;;;
;;; data.imap - immutable tree map
;;;
;;;   Copyright (c) 2015-2024  Shiro Kawai  <shiro@acm.org>
;;;
;;;   Redistribution and use in source and binary forms, with or without
;;;   modification, are permitted provided that the following conditions
;;;   are met:
;;;
;;;   1. Redistributions of source code must retain the above copyright
;;;      notice, this list of conditions and the following disclaimer.
;;;
;;;   2. Redistributions in binary form must reproduce the above copyright
;;;      notice, this list of conditions and the following disclaimer in the
;;;      documentation and/or other materials provided with the distribution.
;;;
;;;   3. Neither the name of the authors nor the names of its contributors
;;;      may be used to endorse or promote products derived from this
;;;      software without specific prior written permission.
;;;
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;;;

;; This implements functional red-black tree
;; as described in Chris Okasaki's Purely Functional Data Structures.

(define-module data.imap
  (use gauche.sequence)
  (use gauche.dictionary)
  (use gauche.record)
  (use data.queue)
  (use util.match)
  (use srfi.114)
  (export <imap> <imap-meta>
          make-imap alist->imap tree-map->imap
          imap? imap-empty?
          imap-exists? imap-get imap-put
          imap-delete
          imap-min imap-max)
  )
(select-module data.imap)

;;
;; Internal implementation
;;

(define-record-type (T (pseudo-rtd <vector>)) #t #t color left elem right)
;; NB: We just use #f as E node.
(define (E? x) (not x))

(define balance
  (match-lambda*
    [(or ('B #('R #('R a x b) y c) z d)
         ('B #('R a x #('R b y c)) z d)
         ('B a x #('R #('R b y c) z d))
         ('B a x #('R b y #('R c z d))))
     (make-T 'R (make-T 'B a x b) y (make-T 'B c z d))]
    [(color a x b)
     (make-T color a x b)]))

(define (get key tree cmpr)
  (and (T? tree)
       (match-let1 #(_ a p b) tree
         (if3 (comparator-compare cmpr key (car p))
              (get key a cmpr)
              p
              (get key b cmpr)))))

(define (insert key val tree cmpr)
  (define (ins tree)
    (if (E? tree)
      (make-T 'R #f (cons key val) #f)
      (match-let1 #(color a p b) tree
        (if3 (comparator-compare cmpr key (car p))
             (balance color (ins a) p b)
             (make-T color a (cons key val) b)
             (balance color a p (ins b))))))
  (match-let1 #(_ a p b) (ins tree)
    (make-T 'B a p b)))

(define (populate tree alist cmpr)
  ((rec (f tree alist)
     (if (null? alist)
       tree
       (f (insert (caar alist) (cdar alist) tree cmpr) (cdr alist))))
   tree alist))

(define (delete key tree cmpr)
  (define (del-min tree)
    (match tree
      [#(color #f p #f) (values p #f)]
      [#(color #f p b)  (values p b)]
      [#(color a p b)   (receive (min-p a.) (del-min a)
                          (values min-p (balance color a. p b)))]))
  (define (del tree)
    (if (E? tree)
      tree
      (match-let1 #(color a p b) tree
        (if3 (comparator-compare cmpr key (car p))
             (balance color (del a) p b)
             (if a
               (if b
                 (receive (min-p b.) (del-min b)
                   (balance color a min-p b.))
                 a)
               b)
             (balance color a p (del b))))))
  (match (del tree)
    [#f  #f] ;empty
    [#(_ a p b) (make-T 'B a p b)]))

;; aux fn
(define (%key-proc->comparator key=? key<?)
  (make-comparator #t key=? key<? #f))

;;
;; External interface
;;
(define-class <imap-meta> (<class>) ())

(define-class <imap> (<ordered-dictionary>)
  ((comparator :init-keyword :comparator)
   (tree :init-keyword :tree :init-form #f))
  :metaclass <imap-meta>)

;; API
(define (imap? x) (is-a? x <imap>))

;; API
(define make-imap
  (case-lambda
    [() (make-imap default-comparator)]
    [(cmpr)
     (unless (comparator? cmpr)
       (error "comparator required, but got:" cmpr))
     (make <imap> :comparator cmpr)]
    [(key=? key<?) (make-imap (%key-proc->comparator key=? key<?))]))

;; API
(define alist->imap
  (case-lambda
    [(alist) (alist->imap alist default-comparator)]
    [(alist cmpr)
     (unless (comparator? cmpr)
       (error "comparator required, but got:" cmpr))
     (make <imap>
       :comparator cmpr :tree (populate #f alist cmpr))]
    [(alist key=? key<?)
     (alist->imap alist (%key-proc->comparator key=? key<?))]))

;; API
(define (tree-map->imap tree-map)
  (alist->imap (tree-map->alist tree-map)
                        (tree-map-comparator tree-map)))

;; API
(define (imap-empty? immap) (E? (~ immap'tree)))

;; API
(define (imap-exists? immap key)
  (boolean (get key (~ immap'tree) (~ immap'comparator))))

;; API
(define (imap-get immap key :optional default)
  (if-let1 p (get key (~ immap'tree) (~ immap'comparator))
    (cdr p)
    (if (undefined? default)
      (errorf "No such key in a imap ~s: ~s" immap key)
      default)))

;; API
(define (imap-put immap key val)
  (make <imap>
    :comparator (~ immap'comparator)
    :tree (insert key val (~ immap'tree) (~ immap'comparator))))

;; API
(define (imap-delete immap key)
  (make <imap>
    :comparator (~ immap'comparator)
    :tree (delete key (~ immap'tree) (~ immap'comparator))))

;; API
(define (imap-min immap)
  (define (descend tree)
    (match-let1 #(_ a p b) tree
      (if (E? a) p (descend a))))
  (let1 t (~ immap'tree)
    (and (not (E? t)) (descend t))))

;; API
(define (imap-max immap)
  (define (descend tree)
    (match-let1 #(_ a p b) tree
      (if (E? b) p (descend b))))
  (let1 t (~ immap'tree)
    (and (not (E? t)) (descend t))))

;; Fundamental iterators
(define (%imap-fold immap proc seed)
  (define (rec tree seed)
    (if (E? tree)
      seed
      (match-let1 #(_ a p b) tree
        (rec b (proc p (rec a seed))))))
  (rec (~ immap'tree) seed))

(define (%imap-fold-right immap proc seed)
  (define (rec tree seed)
    (if (E? tree)
      seed
      (match-let1 #(_ a p b) tree
        (rec a (proc p (rec b seed))))))
  (rec (~ immap'tree) seed))

;; Collection framework
(define-method call-with-iterator ((coll <imap>) proc :allow-other-keys)
  (if (imap-empty? coll)
    (proc (^[] #t) (^[] #t))
    (let1 q (make-queue)  ; only contains T
      (enqueue! q (~ coll'tree))
      (proc (^[] (queue-empty? q))
            (rec (next)
              (match-let1 #(c a p b) (dequeue! q)
                (if (E? a)
                  (if (E? b)
                    p
                    (begin (queue-push! q b) p))
                  (begin (queue-push! q (make-T c #f p b))
                         (queue-push! q a)
                         (next)))))))))

(define-method call-with-builder ((class <imap-meta>) proc
                                  :key (comparator default-comparator)
                                  :allow-other-keys)
  (define alist '())
  (proc (^p (push! alist p))
        (^[] (alist->imap alist comparator))))

;; Key comparator
(define (imap-comparator imap)
  (slot-ref imap 'comparator))

;; A couple of conversion methods for the efficiency
(define-method coerce-to ((c <imap-meta>) (src <list>))
  (alist->imap src))
(define-method coerce-to ((c <imap-meta>) (src <tree-map>))
  (tree-map->imap src))

;; Dictionary interface
;; As a dictionary, it behaves as immutable dictionary.
(define-dict-interface <imap>
  :get imap-get
  :put! (^[m k v] (error "imap is immutable:" m))
  :immutable? (^m #t)
  :fold       (^[m proc seed]
                (%imap-fold m (^[p s] (proc (car p) (cdr p) s)) seed))
  :fold-right (^[m proc seed]
                (%imap-fold-right m (^[p s] (proc (car p) (cdr p) s)) seed))
  :exists? imap-exists?
  :comparator imap-comparator)
