GTL/src/fixed_slot_map.zig

//! ----------------------------------------------------
//! ----------------------------------------------------

const std = @import("std");
const FixedArrayList = @import("fixed_array_list.zig").FixedArrayList;
const Handle = @import("handle.zig").Handle;

/// ----------------------------------------------------
/// # Example
///
/// ```zig
/// var materials: FixedSlotMap(Material, 3) = .empty;
///
/// _ = try materials.put(.{});
/// _ = try materials.put(.{});
/// const handle = try materials.put(.{ .unique = .{ .topology = .line_list } });
///
/// if (materials.get(handle)) |v| {
///     std.debug.print("Material: {}\n", .{v.unique.topology});
/// }
/// ```
/// ----------------------------------------------------
pub fn FixedSlotMap(
    comptime T: type,
    comptime Capacity: usize,
) type {
    return struct {
        const Self = @This();

        //
        // FIELDS
        //

        // --- VALUES ---
        values: FixedArrayList(T, Capacity),

        // --- LIFE TIMES ---
        generations: FixedArrayList(u32, Capacity),
        free_list: FixedArrayList(u32, Capacity),

        // --- GENERATION COUNTER ---
        next_gen: u32 = 0,

        /// ----------------------------------------------------
        /// ----------------------------------------------------
        pub const empty = Self{
            .values = .empty,
            .generations = .empty,
            .free_list = .empty,
        };

        /// ----------------------------------------------------
        /// ----------------------------------------------------
        pub fn put(
            self: *Self,
            value: T,
        ) error{Full}!Handle(T) {
            // --- REUSE A FREED SLOT ---
            if (self.free_list.pop()) |idx| {
                const gen = self.mint();
                self.values.items()[idx] = value;
                self.generations.items()[idx] = gen;

                return .{
                    .idx = idx,
                    .gen = gen,
                };
            }

            // --- METADATA ---
            const idx: u32 = @intCast(self.values.items().len);
            const gen = self.mint();

            // --- NEW VALUE ---
            try self.values.append(value);
            errdefer _ = self.values.pop();
            try self.generations.append(gen);

            // --- RESULT ---
            return .{
                .idx = idx,
                .gen = gen,
            };
        }

        /// ----------------------------------------------------
        /// Returns an error if the free list could not grow
        /// Stale handles are silently ignored
        /// ----------------------------------------------------
        pub fn remove(
            self: *Self,
            handle: Handle(T),
        ) error{OutOfMemory}!void {
            const idx: u32 = handle.idx;

            // --- CHECK IF STALE ---
            if (idx >= self.generations.items().len) return;
            if (self.generations.items()[idx] != handle.gen) return;

            // --- FREE ---
            try self.free_list.append(idx);
            self.generations.items()[idx] = self.mint();
        }

        /// ----------------------------------------------------
        /// ----------------------------------------------------
        pub fn get(self: *Self, handle: Handle(T)) ?*T {
            if (handle.idx >= self.values.len()) return null;
            if (self.generations.items()[handle.idx] != handle.gen) return null;
            return &self.values.items()[handle.idx];
        }

        /// ----------------------------------------------------
        /// ----------------------------------------------------
        pub fn clear(self: *Self) void {
            self.values.clear();
            self.generations.clear();
            self.free_list.clear();
        }

        /// ----------------------------------------------------
        /// ----------------------------------------------------
        fn mint(self: *Self) u32 {
            const gen = self.next_gen +% 1;
            self.next_gen = gen;
            return gen;
        }
    };
}