//! vcs-core: the vcs data model with no I/O.
//!
//! Content is addressed by SHA-256. A project is a set of objects (blobs,
//! chunk lists for big files, trees) plus a numbered line of versions (main)
//! and changes that land on it. Hosts (the CLI, the self-hosted server and the
//! hosted server in Wasm) own storage and the network; everything here works
//! on bytes and on a [`Source`] that hands objects back by ID.

pub mod bundle;
pub mod chunk;
pub mod diff;
pub mod filter;
pub mod id;
pub mod merge;
pub mod object;
pub mod secrets;
pub mod tree;
pub mod version;

pub use id::Id;
pub use object::{Kind, Object};

#[derive(Debug, thiserror::Error)]
pub enum Error {
    #[error("missing object {0}")]
    Missing(Id),
    #[error("{0}")]
    Corrupt(String),
    #[error("limit exceeded: {0}")]
    Limit(String),
}

pub type Result<T, E = Error> = std::result::Result<T, E>;

pub(crate) fn corrupt(message: impl Into<String>) -> Error {
    Error::Corrupt(message.into())
}

/// Where objects come from. Hosts implement it over their storage.
pub trait Source {
    fn get(&self, id: &Id) -> Result<Option<Object>>;

    fn must(&self, id: &Id) -> Result<Object> {
        self.get(id)?.ok_or(Error::Missing(*id))
    }
}

/// An in-memory source, for tests and for objects a merge creates.
#[derive(Default, Clone)]
pub struct MemSource {
    pub objects: std::collections::HashMap<Id, Object>,
}

impl MemSource {
    pub fn put(&mut self, object: Object) -> Id {
        let id = object.id();
        self.objects.insert(id, object);
        id
    }
}

impl Source for MemSource {
    fn get(&self, id: &Id) -> Result<Option<Object>> {
        Ok(self.objects.get(id).cloned())
    }
}

/// Reads from `first`, then `second` (new objects over a store).
pub struct Layered<'a> {
    pub first: &'a dyn Source,
    pub second: &'a dyn Source,
}

impl Source for Layered<'_> {
    fn get(&self, id: &Id) -> Result<Option<Object>> {
        match self.first.get(id)? {
            Some(o) => Ok(Some(o)),
            None => self.second.get(id),
        }
    }
}