use std::collections::HashMap;
use std::io;
use std::path::PathBuf;
use std::sync::atomic::{AtomicU64, Ordering};
use std::sync::{Arc, Mutex, MutexGuard};

use avril_caps::{Capabilities, PathTarget};
use avril_kv::{Access, Store};

fn denied(label: &str, write: bool) -> io::Error {
    io::Error::new(
        io::ErrorKind::PermissionDenied,
        format!("{} {label}", if write { "write" } else { "read" }),
    )
}

fn authorize(caps: &Capabilities, target: &PathTarget, label: &str, write: bool) -> io::Result<()> {
    if write {
        caps.check_write(target)
    } else {
        caps.check_read(target)
    }
    .map_err(|_| denied(label, write))
}

fn absolute(caps: &Capabilities, label: &str) -> io::Result<(PathBuf, PathTarget)> {
    if label.is_empty() {
        return Err(io::Error::new(
            io::ErrorKind::NotFound,
            "Empty kv database path",
        ));
    }
    let path = caps.cwd().join(label);
    // Resolve the existing prefix like ext:file does; a missing tail is
    // expected when creating, so refusal to resolve only denies when the
    // grant could not have covered the path anyway.
    let target = match PathTarget::for_creation(&path) {
        Ok(target) => target,
        Err(_) if !caps.unrestricted_read() && !caps.unrestricted_write() => {
            return Err(denied(label, false));
        }
        Err(error) => return Err(error),
    };
    Ok((path, target))
}

struct Auth {
    caps: Arc<Capabilities>,
    target: PathTarget,
    label: String,
}

pub struct Entry {
    state: Mutex<EntryState>,
}

struct EntryState {
    store: Option<Store>,
    auth: Auth,
}

pub struct Registry {
    next: AtomicU64,
    entries: Mutex<HashMap<u64, Arc<Entry>>>,
}

impl Default for Registry {
    fn default() -> Self {
        Self::new()
    }
}

fn closed_handle() -> io::Error {
    io::Error::new(io::ErrorKind::NotConnected, "kv database handle is closed")
}

impl Registry {
    pub fn new() -> Self {
        Self {
            next: AtomicU64::new(1),
            entries: Mutex::new(HashMap::new()),
        }
    }

    fn lock_entries(&self) -> MutexGuard<'_, HashMap<u64, Arc<Entry>>> {
        self.entries
            .lock()
            .unwrap_or_else(|error| error.into_inner())
    }

    fn entry(&self, id: u64) -> io::Result<Arc<Entry>> {
        self.lock_entries().get(&id).cloned().ok_or_else(|| {
            io::Error::new(
                io::ErrorKind::NotConnected,
                "kv database handle is closed or unknown",
            )
        })
    }

    pub fn with_entry<T>(
        &self,
        id: u64,
        write: bool,
        work: impl FnOnce(&Store) -> io::Result<T>,
    ) -> io::Result<T> {
        let entry = self.entry(id)?;
        let state = entry
            .state
            .lock()
            .unwrap_or_else(|error| error.into_inner());
        let Some(store) = state.store.as_ref() else {
            return Err(closed_handle());
        };
        // Re-authorize against the live Capabilities on every operation so a
        // pledge() narrowing applies to already-open handles. Writes need
        // read access too: a write transaction reads internally.
        let auth = &state.auth;
        authorize(&auth.caps, &auth.target, &auth.label, false)?;
        if write {
            authorize(&auth.caps, &auth.target, &auth.label, true)?;
        }
        // The entry mutex stays held for the whole operation: close() takes
        // the store under the same mutex, so nothing is in flight when the
        // last Arc drops and the database file is physically released.
        work(store)
    }

    pub fn open(
        &self,
        caps: &Arc<Capabilities>,
        label: &str,
        access: &str,
        create: bool,
    ) -> io::Result<u64> {
        let access = Access::parse(access)?;
        let (path, target) = absolute(caps, label)?;
        authorize(caps, &target, label, false)?;
        if access.writes() {
            authorize(caps, &target, label, true)?;
        }
        let store = Store::open(&path, access, create)?;
        let id = self.next.fetch_add(1, Ordering::Relaxed);
        self.lock_entries().insert(
            id,
            Arc::new(Entry {
                state: Mutex::new(EntryState {
                    store: Some(store),
                    auth: Auth {
                        caps: caps.clone(),
                        target,
                        label: label.to_owned(),
                    },
                }),
            }),
        );
        Ok(id)
    }

    // Idempotent: closing an unknown or already-closed id succeeds. Taking
    // the store under the entry mutex physically releases the database:
    // with_entry holds that mutex for its whole operation, so nothing is in
    // flight when close returns.
    pub fn close(&self, id: u64) -> io::Result<()> {
        let entry = self.lock_entries().remove(&id);
        if let Some(entry) = entry {
            let mut state = entry
                .state
                .lock()
                .unwrap_or_else(|error| error.into_inner());
            state.store.take();
            drop(state);
        }
        Ok(())
    }
}