// SPDX-License-Identifier: LGPL-3.0-only
// SPDX-FileCopyrightText: Copyright (C) 2026 Catcrafts®

// Readers scanning (with tombstones present, so the per-row flag test is on)
// while one writer appends and erases. Every row a scan yields must be a
// published, live-at-some-point row with increasing ids, and nothing may
// read past the published slots. Guards the sentinel-based iteration and the
// tombstone flag against the Append race.

import Crafter.DB;
import std;

using namespace Crafter;
namespace fs = std::filesystem;

namespace {
    int failures = 0;

    void Check(bool condition, std::string_view what) {
        if (!condition) {
            std::println("FAIL: {}", what);
            ++failures;
        }
    }

    // 16-byte rows: a power-of-two size, so a full extent ends exactly on a
    // row boundary and the slot after the last published one can be the
    // first byte of the unmapped reservation.
    struct Tick {
        std::uint64_t sequence;
        std::uint64_t value;
        static constexpr std::uint32_t version = 1;
        static constexpr auto orderedBy = &Tick::sequence;
    };
    static_assert(DB::Layout<Tick>::rowSize == 24 || DB::Layout<Tick>::rowSize == 32);
}

int main() {
    const fs::path dir = fs::temp_directory_path() / "crafter-db-concurrent";
    fs::remove_all(dir);
    DB::Table<Tick> table(dir / "ticks.cdb");
    table.Append({ 0, 0 });
    table.Append({ 1, 1 });
    table.Erase(0);   // a tombstone from the start: scans run with the flag test

    std::atomic<bool> stop{ false };
    std::atomic<std::uint64_t> scans{ 0 };
    std::atomic<std::uint64_t> badOrder{ 0 };
    std::atomic<std::uint64_t> badId{ 0 };
    std::atomic<std::uint64_t> sawErased{ 0 };

    const auto reader = [&] {
        while (!stop.load(std::memory_order_relaxed)) {
            std::uint64_t last = 0;
            bool first = true;
            const std::uint64_t slots = table.SlotCount();
            for (DB::Row<Tick> row : table) {
                const std::uint64_t id = row.Id();
                if (!first && id <= last) badOrder.fetch_add(1, std::memory_order_relaxed);
                if (id >= table.SlotCount()) badId.fetch_add(1, std::memory_order_relaxed);
                if (row[&Tick::sequence] != id) badId.fetch_add(1, std::memory_order_relaxed);
                first = false;
                last = id;
            }
            if (slots > 2 && table.Rows(0, 1).Any()) sawErased.fetch_add(1, std::memory_order_relaxed);
            scans.fetch_add(1, std::memory_order_relaxed);
        }
    };

    std::vector<std::jthread> readers;
    for (std::int32_t i = 0; i < 4; i++) readers.emplace_back(reader);

    std::vector<Tick> batch;
    for (std::uint64_t sequence = 2; sequence < 400'000; sequence++) {
        batch.push_back({ sequence, sequence });
        if (batch.size() == 64) {
            table.Append(batch);
            batch.clear();
            if (sequence % 1024 == 1) table.Erase(sequence - 500);
        }
    }
    stop.store(true);
    readers.clear();

    Check(scans.load() > 0, "readers ran");
    Check(badOrder.load() == 0, std::format("ids strictly increase within a scan ({} violations)", badOrder.load()));
    Check(badId.load() == 0, std::format("every yielded row is a published row ({} violations)", badId.load()));
    Check(sawErased.load() == 0, std::format("the erased first row never shows up ({} times)", sawErased.load()));
    Check(table.Verify().Ok(), "table verifies after the run");

    fs::remove_all(dir);
    if (failures == 0) std::println("ok");
    return failures == 0 ? 0 : 1;
}