simplify HistoryIterator message ordering

rename reverse -> oldest_first, which is more obvious what it does.
Then, honor it entirely - if you specify no `after` endpoint, we default
to the beginning of message history, similar to how `before` defaults to
the end of message history.

This is a breaking change, and will change the behavior of any iterator
that previously would have been returning messages in a weird order for
limits over 100

`for msg in history(reversed=True, limit=300)` would return the newest
300 messages, in a messed up order (100..0, 200..100, 300..200).
`for msg in history(oldest_first=True, limit=300)` will now return the
oldest 300 messages in order. And so on.

`for msg in history(after=msg)` is unchanged, this previously would
return the oldest 100 messages after `msg`, oldest->newest order, and
still will.

4 files changed, 29 insertions, 32 deletions

diff --git a/discord/abc.py b/discord/abc.py
index 4b34e566..86466d3d 100644
--- a/discord/abc.py
+++ b/discord/abc.py
@@ -863,7 +863,7 @@ class Messageable(metaclass=abc.ABCMeta):
         data = await state.http.pins_from(channel.id)
         return [state.create_message(channel=channel, data=m) for m in data]
 
-    def history(self, *, limit=100, before=None, after=None, around=None, reverse=None):
+    def history(self, *, limit=100, before=None, after=None, around=None, oldest_first=None):
         """Return an :class:`.AsyncIterator` that enables receiving the destination's message history.
 
         You must have :attr:`~.Permissions.read_message_history` permissions to use this.
@@ -902,11 +902,9 @@ class Messageable(metaclass=abc.ABCMeta):
             If a date is provided it must be a timezone-naive datetime representing UTC time.
             When using this argument, the maximum limit is 101. Note that if the limit is an
             even number then this will return at most limit + 1 messages.
-        reverse: Optional[:class:`bool`]
-            If set to true, return messages in oldest->newest order. If unspecified,
-            this defaults to ``False`` for most cases. However if passing in a
-            ``after`` parameter then this is set to ``True``. This avoids getting messages
-            out of order in the ``after`` case.
+        oldest_first: Optional[:class:`bool`]
+            If set to true, return messages in oldest->newest order. Defaults to True if
+            ``after`` is specified, otherwise False.
 
         Raises
         ------
@@ -920,7 +918,7 @@ class Messageable(metaclass=abc.ABCMeta):
         :class:`.Message`
             The message with the message data parsed.
         """
-        return HistoryIterator(self, limit=limit, before=before, after=after, around=around, reverse=reverse)
+        return HistoryIterator(self, limit=limit, before=before, after=after, around=around, oldest_first=oldest_first)
 
 
 class Connectable(metaclass=abc.ABCMeta):
diff --git a/discord/channel.py b/discord/channel.py
index 67b8981c..86d93abf 100644
--- a/discord/channel.py
+++ b/discord/channel.py
@@ -265,7 +265,7 @@ class TextChannel(discord.abc.Messageable, discord.abc.GuildChannel, Hashable):
         message_ids = [m.id for m in messages]
         await self._state.http.delete_messages(self.id, message_ids)
 
-    async def purge(self, *, limit=100, check=None, before=None, after=None, around=None, reverse=False, bulk=True):
+    async def purge(self, *, limit=100, check=None, before=None, after=None, around=None, oldest_first=False, bulk=True):
         """|coro|
 
         Purges a list of messages that meet the criteria given by the predicate
@@ -306,8 +306,8 @@ class TextChannel(discord.abc.Messageable, discord.abc.GuildChannel, Hashable):
             Same as ``after`` in :meth:`history`.
         around
             Same as ``around`` in :meth:`history`.
-        reverse
-            Same as ``reverse`` in :meth:`history`.
+        oldest_first
+            Same as ``oldest_first`` in :meth:`history`.
         bulk: class:`bool`
             If True, use bulk delete. bulk=False is useful for mass-deleting
             a bot's own messages without manage_messages. When True, will fall
@@ -330,7 +330,7 @@ class TextChannel(discord.abc.Messageable, discord.abc.GuildChannel, Hashable):
         if check is None:
             check = lambda m: True
 
-        iterator = self.history(limit=limit, before=before, after=after, reverse=reverse, around=around)
+        iterator = self.history(limit=limit, before=before, after=after, oldest_first=oldest_first, around=around)
         ret = []
         count = 0
 
diff --git a/discord/iterators.py b/discord/iterators.py
index 015620bc..73216ccf 100644
--- a/discord/iterators.py
+++ b/discord/iterators.py
@@ -28,10 +28,12 @@ import asyncio
 import datetime
 
 from .errors import NoMoreItems
-from .utils import time_snowflake, maybe_coroutine
+from .utils import DISCORD_EPOCH, time_snowflake, maybe_coroutine
 from .object import Object
 from .audit_logs import AuditLogEntry
 
+OLDEST_MESSAGE = Object(id=0)
+
 class _AsyncIterator:
     __slots__ = ()
 
@@ -196,14 +198,13 @@ class HistoryIterator(_AsyncIterator):
     around: :class:`abc.Snowflake`
         Message around which all messages must be. Limit max 101. Note that if
         limit is an even number, this will return at most limit+1 messages.
-    reverse: :class:`bool`
-        If set to true, return messages in oldest->newest order. Recommended
-        when using with "after" queries with limit over 100, otherwise messages
-        will be out of order.
+    oldest_first: :class:`bool`
+        If set to true, return messages in oldest->newest order. Defaults to 
+        True if ``after`` is specified, otherwise False.
     """
 
     def __init__(self, messageable, limit,
-                 before=None, after=None, around=None, reverse=None):
+                 before=None, after=None, around=None, oldest_first=None):
 
         if isinstance(before, datetime.datetime):
             before = Object(id=time_snowflake(before, high=False))
@@ -212,17 +213,17 @@ class HistoryIterator(_AsyncIterator):
         if isinstance(around, datetime.datetime):
             around = Object(id=time_snowflake(around))
 
+        if oldest_first is None:
+            self.reverse = after is not None
+        else:
+            self.reverse = oldest_first
+
         self.messageable = messageable
         self.limit = limit
         self.before = before
-        self.after = after
+        self.after = after or OLDEST_MESSAGE
         self.around = around
 
-        if reverse is None:
-            self.reverse = after is not None
-        else:
-            self.reverse = reverse
-
         self._filter = None  # message dict -> bool
 
         self.state = self.messageable._state
@@ -246,17 +247,15 @@ class HistoryIterator(_AsyncIterator):
                 self._filter = lambda m: int(m['id']) < self.before.id
             elif self.after:
                 self._filter = lambda m: self.after.id < int(m['id'])
-        elif self.before and self.after:
+        else:
             if self.reverse:
                 self._retrieve_messages = self._retrieve_messages_after_strategy
-                self._filter = lambda m: int(m['id']) < self.before.id
+                if (self.before):
+                    self._filter = lambda m: int(m['id']) < self.before.id
             else:
                 self._retrieve_messages = self._retrieve_messages_before_strategy
-                self._filter = lambda m: int(m['id']) > self.after.id
-        elif self.after:
-            self._retrieve_messages = self._retrieve_messages_after_strategy
-        else:
-            self._retrieve_messages = self._retrieve_messages_before_strategy
+                if (self.after and self.after != OLDEST_MESSAGE):
+                    self._filter = lambda m: int(m['id']) > self.after.id
 
     async def next(self):
         if self.messages.empty():
diff --git a/docs/locale/ja/LC_MESSAGES/api.po b/docs/locale/ja/LC_MESSAGES/api.po
index c947dbda..f381f28d 100644
--- a/docs/locale/ja/LC_MESSAGES/api.po
+++ b/docs/locale/ja/LC_MESSAGES/api.po
@@ -7361,8 +7361,8 @@ msgid "Same as ``around`` in :meth:`history`."
 msgstr ":meth:`history` での ``around`` と同じです。"
 
 #: discord.TextChannel.purge:25 of
-msgid "Same as ``reverse`` in :meth:`history`."
-msgstr ":meth:`history` での ``reverse`` と同じです。"
+msgid "Same as ``oldest_first`` in :meth:`history`."
+msgstr ":meth:`history` での ``oldest_first`` と同じです。"
 
 #: discord.TextChannel.purge:26 of
 msgid "If True, use bulk delete. bulk=False is useful for mass-deleting a bot's own messages without manage_messages. When True, will fall back to single delete if current account is a user bot, or if messages are older than two weeks."