//! `NullStore`: a no-op `SessionStore` for embedders who opt out of
//! persistence (and the default backing for an `Agent` constructed without
//! a real store).
//!
//! Every append is dropped (but its messages are freed, honoring the store
//! ownership contract — appends consume their messages). `loadConversation`
//! returns an empty conversation with no dangling prompt. `activeModel` is
//! null and the session id is the empty string.
//!
//! Because freeing the dropped messages needs an allocator, `NullStore`
//! carries one. Construct it with `init(alloc)` and call `.store()` for the
//! interface handle. The struct is trivially copyable and holds no other
//! state, so the handle may borrow it for the agent's lifetime.

const std = @import("std");
const Allocator = std.mem.Allocator;

const session_store_mod = @import("session_store.zig");
const conversation_mod = @import("conversation.zig");

const SessionStore = session_store_mod.SessionStore;
const LoadedSession = session_store_mod.LoadedSession;
const DiskMessage = session_store_mod.DiskMessage;
const ActiveModel = session_store_mod.ActiveModel;

pub const NullStore = struct {
    allocator: Allocator,

    pub fn init(allocator: Allocator) NullStore {
        return .{ .allocator = allocator };
    }

    fn appendMessagesVT(
        ctx: *anyopaque,
        messages: []DiskMessage,
        _: []const ?[]const u8,
        _: []const ?[]const u8,
    ) anyerror!void {
        const self: *NullStore = @ptrCast(@alignCast(ctx));
        // Appends consume their messages; we drop them, so free them.
        for (messages) |m| m.deinit(self.allocator);
    }

    fn loadConversationVT(ctx: *anyopaque, alloc: Allocator) anyerror!LoadedSession {
        _ = ctx;
        return .{
            .conversation = conversation_mod.Conversation.init(alloc),
            .dangling_user = null,
        };
    }

    fn sessionIdVT(_: *anyopaque) []const u8 {
        return "";
    }

    fn activeModelVT(_: *anyopaque) ?ActiveModel {
        return null;
    }

    const vtable: SessionStore.VTable = .{
        .appendMessages = appendMessagesVT,
        .loadConversation = loadConversationVT,
        .sessionId = sessionIdVT,
        .activeModel = activeModelVT,
    };

    /// Wrap this `NullStore` as a `SessionStore`. The handle borrows
    /// `self`; `self` must outlive it.
    pub fn store(self: *NullStore) SessionStore {
        return .{ .ptr = self, .vtable = &vtable };
    }
};

const testing = std.testing;

test "NullStore: appends are dropped (and freed) and load returns empty" {
    var ns = NullStore.init(testing.allocator);
    const s = ns.store();

    var msg_content = try testing.allocator.alloc(session_store_mod.DiskContentBlock, 1);
    msg_content[0] = .{ .text = .{ .text = try testing.allocator.dupe(u8, "hi") } };
    var messages = [_]DiskMessage{.{ .role = .user, .content = msg_content }};
    // No defer-free: the store consumes (frees) the message.
    try s.appendMessages(&messages, &.{null}, &.{null});

    var loaded = try s.loadConversation(testing.allocator);
    defer loaded.deinit(testing.allocator);
    try testing.expectEqual(@as(usize, 0), loaded.conversation.messages.items.len);
    try testing.expect(loaded.dangling_user == null);
    try testing.expect(s.activeModel() == null);
    try testing.expectEqualStrings("", s.sessionId());
}