stet_core/

entity_table.rs

// stet - A PostScript Interpreter
// Copyright (c) 2026 Scott Bowman
// SPDX-License-Identifier: Apache-2.0 OR MIT

//! Entity table: indirection layer for arena stores.
//!
//! Each composite object (string, array, dict) is identified by an `EntityId`.
//! The entity table maps `EntityId → EntityMeta`, which records the offset
//! into the backing store's data vec, the allocated length, the save level
//! at creation/last COW copy, and flags (global, gc_mark).

use crate::object::EntityId;

/// Metadata for one entity in an arena store.
#[derive(Clone, Debug)]
pub struct EntityMeta {
    /// Offset into the backing store's data vec.
    pub offset: u32,
    /// Allocated capacity (number of elements/bytes).
    pub len: u32,
    /// Save level when created or last COW-copied.
    pub save_level: u16,
    /// Bit 0: is_global, Bit 1: gc_mark (reserved for future use),
    /// Bit 2: cow_backup.
    pub flags: u8,
    /// Save ID that was active when this entity was created (0 = before any save).
    /// Used for invalidrestore: entities with created_after_save >= target_save_id
    /// are "newer than the snapshot being restored."
    pub created_after_save: u32,
}

impl EntityMeta {
    const FLAG_GLOBAL: u8 = 1;
    const FLAG_COW_BACKUP: u8 = 1 << 2;

    /// Check if this entity is in global VM.
    pub fn is_global(&self) -> bool {
        self.flags & Self::FLAG_GLOBAL != 0
    }

    /// Set the global flag.
    pub fn set_global(&mut self, global: bool) {
        if global {
            self.flags |= Self::FLAG_GLOBAL;
        } else {
            self.flags &= !Self::FLAG_GLOBAL;
        }
    }

    /// Whether this entity is a copy-on-write backup rather than live data.
    ///
    /// `cow_copy` allocates one of these to hold a composite's pre-mutation
    /// contents so `restore` can swap them back. It is never reachable from
    /// PostScript in either state: before the restore it holds the snapshot,
    /// after it holds the discarded post-save data. Whole-arena sweeps that
    /// reason about reachability — see [`crate::vm_audit`] — must skip it.
    pub fn is_cow_backup(&self) -> bool {
        self.flags & Self::FLAG_COW_BACKUP != 0
    }

    /// Mark this entity as a copy-on-write backup.
    pub fn set_cow_backup(&mut self) {
        self.flags |= Self::FLAG_COW_BACKUP;
    }
}

/// Indirection table mapping `EntityId` to metadata about stored data.
pub struct EntityTable {
    entries: Vec<EntityMeta>,
}

impl EntityTable {
    /// Create an empty entity table.
    pub fn new() -> Self {
        Self {
            entries: Vec::new(),
        }
    }

    /// Allocate a new entity, returning its `EntityId`.
    /// The returned EntityId is tagged with the global bit based on the `global` param.
    pub fn allocate(
        &mut self,
        offset: u32,
        len: u32,
        save_level: u16,
        global: bool,
        created_after_save: u32,
    ) -> EntityId {
        let index = self.entries.len() as u32;
        let id = if global {
            EntityId::global(index)
        } else {
            EntityId::local(index)
        };
        let mut flags = 0u8;
        if global {
            flags |= EntityMeta::FLAG_GLOBAL;
        }
        self.entries.push(EntityMeta {
            offset,
            len,
            save_level,
            flags,
            created_after_save,
        });
        id
    }

    /// Get metadata for an entity (read-only).
    #[inline]
    pub fn get(&self, id: EntityId) -> &EntityMeta {
        &self.entries[id.raw_index()]
    }

    /// Get mutable metadata for an entity.
    pub fn get_mut(&mut self, id: EntityId) -> &mut EntityMeta {
        &mut self.entries[id.raw_index()]
    }

    /// Get metadata by raw table index, without needing a tagged `EntityId`.
    ///
    /// Callers that sweep the whole table (the VM audit) do not have an
    /// `EntityId` in hand — they need the metadata in order to build one with
    /// the correct global tag.
    #[inline]
    pub fn get_by_index(&self, index: usize) -> &EntityMeta {
        &self.entries[index]
    }

    /// Number of entities allocated.
    pub fn len(&self) -> usize {
        self.entries.len()
    }

    /// Drop every entity from index `n` onward, so their ids become available
    /// for reuse.
    ///
    /// Only sound when no reachable object still refers to those ids. `restore`
    /// establishes that: PLRM 3.7.3.2 forbids a surviving reference to a
    /// composite created after the save (enforced by `check_invalidrestore`),
    /// and COW reverts any pre-save composite that was mutated to point at one.
    ///
    /// Note that this makes `EntityId`s **reusable**. Anything keyed by
    /// `EntityId` that outlives a restore must be purged in the same step, or a
    /// later entity reusing the index will collide with the stale entry.
    pub fn truncate(&mut self, n: usize) {
        self.entries.truncate(n);
    }

    /// Whether the table is empty.
    pub fn is_empty(&self) -> bool {
        self.entries.is_empty()
    }
}

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

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_allocate_and_get() {
        let mut table = EntityTable::new();
        let id = table.allocate(0, 10, 0, false, 0);
        assert_eq!(id, EntityId::local(0));
        assert!(!id.is_global());
        let meta = table.get(id);
        assert_eq!(meta.offset, 0);
        assert_eq!(meta.len, 10);
        assert_eq!(meta.save_level, 0);
        assert!(!meta.is_global());
    }

    #[test]
    fn test_multiple_allocations() {
        let mut table = EntityTable::new();
        let id0 = table.allocate(0, 5, 0, false, 0);
        let id1 = table.allocate(5, 10, 0, true, 0);
        assert_eq!(id0, EntityId::local(0));
        assert_eq!(id1, EntityId::global(1));
        assert_eq!(table.len(), 2);
        assert!(!id0.is_global());
        assert!(id1.is_global());
    }

    #[test]
    fn test_get_mut() {
        let mut table = EntityTable::new();
        let id = table.allocate(0, 5, 0, false, 0);
        table.get_mut(id).offset = 100;
        assert_eq!(table.get(id).offset, 100);
    }

    #[test]
    fn test_global_flag() {
        let mut table = EntityTable::new();
        let id = table.allocate(0, 5, 0, false, 0);
        assert!(!table.get(id).is_global());
        table.get_mut(id).set_global(true);
        assert!(table.get(id).is_global());
        table.get_mut(id).set_global(false);
        assert!(!table.get(id).is_global());
    }

    #[test]
    fn test_save_level_tracking() {
        let mut table = EntityTable::new();
        let id = table.allocate(0, 5, 1, false, 0);
        assert_eq!(table.get(id).save_level, 1);
        table.get_mut(id).save_level = 2;
        assert_eq!(table.get(id).save_level, 2);
    }

    #[test]
    fn test_empty_table() {
        let table = EntityTable::new();
        assert_eq!(table.len(), 0);
        assert!(table.is_empty());
    }

    #[test]
    fn test_default() {
        let table = EntityTable::default();
        assert!(table.is_empty());
    }

    #[test]
    fn test_len_after_allocations() {
        let mut table = EntityTable::new();
        table.allocate(0, 1, 0, false, 0);
        table.allocate(1, 2, 0, false, 0);
        table.allocate(3, 3, 0, false, 0);
        assert_eq!(table.len(), 3);
        assert!(!table.is_empty());
    }
}