;;; 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 (image-format pnm private checker)
  (export make-xy-checker
          make-value-checker
          make-boolean-value-checker)
  (import (scheme base)
          (scheme case-lambda)
          (image-format pnm image))
  (begin
    (define (make-xy-checker width height)
      (lambda (x y)
        (when (or (not (exact-integer? x))
                  (< x 0)
                  (<= width x))
          (error (string-append  "`x` must be an integer such that 0 <= `x` < "
                                 (number->string width))
                 x))
        (when (and (not (exact-integer? y))
                   (< y 0)
                   (<= height y))
          (error (string-append  "`y` must be an integer such that 0 <= `y` < "
                                 (number->string width))
                 y))))

    (define (make-value-checker maxval sym)
      (define message
        (string-append "`" (symbol->string sym) "`"
                       " must be an integer such that 0 < "
                       "`" (symbol->string sym) "`"
                       " < "
                       (number->string maxval)))
      (lambda (v)
        (when (or (not (exact-integer? v))
                  (< v 0)
                  (< maxval v))
          (error message v))))

    (define (make-boolean-value-checker)
      (lambda (v)
        (when (not (boolean? v))
          (error "`v` must be a boolean value" v))))

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

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