rdrecord: correctly infer length for signal files with a prolog.

If the header file does not specify the record length, it must be
inferred from the size of the signal file (dividing the data length by
the number of samples per frame.)

Previously, this calculation assumed that the file did not contain a
prolog.  Correctly calculate the data length by subtracting the prolog
length (byte offset) from the size of the file.

Author: Benjamin Moody

wfdb/io/_signal.py

@@ -2059,7 +2059,8 @@ def describe_list_indices(full_list):
     return unique_elements, element_indices
 
 
-def _infer_sig_len(file_name, fmt, tsamps_per_frame, dir_name, pn_dir=None):
+def _infer_sig_len(file_name, fmt, tsamps_per_frame, byte_offset,
+                   dir_name, pn_dir=None):
     """
     Infer the length of a signal from a dat file.
 
@@ -2071,6 +2072,8 @@ def _infer_sig_len(file_name, fmt, tsamps_per_frame, dir_name, pn_dir=None):
         WFDB fmt of the dat file.
     tsamps_per_frame : int
         Total number of samples per frame contained in the dat file.
+    byte_offset : int or None
+        The byte offset of the dat file.  None is equivalent to zero.
     dir_name : str
         The full directory where the dat file(s) are located, if the dat
         file(s) are local.
@@ -2094,7 +2097,10 @@ def _infer_sig_len(file_name, fmt, tsamps_per_frame, dir_name, pn_dir=None):
         file_size = download._remote_file_size(file_name=file_name,
                                                pn_dir=pn_dir)
 
-    sig_len = int(file_size / (BYTES_PER_SAMPLE[fmt] * tsamps_per_frame))
+    if byte_offset is None:
+        byte_offset = 0
+    data_size = file_size - byte_offset
+    sig_len = int(data_size / (BYTES_PER_SAMPLE[fmt] * tsamps_per_frame))
 
     return sig_len
 

wfdb/io/record.py

@@ -3453,6 +3453,7 @@ def rdrecord(record_name, sampfrom=0, sampto=None, channels=None,
                 record.sig_len = _signal._infer_sig_len(
                     file_name=record.file_name[0], fmt=record.fmt[0],
                     tsamps_per_frame=tsamps_per_frame,
+                    byte_offset=record.byte_offset[0],
                     dir_name=dir_name, pn_dir=pn_dir)
         sampto = record.sig_len