perf(csharp): resume past the \$/@ prefix run so malformed files stay linear (#3376)

(cherry picked from commit 7f0fd6ddfe)

graphify/extract.py

@@ -3167,8 +3167,14 @@ def extract_ruby(path: Path) -> dict:
     return _extract_generic(path, _RUBY_CONFIG)
 
 
-def _csharp_string_opener(source: bytes, i: int) -> "tuple[int, int, bool, bool] | None":
-    """If a string literal starts at ``i``, return ``(quote_pos, quote_run, interpolated, verbatim)``.
+def _csharp_string_opener(source: bytes, i: int) -> "tuple[int, int, bool, bool]":
+    """Read a possible string-literal opener at ``i``.
+
+    Returns ``(prefix_end, quote_run, interpolated, verbatim)``. ``quote_run`` is
+    0 when no string starts here, and ``prefix_end`` is always where the run of
+    ``$``/``@`` characters stopped — the caller needs that even on a miss, so it
+    can resume past the run instead of re-walking it from the next byte (a long
+    run in malformed source would otherwise cost O(n^2)).
 
     C# lets the ``$`` and ``@`` prefixes appear in either order and repeat, and
     a raw string literal opens with a run of three or more quotes, so the prefix
@@ -3187,7 +3193,7 @@ def _csharp_string_opener(source: bytes, i: int) -> "tuple[int, int, bool, bool]
             break
         j += 1
     if source[j:j + 1] != b'"':
-        return None
+        return j, 0, interpolated, verbatim
     run = 0
     while source[j + run:j + run + 1] == b'"':
         run += 1
@@ -3229,11 +3235,13 @@ def _csharp_broken_escape_offsets(source: bytes) -> list[int]:
                 j += 1
             i = j + 1
             continue
-        opener = _csharp_string_opener(source, i)
-        if opener is None:
-            i += 1
+        quote, run, interpolated, verbatim = _csharp_string_opener(source, i)
+        if run == 0:
+            # No string opens here. Resume at the end of the `$`/`@` run rather
+            # than at i + 1: re-walking a long run from every offset is what
+            # makes a malformed file quadratic.
+            i = quote if quote > i else i + 1
             continue
-        quote, run, interpolated, verbatim = opener
         if run >= 3:
             # Raw string literal: closed by a quote run of the same length.
             end = source.find(b'"' * run, quote + run)

tests/test_csharp_verbatim_interpolation.py

@@ -16,7 +16,11 @@ from __future__ import annotations
 import os
 from pathlib import Path
 
-from graphify.extract import _normalize_csharp_verbatim_interpolation, extract
+from graphify.extract import (
+    _csharp_broken_escape_offsets,
+    _normalize_csharp_verbatim_interpolation,
+    extract,
+)
 
 # The offending literal, kept in one place: `}` then an escaped quote then the
 # terminator. Written as a plain string so the C# quoting stays readable.
@@ -164,3 +168,20 @@ def test_two_offending_literals_in_one_file(tmp_path):
     labels = {str(n.get("label", "")).lstrip(".") for n in result["nodes"]}
     for member in ("First()", "Between()", "Second()", "Last()"):
         assert member in labels, f"{member} missing from {sorted(labels)}"
+
+
+def test_long_prefix_run_does_not_blow_up(tmp_path):
+    """A malformed file is the normal input here -- the scan only runs on files
+    the grammar already rejected -- so a long run of `$`/`@` must stay linear.
+
+    Resuming one byte past the start of such a run re-walks it from every offset:
+    4k characters took 0.8s that way, and the cost quadruples each doubling.
+    """
+    import time
+
+    src = b"class C { " + (b"$" * 1_000_000) + b" }"
+    started = time.perf_counter()
+    assert _csharp_broken_escape_offsets(src) == []
+    elapsed = time.perf_counter() - started
+    # Linear finishes in well under a second; the quadratic form needs hours.
+    assert elapsed < 5.0, f"prefix scan took {elapsed:.1f}s -- quadratic again?"