1 files changed, 29 insertions, 0 deletions
diff --git a/image-format/pnm/private/double-byte.scm b/image-format/pnm/private/double-byte.scm
new file mode 100644
index 0000000..156b409
--- /dev/null
+++ b/image-format/pnm/private/double-byte.scm
@@ -0,0 +1,29 @@
+;;; 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 double-byte)
+  (export split-double-byte combine-bytes)
+  (import (scheme base)
+          (image-format pnm private bitwise))
+  (begin
+    (define (split-double-byte v)
+      (values (arithmetic-shift v -8)
+              (bitwise-and v 255)))
+
+    (define (combine-bytes l r)
+      (bitwise-ior (arithmetic-shift l 8) r))))