new os.isRelativeTo (#13212)

3 files changed, 26 insertions, 0 deletions

diff --git a/changelog.md b/changelog.md
index 7e08133a0..8fac8dab6 100644
--- a/changelog.md
+++ b/changelog.md
@@ -53,6 +53,8 @@
   Eg: `echo ?.n.typ.kind`
 - Added `minIndex` and `maxIndex` to the `sequtils` module
 
+- Added `os.isRelativeTo` to tell whether a path is relative to another
+
 ## Library changes
 
 - `asyncdispatch.drain` now properly takes into account `selector.hasPendingOperations`
diff --git a/lib/pure/os.nim b/lib/pure/os.nim
index 3b1421cb5..12170aa40 100644
--- a/lib/pure/os.nim
+++ b/lib/pure/os.nim
@@ -406,6 +406,18 @@ proc relativePath*(path, base: string; sep = DirSep): string {.
     if not f.hasNext(path): break
     ff = f.next(path)
 
+proc isRelativeTo*(path: string, base: string): bool {.since: (1, 1).} =
+  ## Returns true if `path` is relative to `base`.
+  runnableExamples:
+    doAssert isRelativeTo("./foo//bar", "foo")
+    doAssert isRelativeTo("foo/bar", ".")
+    doAssert isRelativeTo("/foo/bar.nim", "/foo/bar.nim")
+    doAssert not isRelativeTo("foo/bar.nims", "foo/bar.nim")
+  let path = path.normalizePath
+  let base = base.normalizePath
+  let ret = relativePath(path, base)
+  result = path.len > 0 and not ret.startsWith ".."
+
 proc parentDirPos(path: string): int =
   var q = 1
   if len(path) >= 1 and path[len(path)-1] in {DirSep, AltSep}: q = 2
diff --git a/tests/stdlib/tos.nim b/tests/stdlib/tos.nim
index 48202157a..94f0d657e 100644
--- a/tests/stdlib/tos.nim
+++ b/tests/stdlib/tos.nim
@@ -382,3 +382,15 @@ block osenv:
     doAssert existsEnv(dummyEnvVar) == false
     delEnv(dummyEnvVar)         # deleting an already deleted env var
     doAssert existsEnv(dummyEnvVar) == false
+
+block isRelativeTo:
+  doAssert isRelativeTo("/foo", "/")
+  doAssert isRelativeTo("/foo/bar", "/foo")
+  doAssert isRelativeTo("foo/bar", "foo")
+  doAssert isRelativeTo("/foo/bar.nim", "/foo/bar.nim")
+  doAssert isRelativeTo("./foo/", "foo")
+  doAssert isRelativeTo("foo", "./foo/")
+  doAssert isRelativeTo(".", ".")
+  doAssert isRelativeTo("foo/bar", ".")
+  doAssert not isRelativeTo("foo/bar.nims", "foo/bar.nim")
+  doAssert not isRelativeTo("/foo2", "/foo")