Zig VTable example

vtable.zig

pub fn HeapPage_init(self: *HeapPage) void {
    self.page.header.lsn = 0;
    self.page.header.num_records = 0;
    self.page.header.free_space_end = @intCast(u16, PAGE_SIZE);
}

pub fn HeapPage_getFreeSpace(self: *HeapPage) u16 {
    return self.page.header.free_space_end - HEADER_SZ - self.page.header.num_records * SLOT_SZ;
}

pub fn HeapPage_getRecord(self: *HeapPage, slot: u16) []u8 {
    assert(slot < self.page.header.num_records);
    const slot_ptr = &self.page.slots[slot];
    return self.page.data[slot_ptr.page_offset .. slot_ptr.page_offset + slot_ptr.record_length];
}

const IHeapPage = struct {
    const Self = @This();

    page: HeapPage,
    vtable: *const VTable,

    const VTable = struct {
        init: fn (self: *Self) void,
        getFreeSpace: fn (self: *Self) u16,
        getRecord: fn (self: *Self, slot: u16) []u8,
    };

    pub fn init(self: *Self) void {
        self.vtable.init(self);
    }

    pub fn getFreeSpace(self: *Self) u16 {
        return self.vtable.getFreeSpace(self);
    }

    pub fn getRecord(self: *Self, slot: u16) []u8 {
        return self.vtable.getRecord(self, slot);
    }
};

const HeapPageVTable = struct {
    const Self = @This();

    vtable: IHeapPage.VTable,

    pub fn init(self: *Self) void {
        self.vtable.init = HeapPage_init;
        self.vtable.getFreeSpace = HeapPage_getFreeSpace;
        self.vtable.getRecord = HeapPage_getRecord;
    }
};

const HeapPageVTableInstance = HeapPageVTable.init();
const HeapPageInstance = IHeapPage{
    .page = HeapPage.init(),
    .vtable = &HeapPageVTableInstance.vtable,
};