//! Process identity is minted from a kernel owned process ID.
//! User requests may inspect an assigned identity, but cannot replace it.

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Principal(u64);

impl Principal {
    pub const fn id(self) -> u64 { self.0 }
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Error {
    Full,
    Duplicate,
    Unknown,
    Access,
    Exhausted,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
struct Binding<P> {
    process: P,
    principal: Principal,
}

pub struct Registry<P: Copy + Eq, const N: usize> {
    next: u64,
    bindings: [Option<Binding<P>>; N],
}

impl<P: Copy + Eq, const N: usize> Registry<P, N> {
    pub fn new() -> Self {
        Self { next: 1, bindings: [None; N] }
    }

    /// Called only after the kernel has obtained a genuine ProcessId.
    pub fn bind_kernel(&mut self, process: P) -> Result<Principal, Error> {
        if self.bindings.iter().flatten().any(|row| row.process == process) {
            return Err(Error::Duplicate);
        }
        let slot = self.bindings.iter_mut().find(|slot| slot.is_none())
            .ok_or(Error::Full)?;
        let following = self.next.checked_add(1).ok_or(Error::Exhausted)?;
        let principal = Principal(self.next);
        *slot = Some(Binding { process, principal });
        self.next = following;
        Ok(principal)
    }

    pub fn principal(&self, process: P) -> Result<Principal, Error> {
        self.bindings.iter().flatten()
            .find(|row| row.process == process)
            .map(|row| row.principal).ok_or(Error::Unknown)
    }

    /// The syscall path always returns an explicit denial for a live process.
    /// `requested` is deliberately never used to construct a Principal.
    pub fn request_user_change(&self, process: P, _requested: u64) -> Result<(), Error> {
        self.principal(process)?;
        Err(Error::Access)
    }

    pub fn retire_kernel(&mut self, process: P) -> Result<(), Error> {
        let slot = self.bindings.iter_mut()
            .find(|slot| slot.is_some_and(|row| row.process == process))
            .ok_or(Error::Unknown)?;
        *slot = None;
        Ok(())
    }

    pub fn snapshot(&self) -> (u64, [Option<(P, u64)>; N]) {
        (self.next, self.bindings.map(|row| row.map(|row| (row.process, row.principal.id()))))
    }
}

impl<P: Copy + Eq, const N: usize> Default for Registry<P, N> {
    fn default() -> Self { Self::new() }
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Handle(u64);

impl Handle {
    pub const fn id(self) -> u64 { self.0 }
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Capability {
    ReadOnlyFile { object: u64, allow_child: bool },
    KernelDevice { object: u64 },
}

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
struct Grant<P> {
    handle: Handle,
    owner: P,
    capability: Capability,
}

pub struct HandleSet<P: Copy + Eq, const M: usize> {
    next: u64,
    grants: [Option<Grant<P>>; M],
}

impl<P: Copy + Eq, const M: usize> HandleSet<P, M> {
    pub fn new() -> Self {
        Self { next: 1, grants: [None; M] }
    }

    pub fn issue_kernel<const N: usize>(
        &mut self, identities: &Registry<P, N>, owner: P, capability: Capability,
    ) -> Result<Handle, Error> {
        identities.principal(owner)?;
        let slot = self.grants.iter_mut().find(|slot| slot.is_none())
            .ok_or(Error::Full)?;
        let following = self.next.checked_add(1).ok_or(Error::Exhausted)?;
        let handle = Handle(self.next);
        *slot = Some(Grant { handle, owner, capability });
        self.next = following;
        Ok(handle)
    }

    pub fn lookup(&self, owner: P, handle: u64) -> Result<Capability, Error> {
        self.grants.iter().flatten()
            .find(|grant| grant.owner == owner && grant.handle.id() == handle)
            .map(|grant| grant.capability).ok_or(Error::Unknown)
    }

    /// The caller supplies `actual_parent` from the live process table record.
    /// A child receives only a named, explicitly inheritable read-only file.
    pub fn inherit_explicit<const N: usize>(
        &mut self, identities: &Registry<P, N>, parent: P, child: P,
        actual_parent: Option<P>, requested: u64,
    ) -> Result<Handle, Error> {
        identities.principal(parent)?;
        identities.principal(child)?;
        if parent == child || actual_parent != Some(parent) {
            return Err(Error::Access);
        }
        let inherited = match self.lookup(parent, requested)? {
            Capability::ReadOnlyFile { object, allow_child: true } =>
                Capability::ReadOnlyFile { object, allow_child: false },
            _ => return Err(Error::Access),
        };
        let slot = self.grants.iter_mut().find(|slot| slot.is_none())
            .ok_or(Error::Full)?;
        let following = self.next.checked_add(1).ok_or(Error::Exhausted)?;
        let handle = Handle(self.next);
        *slot = Some(Grant { handle, owner: child, capability: inherited });
        self.next = following;
        Ok(handle)
    }

    pub fn close_owner(&mut self, owner: P) -> usize {
        let mut closed = 0;
        for slot in &mut self.grants {
            if slot.is_some_and(|grant| grant.owner == owner) {
                *slot = None;
                closed += 1;
            }
        }
        closed
    }

    pub fn snapshot(&self) -> (u64, [Option<(u64, P, Capability)>; M]) {
        (self.next, self.grants.map(|grant| grant.map(|grant|
            (grant.handle.id(), grant.owner, grant.capability))))
    }
}

impl<P: Copy + Eq, const M: usize> Default for HandleSet<P, M> {
    fn default() -> Self { Self::new() }
}