_rd_dat_signals: fix conversion of format 8.

Benjamin Moody · Benjamin Moody · commit 6bede91f2fbe · 2021-09-24T15:22:18.000-04:00
In signal format 8, each sample is stored as an 8-bit signed
difference from the previous sample.  This means that after reading
the raw byte values, they must be translated to absolute sample values
by calling cumsum() and adding the initial value.
diff --git a/wfdb/io/_signal.py b/wfdb/io/_signal.py
@@ -1182,6 +1182,32 @@ def _rd_dat_signals(file_name, dir_name, pn_dir, fmt, n_sig, sig_len,
         elif fmt == '160':
             sig_data = (sig_data.astype('int32') - 32768).astype('int16')
 
+    # For format 8, convert sample differences to absolute samples.  Note
+    # that if sampfrom is not 0, the results will be wrong, since we can't
+    # know the starting value without reading the entire record from the
+    # beginning - an inherent limitation of the format, and the use of
+    # format 8 is discouraged for this reason!  However, the following is
+    # consistent with the behavior of the WFDB library: the initial value
+    # specified by the header file is used as the starting sample value,
+    # regardless of where in the record we begin reading.  Therefore, the
+    # following should give the same results as rdsamp.
+    if fmt == '8':
+        dif_frames = sig_data.reshape(-1, tsamps_per_frame)
+        abs_frames = np.empty(dif_frames.shape, dtype='int32')
+        ch_start = 0
+        for ch in range(n_sig):
+            ch_end = ch_start + samps_per_frame[ch]
+            # Extract sample differences as a 2D array
+            ch_dif_signal = dif_frames[:, ch_start:ch_end]
+            # Convert to a 1D array of absolute samples
+            ch_abs_signal = ch_dif_signal.cumsum(dtype=abs_frames.dtype)
+            ch_abs_signal += init_value[ch]
+            # Transfer to the output array
+            ch_abs_signal = ch_abs_signal.reshape(ch_dif_signal.shape)
+            abs_frames[:, ch_start:ch_end] = ch_abs_signal
+            ch_start = ch_end
+        sig_data = abs_frames.reshape(-1)
+
     # At this point, dtype of sig_data is the minimum integer format
     # required for storing the final digital samples.
 
