[3.13] gh-119034, REPL: Change page up/down keys to search in history (GH-123607) (GH-123773)

Change <page up> and <page down> keys of the Python REPL to history
search forward/backward.

(cherry picked from commit 8311b11800)

Co-authored-by: Victor Stinner <vstinner@python.org>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/_pyrepl/historical_reader.py

@@ -71,6 +71,18 @@ class previous_history(commands.Command):
         r.select_item(r.historyi - 1)
 
 
+class history_search_backward(commands.Command):
+    def do(self) -> None:
+        r = self.reader
+        r.search_next(forwards=False)
+
+
+class history_search_forward(commands.Command):
+    def do(self) -> None:
+        r = self.reader
+        r.search_next(forwards=True)
+
+
 class restore_history(commands.Command):
     def do(self) -> None:
         r = self.reader
@@ -234,6 +246,8 @@ class HistoricalReader(Reader):
             isearch_forwards,
             isearch_backwards,
             operate_and_get_next,
+            history_search_backward,
+            history_search_forward,
         ]:
             self.commands[c.__name__] = c
             self.commands[c.__name__.replace("_", "-")] = c
@@ -251,8 +265,8 @@ class HistoricalReader(Reader):
             (r"\C-s", "forward-history-isearch"),
             (r"\M-r", "restore-history"),
             (r"\M-.", "yank-arg"),
-            (r"\<page down>", "last-history"),
-            (r"\<page up>", "first-history"),
+            (r"\<page down>", "history-search-forward"),
+            (r"\<page up>", "history-search-backward"),
         )
 
     def select_item(self, i: int) -> None:
@@ -305,6 +319,59 @@ class HistoricalReader(Reader):
         else:
             return super().get_prompt(lineno, cursor_on_line)
 
+    def search_next(self, *, forwards: bool) -> None:
+        """Search history for the current line contents up to the cursor.
+
+        Selects the first item found. If nothing is under the cursor, any next
+        item in history is selected.
+        """
+        pos = self.pos
+        s = self.get_unicode()
+        history_index = self.historyi
+
+        # In multiline contexts, we're only interested in the current line.
+        nl_index = s.rfind('\n', 0, pos)
+        prefix = s[nl_index + 1:pos]
+        pos = len(prefix)
+
+        match_prefix = len(prefix)
+        len_item = 0
+        if history_index < len(self.history):
+            len_item = len(self.get_item(history_index))
+        if len_item and pos == len_item:
+            match_prefix = False
+        elif not pos:
+            match_prefix = False
+
+        while 1:
+            if forwards:
+                out_of_bounds = history_index >= len(self.history) - 1
+            else:
+                out_of_bounds = history_index == 0
+            if out_of_bounds:
+                if forwards and not match_prefix:
+                    self.pos = 0
+                    self.buffer = []
+                    self.dirty = True
+                else:
+                    self.error("not found")
+                return
+
+            history_index += 1 if forwards else -1
+            s = self.get_item(history_index)
+
+            if not match_prefix:
+                self.select_item(history_index)
+                return
+
+            len_acc = 0
+            for i, line in enumerate(s.splitlines(keepends=True)):
+                if line.startswith(prefix):
+                    self.select_item(history_index)
+                    self.pos = pos + len_acc
+                    return
+                len_acc += len(line)
+
     def isearch_next(self) -> None:
         st = self.isearch_term
         p = self.pos