Record.wrsamp: always set checksums to correct values.

When writing a record, the application generally shouldn't have to
worry about details like calculating checksums.  The wfdb
package *should* ensure that the output files it generates are
correct, and *should not* trust applications to calculate checksums
correctly.

Moreover, although it was originally intended for SignalMixin.wr_dats
to verify that the application-supplied checksums were correct, this
never actually worked, and existing test cases depend on that fact.
So it is not reasonable for wr_dats to raise an exception if the
checksums are wrong.

Instead, update the checksums in the Record object when wrsamp is
called (and note that we have to do this before calling wrheader,
which is called before wr_dats).

For compatibility with old test cases, that expect to be able to read
and write records quasi-unmodified, we don't set checksums for signals
that previously had a checksum of None, or that previously had a
checksum that was correct mod 2**16.

Author: Benjamin Moody

wfdb/io/record.py

@@ -922,6 +922,16 @@ def wrsamp(self, expanded=False, write_dir=""):
         N/A
 
         """
+        # Update the checksum field (except for channels that did not have
+        # a checksum to begin with, or where the checksum was already
+        # valid.)
+        if self.checksum is not None:
+            checksums = self.calc_checksum(expanded=expanded)
+            for ch, old_val in enumerate(self.checksum):
+                if old_val is None or (checksums[ch] - old_val) % 65536 == 0:
+                    checksums[ch] = old_val
+            self.checksum = checksums
+
         # Perform field validity and cohesion checks, and write the
         # header file.
         self.wrheader(write_dir=write_dir, expanded=expanded)