unit std.arena;

import std.mem;

// An arena of `T` reached by handle, and the handle that reaches it.
//
// This is the shape SPEC R11 asks for when data points at data: the arena owns
// the values and a handle is a number, so nothing here is a reference and R4
// has nothing to object to. `mem.Arena` next door is a different structure --
// a block of bytes handed out by offset, for data whose size varies. This one
// holds a fixed `T` per slot and can take slots back.
//
// A handle is checked, not trusted. It carries the generation of the slot it
// was made from, the epoch of the arena, and which arena it came from, so
// using it after the slot was freed, after the arena was reset, or against
// some other arena all answer `null` rather than a neighbouring value.
//
// `T` is expected to be Copy -- integers, handles, small records. `get`
// answers with a copy, which is the whole reason a borrow never outlives the
// statement it was taken in. For a `T` that owns something, `take` moves it
// out and leaves a replacement (SPEC 5 R7).

/// No slot. Every real index is smaller.
pub const NONE: u32 = 4294967295;

/// Eight bytes, and the same eight in every build: `--no-checks` skips the
/// comparison, never the layout. A handle that changed shape with a flag
/// would not survive being written down.
pub struct Handle(T) {
    pub index: u32,
    /// `arena 8 | epoch 8 | generation 16`
    pub tag: u32,

    pub fn none() -> Self { return Self{ index: NONE, tag: 0 }; }

    pub fn is_none(self: &Self) -> bool { return self.index == NONE; }

    pub fn same(self: &Self, other: Handle(T)) -> bool {
        return self.index == other.index and self.tag == other.tag;
    }
}

struct Slot(T) {
    value: T,
    /// Bumped every time the slot is freed, so old handles stop matching.
    gen: u32,
    live: bool,
    /// The next slot on the free list, or `NONE`.
    next: u32,
}

pub struct Arena(T) {
    slots: ^[]mut Slot(T),
    /// How many slots have ever been handed out; slots past this are untouched.
    high: usize,
    /// How many are live right now.
    count: usize,
    free: u32,
    id: u32,
    epoch: u32,

    /// `id` tells one arena from another in a handle. A program with a handful
    /// of arenas numbers them itself; eight bits is more than that needs.
    pub fn with_capacity(id: u32, n: usize) -> !Self {
        let room: ^[]mut Slot(T) = try mem.alloc_slice(Slot(T), n);
        return Self{ slots: room, high: 0, count: 0, free: NONE,
                     id: id % 256, epoch: 0 };
    }

    pub fn len(self: &Self) -> usize { return self.count; }

    pub fn room(self: &Self) -> usize { return self.slots.^.n; }

    fn tag_of(self: &Self, gen: u32) -> u32 {
        return (self.id * 16777216) + (self.epoch * 65536) + gen;
    }

    /// Is this handle still talking about a live slot in this arena?
    pub fn valid(self: &Self, h: Handle(T)) -> bool {
        if h.index == NONE { return false; }
        if (h.index as usize) >= self.high { return false; }
        if not self.slots.^[h.index as usize].live { return false; }
        return self.slots.^[h.index as usize].gen == h.tag;
    }

    pub fn alloc(self: &mut Self, v: T) -> !Handle(T) {
        var at: usize = 0;
        if self.free != NONE {
            at = self.free as usize;
            self.free = self.slots.^[at].next;
        } else {
            if self.high == self.slots.^.n { return error.OutOfMemory; }
            at = self.high;
            self.high = self.high + 1;
            self.slots.^[at].gen = self.tag_of(0);
        }
        self.slots.^[at].value = v;
        self.slots.^[at].live = true;
        self.slots.^[at].next = NONE;
        self.count = self.count + 1;
        return Handle(T){ index: at as u32, tag: self.slots.^[at].gen };
    }

    /// A copy of what the slot holds, or nothing when the handle is stale.
    /// The borrow of the arena ends with this statement, which is what lets a
    /// caller read one slot while writing another.
    pub fn get(self: &Self, h: Handle(T)) -> ?T {
        if not self.valid(h) { return null; }
        return self.slots.^[h.index as usize].value;
    }

    /// Overwrite in place. Says whether the handle was good.
    pub fn set(self: &mut Self, h: Handle(T), v: T) -> bool {
        if not self.valid(h) { return false; }
        self.slots.^[h.index as usize].value = v;
        return true;
    }

    /// Move the value out and leave `replacement` behind (SPEC 5 R7). This is
    /// how a `T` that owns something leaves the arena.
    pub fn take(self: &mut Self, h: Handle(T), replacement: T) -> ?T {
        if not self.valid(h) { return null; }
        return mem.replace(&mut self.slots.^[h.index as usize].value,
                           replacement);
    }

    /// Exchange what two slots hold. Both handles have to be good.
    pub fn swap(self: &mut Self, a: Handle(T), b: Handle(T)) -> bool {
        if not self.valid(a) { return false; }
        if not self.valid(b) { return false; }
        let ai: usize = a.index as usize;
        let bi: usize = b.index as usize;
        if ai == bi { return true; }
        let first: T = self.slots.^[ai].value;
        let second: T = mem.replace(&mut self.slots.^[bi].value, first);
        self.slots.^[ai].value = second;
        return true;
    }

    /// Give the slot back. Every handle to it stops matching. A slot whose
    /// generation has run out is retired rather than reused -- wrapping around
    /// would make an old handle valid again, which is the one thing the
    /// generation is there to prevent.
    pub fn free(self: &mut Self, h: Handle(T)) -> bool {
        if not self.valid(h) { return false; }
        let at: usize = h.index as usize;
        self.slots.^[at].live = false;
        self.count = self.count - 1;
        let gen: u32 = self.slots.^[at].gen % 65536;
        if gen == 65535 { return true; }
        self.slots.^[at].gen = self.slots.^[at].gen + 1;
        self.slots.^[at].next = self.free;
        self.free = h.index;
        return true;
    }

    /// Forget everything at once. The epoch moves, so every handle made before
    /// now is stale without having to touch a single slot.
    pub fn reset(self: &mut Self) -> void {
        self.epoch = (self.epoch + 1) % 256;
        self.high = 0;
        self.count = 0;
        self.free = NONE;
        return;
    }
}