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;;; Copyright © 2021 Gerald Sussman and Chris Hanson
;;; Copyright © 2022 David Thompson <dthompson2@worcester.edu>
;;;
;;; This program is free software: you can redistribute it and/or
;;; modify it under the terms of the GNU General Public License as
;;; published by the Free Software Foundation, either version 3 of the
;;; License, or (at your option) any later version.
;;;
;;; This program is distributed in the hope that it will be useful,
;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
;;; General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with this program. If not, see
;;; <http://www.gnu.org/licenses/>.
;; 2.4.1 A monolithic implementation
(use-modules (srfi srfi-1)
(srfi srfi-9))
;; A checkers domain model
(define %checkers-board-width 8)
(define %checkers-board-height 8)
(define-record-type <coords>
(coords x y)
coords?
(x coords-x)
(y coords-y))
(define (coords+ a b)
(coords (+ (coords-x a) (coords-x b))
(+ (coords-y a) (coords-y b))))
(define-record-type <piece>
(make-piece coords owner king?)
piece?
(coords piece-coords)
(owner piece-owner)
(king? piece-king?))
(define (player-piece? piece)
(eq? (piece-owner piece) 'player))
(define (opponent-piece? piece)
(eq? (piece-owner piece) 'opponent))
(define (should-be-crowned? piece)
(and (not (piece-king? piece))
(= (coords-y (piece-coords piece))
(if (player-piece? piece)
(- %checkers-board-height 1)
0))))
(define (crown-piece piece)
(make-piece (piece-coords piece)
(piece-owner piece)
#t))
(define (possible-directions piece)
(cond
((piece-king? piece)
(list (coords 1 1)
(coords -1 1)
(coords -1 -1)
(coords 1 -1)))
((player-piece? piece)
(list (coords 1 1)
(coords -1 1)))
(else
(list (coords 1 -1)
(coords -1 -1)))))
(define (move-piece piece new-coords)
(make-piece new-coords
(piece-owner piece)
(piece-king? piece)))
(define-record-type <board>
(make-board pieces)
board?
(pieces board-pieces))
(define (current-pieces board)
(filter player-piece?
(board-pieces board)))
(define (is-position-on-board? coords board)
(and (>= (coords-x coords) 0)
(< (coords-x coords) %checkers-board-width)
(>= (coords-y coords) 0)
(< (coords-y coords) %checkers-board-height)))
(define (board-get coords board)
(find (lambda (piece)
(equal? coords (piece-coords piece)))
(board-pieces board)))
(define (position-info coords board)
(let ((piece (board-get coords board)))
(cond
((not piece) 'unoccupied)
((player-piece? piece) 'occupied-by-self)
((opponent-piece? piece) 'occupied-by-opponent))))
(define (is-position-unoccupied? coords board)
(eq? (position-info coords board) 'unoccupied))
(define (is-position-occupied-by-self? coords board)
(eq? (position-info coords board) 'occupied-by-self))
(define (is-position-occupied-by-opponent? coords board)
(eq? (position-info coords board) 'occupied-by-opponent))
;; A checkers referee
(define-record-type <step>
(make-step from to board jump?)
step?
(from step-from)
(to step-to)
(board step-board)
(jump? step-jump?))
(define (replace-piece new-piece old-piece board)
(let ((new-board (make-board (cons new-piece (delq old-piece (board-pieces board))))))
(make-step old-piece new-piece new-board #f)))
(define (make-simple-move new-coords piece board)
(replace-piece (move-piece piece new-coords) piece board))
(define (make-jump new-coords jumped-coords piece board)
(let* ((new-piece (move-piece piece new-coords))
(new-board (make-board (remove (lambda (piece)
(equal? (piece-coords piece)
jumped-coords))
(board-pieces board)))))
(make-step piece new-piece new-board #t)))
(define (path-contains-jumps? path)
(any step-jump? path))
(define (try-step piece board direction path)
(let ((new-coords
(coords+ (piece-coords piece) direction)))
(and (is-position-on-board? new-coords board)
(case (position-info new-coords board)
((unoccupied)
(and (not (path-contains-jumps? path))
(cons (make-simple-move new-coords piece board)
path)))
((occupied-by-opponent)
(let ((landing (coords+ new-coords direction)))
(and (is-position-on-board? landing board)
(is-position-unoccupied? landing board)
(cons (make-jump landing new-coords piece board)
path))))
((occupied-by-self) #f)
(else (error "Unknown position info"))))))
(define (compute-next-steps piece board path)
(filter-map (lambda (direction)
(try-step piece board direction path))
(possible-directions piece)))
(define (evolve-paths piece board)
(let* ((paths (compute-next-steps piece board '()))
(jumps (filter path-contains-jumps? paths)))
(if (null? jumps)
paths
(evolve-jumps jumps))))
(define (evolve-jumps paths)
(append-map (lambda (path)
(let ((paths (let ((step (car path)))
(compute-next-steps (step-to step)
(step-board step)
path))))
(if (null? paths)
(list path)
;; continue jumping if possible
(evolve-jumps paths))))
paths))
(define (mandate-jumps paths)
(let ((jumps (filter path-contains-jumps? paths)))
(if (null? jumps)
paths
jumps)))
(define (crown-kings paths)
(map (lambda (path)
(let ((piece (step-to (car path))))
(if (should-be-crowned? piece)
(cons (replace-piece (crown-piece piece)
piece
(step-board (car path)))
path)
path)))
paths))
(define (generate-moves board)
(crown-kings
(mandate-jumps
(append-map (lambda (piece)
(evolve-paths piece board))
(current-pieces board)))))
;; A quick thing I made up to make verifying that the paths are what I
;; expect a little easier.
(define (describe-path path)
(display (length path))
(display " steps:")
(newline)
(for-each (lambda (step)
(let ((from (step-from step))
(to (step-to step)))
(unless (eq? (piece-coords from) (piece-coords to))
(display "piece moved from (")
(display (coords-x (piece-coords from)))
(display ", ")
(display (coords-y (piece-coords from)))
(display ") to (")
(display (coords-x (piece-coords to)))
(display ", ")
(display (coords-y (piece-coords to)))
(display "); "))
(when (and (not (piece-king? from)) (piece-king? to))
(display "king; "))
(when (step-jump? step)
(display "jump; "))
(newline)))
(reverse path)))
;; Example:
;;
;; $: player piece
;; o: opponent piece
;;
;; ______o_
;; ___o_o__
;; ____$___
;; ________
;; ________
;; ________
;; ________
;; ________
;;
;; Should result in 1 path of 2 steps: a jump and then a king.
(for-each describe-path
(generate-moves
(make-board (list (make-piece (coords 4 5) 'player #f)
(make-piece (coords 6 7) 'opponent #f)
(make-piece (coords 3 6) 'opponent #f)
(make-piece (coords 5 6) 'opponent #f)))))
;; 2.4.2 Factoring out the domain
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