//! Pluggable remote storage for DarkRoom.
//!
//! Defines the [`RemoteBackend`] trait and the capability model the sync
//! engine adapts to. Only the Nextcloud connector is implemented
//! (`dr-sync-nextcloud`), but the boundary is designed so other backends can
//! be added without touching the engine.
//!
//! # Why capabilities rather than a common denominator
//!
//! Nextcloud's fast path relies on a behaviour that is *not* a WebDAV
//! guarantee: directory ETags propagate up the tree, so an unchanged root
//! ETag proves nothing anywhere in the library changed. That single property
//! turns a no-op sync over 50k images into one HTTP request.
//!
//! A trait built to what every backend can do would force full enumeration
//! every time — the exact cost the design exists to avoid. So backends
//! declare what they support and the engine picks a strategy (ARCH §8.1).

use std::ops::Range;

use async_trait::async_trait;

pub mod capability;
pub mod error;
pub mod types;

pub use capability::{Capabilities, ChangeDetection, ChunkConstraints, ServerPreviews};
pub use error::RemoteError;
pub use types::{
    Cursor, EntryKind, Identity, Precondition, RemoteChange, RemoteEntry, RemoteId, RemotePath,
    Validator,
};

/// TRACES: FR-NC-12
/// A remote storage backend.
///
/// Implementations are expected to be cheap to clone or to be used behind an
/// `Arc`; the engine may call them concurrently.
#[async_trait]
pub trait RemoteBackend: Send + Sync {
    /// What this backend supports. Read once at connect time and used to pick
    /// a sync strategy.
    fn capabilities(&self) -> &Capabilities;

    /// Human-readable backend name, for logs and the UI.
    fn name(&self) -> &str;

    // ---- discovery --------------------------------------------------------

    /// List one directory level.
    ///
    /// `since` carries the validator the caller last saw, so backends able to
    /// skip unchanged entries may do so. Backends that cannot simply ignore
    /// it.
    async fn list(
        &self,
        dir: &RemotePath,
        since: Option<&Validator>,
    ) -> Result<Vec<RemoteEntry>, RemoteError>;

    /// Fetch a directory's validator without listing its contents.
    ///
    /// The cheap probe that makes ETag pruning work: one request against the
    /// root answers "did anything change?". Backends without
    /// [`ChangeDetection::PropagatingEtags`] return
    /// [`RemoteError::Unsupported`].
    async fn dir_validator(&self, dir: &RemotePath) -> Result<Validator, RemoteError>;

    /// Ask what changed since a cursor.
    ///
    /// Only meaningful for [`ChangeDetection::DeltaCursor`] backends; others
    /// return [`RemoteError::Unsupported`].
    async fn delta(&self, cursor: &Cursor) -> Result<(Vec<RemoteChange>, Cursor), RemoteError>;

    // ---- transfer ---------------------------------------------------------

    /// Fetch an object, optionally a byte range.
    ///
    /// The range is a hint, not a guarantee: backends without range support
    /// may return the whole object, and the caller slices. Correctness holds
    /// either way; [`Capabilities::range_reads`] says whether it was cheap.
    async fn get(&self, id: &RemoteId, range: Option<Range<u64>>) -> Result<Vec<u8>, RemoteError>;

    /// Upload, optionally guarded by a precondition.
    ///
    /// Backends handle chunking internally based on body size — chunked
    /// upload is an implementation detail, not part of this interface, since
    /// exposing it would leak one server's protocol into the abstraction.
    async fn put(
        &self,
        path: &RemotePath,
        body: Vec<u8>,
        precond: Option<Precondition>,
    ) -> Result<Validator, RemoteError>;

    /// Upload many small objects.
    ///
    /// Defaults to sequential [`put`](Self::put) calls; backends with a bulk
    /// endpoint override it. Sidecars are the motivating case — hundreds of
    /// a few KB each.
    async fn put_many(
        &self,
        items: Vec<(RemotePath, Vec<u8>)>,
    ) -> Result<Vec<Result<Validator, RemoteError>>, RemoteError> {
        let mut out = Vec::with_capacity(items.len());
        for (path, body) in items {
            out.push(self.put(&path, body, None).await);
        }
        Ok(out)
    }

    /// Delete an object.
    async fn delete(&self, id: &RemoteId, precond: Option<Precondition>)
        -> Result<(), RemoteError>;

    // ---- optional ---------------------------------------------------------

    /// Server-rendered thumbnail, where available.
    ///
    /// `Ok(None)` means the server has no preview for this object — which is
    /// common for RAW, since stock Nextcloud ships no RAW preview provider
    /// (ARCH §6.7). Callers must have a local fallback.
    async fn thumbnail(&self, _id: &RemoteId, _size: u32) -> Result<Option<Vec<u8>>, RemoteError> {
        Ok(None)
    }
}

/// TRACES: FR-NC-4 | FR-NC-12
/// Which sync strategy the engine should use for a backend.
///
/// Derived from capabilities at connect time. Reported to the user so a slow
/// backend is visibly slow rather than mysteriously slow (FR-NC-12).
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SyncStrategy {
    /// Ask the server what changed. Cheapest.
    Delta,
    /// Probe the root ETag; recurse only where it differs. One request when
    /// nothing changed.
    EtagPruning,
    /// Enumerate the tree, using per-entry ETags to avoid re-downloading.
    FullListing,
    /// Enumerate and compare modification times. Degraded — clock skew and
    /// second-granularity timestamps both cause misses.
    TimestampCompare,
}

impl SyncStrategy {
    pub fn for_capabilities(caps: &Capabilities) -> Self {
        match caps.change_detection {
            ChangeDetection::DeltaCursor => SyncStrategy::Delta,
            ChangeDetection::PropagatingEtags => SyncStrategy::EtagPruning,
            ChangeDetection::LocalEtags => SyncStrategy::FullListing,
            ChangeDetection::Timestamps => SyncStrategy::TimestampCompare,
        }
    }

    /// A short description for the UI.
    pub fn describe(self) -> &'static str {
        match self {
            SyncStrategy::Delta => "server change feed",
            SyncStrategy::EtagPruning => "incremental (ETag pruning)",
            SyncStrategy::FullListing => "full listing, cached by ETag",
            SyncStrategy::TimestampCompare => "full listing by timestamp (degraded)",
        }
    }

    /// Whether this strategy can prove "nothing changed" cheaply.
    ///
    /// Where false, every sync costs at least one request per folder, which
    /// the UI should warn about on large libraries.
    pub fn has_cheap_noop(self) -> bool {
        matches!(self, SyncStrategy::Delta | SyncStrategy::EtagPruning)
    }
}

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

    #[test]
    fn strategy_follows_capability() {
        let mut caps = Capabilities::minimal();
        assert_eq!(
            SyncStrategy::for_capabilities(&caps),
            SyncStrategy::TimestampCompare
        );

        caps.change_detection = ChangeDetection::PropagatingEtags;
        assert_eq!(
            SyncStrategy::for_capabilities(&caps),
            SyncStrategy::EtagPruning
        );
    }

    #[test]
    fn only_delta_and_pruning_have_cheap_noop() {
        assert!(SyncStrategy::Delta.has_cheap_noop());
        assert!(SyncStrategy::EtagPruning.has_cheap_noop());
        // These cost at least one request per folder, every time.
        assert!(!SyncStrategy::FullListing.has_cheap_noop());
        assert!(!SyncStrategy::TimestampCompare.has_cheap_noop());
    }
}