//! Keeping the files and the settings between sessions.
//!
//! The engine does not know where the data goes: the host (the browser, a desktop app...)
//! implements [`Storage`], and a [`Persister`] works out what to write after each change.
//!
//! The files are not stored as a whole. Their trackpoints and waypoints are stored in chunks (see
//! [`crate::Chunked`]) that are written once, under their identity, and a file is a small record
//! that lists the chunks it is made of. An edit only writes the new chunks and the records of the
//! files that changed; the chunks that nothing refers to anymore are deleted at the same time.
//!
//! Several engines can write to a storage (tabs, a server). It has a head, a revision that each
//! commit advances, and a commit only applies if it is based on the current head: an engine that
//! is behind is refused, and has to read the storage again, which replaces what it holds, before
//! it writes. So what an engine knows of the storage is exact whenever it writes, which is what
//! lets it write only what changed and delete what it does not use anymore.
//!
//! The surface, highway and scales of the trackpoints are stored as names, in the chunk that has
//! them: the codes that the engine gives them (see [`crate::TrackpointCategories`]) depend on the
//! order in which it learned the names, so they mean nothing outside of it. A storage that two
//! engines (tabs, a server) write to therefore has nothing to agree on but the chunks.

mod codec;
mod memory;
mod persister;

pub use memory::*;
pub use persister::*;

use uuid::Uuid;

use crate::FileId;

#[derive(Debug, Clone, PartialEq, Eq)]
pub enum StorageError {
    /// The storage could not be read or written.
    Failed(String),
    /// Some of what is stored was written by a newer version of the app, which this one cannot
    /// read. Nothing is written then, so that the newer version finds its data again.
    Newer,
}

impl std::fmt::Display for StorageError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Failed(message) => write!(f, "storage error: {message}"),
            Self::Newer => write!(f, "stored by a newer version"),
        }
    }
}

impl std::error::Error for StorageError {}

/// A state of the storage: the head is the revision of the last commit, 0 for a storage that was
/// never written to.
#[derive(Debug, Default, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct Revision(pub u64);

impl Revision {
    pub fn next(self) -> Self {
        Self(self.0 + 1)
    }
}

/// Why a commit did not happen.
#[derive(Debug, Clone, PartialEq, Eq)]
pub enum CommitError {
    /// The head is not the one the commit is based on: another engine wrote since.
    Stale {
        head: Revision,
    },
    Failed(StorageError),
}

impl std::fmt::Display for CommitError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::Stale { head } => write!(f, "the storage changed (now at revision {})", head.0),
            Self::Failed(error) => error.fmt(f),
        }
    }
}

impl std::error::Error for CommitError {}

impl From<StorageError> for CommitError {
    fn from(error: StorageError) -> Self {
        Self::Failed(error)
    }
}

/// What a chunk holds.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum ChunkKind {
    Trackpoints,
    Waypoints,
}

/// Where a chunk is stored: chunks are told apart by their identity within their kind.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub struct ChunkKey {
    pub kind: ChunkKind,
    pub id: Uuid,
}

impl ChunkKey {
    pub fn trackpoints(id: Uuid) -> Self {
        Self {
            kind: ChunkKind::Trackpoints,
            id,
        }
    }

    pub fn waypoints(id: Uuid) -> Self {
        Self {
            kind: ChunkKind::Waypoints,
            id,
        }
    }
}

/// Everything that is stored about the files, as opaque bytes (see the module documentation).
#[derive(Debug, Default, Clone, PartialEq, Eq)]
pub struct StoredData {
    /// The record of each file.
    pub files: Vec<(FileId, Vec<u8>)>,
    pub chunks: Vec<(ChunkKey, Vec<u8>)>,
    /// The order of the files.
    pub order: Option<Vec<u8>>,
    /// The revision of the storage that this is the state of.
    pub head: Revision,
}

/// A change of the stored data, to be applied all at once: a stored state is always one that the
/// engine had.
#[derive(Debug, Default, Clone, PartialEq, Eq)]
pub struct Batch {
    pub put_files: Vec<(FileId, Vec<u8>)>,
    pub delete_files: Vec<FileId>,
    pub put_chunks: Vec<(ChunkKey, Vec<u8>)>,
    pub delete_chunks: Vec<ChunkKey>,
    pub order: Option<Vec<u8>>,
    /// The head that the batch is based on: it is only applied if the head still is that one.
    pub expect_head: Revision,
}

impl Batch {
    pub fn is_empty(&self) -> bool {
        self == &Self::default()
    }

    /// How many chunks of the kind the batch writes.
    pub fn puts(&self, kind: ChunkKind) -> usize {
        self.put_chunks
            .iter()
            .filter(|(key, _)| key.kind == kind)
            .count()
    }

    /// How many chunks of the kind the batch deletes.
    pub fn deletes(&self, kind: ChunkKind) -> usize {
        self.delete_chunks
            .iter()
            .filter(|key| key.kind == kind)
            .count()
    }
}

/// Where the data is kept. The implementation belongs to the host; [`MemoryStorage`] is one that
/// keeps nothing after the session.
///
/// The methods are async because storage often is (IndexedDB), the engine itself is not: it
/// changes its files at once and the changes are written afterwards, see [`Persister`].
#[allow(async_fn_in_trait)]
pub trait Storage {
    /// Reads the files.
    async fn load(&self) -> Result<StoredData, StorageError>;

    /// The head of the storage.
    async fn head(&self) -> Result<Revision, StorageError>;

    /// Applies a batch if the head is the one it is based on (`Stale` otherwise): either all of it
    /// is stored, or none. The head advances. Gives the new head.
    async fn commit(&self, batch: &Batch) -> Result<Revision, CommitError>;

    /// Reads the settings, as `(key, value)`. The engine does not look into them.
    async fn load_settings(&self) -> Result<Vec<(String, String)>, StorageError>;

    async fn put_setting(&self, key: &str, value: &str) -> Result<(), StorageError>;

    async fn delete_setting(&self, key: &str) -> Result<(), StorageError>;
}

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

    #[test]
    fn test_batch_is_empty_until_it_holds_something() {
        assert!(Batch::default().is_empty());
        let holds = [
            Batch {
                put_files: vec![(FileId::default(), vec![])],
                ..Default::default()
            },
            Batch {
                delete_files: vec![FileId::default()],
                ..Default::default()
            },
            Batch {
                put_chunks: vec![(ChunkKey::waypoints(Uuid::new_v4()), vec![])],
                ..Default::default()
            },
            Batch {
                delete_chunks: vec![ChunkKey::trackpoints(Uuid::new_v4())],
                ..Default::default()
            },
            Batch {
                order: Some(vec![]),
                ..Default::default()
            },
        ];
        assert!(holds.iter().all(|batch| !batch.is_empty()));
    }

    #[test]
    fn test_storage_error_is_readable() {
        let error = StorageError::Failed("disk full".into());
        assert_eq!(error.to_string(), "storage error: disk full");
        let _: &dyn std::error::Error = &error;
    }
}