//! `NullStore`: a no-op `SessionStore` for embedders who opt out of
//! persistence (and the default backing for an `Agent` constructed without
//! an explicit store).
//!
//! Every append is dropped. `loadConversation` returns an empty
//! conversation with no dangling prompt. `activeModel` is null and the
//! session id is the empty string. The store is stateless, so a single
//! process-global instance backs every `NullStore` handle.

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;

/// Stateless singleton context. The vtable ignores `ctx` entirely; we hand
/// out a pointer to this so the `*anyopaque` is always valid.
var singleton: u8 = 0;

fn appendMessagesVT(
    _: *anyopaque,
    _: []DiskMessage,
    _: []const ?[]const u8,
    _: []const ?[]const u8,
) anyerror!void {}

fn loadConversationVT(_: *anyopaque, alloc: Allocator) anyerror!LoadedSession {
    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,
};

/// A no-op `SessionStore`. Cheap to call repeatedly; all handles share one
/// stateless context.
pub fn store() SessionStore {
    return .{ .ptr = &singleton, .vtable = &vtable };
}

const testing = std.testing;

test "NullStore: appends are dropped and load returns empty" {
    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 }};
    defer messages[0].deinit(testing.allocator);

    const s = store();
    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());
}