//! Whole transaction reservation in logical journal slots, before any I/O.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum Error { Capacity, RecordCount, Full }

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Reservation { pub start: u16, pub slots: u16, pub records: u16 }

#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct Space { capacity: u16, used: u16 }

impl Space {
    pub fn new(capacity: u16, used: u16) -> Result<Self, Error> {
        if capacity < 5 || used > capacity { return Err(Error::Capacity); }
        Ok(Self {capacity, used})
    }
    pub const fn used(&self) -> u16 { self.used }
    pub const fn remaining(&self) -> u16 { self.capacity - self.used }

    pub fn reserve(&mut self, records: u16) -> Result<Reservation, Error> {
        if !(1..=3).contains(&records) { return Err(Error::RecordCount); }
        let needed = records + 2; // Header and commit each require a slot.
        if needed > self.capacity - self.used { return Err(Error::Full); }
        let reservation = Reservation {start: self.used, slots: needed, records};
        self.used += needed;
        Ok(reservation)
    }
}