NPI-3668 update sp3 header parsing to store the number of SVs found i…

…n the header, and the number the header claims are there, in dataframe header metadata, so it can be easily checked by the header generator to validate consistency with the number of SVs in the dataframe. Added some clarifying comments to header generation

gnssanalysis/gn_io/sp3.py

@@ -420,26 +420,6 @@ def parse_sp3_header(header: bytes, warn_on_negative_sv_acc_values: bool = True)
     :param bytes header: The header of the SP3 file (as a byte string).
     :return pandas.Series: A Series containing the parsed information from the SP3 header.
     """
-    try:
-        sp3_heading = _pd.Series(
-            data=_np.asarray(_RE_SP3_HEAD.search(header).groups() + _RE_SP3_HEAD_FDESCR.search(header).groups()).astype(
-                str
-            ),
-            index=[
-                "VERSION",
-                "PV_FLAG",
-                "DATETIME",
-                "N_EPOCHS",
-                "DATA_USED",
-                "COORD_SYS",
-                "ORB_TYPE",
-                "AC",
-                "FILE_TYPE",
-                "TIME_SYS",
-            ],
-        )
-    except AttributeError as e:  # Make the exception slightly clearer.
-        raise AttributeError("Failed to parse SP3 header. Regex likely returned no match.", e)
 
     # Find all Satellite Vehicle (SV) entries
     # Updated to also extract the count of expected SVs from the header, and compare that to the number of SVs we get.
@@ -491,6 +471,34 @@ def parse_sp3_header(header: bytes, warn_on_negative_sv_acc_values: bool = True)
             f"Parsed SVs and ACCs: {sv_tbl}"
         )
 
+    try:
+        claimed_sv_count_str = str(head_sv_expected_count) if head_sv_expected_count is not None else ""
+        header_array = _np.asarray(
+            _RE_SP3_HEAD.search(header).groups()
+            + _RE_SP3_HEAD_FDESCR.search(header).groups()
+            + (bytes(str(found_sv_count), "utf-8"),)  # Number of SVs listed in header
+            + (bytes(claimed_sv_count_str, "utf-8"),)  # Number of SVs header states should be there
+        ).astype(str)
+        sp3_heading = _pd.Series(
+            data=header_array,
+            index=[
+                "VERSION",
+                "PV_FLAG",
+                "DATETIME",
+                "N_EPOCHS",
+                "DATA_USED",
+                "COORD_SYS",
+                "ORB_TYPE",
+                "AC",
+                "FILE_TYPE",
+                "TIME_SYS",
+                "SV_COUNT_ACTUAL",  # (Here for convenience) Calculated, not parsed from header by above regex
+                "SV_COUNT_STATED",  # (Here for convenience) Parsed earlier, not by above regex
+            ],
+        )
+    except AttributeError as e:  # Make the exception slightly clearer.
+        raise AttributeError("Failed to parse SP3 header. Regex likely returned no match.", e)
+
     return _pd.concat([sp3_heading, sv_tbl], keys=["HEAD", "SV_INFO"], axis=0)
 
 
@@ -587,13 +595,35 @@ def gen_sp3_header(sp3_df: _pd.DataFrame) -> str:
     sats = sv_tbl.index.to_list()
     n_sats = sv_tbl.shape[0]
 
+    # Check that number of SVs the header metadata says should be here, matches the number of SVs *listed* in
+    # header metadata (which we use to output the new header). Then check that this in turn matches the number of
+    # unique SVs in the SP3 DataFrame itself.
+    dataframe_sv_count = sp3_df.index.get_level_values(1).unique().size
+    header_sv_stated_count = int(head["SV_COUNT_STATED"])
+    header_sv_actual_count = int(n_sats)
+
+    # Check header internal consistency, regarding number of SVs
+    if header_sv_actual_count != header_sv_stated_count:
+        raise AttributeError(
+            f"Number of SVs listed in SP3 header ({str(header_sv_actual_count)}), did not match "
+            f"the number of SVs the header says are there ({header_sv_stated_count})"
+        )
+    # Check header vs DataFrame content regarding number of SVs
+    if header_sv_actual_count != dataframe_sv_count:
+        raise AttributeError(
+            f"Number of SVs listed in SP3 header ({header_sv_actual_count}) did not match "
+            f"SP3 DataFrame contents ({dataframe_sv_count})!"
+        )
+
+    # Alternatively: max(n_sats // 17, 5) # As many lines as needed, minimum 5
     sats_rows = (n_sats // 17) + 1 if n_sats > (17 * 5) else 5  # should be 5 but MGEX need more lines (e.g. CODE sp3)
     sats_header = (
         _np.asarray(sats + ["  0"] * (17 * sats_rows - n_sats), dtype=object).reshape(sats_rows, -1).sum(axis=1) + "\n"
     )
 
+    # Add *calculated, not stated* SV count within the lead-in / padding of the first row of SVs
     sats_header[0] = "+ {:4}   ".format(n_sats) + sats_header[0]
-    sats_header[1:] = "+        " + sats_header[1:]
+    sats_header[1:] = "+        " + sats_header[1:]  # Format remaining rows of SVs with just their lead-in string
 
     sv_orb_head = (
         _np.asarray(sv_tbl.astype(str).str.rjust(3).to_list() + ["  0"] * (17 * sats_rows - n_sats), dtype=object)