use chrono::{DateTime, Utc};
use uuid::Uuid;

/// A representation of stored metadata about a client.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct Client {
    /// The latest version for this client (may be the nil version)
    pub latest_version_id: Uuid,
    /// Data about the latest snapshot for this client
    pub snapshot: Option<Snapshot>,
}

/// Metadata about a snapshot, not including the snapshot data itself.
#[derive(Clone, PartialEq, Eq, Debug)]
pub struct Snapshot {
    /// ID of the version at which this snapshot was made
    pub version_id: Uuid,

    /// Timestamp at which this snapshot was set
    pub timestamp: DateTime<Utc>,

    /// Number of versions since this snapshot was made
    pub versions_since: u32,
}

#[derive(Clone, PartialEq, Eq, Debug)]
pub struct Version {
    /// The uuid identifying this version.
    pub version_id: Uuid,
    /// The uuid identifying this version's parent.
    pub parent_version_id: Uuid,
    /// The data carried in this version.
    pub history_segment: Vec<u8>,
}

/// A transaction in the storage backend.
///
/// Transactions must be sequentially consistent. That is, the results of transactions performed
/// in storage must be as if each were executed sequentially in some order. In particular,
/// un-committed changes must not be read by another transaction, but committed changes must
/// be visible to subequent transations. Together, this guarantees that `add_version` reliably
/// constructs a linear sequence of versions.
///
/// Transactions with different client IDs cannot share any data, so it is safe to handle them
/// concurrently.
///
/// Changes in a transaction that is dropped without calling `commit` must not appear in any other
/// transaction.
#[async_trait::async_trait(?Send)]
pub trait StorageTxn {
    /// Get information about the client for this transaction
    async fn get_client(&mut self) -> anyhow::Result<Option<Client>>;

    /// Create the client for this transaction, with the given latest_version_id. The client must
    /// not already exist.
    async fn new_client(&mut self, latest_version_id: Uuid) -> anyhow::Result<()>;

    /// Set the client's most recent snapshot.
    async fn set_snapshot(&mut self, snapshot: Snapshot, data: Vec<u8>) -> anyhow::Result<()>;

    /// Get the data for the most recent snapshot.  The version_id
    /// is used to verify that the snapshot is for the correct version.
    async fn get_snapshot_data(&mut self, version_id: Uuid) -> anyhow::Result<Option<Vec<u8>>>;

    /// Get a version, indexed by parent version id
    async fn get_version_by_parent(
        &mut self,
        parent_version_id: Uuid,
    ) -> anyhow::Result<Option<Version>>;

    /// Get a version, indexed by its own version id
    async fn get_version(&mut self, version_id: Uuid) -> anyhow::Result<Option<Version>>;

    /// Add a version (that must not already exist), and
    ///  - update latest_version_id from parent_version_id to version_id
    ///  - increment snapshot.versions_since
    /// Fails if the existing `latest_version_id` is not equal to `parent_version_id`. Check
    /// this by calling `get_client` earlier in the same transaction.
    async fn add_version(
        &mut self,
        version_id: Uuid,
        parent_version_id: Uuid,
        history_segment: Vec<u8>,
    ) -> anyhow::Result<()>;

    /// Commit any changes made in the transaction.  It is an error to call this more than
    /// once.  It is safe to skip this call for read-only operations.
    async fn commit(&mut self) -> anyhow::Result<()>;
}

/// A trait for objects able to act as storage.  Most of the interesting behavior is in the
/// [`crate::storage::StorageTxn`] trait.
#[async_trait::async_trait]
pub trait Storage: Send + Sync {
    /// Begin a transaction for the given client ID.
    async fn txn(&self, client_id: Uuid) -> anyhow::Result<Box<dyn StorageTxn + '_>>;
}