attachments: where a new attachment goes, as Obsidian decides

attachments::folder reads Obsidian's "Default location for new
attachments" (attachmentFolderPath in .obsidian/app.json) -- root, the
note's own folder, a subfolder of it, or a fixed folder -- so a file
another app adds lands where Obsidian would put it. unique_path numbers a
clash the way Obsidian does ("pic 1.png").

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

 src/attachments.rs | 86 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs         |  1 +
 2 files changed, 87 insertions(+)

diff --git a/src/attachments.rs b/src/attachments.rs
new file mode 100644
index 0000000..f2b56f4
--- /dev/null
+++ b/src/attachments.rs
@@ -0,0 +1,86 @@
+//! Where a new attachment goes: Obsidian's "Default location for new
+//! attachments" (Settings → Files and links), stored as
+//! `attachmentFolderPath` in `.obsidian/app.json`, so a file another app
+//! adds to the vault lands where Obsidian would have put it.
+//!
+//! The setting's forms, as Obsidian reads them:
+//!
+//! | value | folder |
+//! | --- | --- |
+//! | missing, `""` or `/` | the vault root (Obsidian's default) |
+//! | `./` | the folder of the note (or canvas) it is for |
+//! | `./sub` | `sub` inside that folder |
+//! | `Some/Folder` | that folder, from the vault root |
+
+use std::path::{Path, PathBuf};
+
+/// The vault folder (vault-relative, `""` for the root) that a new
+/// attachment for the vault file `for_file` goes in.
+pub fn folder(root: &Path, for_file: &str) -> String {
+    let setting = std::fs::read_to_string(root.join(".obsidian/app.json"))
+        .ok()
+        .and_then(|t| serde_json::from_str::<serde_json::Value>(&t).ok())
+        .and_then(|v| v.get("attachmentFolderPath")?.as_str().map(str::to_string))
+        .unwrap_or_default();
+    folder_for(&setting, for_file)
+}
+
+/// [`folder`] for a setting already read.
+pub fn folder_for(setting: &str, for_file: &str) -> String {
+    let s = setting.trim();
+    let here = crate::index::parent(for_file);
+    let joined = match s.strip_prefix("./") {
+        Some(sub) => format!("{here}/{sub}"),
+        None if s == "." => here.to_string(),
+        None => s.to_string(),
+    };
+    joined.split('/').filter(|p| !p.is_empty() && *p != ".").collect::<Vec<_>>().join("/")
+}
+
+/// An absolute path in `dir` for a file named `name` that does not exist
+/// yet: `name`, else `name 1`, `name 2`, … before the extension, the way
+/// Obsidian numbers a clash.
+pub fn unique_path(dir: &Path, name: &str) -> PathBuf {
+    let first = dir.join(name);
+    if !first.exists() {
+        return first;
+    }
+    let (stem, ext) = match name.rsplit_once('.') {
+        Some((s, e)) if !s.is_empty() => (s, format!(".{e}")),
+        _ => (name, String::new()),
+    };
+    (1..)
+        .map(|n| dir.join(format!("{stem} {n}{ext}")))
+        .find(|p| !p.exists())
+        .expect("an unused name exists")
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn setting_forms() {
+        assert_eq!(folder_for("", "Desktop.canvas"), "");
+        assert_eq!(folder_for("/", "Notes/a.md"), "");
+        assert_eq!(folder_for("./", "Notes/a.md"), "Notes");
+        assert_eq!(folder_for("./", "a.md"), "");
+        assert_eq!(folder_for("./assets", "Notes/a.md"), "Notes/assets");
+        assert_eq!(folder_for("Attachments/", "Notes/a.md"), "Attachments");
+        assert_eq!(folder_for("Media/Img", "a.md"), "Media/Img");
+    }
+
+    #[test]
+    fn reads_app_json_and_numbers_clashes() {
+        let dir = tempfile::tempdir().unwrap();
+        let root = dir.path();
+        assert_eq!(folder(root, "Desktop.canvas"), "");
+        std::fs::create_dir_all(root.join(".obsidian")).unwrap();
+        std::fs::write(root.join(".obsidian/app.json"), r#"{"attachmentFolderPath":"Attachments"}"#).unwrap();
+        assert_eq!(folder(root, "Desktop.canvas"), "Attachments");
+        std::fs::write(root.join("pic.png"), b"x").unwrap();
+        std::fs::write(root.join("pic 1.png"), b"x").unwrap();
+        assert_eq!(unique_path(root, "pic.png"), root.join("pic 2.png"));
+        assert_eq!(unique_path(root, "new.png"), root.join("new.png"));
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index c8b3b84..fd9ce01 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -31,6 +31,7 @@
 //! # Ok::<(), Box<dyn std::error::Error>>(())
 //! ```
 
+pub mod attachments;
 pub mod canvas;
 pub mod config;
 pub mod daily;