Refactors conditional to polymorphism in HeaderPacker. About 5% faster at reading trace headers.

segpy/packer.py

@@ -112,19 +112,28 @@ def compile_struct(header_format_class, start_offset=0, length_in_bytes=None, en
     return cformat, field_name_allocations
 
 
+def make_header_packer(header_format_class, endian='>'):
+    cformat, field_name_allocations = compile_struct(
+        header_format_class,
+        header_format_class.START_OFFSET_IN_BYTES,
+        header_format_class.LENGTH_IN_BYTES,
+        endian)
+    structure = Struct(cformat)
+
+    one_to_one = all(len(fields) == 1 for fields in field_name_allocations)
+
+    if one_to_one:
+        return BijectiveHeaderPacker(header_format_class, structure, field_name_allocations)
+    return SurjectiveHeaderPacker(header_format_class, structure, field_name_allocations)
+
+
 class HeaderPacker:
     """Packing and unpacking header instances."""
 
-    def __init__(self, header_format_class, endian='>'):
+    def __init__(self, header_format_class, structure, field_name_allocations):
         self._header_format_class = header_format_class
-        self._format, self._field_name_allocations = compile_struct(
-            header_format_class,
-            header_format_class.START_OFFSET_IN_BYTES,
-            header_format_class.LENGTH_IN_BYTES,
-            endian)
-        self._struct = Struct(self._format)
-        self._one_to_one = all(len(fields) == 1 for fields in self._field_name_allocations)
-        pass
+        self._structure = structure
+        self._field_name_allocations = field_name_allocations
 
     @property
     def header_format_class(self):
@@ -140,7 +149,16 @@ class HeaderPacker:
                 header.__class__.__name__
             ))
         values = [getattr(header, names[0]) for names in self._field_name_allocations]
-        return self._struct.pack(*values)
+        return self._structure.pack(*values)
+
+    def __repr__(self):
+        return "{}({})".format(
+            self.__class__.__name__,
+            self._header_format_class.__name__)
+
+
+class BijectiveHeaderPacker(HeaderPacker):
+    """One-to-one packing/unpacking of serialised values to header fields."""
 
     def unpack(self, buffer):
         """Unpack a header into a header object.
@@ -151,10 +169,23 @@ class HeaderPacker:
         Returns:
             The header object.
         """
-        values = self._struct.unpack(buffer)
+        values = self._structure.unpack(buffer)
+        return self._header_format_class(*values)
 
-        if self._one_to_one:
-            return self._header_format_class(*values)
+
+class SurjectiveHeaderPacker(HeaderPacker):
+    """One-to-many unpacking of serialised values to header fields."""
+
+    def unpack(self, buffer):
+        """Unpack a header into a header object.
+
+        Overwrites any existing header field values with new values
+        obtained from the buffer.
+
+        Returns:
+            The header object.
+        """
+        values = self._structure.unpack(buffer)
 
         kwargs = {name: value
                   for names, value in zip(self._field_name_allocations, values)
@@ -162,11 +193,6 @@ class HeaderPacker:
 
         return self._header_format_class(**kwargs)
 
-    def __repr__(self):
-        return "{}({})".format(
-            self.__class__.__name__,
-            self._header_format_class.__name__)
-
 
 def main():
     from segpy.trace_header import TraceHeaderRev0

segpy/reader.py

@@ -1,6 +1,6 @@
 from __future__ import print_function
 from segpy.encoding import ASCII
-from segpy.packer import HeaderPacker
+from segpy.packer import make_header_packer
 
 from segpy.trace_header import TraceHeaderRev1
 from segpy.util import file_length, filename_from_handle
@@ -175,7 +175,7 @@ class SegYReader(object):
         self._binary_reel_header = binary_reel_header
         self._extended_textual_headers = extended_textual_headers
 
-        self._trace_header_packer = HeaderPacker(trace_header_format, endian)
+        self._trace_header_packer = make_header_packer(trace_header_format, endian)
 
         self._trace_offset_catalog = trace_offset_catalog
         self._trace_length_catalog = trace_length_catalog

segpy/toolkit.py

@@ -15,7 +15,7 @@ from segpy.catalog import CatalogBuilder
 from segpy.datatypes import SEG_Y_TYPE_TO_CTYPE, size_in_bytes, DATA_SAMPLE_FORMAT_TO_SEG_Y_TYPE, CTYPE_TO_SIZE
 from segpy.encoding import guess_encoding, is_supported_encoding, UnsupportedEncodingError
 from segpy.ibm_float import IBMFloat
-from segpy.packer import HeaderPacker
+from segpy.packer import make_header_packer
 from segpy.revisions import canonicalize_revision
 from segpy.trace_header import TraceHeaderRev1
 from segpy.util import file_length, batched, pad, complementary_intervals, NATIVE_ENDIANNESS, EMPTY_BYTE_STRING
@@ -168,7 +168,7 @@ def read_binary_reel_header(fh, endian='>'):
         endian: '>' for big-endian data (the standard and default), '<' for
             little-endian (non-standard)
     """
-    header_packer = HeaderPacker(BinaryReelHeader, endian)
+    header_packer = make_header_packer(BinaryReelHeader, endian)
     fh.seek(TEXTUAL_HEADER_NUM_BYTES)  # Consider using from_one_based(BinaryReelHeader.START_OFFSET_IN_BYTES)
     buffer = fh.read(BinaryReelHeader.LENGTH_IN_BYTES)
     reel_header = header_packer.unpack(buffer)
@@ -336,7 +336,7 @@ def catalog_traces(fh, bps, trace_header_format=TraceHeaderRev1, endian='>', pro
     if not callable(progress_callback):
         raise TypeError("catalog_traces(): progress callback must be callable")
 
-    trace_header_packer = HeaderPacker(trace_header_format, endian)
+    trace_header_packer = make_header_packer(trace_header_format, endian)
 
     length = file_length(fh)
 
@@ -637,7 +637,7 @@ def write_binary_reel_header(fh, binary_reel_header, endian='>'):
         The file pointer for fh will be positioned at the first byte following
         the binary reel header.
     """
-    header_packer = HeaderPacker(BinaryReelHeader, endian)  # TODO: Hard wiring
+    header_packer = make_header_packer(BinaryReelHeader, endian)  # TODO: Hard wiring
     buffer = header_packer.pack(binary_reel_header)
     fh.write(buffer)
     fh.seek(REEL_HEADER_NUM_BYTES)