Begin move to columnar types

This will allow making columns optional, as desired, and make
adding new columns straightforward without impacting existing
behavior.

Signed-off-by: Will Beason <willbeason@gmail.com>

tables.py

@@ -0,0 +1,136 @@
+from abc import abstractmethod, ABC
+from datetime import datetime, timezone
+from hashlib import sha1
+from typing import Generic, TypeVar
+
+import mwtypes
+import mwxml
+
+import pyarrow as pa
+
+T = TypeVar('T')
+
+
+class RevisionField(ABC, Generic[T]):
+    """
+    Abstract type which represents a field in a table of page revisions.
+    """
+
+    def __init__(self, field: pa.Field):
+        self.field = field
+
+    @abstractmethod
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> T:
+        """
+        :param page: The page for this set of revisions.
+        :param revisions: The set of revisions to compute the field from.
+        Revisions are passed in chronological order, so use revisions[-1] to
+        access the most recent revision in the set.
+        """
+        pass
+
+
+class RevisionTableColumn(Generic[T]):
+    def __init__(self, field: RevisionField[T]):
+        self.field: RevisionField = field
+        self.data: list[T] = []
+
+    def add(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> None:
+        self.data.append(self.field.extract(page, revisions))
+
+    def pop_column(self) -> list[T]:
+        data = self.data
+        self.data = []
+        return data
+
+
+class RevisionTable:
+    columns: list[RevisionTableColumn]
+
+    def add_revision_set(self, page: mwtypes.Page, revisions: list[mwxml.Revision]):
+        for column in self.columns:
+            column.add(page, revisions)
+
+
+class RevisionId(RevisionField[int]):
+    def extract(self, _: mwtypes.Page, revisions: list[mwxml.Revision]) -> int:
+        revision = revisions[-1]
+        return revision.id
+
+
+class RevisionTimestamp(RevisionField[datetime]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> datetime:
+        revision = revisions[-1]
+        return revision.timestamp
+
+
+class RevisionEditorId(RevisionField[int | None]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> int | None:
+        revision = revisions[-1]
+        if revision.deleted.user or revision.user.id is None:
+            return None
+
+        return revision.user.id
+
+
+class RevisionAnon(RevisionField[bool | None]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> bool | None:
+        revision = revisions[-1]
+        if revision.deleted.user:
+            return None
+
+        return revision.user.id is None
+
+
+class RevisionEditorText(RevisionField[str | None]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> str | None:
+        revision = revisions[-1]
+        if revision.deleted.user:
+            return None
+
+        return revision.user.text
+
+
+class RevisionPageTitle(RevisionField[str]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> str:
+        return page.title
+
+
+class RevisionDeleted(RevisionField[bool]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> bool:
+        revision = revisions[-1]
+        return revision.deleted.text
+
+
+class RevisionNamespace(RevisionField[int]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> int:
+        return page.namespace
+
+
+class RevisionSha1(RevisionField[str]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> str:
+        revision = revisions[-1]
+        if revision.sha1:
+            return revision.sha1
+
+        return sha1(revision.sha1).hexdigest()
+
+
+class RevisionTextChars(RevisionField[int]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> int:
+        revision = revisions[-1]
+        return len(revision.text)
+
+
+class RevisionMinor(RevisionField[bool]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> bool:
+        revision = revisions[-1]
+        return revision.minor
+
+
+class RevisionCollapse(RevisionField[int]):
+    def extract(self, page: mwtypes.Page, revisions: list[mwxml.Revision]) -> int:
+        return len(revisions)
+
+
+