Read chunks, identify their types and check crc.

basic-chunks.lisp

@@ -12,4 +12,32 @@
     (cond
       ((every #'eql *png-header* header)
        (read-png-chunks png-stream))
-      (t (error "Not PNG file.")))))
+      (t (error "Not PNG file.")))))
+
+(defun big-endian-vector-to-integer (byte-vector)
+  (check-type byte-vector (vector (unsigned-byte 8)))
+  (iter (for i from (1- (length byte-vector)) downto 0)
+	(for j from 0)
+	(summing (ash (aref byte-vector j) i))))
+
+
+(defun read-png-chunks (png-stream)
+  (let ((length-field (make-array 4 :element-type '(unsigned-byte 8)))
+	(type-field (make-array 4 :element-type '(unsigned-byte 8)))
+	(crc-field (make-array 4 :element-type '(unsigned-byte 8))))
+   (iter
+     (for read-status next (read-sequence length-field png-stream))
+     (for type-status next (read-sequence type-field png-stream))
+     (until (zerop read-status))
+     (assert (eql read-status 4))
+     (assert (eql type-status 4))
+     (let ((chunk-length (big-endian-vector-to-integer length-field))
+	   (type-string (map 'string #'code-char type-field)))
+       (let ((chunk-data (make-array chunk-length)))
+	 (let ((data-status (read-sequence chunk-data png-stream)))
+	   (assert (eql data-status chunk-length))
+	   (let ((crc-status (read-sequence crc-field png-stream)))
+	     (assert (eql crc-status 4))
+	     (let ((read-crc (big-endian-vector-to-integer crc-field))
+		   (computed-crc (updated-crc (crc type-field) chunk-data)))
+	      (collecting (list type-string chunk-data (eql read-crc computed-crc) read-crc computed-crc))))))))))