changelog-v2: use helper constant in the code to pack/unpack entries

changelog-v2: use helper constant in the code to pack/unpack entries

This makes the code simpler to read and modify. This is probably less
efficient, but we are talking about the Python implementation here, so
performance is concerne secondary to readability.

Differential Revision: https://phab.mercurial-scm.org/D11934

--- a/mercurial/pure/parsers.py	Wed Dec 15 15:28:30 2021 +0100
+++ b/mercurial/pure/parsers.py	Tue Dec 14 19:24:24 2021 +0100
@@ -880,18 +880,43 @@
 
     def _unpack_entry(self, rev, data, r=True):
         items = self.index_format.unpack(data)
-        entry = items[:3] + (rev, rev) + items[3:8]
-        data_comp = items[8] & 3
-        sidedata_comp = (items[8] >> 2) & 3
-        return entry + (data_comp, sidedata_comp)
+        return (
+            items[revlog_constants.INDEX_ENTRY_V2_IDX_OFFSET],
+            items[revlog_constants.INDEX_ENTRY_V2_IDX_COMPRESSED_LENGTH],
+            items[revlog_constants.INDEX_ENTRY_V2_IDX_UNCOMPRESSED_LENGTH],
+            rev,
+            rev,
+            items[revlog_constants.INDEX_ENTRY_V2_IDX_PARENT_1],
+            items[revlog_constants.INDEX_ENTRY_V2_IDX_PARENT_2],
+            items[revlog_constants.INDEX_ENTRY_V2_IDX_NODEID],
+            items[revlog_constants.INDEX_ENTRY_V2_IDX_SIDEDATA_OFFSET],
+            items[
+                revlog_constants.INDEX_ENTRY_V2_IDX_SIDEDATA_COMPRESSED_LENGTH
+            ],
+            items[revlog_constants.INDEX_ENTRY_V2_IDX_COMPRESSION_MODE] & 3,
+            (items[revlog_constants.INDEX_ENTRY_V2_IDX_COMPRESSION_MODE] >> 2)
+            & 3,
+        )
 
     def _pack_entry(self, rev, entry):
-        assert entry[3] == rev, entry[3]
-        assert entry[4] == rev, entry[4]
-        data = entry[:3] + entry[5:10]
-        data_comp = entry[10] & 3
-        sidedata_comp = (entry[11] & 3) << 2
-        data += (data_comp | sidedata_comp,)
+
+        base = entry[revlog_constants.ENTRY_DELTA_BASE]
+        link_rev = entry[revlog_constants.ENTRY_LINK_REV]
+        assert base == rev, (base, rev)
+        assert link_rev == rev, (link_rev, rev)
+        data = (
+            entry[revlog_constants.ENTRY_DATA_OFFSET],
+            entry[revlog_constants.ENTRY_DATA_COMPRESSED_LENGTH],
+            entry[revlog_constants.ENTRY_DATA_UNCOMPRESSED_LENGTH],
+            entry[revlog_constants.ENTRY_PARENT_1],
+            entry[revlog_constants.ENTRY_PARENT_2],
+            entry[revlog_constants.ENTRY_NODE_ID],
+            entry[revlog_constants.ENTRY_SIDEDATA_OFFSET],
+            entry[revlog_constants.ENTRY_SIDEDATA_COMPRESSED_LENGTH],
+            entry[revlog_constants.ENTRY_DATA_COMPRESSION_MODE] & 3
+            | (entry[revlog_constants.ENTRY_SIDEDATA_COMPRESSION_MODE] & 3)
+            << 2,
+        )
         return self.index_format.pack(*data)
 
 

--- a/mercurial/revlogutils/constants.py	Wed Dec 15 15:28:30 2021 +0100
+++ b/mercurial/revlogutils/constants.py	Tue Dec 14 19:24:24 2021 +0100
@@ -184,6 +184,15 @@
 #  27 bytes: Padding to align to 96 bytes (see RevlogV2Plan wiki page)
 INDEX_ENTRY_CL_V2 = struct.Struct(b">Qiiii20s12xQiB27x")
 assert INDEX_ENTRY_CL_V2.size == 32 * 3, INDEX_ENTRY_V2.size
+INDEX_ENTRY_V2_IDX_OFFSET = 0
+INDEX_ENTRY_V2_IDX_COMPRESSED_LENGTH = 1
+INDEX_ENTRY_V2_IDX_UNCOMPRESSED_LENGTH = 2
+INDEX_ENTRY_V2_IDX_PARENT_1 = 3
+INDEX_ENTRY_V2_IDX_PARENT_2 = 4
+INDEX_ENTRY_V2_IDX_NODEID = 5
+INDEX_ENTRY_V2_IDX_SIDEDATA_OFFSET = 6
+INDEX_ENTRY_V2_IDX_SIDEDATA_COMPRESSED_LENGTH = 7
+INDEX_ENTRY_V2_IDX_COMPRESSION_MODE = 8
 
 # revlog index flags