-max heap; whatever gain earned by limiting to min-heap
;; or max-heap would be small compared to other overhead.

;; M.D. Atkinson, J.-R. Sack, N. Santoro, T. Strothotte,
;; Min-max heaps and generalized priority queues, CACM 29(10) pp.996-1000
;; Oct. 1986, http://dx.doi.org/10.1145/6617.6621

;; Binary heap is stored in an array, S[].
;;
;; - S[1] is the root.
;; - S[i]'s child is S[2i] and S[2i+1]
;; - S[i]'s parent is S[floor(i/2)] when i > 1.
;; - Stored data can be anything comparable.
;;
;; Since S[0] won't be used, we treat the backing storage as 1-based;
;; that is, S[1] is stored in the first element, and so on.
;;
;; Backing storage can be either a flat vector (vector, uvector) or
;; a sparse vector.  If it is a flat vector, the heap has maximum size
;; (we don't extend the buffer automatically).

(define-inline (Ix x) (- x 1)) ; convert 1-base to 0-base

(define-syntax swap!
  (syntax-rules ()
    [(_ storage i j)
     (let1 v (~ storage (Ix i))
       (set! (~ storage (Ix i)) (~ storage (Ix j)))
       (set! (~ storage (Ix j)) v))]))

;; When an item is removed, we fill the slot with this value to be GC friendly.
;; 0 is chosen that all supported types of backing storage can hold it; if we
;; extend the support of backing storage, we may need to reconsider this.
(define-constant *filler* 0)

(define-class <binary-heap> ()
  ((comparator :init-keyword :comparator)
   (key        :init-keyword :key)
   (capacity   :init-keyword :capacity)  ; +inf.0 if storage is a sparse vector
   ;; internal
   (<:         :init-keyword :<:) ; cached less-than proc
   (>:         :init-keyword :>:) ; cached greater-than proc
   (storage    :init-keyword :storage)
   (next-leaf  :init-keyword :next-leaf :init-value 1)  ; next leaf index
   ))

(define (make-binary-heap :key (comparator default-comparator)
                               (storage (make-sparse-vector))
                               (key identity))
  (unless (comparator-ordered? comparator)
    (error "make-binary-heap requires ordered comparator, \
            but got:" comparator))
  (receive (<: >:)
      (ecase (comparator-flavor comparator)
        [(ordering) (values (comparator-ordering-predicate comparator)
                            (^[a b] (>? comparator a b)))]
        [(comparison) (values (^[a b] (<? comparator a b))
                              (^[a b] (>? comparator a b)))])
    (make <binary-heap> :comparator comparator
          :storage (if (or (vector? storage) (uvector? storage)
                           (is-a? storage <sparse-vector-base>))
                     storage
                     (error "make-binary-heap requires a vector, a uvector \
                             or a sparse vector as a storage, but got:"
                            storage))
          :key key
          :capacity (cond [(vector? storage) (vector-length storage)]
                          [(uvector? storage) (uvector-length storage)]
                          [else +inf.0])
          :<: <:
          :>: >:)))

;; heapify
(define (build-binary-heap storage :key (num-entries #f)
                                        (comparator default-comparator)
                                        (key identity))
  (unless (comparator-ordered? comparator)
    (error "build-binary-heap requires ordered comparator, \
            but got:" comparator))
  (let* ([xlen (cond
                [(vector? storage) (vector-length storage)]
                [(uvector? storage) (uvector-length storage)]
                [(is-a? storage <sparse-vector-base>)
                 (sparse-vector-num-entries storage)]
                [else (error "build-binary-heap requires a vector, a uvector \
                              or a sparse vector as a storage, but got:"
                              storage)])]
         [size (cond [(not num-entries) xlen]
                     [(and (integer? num-entries) (>= num-entries 0))
                      (min num-entries xlen)]
                     [else
                      (error "invalid num-entries value for build-binary-heap:"
                             num-entries)])])
    (receive (<: >:)
        (ecase (comparator-flavor comparator)
          [(ordering) (values (comparator-ordering-predicate comparator)
                              (^[a b] (>? comparator a b)))]
          [(comparison) (values (^[a b] (<? comparator a b))
                                (^[a b] (>? comparator a b)))])
      (bh-heapify! storage <: >: size)
      (make <binary-heap> :comparator comparator :storage storage :key key
            :<: <: :>: >: :next-leaf (+ size 1)
            :capacity (cond [(vector? storage) (vector-length storage)]
                            [(uvector? storage) (uvector-length storage)]
                            [else +inf.0])))))

(define (binary-heap-copy hp)
  (make <binary-heap>
    :comparator (~ hp'comparator)
    :storage (let1 s (~ hp'storage)
               (cond [(vector? s) (vector-copy s)]
                     [(uvector? s) (uvector-copy s)]
                     [(is-a? s <sparse-vector-base>) (sparse-vector-copy s)]
                     [else (error "[internal] binary-heap-copy: invalid storage:" s)]))
    :key (~ hp'key)
    :capaticy (~ hp'capacity)
    :<: (~ hp'<:)
    :>: (~ hp'>:)
    :next-leaf (~ hp'next-leaf)))

(define (binary-heap-comparator hp) (~ hp'comparator))
(define (binary-heap-key-procedure hp) (~ hp'key))
(define (binary-heap-capacity hp) (~ hp'capacity))

(define (binary-heap-clear! hp)
  (set! (~ hp'next-leaf) 1)
  ;; These are theoretically unnecessary, but works nicely with GC.
  (let1 st (~ hp'storage)
    (cond [(vector? st) (vector-fill! st #f)]
          [(is-a? st <sparse-vector-base>) (sparse-vector-clear! st)])))

(define (binary-heap-push! hp item)
  (let1 next (~ hp'next-leaf)
    (when (>= (- next 1) (~ hp'capacity))
      (errorf "binary heap ~s is full: couldn't insert ~s" hp item))
    (comparator-check-type (~ hp'comparator) ((~ hp'key) item))
    (set! (~ hp'storage (Ix next)) item)
    (set! (~ hp'next-leaf) (+ next 1))
    (when (> next 1)
      (bh-bubble-up (~ hp'storage) (~ hp'<:) (~ hp'>:) next))))

(define (binary-heap-num-entries hp) (- (~ hp'next-leaf) 1))

(define (binary-heap-empty? hp) (= (~ hp'next-leaf) 1))

(define (binary-heap-find-min hp :optional (fallback (undefined)))
  (if (= (~ hp'next-leaf) 1)
    (if (undefined? fallback)
      (error "binary heap is empty:" hp)
      fallback)
    (~ hp'storage (Ix 1))))

(define (binary-heap-find-max hp :optional (fallback (undefined)))
  (case (~ hp'next-leaf)
    [(1) (if (undefined? fallback)
           (error "binary heap is empty:" hp)
           fallback)]
    [(2) (~ hp'storage (Ix 1))]
    [(3) (~ hp'storage (Ix 2))]
    [else (let ([a (~ hp'storage (Ix 2))]
                [b (~ hp'storage (Ix 3))])
            (if ((~ hp'>:) a b) a b))]))

(define (binary-heap-pop-min! hp)
  (let ([nelts (binary-heap-num-entries hp)]
        [storage (~ hp'storage)])
    (when (= nelts 0) (error "binary heap is empty:" hp))
    (rlet1 r (~ storage (Ix 1))
      (set! (~ storage (Ix 1)) (~ storage (Ix nelts)))
      (set! (~ storage (Ix nelts)) *filler*)
      (set! (~ hp'next-leaf) nelts)
      (bh-trickle-down storage (~ hp'<:) (~ hp'>:) 1 nelts))))

(define (binary-heap-pop-max! hp)
  (let ([nelts (binary-heap-num-entries hp)]
        [storage (~ hp'storage)])
    (define (swap-and-adjust index)
      (set! (~ storage (Ix index)) (~ storage (Ix nelts)))
      (set! (~ storage (Ix nelts)) *filler*)
      (set! (~ hp'next-leaf) nelts)
      (bh-trickle-down storage (~ hp'<:) (~ hp'>:) index nelts))
    (case nelts
      [(0) (error "binary heap is empty:" hp)]
      [(1 2) (set! (~ hp'next-leaf) nelts) (~ storage (Ix nelts))]
      [else
       (let ([a (~ storage (Ix 2))]
             [b (~ storage (Ix 3))])
         (if ((~ hp'>:) a b)
           (begin (swap-and-adjust 2) a)
           (begin (swap-and-adjust 3) b)))])))

;; (binary-heap-swap-* hp item) is operationally equivalent to
;; (rlet1 entry (binary-heap-pop-* hp) (binary-heap-push! hp item))
;; but it is more efficient.
(define (binary-heap-swap-min! hp item)
  (let1 storage (~ hp'storage)
    (when (binary-heap-empty? hp) (error "binary heap is empty:" hp))
    (rlet1 r (~ storage (Ix 1))
      (set! (~ storage (Ix 1)) item)
      (let1 n (binary-heap-num-entries hp)
        (when (> n 1)
          (bh-trickle-down storage (~ hp'<:) (~ hp'>:) 1 (+ n 1)))))))

(define (binary-heap-swap-max! hp item)
  (let ([storage (~ hp'storage)]
        [nelts (binary-heap-num-entries hp)])
    (define (store-and-adjust index)
      (let1 a (~ storage (Ix 1))
        (set! (~ storage (Ix index)) item)
        (when ((~ hp'>:) a item)
          (swap! storage 1 index))
        (bh-trickle-down storage (~ hp'<:) (~ hp'>:) index (+ nelts 1))))
    (case nelts
      [(0) (error "binary heap is empty:" hp)]
      [(1) (rlet1 r (~ storage (Ix 1))
             (set! (~ storage (Ix 1)) item))]
      [(2) (rlet1 r (~ storage (Ix 2))
             (let1 a (~ storage (Ix 1))
               (set! (~ storage (Ix 2)) item)
               (when ((~ hp'>:) a item)
                 (swap! storage 1 2))))]
      [else
       (let ([a (~ storage (Ix 2))]
             [b (~ storage (Ix 3))])
         (if ((~ hp'>:) a b)
           (begin (store-and-adjust 2) a)
           (begin (store-and-adjust 3) b)))])))

;; not exactly heap operations, but useful...

;; Up to 0.9.10, we have (binary-heap-find <heap> <pred>) but that doesn't
;; align with other *-find procedures.  In 0.9.11 we change it to
;; (binary-heap-find <pred> <heap> :optional <failure>), while supporting
;; old API as well.
(define (binary-heap-find pred hp :optional (failure (^[] #f)))
  (if (is-a? pred <binary-heap>)
    (binary-heap-find hp pred failure) ; backward compatibility
    (or (and-let* ([storage (~ hp'storage)]
                   [i (find-index (^i (pred (~ storage i)))
                                  (liota (binary-heap-num-entries hp)))])
          ;; NB: This i is 0-base, so we don't need Ix.
          (~ storage i))
        (failure))))

(define (binary-heap-remove! hp pred)
  (let1 storage (~ hp'storage)

    (define (finish-up next)
      (unless (= next (~ hp'next-leaf)) ;; some keys are removed
        (bh-heapify! storage (~ hp'<:) (~ hp'>:) (- next 1))
        (do ([i next (+ i 1)]
             [lim (~ hp'next-leaf)])
            [(>= i lim)]
          (set! (~ hp'storage (Ix i)) *filler*))
        (set! (~ hp'next-leaf) next))
      hp)

    (let loop ([i 1] [next (~ hp'next-leaf)])
      (if (>= i next)
        (finish-up next)
        (if (pred (~ storage (Ix i)))
          (if (= (+ i 1) next)
            (finish-up i)  ; this is the last item
            (begin
              (swap! storage i (- next 1))
              (loop i (- next 1))))
          (loop (+ i 1) next))))))

(define (binary-heap-delete! hp item)
  (let ([cmp (~ hp'comparator)]
        [key (~ hp'key)])
    (binary-heap-remove! hp (^e (=? cmp (key item) (key e))))))

;; Internal procedures
(define-inline (min-node? index) (odd? (integer-length index)))

(define (binary-heap-check hp)
  (unless (>= (~ hp'next-leaf) 1)
    (error "next-leaf index is less than 1"))
  (let rec ((i 1))
    (let ([kids  (take-while (cute < <> (~ hp'next-leaf)) (iota 2 (* i 2)))]
          [gkids (take-while (cute < <> (~ hp'next-leaf)) (iota 4 (* i 4)))]
          [cmp (if (min-node? i) (~ hp'<:) (~ hp'>:))])
      (unless (every (^k (cmp (~ hp'storage(Ix i)) (~ hp'storage(Ix k)))) kids)
        (errorf "parent-kid relation violated: ~a-node[~a]=~s, kids[~a]=~s"
               (if (min-node? i) 'min 'max)
               i (~ hp'storage (Ix i)) kids
               (map (^k (~ hp'storage (Ix k))) kids)))
      (unless (every (^k (cmp (~ hp'storage(Ix i)) (~ hp'storage(Ix k)))) gkids)
        (errorf "grandparent-grandkid relation violated: ~a-node[~a]=~s, grandkids[~a]=~s"
               (if (min-node? i) 'min 'max)
               i (~ hp'storage (Ix i)) gkids
               (map (^k (~ hp'storage (Ix k))) gkids)))
      (for-each rec kids)
      #t)))

;; index
;;  1   : min(2,3)
;;  2,3 : max(4,5) and max(6,7)
;;  4-7 : min(8,9) ... min(14,15)
;;  8-15: max(16,17) ... max(30,31)
;; etc.
;; if (integer-length i) is odd, we have min node
;; else we have max node

;; called with index > 1
(define (bh-bubble-up storage <: >: index)

  (define (bubble-up-rec >< index)
    (when (> index 3)
      (let1 grandparent-index (ash index -2)
        (unless (>< (~ storage (Ix grandparent-index)) (~ storage (Ix index)))
          (swap! storage grandparent-index index)
          (bubble-up-rec >< grandparent-index)))))

  (let1 parent-index (ash index -1)
    (if (min-node? parent-index)
      (if (<: (~ storage (Ix parent-index)) (~ storage (Ix index)))
        (bubble-up-rec >: index)
        (begin
          (swap! storage parent-index index)
          (bubble-up-rec <: parent-index)))
      (if (>: (~ storage (Ix parent-index)) (~ storage (Ix index)))
        (bubble-up-rec <: index)
        (begin
          (swap! storage parent-index index)
          (bubble-up-rec >: parent-index))))))

(define (bh-heapify! storage <: >: size)
  (do ([i 2 (+ i 1)])
      [(>= i (+ 1 size))]
    (when (is-a? storage <sparse-vector-base>)
      (unless (sparse-vector-exists? storage (Ix i))
        (errorf "can't heapify a sparse vector with a hole (at index ~s): ~s"
                (Ix i) storage)))
    (bh-bubble-up storage <: >: i)))

(define (bh-trickle-down storage <: >: index size)

  (define-syntax getval (syntax-rules () [(getval i) (~ storage (Ix i))]))
  (define-syntax in-bound? (syntax-rules () [(in-bound? i) (< i size)]))

  ;; We need to check up to 2 kids and 4 grandkids.  This returns appropriate
  ;; index for n in 0..5; that is, n=0,1 for kids, n=2,3,4,5 for grandkids.
  (define (descendant n index)
    (if (< n 2)
      (+ (ash index 1) n)
      (+ (ash index 2) (- n 2))))

  ;; Among self, children and grandchildren, find an index that has
  ;; the minimum or maximum value.
  ;; NB: This can be witten more cleanly by making the list of kids index
  ;; and folding over it; but we want to avoid allocation in it, for this
  ;; will be called log(n) times in average for every deletion.
  (define (find-extreme >< index)
    (let loop ([minval (getval index)]
               [minidx index]
               [n 0])
      (if (> n 5)
        minidx
        (let1 i (descendant n index)
          (if (not (in-bound? i))
            minidx
            (let1 v (getval i)
              (if (>< v minval)
                (loop v i (+ n 1))
                (loop minval minidx (+ n 1)))))))))

  (define (trickle-down-rec >< index)
    (let1 pick (find-extreme >< index)
      (unless (= pick index)
        (swap! storage pick index)
        (when (>= pick (ash index 2)) ; grandchild
          (unless (>< (getval pick) (getval (ash pick -1)))
            (swap! storage (ash pick -1) pick))
          (trickle-down-rec >< pick)))))

  (trickle-down-rec (if (min-node? index) <: >:) index))
