gh-102511: Speed up os.path.splitroot() with native helpers (GH-118089)

Python/fileutils.c

@@ -2295,6 +2295,99 @@ PathCchCombineEx(wchar_t *buffer, size_t bufsize, const wchar_t *dirname,
 
 #endif /* defined(MS_WINDOWS_GAMES) && !defined(MS_WINDOWS_DESKTOP) */
 
+void
+_Py_skiproot(const wchar_t *path, Py_ssize_t size, Py_ssize_t *drvsize,
+             Py_ssize_t *rootsize)
+{
+    assert(drvsize);
+    assert(rootsize);
+#ifndef MS_WINDOWS
+#define IS_SEP(x) (*(x) == SEP)
+    *drvsize = 0;
+    if (!IS_SEP(&path[0])) {
+        // Relative path, e.g.: 'foo'
+        *rootsize = 0;
+    }
+    else if (!IS_SEP(&path[1]) || IS_SEP(&path[2])) {
+        // Absolute path, e.g.: '/foo', '///foo', '////foo', etc.
+        *rootsize = 1;
+    }
+    else {
+        // Precisely two leading slashes, e.g.: '//foo'. Implementation defined per POSIX, see
+        // https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap04.html#tag_04_13
+        *rootsize = 2;
+    }
+#undef IS_SEP
+#else
+    const wchar_t *pEnd = size >= 0 ? &path[size] : NULL;
+#define IS_END(x) (pEnd ? (x) == pEnd : !*(x))
+#define IS_SEP(x) (*(x) == SEP || *(x) == ALTSEP)
+#define SEP_OR_END(x) (IS_SEP(x) || IS_END(x))
+    if (IS_SEP(&path[0])) {
+        if (IS_SEP(&path[1])) {
+            // Device drives, e.g. \\.\device or \\?\device
+            // UNC drives, e.g. \\server\share or \\?\UNC\server\share
+            Py_ssize_t idx;
+            if (path[2] == L'?' && IS_SEP(&path[3]) &&
+                (path[4] == L'U' || path[4] == L'u') &&
+                (path[5] == L'N' || path[5] == L'n') &&
+                (path[6] == L'C' || path[6] == L'c') &&
+                IS_SEP(&path[7]))
+            {
+                idx = 8;
+            }
+            else {
+                idx = 2;
+            }
+            while (!SEP_OR_END(&path[idx])) {
+                idx++;
+            }
+            if (IS_END(&path[idx])) {
+                *drvsize = idx;
+                *rootsize = 0;
+            }
+            else {
+                idx++;
+                while (!SEP_OR_END(&path[idx])) {
+                    idx++;
+                }
+                *drvsize = idx;
+                if (IS_END(&path[idx])) {
+                    *rootsize = 0;
+                }
+                else {
+                    *rootsize = 1;
+                }
+            }
+        }
+        else {
+            // Relative path with root, e.g. \Windows
+            *drvsize = 0;
+            *rootsize = 1;
+        }
+    }
+    else if (!IS_END(&path[0]) && path[1] == L':') {
+        *drvsize = 2;
+        if (IS_SEP(&path[2])) {
+            // Absolute drive-letter path, e.g. X:\Windows
+            *rootsize = 1;
+        }
+        else {
+            // Relative path with drive, e.g. X:Windows
+            *rootsize = 0;
+        }
+    }
+    else {
+        // Relative path, e.g. Windows
+        *drvsize = 0;
+        *rootsize = 0;
+    }
+#undef SEP_OR_END
+#undef IS_SEP
+#undef IS_END
+#endif
+}
+
 // The caller must ensure "buffer" is big enough.
 static int
 join_relfile(wchar_t *buffer, size_t bufsize,
@@ -2411,49 +2504,39 @@ _Py_normpath_and_size(wchar_t *path, Py_ssize_t size, Py_ssize_t *normsize)
 #endif
 #define SEP_OR_END(x) (IS_SEP(x) || IS_END(x))
 
-    // Skip leading '.\'
     if (p1[0] == L'.' && IS_SEP(&p1[1])) {
+        // Skip leading '.\'
         path = &path[2];
-        while (IS_SEP(path) && !IS_END(path)) {
+        while (IS_SEP(path)) {
             path++;
         }
         p1 = p2 = minP2 = path;
         lastC = SEP;
     }
-#ifdef MS_WINDOWS
-    // Skip past drive segment and update minP2
-    else if (p1[0] && p1[1] == L':') {
-        *p2++ = *p1++;
-        *p2++ = *p1++;
-        minP2 = p2;
-        lastC = L':';
-    }
-    // Skip past all \\-prefixed paths, including \\?\, \\.\,
-    // and network paths, including the first segment.
-    else if (IS_SEP(&p1[0]) && IS_SEP(&p1[1])) {
-        int sepCount = 2;
-        *p2++ = SEP;
-        *p2++ = SEP;
-        p1 += 2;
-        for (; !IS_END(p1) && sepCount; ++p1) {
-            if (IS_SEP(p1)) {
-                --sepCount;
-                *p2++ = lastC = SEP;
-            } else {
-                *p2++ = lastC = *p1;
-            }
-        }
-        minP2 = p2 - 1;
-    }
+    else {
+        Py_ssize_t drvsize, rootsize;
+        _Py_skiproot(path, size, &drvsize, &rootsize);
+        if (drvsize || rootsize) {
+            // Skip past root and update minP2
+            p1 = &path[drvsize + rootsize];
+#ifndef ALTSEP
+            p2 = p1;
 #else
-    // Skip past two leading SEPs
-    else if (IS_SEP(&p1[0]) && IS_SEP(&p1[1]) && !IS_SEP(&p1[2])) {
-        *p2++ = *p1++;
-        *p2++ = *p1++;
-        minP2 = p2 - 1;  // Absolute path has SEP at minP2
-        lastC = SEP;
+            for (; p2 < p1; ++p2) {
+                if (*p2 == ALTSEP) {
+                    *p2 = SEP;
+                }
+            }
+#endif
+            minP2 = p2 - 1;
+            lastC = *minP2;
+#ifdef MS_WINDOWS
+            if (lastC != SEP) {
+                minP2++;
+            }
+#endif
+        }
     }
-#endif /* MS_WINDOWS */
 
     /* if pEnd is specified, check that. Else, check for null terminator */
     for (; !IS_END(p1); ++p1) {