;;; R7RS-PNM --- Library for reading and writing PNM (Portable Any Map) files for R7RS
;;; Copyright © 2024 Masaya Tojo <masaya@tojo.tokyo>
;;;
;;; This file is part of R7RS-PNM.
;;;
;;; R7RS-PNM is free software: you can redistribute it and/or modify it
;;; under the terms of the GNU Lesser General Public License as published
;;; by the Free Software Foundation, either version 3 of the License, or
;;; (at your option) any later version.
;;;
;;; R7RS-PNM 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 Lesser General Public License for more details.
;;;
;;; You should have received a copy of the GNU Lesser General Public License
;;; along with R7RS-PNM. If not, see <https://www.gnu.org/licenses/>.

(define-library (pnm pgm)
  (export make-pgm-image
          %make-pgm-image)
  (import (scheme base)
          (scheme case-lambda)
          (pnm image))
  (begin
    (define make-pgm-image
      (case-lambda
        ((width height)
         (make-pgm-image width height 255))
        ((width height maxval)
         (when (or (< maxval 0)
                   (< 65536 maxval))
           (error "(pnm pgm) make-pgm: maxval is out of range"))
         (let* ((byte-count (* width height
                               (if (< maxval 256)
                                   1
                                   2)))
                (data (make-bytevector byte-count 0)))
           (%make-pgm-image width height maxval data)))))

    (define (%make-pgm-image width height maxval data)
      (when (or (< maxval 0)
                (< 65536 maxval))
        (error "(pnm pgm) make-pgm: maxval is out of range"))
      (if (< maxval 256)
          (let ((byte-count (* width height)))
            (define (xy->idx x y) (+ x (* y width)))
            (define (pixel-getter x y)
              (let ((idx (xy->idx x y)))
                (bytevector-u8-ref data idx)))
            (define (pixel-setter x y v)
              (let ((idx (xy->idx x y)))
                (bytevector-u8-set! data idx v)))
            (unless (= byte-count (bytevector-length data))
              (error (string-append "(pnm pbm) make-pbm-image: Invalid bytevector length" byte-count)))
            (make-image 'pgm width height maxval data pixel-getter pixel-setter))
          (let ((byte-count (* width height 2)))
            (define (xy->idx x y) (+ x (* y width)))
            (define (pixel-getter x y)
              (let ((idx (xy->idx x y)))
                (combine-values (bytevector-u8-ref data idx)
                                (bytevector-u8-ref data (+ idx 1)))))
            (define (pixel-setter x y v)
              (let ((idx (xy->idx x y)))
                (let-values (((v1 v2) (split-value v)))
                  (bytevector-u8-set! data idx v1)
                  (bytevector-u8-set! data (+ idx 1) v2))))
            (unless (= byte-count (bytevector-length data))
              (error (string-append "(pnm pbm) make-pbm-image: Invalid bytevector length" byte-count)))
            (make-image 'pgm width height maxval data pixel-getter pixel-setter))))

    (define (split-value v)
      (values (modulo (quotient v 256) 256)
              (modulo v 256)))

    (define (combine-values l r)
      (+ (* 256 l) r))))