Fix absolute and volume name checks on Windows

core/os/os2/path_windows.odin

@@ -283,6 +283,10 @@ _is_absolute_path :: proc(path: string) -> bool {
 	if _is_reserved_name(path) {
 		return true
 	}
+	if len(path) > 0 && _is_path_separator(path[0]) {
+		return true
+	}
+
 	l := _volume_name_len(path)
 	if l == 0 {
 		return false

core/path/filepath/path.odin

@@ -49,35 +49,66 @@ volume_name_len :: proc(path: string) -> int {
 		if len(path) < 2 {
 			return 0
 		}
-		c := path[0]
+
 		if path[1] == ':' {
-			switch c {
+			switch path[0] {
 			case 'a'..='z', 'A'..='Z':
 				return 2
 			}
 		}
 
-		// URL: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
-		if l := len(path); l >= 5 && is_slash(path[0]) && is_slash(path[1]) &&
-			!is_slash(path[2]) && path[2] != '.' {
-			for n := 3; n < l-1; n += 1 {
-				if is_slash(path[n]) {
-					n += 1
-					if !is_slash(path[n]) {
-						if path[n] == '.' {
-							break
-						}
-					}
-					for ; n < l; n += 1 {
-						if is_slash(path[n]) {
-							break
-						}
-					}
-					return n
+		/*
+			See: URL: https://msdn.microsoft.com/en-us/library/windows/desktop/aa365247(v=vs.85).aspx
+			Further allowed paths can be of the form of:
+			- \\server\share or \\server\share\more\path
+			- \\?\C:\...
+			- \\.\PhysicalDriveX
+		*/
+		// Any remaining kind of path has to start with two slashes.
+		if !is_separator(path[0]) || !is_separator(path[1]) {
+			return 0
+		}
+
+		// Device path. The volume name is the whole string
+		if len(path) >= 5 && path[2] == '.' && is_separator(path[3]) {
+			return len(path)
+		}
+
+		// We're a UNC share `\\host\share`, file namespace `\\?\C:` or UNC in file namespace `\\?\\host\share`
+		prefix := 2
+
+		// File namespace.
+		if len(path) >= 5 && path[2] == '?' && is_separator(path[3]) {
+			if is_separator(path[4]) {
+				// `\\?\\` UNC path in file namespace
+				prefix = 5
+			}
+
+			if len(path) >= 6 && path[5] == ':' {
+				switch path[4] {
+				case 'a'..='z', 'A'..='Z':
+					return 6
+				case:
+					return 0
 				}
-				break
 			}
 		}
+
+		// UNC path, minimum version of the volume is `\\h\s` for host, share.
+		// Can also contain an IP address in the host position.
+		slash_count := 0
+		for i in prefix..<len(path) {
+			// Host needs to be at least 1 character
+			if is_separator(path[i]) && i > 0 {
+				slash_count += 1
+
+				if slash_count == 2 {
+					return i
+				}
+			}
+		}
+
+		return len(path)
 	}
 	return 0
 }

core/path/filepath/path_windows.odin

@@ -36,6 +36,9 @@ is_abs :: proc(path: string) -> bool {
 	if is_reserved_name(path) {
 		return true
 	}
+	if len(path) > 0 && is_slash(path[0]) {
+		return true
+	}
 	l := volume_name_len(path)
 	if l == 0 {
 		return false