add: common paths finder

src/common_paths_tests.rs

@@ -0,0 +1,29 @@
+use std::path::PathBuf;
+
+#[test]
+fn empty() {
+    assert_eq!(crate::common_parent::<PathBuf>(&[]), PathBuf::new());
+}
+
+#[test]
+fn single1() {
+    assert_eq!(crate::common_parent::<&str>(&["a"]).as_path(), "");
+}
+
+#[test]
+fn single2() {
+    assert_eq!(crate::common_parent::<&str>(&["/a"]).as_path(), "/");
+}
+
+#[test]
+fn double() {
+    assert_eq!(crate::common_parent::<&str>(&["d/a", "d/b"]).as_path(), "d");
+}
+
+#[test]
+fn dir_file() {
+    assert_eq!(
+        crate::common_parent::<&str>(&["./d", "./d/f"]).as_path(),
+        "."
+    );
+}

src/lib.rs

@@ -1,3 +1,4 @@
+use std::path::Path;
 use std::{ffi::OsString, io::Read, path::PathBuf, str::FromStr, string::FromUtf8Error};
 
 use anyhow::bail;
@@ -80,6 +81,27 @@ pub fn actions_token() -> Result<SecretString, ActionsTokenError> {
     Ok(SecretString::new(str.into_boxed_str()))
 }
 
+pub fn common_parent<P: AsRef<Path>>(paths: &[P]) -> PathBuf {
+    let Some((path0, paths)) = paths.split_first() else {
+        return PathBuf::new();
+    };
+    let mut common = PathBuf::new();
+    for comp in path0.as_ref() {
+        common.push(comp);
+        if !paths.iter().all(|p| p.as_ref().starts_with(&common)) {
+            break;
+        }
+    }
+    // first path is common to all the paths, or the last component does not match all the paths
+    //
+    // either way, we need to pop a single component from the end
+    common.pop();
+    common
+}
+
+#[cfg(test)]
+mod common_paths_tests;
+
 #[derive(Debug)]
 pub struct Client {
     http_client: HttpClient,