added os.nativeToUnixPath, os.absolutePrefix

Author: timotheecour

changelog.md

@@ -36,7 +36,8 @@
   (see #13222); instead they now use `getCurrentDir` to resolve those cases,
   and this can now throw in edge cases where `getCurrentDir` throws.
   `relativePath` also now works for js with `-d:nodejs`.
-
+- Added `os.absolutePrefix` to return the root path component for absolute paths
+- Added `os.nativeToUnixPath` to convert a native path to a UNIX path.
 
 ## Language changes
 - In newruntime it is now allowed to assign discriminator field without restrictions as long as case object doesn't have custom destructor. Discriminator value doesn't have to be a constant either. If you have custom destructor for case object and you do want to freely assign discriminator fields, it is recommended to refactor object into 2 objects like this:

lib/pure/os.nim

@@ -276,6 +276,35 @@ proc splitPath*(path: string): tuple[head, tail: string] {.
     result.head = ""
     result.tail = path
 
+proc absolutePrefix*(path: string): string {.since: (1, 1).} =
+  ## returns the root path component for absolute paths, empty for relative paths.
+  ## See also `isAbsolute`.
+  runnableExamples:
+    when defined(posix):
+      doAssert absolutePrefix("//foo") == "//"
+      doAssert absolutePrefix("foo") == ""
+    when defined(windows):
+      doAssert absolutePrefix(r"C:\\\bar") == r"C:\\\"
+  var i = 0
+  let len = len(path)
+  when doslikeFileSystem:
+    while i < len:
+      if path[i] in {DirSep, AltSep}: i.inc
+      elif i == 0 and path[0] in {'a'..'z', 'A'..'Z'} and len > 1 and path[1] == ':': i.inc
+      else: break
+  elif defined(macos):
+    # according to https://perldoc.perl.org/File/Spec/Mac.html `:a` is a relative path
+    while i < len:
+      if path[i] != ':': i.inc
+      else: break
+  elif defined(RISCOS):
+    if len > 0 and path[0] == '$': i = 1
+  elif defined(posix):
+    while i < len:
+      if path[i] == '/': i.inc
+      else: break
+  result = path[0 ..< i]
+
 proc isAbsolute*(path: string): bool {.rtl, noSideEffect, extern: "nos$1", raises: [].} =
   ## Checks whether a given `path` is absolute.
   ##
@@ -817,6 +846,21 @@ proc cmpPaths*(pathA, pathB: string): int {.
     else:
       result = cmpIgnoreCase(a, b)
 
+proc nativeToUnixPath*(path: string): string {.noSideEffect, since: (1, 1).} =
+  ## Converts a native path to a UNIX path; on windows, the drive is converted to
+  ## `/`.
+  ## See also `unixToNativePath`.
+  runnableExamples:
+    when defined(windows):
+      doAssert nativeToUnixPath(r"C:\foo\bar") == "/foo/bar"
+  when defined(posix):
+    result = path
+  else:
+    let prefix = path.absolutePrefix
+    if prefix.len > 0:
+      result.add '/'
+    result.add path.replace('\\', '/')
+
 proc unixToNativePath*(path: string, drive=""): string {.
   noSideEffect, rtl, extern: "nos$1".} =
   ## Converts an UNIX-like path to a native one.
@@ -828,6 +872,7 @@ proc unixToNativePath*(path: string, drive=""): string {.
   ## which drive label to use during absolute path conversion.
   ## `drive` defaults to the drive of the current working directory, and is
   ## ignored on systems that do not have a concept of "drives".
+  ## See also `nativeToUnixPath`.
   when defined(unix):
     result = path
   else: