#include "eventlog.h"

#include <sys/mman.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <time.h>
#include <unistd.h>
#include <string.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdatomic.h>

#define DEFAULT_MAX_ENTRIES 131072

#define EVENTLOG_MAGIC 0x12345678

static int gs_fd;
static struct eventlog_shm_header* gs_header;
static struct eventlog_entry* gs_entries;

static int eventlog_create(const char* path, const size_t entries_num)
{
    gs_fd = shm_open(path, O_CREAT | O_RDWR, 0666);
    if (gs_fd < 0)
    {
        return -1;
    }
    const size_t expected_size = entries_num * sizeof(struct eventlog_entry) +
        sizeof(struct eventlog_shm_header);

    if(ftruncate(gs_fd, expected_size) < 0)
    {
        close(gs_fd);
        return -2;
    }

    void* const addr = mmap(NULL, expected_size, PROT_READ | PROT_WRITE, MAP_SHARED, gs_fd, 0);
    close(gs_fd);
    if(addr == MAP_FAILED)
    {
        return -3;
    }

    gs_header = addr;
    gs_header->magic = EVENTLOG_MAGIC;
    gs_header->version = 1;
    gs_header->size = entries_num;

    gs_entries = (struct eventlog_entry*)(gs_header + 1);

    atomic_store(&gs_header->tail, 0);

    return 0;
}

static int eventlog_attach(const char* path)
{
    gs_fd = shm_open(path, O_CREAT | O_RDWR, 0666);
    if (gs_fd < 0)
    {
        return -1;
    }

    const off_t size = lseek(gs_fd, 0, SEEK_END);
    if (size == -1) {
        close(gs_fd);
        return -1;
    }

    void* const addr = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, gs_fd, 0);
    close(gs_fd);
    if(addr == MAP_FAILED)
    {
        return -1;
    }

    gs_header = addr;

    if (gs_header->magic != EVENTLOG_MAGIC)
    {
        munmap(addr, size);
        return -1;
    }

    if (gs_header->version != 1)
    {
        munmap(addr, size);
        return -1;
    }

    gs_entries = (struct eventlog_entry*)(gs_header + 1);

    return 0;
}

int eventlog_init(const char* path, enum eventlog_mode mode)
{
    if (gs_header != NULL)
    {
        return -1;
    }

    if (mode == EVENTLOG_MODE_CREATE)
    {
        return eventlog_create(path, DEFAULT_MAX_ENTRIES);
    }
    else
    {
        return eventlog_attach(path);
    }
}

int eventlog_shutdown()
{
    // TODO
    return 0;
}

int eventlog_emit_event(uint64_t timestamp, const char* fmt, ...)
{
    va_list args;
    char data[EVENTLOG_DATA_MAX_SIZE];
    va_start(args, fmt);
    const int chars = vsnprintf(data, sizeof(data), fmt, args);
    va_end(args);
    if (chars < 0 || chars >= sizeof(data))
    {
        return -1;
    }

    return eventlog_emit_event_struct(timestamp, EVENTLOG_TYPE_STRING, data, chars + 1);
}

int eventlog_emit_event_struct(uint64_t timestamp, uint32_t type, void* data, size_t data_size)
{
    uint64_t tail = 0;
    uint64_t new_tail = 0;

    if (data_size > EVENTLOG_DATA_MAX_SIZE)
    {
        return -1;
    }

    do
    {
        tail = atomic_load_explicit(&gs_header->tail, memory_order_relaxed);
        new_tail = tail + 1;
        if (new_tail >= gs_header->size)
        {
            new_tail = 0;
        }
    }
    while(!atomic_compare_exchange_weak(&gs_header->tail, &tail, new_tail));

    memcpy(&gs_entries[tail].data, data, data_size);
    gs_entries[tail].datalen = data_size;
    gs_entries[tail].type_id = type;

    atomic_thread_fence(memory_order_release);

    atomic_store_explicit(&gs_entries[tail].timestamp, timestamp, memory_order_release);

    return 0;
}

uint64_t eventlog_get_timestamp()
{
    struct timespec ts;
    clock_gettime(CLOCK_MONOTONIC_COARSE, &ts);
    return (uint64_t)ts.tv_sec * 1000000000ULL + (uint64_t)ts.tv_nsec;
}


int eventlog_get_snapshot(uint64_t* head, struct eventlog_entry* buffer, size_t buffer_size, size_t* actual_size)
{
    const uint64_t tail = atomic_load(&gs_header->tail);
    if (tail == *head)
    {
        *actual_size = 0;
    }
    else if (tail > *head)
    {
        size_t tocopy = tail - *head;
        if (tocopy > buffer_size)
        {
            tocopy = buffer_size;
        }

        memcpy(buffer, &gs_entries[*head], tocopy * sizeof(struct eventlog_entry));
        *head += tocopy;
        *actual_size = tocopy;
    }
    else // tail < *head
    {
        size_t total_left = gs_header->size - (*head - tail) - 1;
        size_t part1 = gs_header->size - *head;
        size_t part2 = tail;

        if (total_left > buffer_size)
        {
            total_left = buffer_size;
        }
        if (part1 > buffer_size)
        {
            part1 = buffer_size;
        }

        memcpy(buffer, &gs_entries[*head], part1 * sizeof(struct eventlog_entry));
        *head += part1;
        total_left -= part1;
        if (part1 == buffer_size)
        {
            *actual_size = part1;
        }
        else
        {
            if (*head == gs_header->size)
            {
                *head = 0;
            }
            if (part2 > total_left)
            {
                part2 = total_left;
            }
            memcpy(buffer + part1, gs_entries, part2);
            *head += part2;
            *actual_size = part1 + part2;
        }
    }
    return 0;
}