Replace quadratic algo in LineDecoder

Leading to enormous speedups when doing things such as
Response(...).iter_lines() as described on issue #2422

httpx/_decoders.py

@@ -5,6 +5,7 @@
 """
 import codecs
 import io
+import re
 import typing
 import zlib
 
@@ -262,49 +263,24 @@ class LineDecoder:
     as line endings, normalizing to `\n`.
     """
 
+    _re = re.compile(r"(.*\n?)")
+
     def __init__(self) -> None:
         self.buffer = ""
 
     def decode(self, text: str) -> typing.List[str]:
-        lines = []
-
-        if text and self.buffer and self.buffer[-1] == "\r":
-            if text.startswith("\n"):
-                # Handle the case where we have an "\r\n" split across
-                # our previous input, and our new chunk.
-                lines.append(self.buffer[:-1] + "\n")
-                self.buffer = ""
-                text = text[1:]
-            else:
-                # Handle the case where we have "\r" at the end of our
-                # previous input.
-                lines.append(self.buffer[:-1] + "\n")
-                self.buffer = ""
-
-        while text:
-            num_chars = len(text)
-            for idx in range(num_chars):
-                char = text[idx]
-                next_char = None if idx + 1 == num_chars else text[idx + 1]
-                if char == "\n":
-                    lines.append(self.buffer + text[: idx + 1])
-                    self.buffer = ""
-                    text = text[idx + 1 :]
-                    break
-                elif char == "\r" and next_char == "\n":
-                    lines.append(self.buffer + text[:idx] + "\n")
-                    self.buffer = ""
-                    text = text[idx + 2 :]
-                    break
-                elif char == "\r" and next_char is not None:
-                    lines.append(self.buffer + text[:idx] + "\n")
-                    self.buffer = ""
-                    text = text[idx + 1 :]
-                    break
-                elif next_char is None:
-                    self.buffer += text
-                    text = ""
-                    break
+        if self.buffer:
+            text = self.buffer + text
+
+        lines = self._re.findall(text)
+        lines.pop()  # always an empty match at the end
+        remainder = ""
+        if lines:
+            final = lines[-1]
+            if not final.endswith("\n"):
+                remainder = lines.pop()
+
+        self.buffer = remainder
 
         return lines