//! TRACES: FR-CAT-9 | FR-NC-10 | FR-CAT-8
//! A local copy of every sidecar this device has seen, and the outbox of the
//! ones the server has not.
//!
//! # Why a cache at all
//!
//! FR-CAT-9: *"edits queue and apply when the source returns."* Before this,
//! a rating or a pasted edit made with no connection was dropped — the
//! judgement survived in the catalog, which is disposable (ARCH §6.12), and a
//! pasted edit survived nowhere at all. An edit that vanishes because the
//! train went into a tunnel is the worst failure an authoritative store can
//! have, and it is silent.
//!
//! So the local file is the **commit point** and the upload is best-effort.
//! Writing is always a local write first; the network attempt follows and, if
//! it fails or was never possible, the entry stays marked and is retried when
//! the server comes back. Online and offline are therefore the same code path
//! differing only in whether the upload is attempted, rather than two paths
//! where one quietly does less.
//!
//! # Why the filesystem is the queue
//!
//! The catalog has a `jobs` table with backoff and coalescing, and
//! `JobKind::WriteSidecar` was reserved for this. It is deliberately not used:
//! the catalog is rebuildable and may be deleted at any time, and a queue of
//! *unuploaded edits* is the one thing in this system that cannot be
//! reconstructed from anywhere else. A pending marker sitting beside the
//! document it refers to survives a catalog deletion, an app reinstall that
//! keeps app data, and a crash — because there is no separate index that could
//! disagree with it.
//!
//! It is also debuggable in the way the sidecar format itself is meant to be:
//! the cache mirrors the server's layout, so the file holding an edit that
//! failed to upload is at the path you would guess, and a `.pending` marker
//! beside it says why it is still there.
//!
//! # Why there is no budget
//!
//! Unlike cached originals (FR-NC-6a), sidecars are hundreds of bytes. A
//! hundred-thousand-image library is tens of megabytes, and evicting one would
//! cost a round-trip to re-read an edit the user is about to open. The
//! originals cache exists because RAW files are 30 MB each; this does not have
//! the problem that motivates one.

use std::path::{Path, PathBuf};

use dr_pipeline::Sidecar;

/// Suffix marking a cached sidecar the server has not yet accepted.
///
/// A separate zero-byte file rather than a flag inside the document: the
/// document's bytes are what gets uploaded, and a marker inside it would have
/// to be stripped on the way out — one more chance to upload something subtly
/// different from what was stored.
const PENDING_SUFFIX: &str = ".pending";

/// The on-disk sidecar cache for one library.
pub struct SidecarCache {
    dir: PathBuf,
}

impl SidecarCache {
    /// Open the cache rooted at `dir`. Nothing is created until a write.
    pub fn open(dir: PathBuf) -> Self {
        Self { dir }
    }

    /// Where a remote sidecar is cached.
    ///
    /// Mirrors the server's layout, so `photos/2024/a.drsc` is cached at
    /// `<dir>/photos/2024/a.drsc`. Keyed on the **sidecar's** remote path
    /// rather than the image's, because that is what an upload addresses —
    /// deriving one from the other in two places is how they come to disagree.
    ///
    /// Returns `None` for a path that would escape the cache directory. The
    /// path comes from a server response, so it is not this process's to
    /// trust: a `..` component would let a hostile or merely broken server
    /// name a file anywhere this app can write.
    pub fn path_for(&self, remote_sidecar_path: &str) -> Option<PathBuf> {
        // No directory means no library is open. Every path would otherwise
        // be relative and land in the process's working directory, which for a
        // desktop launch is wherever the user happened to be standing.
        if self.dir.as_os_str().is_empty() {
            return None;
        }
        let mut out = self.dir.clone();
        let mut depth = 0usize;
        for part in remote_sidecar_path.split('/') {
            match part {
                // Empty from a leading or doubled slash, and `.`, both mean
                // "here" and are simply skipped.
                "" | "." => continue,
                ".." => return None,
                // A Windows-style drive or a backslash cannot appear in a
                // WebDAV path segment and would not mean what it looks like.
                p if p.contains('\\') => return None,
                p => {
                    out.push(p);
                    depth += 1;
                }
            }
        }
        (depth > 0).then_some(out)
    }

    /// The cached document, if there is one.
    ///
    /// An unreadable cache entry answers `None` rather than an error: the
    /// caller's fallback is to treat the photograph as unedited, and a cache
    /// is by definition reconstructible from the server.
    pub fn load(&self, remote_sidecar_path: &str) -> Option<Sidecar> {
        let path = self.path_for(remote_sidecar_path)?;
        let text = std::fs::read_to_string(&path).ok()?;
        match Sidecar::parse(&text) {
            Ok(s) => Some(s),
            Err(e) => {
                log::warn!("cached sidecar at {} is unreadable ({e})", path.display());
                None
            }
        }
    }

    /// Write a document into the cache.
    ///
    /// `pending` records whether the server has this content. Passing `false`
    /// after a successful upload is what takes an entry out of the outbox, so
    /// the marker is *removed* here rather than only in a separate call — one
    /// function owns the pair, and they cannot drift apart.
    pub fn store(
        &self,
        remote_sidecar_path: &str,
        sidecar: &Sidecar,
        pending: bool,
    ) -> Result<(), String> {
        let path = self
            .path_for(remote_sidecar_path)
            .ok_or_else(|| format!("unsafe sidecar path {remote_sidecar_path}"))?;

        if let Some(parent) = path.parent() {
            std::fs::create_dir_all(parent).map_err(|e| e.to_string())?;
        }

        // Write and rename, so an interrupted save cannot truncate an edit
        // that was already safely on disk — the same discipline the settings
        // store and the local sidecar writer use.
        let tmp = path.with_extension("drsc.tmp");
        std::fs::write(&tmp, sidecar.to_text()).map_err(|e| e.to_string())?;
        std::fs::rename(&tmp, &path).map_err(|e| e.to_string())?;

        // The marker is written *after* the document. The other order would
        // leave a window where a crash produced a marker pointing at content
        // that was never stored, and the drain would upload a stale document
        // believing it to be the queued one.
        let marker = marker_for(&path);
        if pending {
            std::fs::write(&marker, b"").map_err(|e| e.to_string())?;
        } else if marker.exists() {
            std::fs::remove_file(&marker).map_err(|e| e.to_string())?;
        }
        Ok(())
    }

    /// Whether this entry is waiting to be uploaded.
    pub fn is_pending(&self, remote_sidecar_path: &str) -> bool {
        self.path_for(remote_sidecar_path)
            .is_some_and(|p| marker_for(&p).exists())
    }

    /// Every sidecar waiting to be uploaded, as remote paths.
    ///
    /// Walks the tree rather than consulting an index, which is the property
    /// that makes the queue survive a catalog deletion: the markers *are* the
    /// queue, so there is nothing to fall out of step with them.
    ///
    /// Sorted, so a drain that is interrupted resumes in the same order rather
    /// than retrying whichever entry the directory happened to yield first.
    pub fn pending(&self) -> Vec<String> {
        let mut out = Vec::new();
        collect_pending(&self.dir, &self.dir, &mut out);
        out.sort();
        out
    }
}

/// The marker path for a cached document.
fn marker_for(path: &Path) -> PathBuf {
    let mut s = path.as_os_str().to_os_string();
    s.push(PENDING_SUFFIX);
    PathBuf::from(s)
}

/// Recursively gather remote paths whose marker exists.
///
/// A missing or unreadable directory contributes nothing rather than failing
/// the walk: a cache that has never been written has no directory at all, and
/// that is the normal state on a fresh install.
fn collect_pending(root: &Path, dir: &Path, out: &mut Vec<String>) {
    let Ok(entries) = std::fs::read_dir(dir) else {
        return;
    };
    for entry in entries.flatten() {
        let path = entry.path();
        if path.is_dir() {
            collect_pending(root, &path, out);
            continue;
        }
        // The marker names the document; the document names the remote path.
        let Some(name) = path.file_name().and_then(|n| n.to_str()) else {
            continue;
        };
        let Some(stem) = name.strip_suffix(PENDING_SUFFIX) else {
            continue;
        };
        let document = path.with_file_name(stem);
        // A marker whose document has gone is not a queued upload; it is
        // debris. Skipped rather than reported, since there is nothing to send
        // and nothing the user could do about it.
        if !document.exists() {
            continue;
        }
        if let Ok(rel) = document.strip_prefix(root) {
            // Back to a remote path: the cache mirrors the server's layout, so
            // the relative path *is* the remote path.
            let remote: Vec<&str> = rel.iter().filter_map(|c| c.to_str()).collect();
            if !remote.is_empty() {
                out.push(remote.join("/"));
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use dr_pipeline::sidecar::Version;

    fn tempdir(name: &str) -> PathBuf {
        let dir = std::env::temp_dir().join(format!(
            "dr-sidecar-cache-{name}-{}-{:?}",
            std::process::id(),
            std::thread::current().id()
        ));
        let _ = std::fs::remove_dir_all(&dir);
        std::fs::create_dir_all(&dir).unwrap();
        dir
    }

    /// A cache and the directory it is rooted at.
    fn cache(name: &str) -> (SidecarCache, PathBuf) {
        let root = tempdir(name).join("sidecars");
        (SidecarCache::open(root.clone()), root)
    }

    fn rated(stars: u8) -> Sidecar {
        let mut s = Sidecar::new();
        s.put(Version {
            uuid: "u1".into(),
            name: "Default".into(),
            is_default: true,
            revision: 1,
            rating: stars,
            ..Default::default()
        });
        s
    }

    #[test]
    fn the_cache_mirrors_the_servers_layout() {
        // What makes an entry findable by hand when an upload has gone wrong,
        // and what lets `pending` recover a remote path with no index.
        let (c, _d) = cache("layout");
        let p = c.path_for("photos/2024/a.drsc").unwrap();
        assert!(
            p.ends_with("sidecars/photos/2024/a.drsc"),
            "{}",
            p.display()
        );
    }

    #[test]
    fn a_document_survives_a_store_and_a_load() {
        let (c, _d) = cache("round-trip");
        c.store("photos/a.drsc", &rated(4), true).unwrap();

        let back = c.load("photos/a.drsc").expect("cached");
        assert_eq!(back.default_version().unwrap().rating, 4);
    }

    #[test]
    fn an_absent_entry_loads_as_nothing() {
        let (c, _d) = cache("absent");
        assert!(c.load("photos/never-seen.drsc").is_none());
    }

    #[test]
    fn a_pending_write_appears_in_the_outbox() {
        // The whole point: an edit made offline is queued rather than dropped.
        let (c, _d) = cache("outbox");
        c.store("photos/a.drsc", &rated(3), true).unwrap();

        assert!(c.is_pending("photos/a.drsc"));
        assert_eq!(c.pending(), vec!["photos/a.drsc".to_string()]);
    }

    #[test]
    fn a_stored_upload_leaves_the_outbox() {
        let (c, _d) = cache("drained");
        c.store("photos/a.drsc", &rated(3), true).unwrap();
        c.store("photos/a.drsc", &rated(3), false).unwrap();

        assert!(!c.is_pending("photos/a.drsc"));
        assert!(c.pending().is_empty());
        // And the content is still cached, so an offline open still shows the
        // edit. Clearing the marker must not clear the document.
        assert_eq!(
            c.load("photos/a.drsc")
                .unwrap()
                .default_version()
                .unwrap()
                .rating,
            3
        );
    }

    #[test]
    fn the_outbox_finds_entries_nested_at_any_depth() {
        // The walk is what stands in for an index, so it has to reach an entry
        // wherever the server's tree put it.
        let (c, _d) = cache("nested");
        c.store("a.drsc", &rated(1), true).unwrap();
        c.store("photos/b.drsc", &rated(1), true).unwrap();
        c.store("photos/2024/spain/c.drsc", &rated(1), true)
            .unwrap();
        c.store("photos/d.drsc", &rated(1), false).unwrap();

        assert_eq!(
            c.pending(),
            vec![
                "a.drsc".to_string(),
                "photos/2024/spain/c.drsc".to_string(),
                "photos/b.drsc".to_string(),
            ]
        );
    }

    #[test]
    fn an_empty_cache_has_an_empty_outbox() {
        // A fresh install has no directory at all; the walk must not fail.
        let (c, _d) = cache("fresh");
        assert!(c.pending().is_empty());
    }

    #[test]
    fn a_marker_without_its_document_is_not_a_queued_upload() {
        // Debris from an interrupted write. There is nothing to send, and
        // reporting it as queued would leave the outbox permanently non-empty.
        let (c, _d) = cache("debris");
        c.store("photos/a.drsc", &rated(1), true).unwrap();
        std::fs::remove_file(c.path_for("photos/a.drsc").unwrap()).unwrap();

        assert!(c.pending().is_empty());
    }

    #[test]
    fn a_path_escaping_the_cache_is_refused() {
        // The remote path comes from a server response and is not this
        // process's to trust.
        let (c, _d) = cache("escape");
        assert!(c.path_for("../../etc/passwd.drsc").is_none());
        assert!(c.path_for("photos/../../../a.drsc").is_none());
        assert!(c.path_for("").is_none());
        assert!(c.store("../evil.drsc", &rated(1), true).is_err());
    }

    #[test]
    fn a_cache_with_no_directory_is_inert() {
        // What a caller holds before a library is open. Writing relative to
        // the working directory would scatter sidecars wherever the app was
        // launched from.
        let c = SidecarCache::open(PathBuf::new());
        assert!(c.path_for("photos/a.drsc").is_none());
        assert!(c.load("photos/a.drsc").is_none());
        assert!(c.store("photos/a.drsc", &rated(1), true).is_err());
        assert!(c.pending().is_empty());
    }

    #[test]
    fn a_leading_slash_is_not_an_absolute_path() {
        // WebDAV paths often arrive with one. Treating it as absolute would
        // push the write to the filesystem root.
        let (c, root) = cache("leading-slash");
        let p = c.path_for("/photos/a.drsc").unwrap();
        assert!(p.starts_with(&root), "{}", p.display());
    }

    #[test]
    fn a_corrupt_cache_entry_reads_as_absent_rather_than_failing() {
        // A cache is reconstructible from the server by definition, so the
        // fallback is to fetch — never to refuse to open the photograph.
        let (c, _d) = cache("corrupt");
        c.store("photos/a.drsc", &rated(1), false).unwrap();
        std::fs::write(c.path_for("photos/a.drsc").unwrap(), "drsc 99\n").unwrap();

        assert!(c.load("photos/a.drsc").is_none());
    }

    #[test]
    fn storing_leaves_no_temporary_file_behind() {
        let (c, root) = cache("no-temp");
        c.store("photos/a.drsc", &rated(1), true).unwrap();

        let leftovers: Vec<_> = std::fs::read_dir(root.join("photos"))
            .unwrap()
            .filter_map(|e| e.ok())
            .map(|e| e.file_name().to_string_lossy().to_string())
            .filter(|n| n.ends_with(".tmp"))
            .collect();
        assert!(leftovers.is_empty(), "left {leftovers:?} behind");
    }
}