#include <ctype.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ioctl.h>
#include <termios.h>
#include <time.h>
#include <unistd.h>

#define TODO_PROGRAM "todo"
#define TODO_VERSION "0.1.0"

#define MAX_ITEMS 4096
#define MAX_TEXT  256

#define LEFT_MARGIN    2
#define SECTION_INDENT 2

enum State
{
    STATE_TODO,
    STATE_DOING,
    STATE_DONE
};

struct Item
{
    enum State state;
    char text[MAX_TEXT];
};

struct List
{
    struct Item items[MAX_ITEMS];
    int count;
};

enum Mode
{
    MODE_NORMAL,
    MODE_INSERT
};

static const char* mode_name(enum Mode mode)
{
    switch (mode)
    {
        case MODE_NORMAL: return "NORMAL";
        case MODE_INSERT: return "INSERT";
    }

    return "?";
}

static void todo_path(char* buffer, size_t size)
{
    const char* env_path = getenv("TODO_FILE");

    if (env_path != NULL && env_path[0] != '\0')
    {
        (void) snprintf(buffer, size, "%s", env_path);
        return;
    }

    const char* home = getenv("HOME");

    (void) snprintf(buffer, size, "%s/.todo.txt", home != NULL ? home : ".");
}

static void strip_newline(char* string)
{
    size_t length = strlen(string);

    while (length > 0 && (string[length - 1] == '\n' || string[length - 1] == '\r'))
        string[--length] = '\0';
}

static void load(struct List* list, const char* path)
{
    list->count = 0;

    FILE* file = fopen(path, "r");

    if (file == NULL)
        return;

    char line[MAX_TEXT + 8];

    while (list->count < MAX_ITEMS && fgets(line, sizeof line, file) != NULL)
    {
        strip_newline(line);

        enum State state = STATE_TODO;
        char* text = line;

        if (line[0] == '[' && line[1] != '\0' && line[2] == ']')
        {
            if (line[1] == 'x' || line[1] == 'X')
                state = STATE_DONE;
            else if (line[1] == '-')
                state = STATE_DOING;

            text = line + 3;

            while (*text == ' ')
                text++;
        }

        if (*text == '\0')
            continue;

        struct Item* item = &list->items[list->count++];
        item->state = state;
        (void) snprintf(item->text, sizeof item->text, "%s", text);
    }

    (void) fclose(file);
}

static int is_section(const struct Item* item)
{
    return item->text[0] == '=';
}

static void save(const struct List* list, const char* path)
{
    FILE* file = fopen(path, "w");

    if (file == NULL)
        return;

    for (int i = 0; i < list->count; i++)
    {
        if (is_section(&list->items[i]))
        {
            (void) fprintf(file, "%s\n", list->items[i].text);
            continue;
        }

        char marker = list->items[i].state == STATE_DONE ? 'x'
                    : list->items[i].state == STATE_DOING ? '-'
                                                          : ' ';

        (void) fprintf(file, "[%c] %s\n", marker, list->items[i].text);
    }

    (void) fclose(file);
}

static void insert_item(struct List* list, int index)
{
    if (list->count >= MAX_ITEMS)
        return;

    if (index < 0)
        index = 0;

    if (index > list->count)
        index = list->count;

    for (int i = list->count; i > index; i--)
        list->items[i] = list->items[i - 1];

    list->items[index].state = STATE_TODO;
    list->items[index].text[0] = '\0';
    list->count++;
}

static void delete_item(struct List* list, int index)
{
    if (index < 0 || index >= list->count)
        return;

    for (int i = index; i < list->count - 1; i++)
        list->items[i] = list->items[i + 1];

    list->count--;
}


static struct termios saved_termios;

static void restore_terminal(void)
{
    static const char reset[] = "\x1b[?25h\x1b[0 q\x1b[?1049l";

    (void) tcsetattr(STDIN_FILENO, TCSAFLUSH, &saved_termios);
    (void) write(STDOUT_FILENO, reset, sizeof reset - 1);
}

static void on_signal(int signal_number)
{
    (void) signal_number;
    exit(EXIT_SUCCESS);
}

static int enter_raw_mode(void)
{
    if (!isatty(STDIN_FILENO))
    {
        (void) fprintf(stderr, "%s: not a terminal\n", TODO_PROGRAM);
        return -1;
    }

    if (tcgetattr(STDIN_FILENO, &saved_termios) != 0)
        return -1;

    struct termios raw = saved_termios;
    raw.c_lflag &= ~((tcflag_t) (ICANON | ECHO | ISIG));
    raw.c_cc[VMIN] = 1;
    raw.c_cc[VTIME] = 0;

    if (tcsetattr(STDIN_FILENO, TCSAFLUSH, &raw) != 0)
        return -1;

    (void) atexit(restore_terminal);
    (void) signal(SIGINT, on_signal);
    (void) signal(SIGTERM, on_signal);

    (void) write(STDOUT_FILENO, "\x1b[?1049h\x1b[?25l", 14);

    return 0;
}

static int screen_rows(void)
{
    struct winsize window;

    if (ioctl(STDOUT_FILENO, TIOCGWINSZ, &window) == 0 && window.ws_row > 0)
        return window.ws_row;

    return 24;
}

static int content_rows(void)
{
    int rows = screen_rows() - 3;

    return rows < 1 ? 1 : rows;
}

static void to_roman(int number, char* buffer, size_t size)
{
    static const int values[] = { 1000, 900, 500, 400, 100, 90, 50, 40, 10, 9, 5, 4, 1 };
    static const char* const symbols[] = { "M", "CM", "D", "CD", "C", "XC", "L", "XL", "X", "IX", "V", "IV", "I" };

    size_t length = 0;
    buffer[0] = '\0';

    for (int i = 0; i < 13; i++)
        while (number >= values[i] && length + 2 < size)
        {
            length += (size_t) snprintf(buffer + length, size - length, "%s", symbols[i]);
            number -= values[i];
        }
}

static void render(const struct List* list, int cursor, int scroll, enum Mode mode)
{
    int rows = screen_rows();
    int visible = content_rows();

    int total_items = 0;
    int done = 0;

    for (int i = 0; i < list->count; i++)
        if (!is_section(&list->items[i]))
        {
            total_items++;
            done += list->items[i].state == STATE_DONE;
        }

    int max_run = 0;
    int run = 0;

    for (int i = 0; i < list->count; i++)
    {
        if (is_section(&list->items[i]))
            run = 0;
        else if (++run > max_run)
            max_run = run;
    }

    int number_width = 1;

    for (int count = max_run; count >= 10; count /= 10)
        number_width++;

    char date[64];
    time_t now = time(NULL);
    struct tm* local = localtime(&now);

    if (local == NULL || strftime(date, sizeof date, "%a %d %b %Y", local) == 0)
        date[0] = '\0';

    char buffer[64 * 1024];
    int length = 0;

    length += snprintf(buffer + length, sizeof buffer - (size_t) length,
                       "\x1b[H%*s\x1b[2m%s\x1b[0m\x1b[K\r\n\x1b[K\r\n", LEFT_MARGIN, "", date);

    int emitted = 0;
    int index = scroll;

    while (emitted < visible && index < list->count)
    {
        const struct Item* item = &list->items[index];

        const char* background       = index == cursor ? "\x1b[48;5;235m" : "";
        const char* background_reset = index == cursor ? "\x1b[49m" : "";

        if (is_section(item))
        {
            if (emitted > 0)
            {
                length += snprintf(buffer + length, sizeof buffer - (size_t) length, "\x1b[K\r\n");
                emitted++;

                if (emitted >= visible)
                    break;
            }

            if (mode == MODE_INSERT && index == cursor)
            {
                length += snprintf(buffer + length, sizeof buffer - (size_t) length,
                                   "%s%*s\x1b[1m%s\x1b[22m\x1b[K%s\r\n",
                                   background, LEFT_MARGIN, "", item->text, background_reset);
            }
            else
            {
                int section_number = 0;

                for (int k = 0; k <= index; k++)
                    if (is_section(&list->items[k]))
                        section_number++;

                char roman[16];
                to_roman(section_number, roman, sizeof roman);

                const char* name = item->text + 1;

                while (*name == ' ')
                    name++;

                length += snprintf(buffer + length, sizeof buffer - (size_t) length,
                                   "%s%*s\x1b[1m%s. %s\x1b[22m\x1b[K%s\r\n",
                                   background, LEFT_MARGIN, "", roman, name, background_reset);
            }

            emitted++;
            index++;
            continue;
        }

        int number = 0;
        int scan = index;

        while (scan >= 0 && !is_section(&list->items[scan]))
        {
            number++;
            scan--;
        }

        int item_indent = LEFT_MARGIN + (scan >= 0 ? SECTION_INDENT : 0);

        const char* box              = item->state == STATE_DONE ? "[x]"
                                     : item->state == STATE_DOING ? "[-]"
                                                                  : "[ ]";
        const char* dim              = item->state == STATE_DONE ? "\x1b[2m" : "";
        const char* dim_reset        = item->state == STATE_DONE ? "\x1b[22m" : "";

        length += snprintf(buffer + length, sizeof buffer - (size_t) length,
                           "%s%*s\x1b[2m%*d.\x1b[22m %s %s%s%s\x1b[K%s\r\n",
                           background, item_indent, "", number_width, number,
                           box, dim, item->text, dim_reset, background_reset);
        emitted++;
        index++;
    }

    while (emitted < visible)
    {
        length += snprintf(buffer + length, sizeof buffer - (size_t) length, "\x1b[K\r\n");
        emitted++;
    }

    length += snprintf(buffer + length, sizeof buffer - (size_t) length,
                       "\x1b[%d;1H%*s\x1b[2m-- %s --  %d/%d done\x1b[0m\x1b[K",
                       rows, LEFT_MARGIN, "", mode_name(mode), done, total_items);

    if (cursor >= 0 && cursor < list->count)
    {
        int rows_before = 0;

        for (int i = scroll; i < cursor; i++)
            rows_before += is_section(&list->items[i]) && i > scroll ? 2 : 1;

        if (is_section(&list->items[cursor]) && cursor > scroll)
            rows_before++;

        int cursor_row = 3 + rows_before;
        int text_length = (int) strlen(list->items[cursor].text);
        int cursor_column;

        if (is_section(&list->items[cursor]))
        {
            cursor_column = mode == MODE_INSERT ? LEFT_MARGIN + text_length + 1 : LEFT_MARGIN + 1;
        }
        else
        {
            int scan = cursor - 1;

            while (scan >= 0 && !is_section(&list->items[scan]))
                scan--;

            int item_indent = LEFT_MARGIN + (scan >= 0 ? SECTION_INDENT : 0);
            int text_start = item_indent + number_width + 7;
            cursor_column = mode == MODE_INSERT ? text_start + text_length : text_start;
        }

        const char* shape = mode == MODE_INSERT ? "\x1b[6 q" : "\x1b[2 q";

        length += snprintf(buffer + length, sizeof buffer - (size_t) length,
                           "\x1b[%d;%dH%s\x1b[?25h", cursor_row, cursor_column, shape);
    }
    else
    {
        length += snprintf(buffer + length, sizeof buffer - (size_t) length, "\x1b[?25l");
    }

    (void) write(STDOUT_FILENO, buffer, (size_t) length);
}

static int normal_key(struct List* list, const char* path, int* cursor, enum Mode* mode, char character)
{
    switch (character)
    {
        case 'q':
            return 0;

        case 'j':
            (*cursor)++;
            break;

        case 'k':
            (*cursor)--;
            break;

        case ' ':
            if (*cursor >= 0 && *cursor < list->count && !is_section(&list->items[*cursor]))
            {
                struct Item* item = &list->items[*cursor];
                item->state = (enum State) ((item->state + 1) % 3);
                save(list, path);
            }

            break;

        case 'x':
            if (*cursor >= 0 && *cursor < list->count && !is_section(&list->items[*cursor]))
            {
                struct Item* item = &list->items[*cursor];
                item->state = item->state == STATE_DONE ? STATE_TODO : STATE_DONE;
                save(list, path);
            }

            break;

        case 'a':
        case 'o':
        case 'i':
            insert_item(list, *cursor + 1);
            *cursor = *cursor + 1 < list->count ? *cursor + 1 : list->count - 1;
            *mode = MODE_INSERT;
            break;

        case 'A':
            if (list->count == 0)
            {
                insert_item(list, 0);
                *cursor = 0;
            }

            if (*cursor >= 0 && *cursor < list->count)
                *mode = MODE_INSERT;

            break;

        case 'R':
            if (list->count == 0)
            {
                insert_item(list, 0);
                *cursor = 0;
            }

            if (*cursor >= 0 && *cursor < list->count)
            {
                list->items[*cursor].text[0] = '\0';
                *mode = MODE_INSERT;
            }

            break;

        case 'g':
        {
            char next;

            if (read(STDIN_FILENO, &next, 1) == 1 && next == 'g')
                *cursor = 0;

            break;
        }

        case 'G':
            *cursor = list->count - 1;
            break;

        case '}':
        {
            int next = *cursor + 1;

            while (next < list->count && !is_section(&list->items[next]))
                next++;

            *cursor = next < list->count ? next : list->count - 1;
            break;
        }

        case '{':
        {
            int previous = *cursor - 1;

            while (previous >= 0 && !is_section(&list->items[previous]))
                previous--;

            *cursor = previous >= 0 ? previous : 0;
            break;
        }

        case 'd':
        {
            char next;

            if (read(STDIN_FILENO, &next, 1) == 1 && next == 'd')
            {
                delete_item(list, *cursor);
                save(list, path);
            }

            break;
        }

        case 'J':
            if (*cursor >= 0 && *cursor + 1 < list->count)
            {
                struct Item moved = list->items[*cursor];
                list->items[*cursor] = list->items[*cursor + 1];
                list->items[*cursor + 1] = moved;
                (*cursor)++;
                save(list, path);
            }

            break;

        case 'K':
            if (*cursor > 0 && *cursor < list->count)
            {
                struct Item moved = list->items[*cursor];
                list->items[*cursor] = list->items[*cursor - 1];
                list->items[*cursor - 1] = moved;
                (*cursor)--;
                save(list, path);
            }

            break;

        case 'D':
        {
            int kept = 0;

            for (int i = 0; i < list->count; i++)
                if (list->items[i].state != STATE_DONE)
                    list->items[kept++] = list->items[i];

            list->count = kept;
            save(list, path);
            break;
        }

        default:
            break;
    }

    return 1;
}

static void insert_key(struct List* list, const char* path, int* cursor, enum Mode* mode, char character)
{
    struct Item* item = &list->items[*cursor];
    size_t length = strlen(item->text);

    if (character == 27 || ((character == '\r' || character == '\n') && item->text[0] == '\0'))
    {
        if (item->text[0] == '\0')
        {
            delete_item(list, *cursor);

            if (*cursor >= list->count)
                *cursor = list->count - 1;

            if (*cursor < 0)
                *cursor = 0;
        }

        save(list, path);
        *mode = MODE_NORMAL;
        return;
    }

    if (character == '\r' || character == '\n')
    {
        save(list, path);
        insert_item(list, *cursor + 1);
        (*cursor)++;
        return;
    }

    if (character == 127 || character == 8)
    {
        if (length > 0)
            item->text[length - 1] = '\0';

        return;
    }

    if (isprint((unsigned char) character) && length + 1 < sizeof item->text)
    {
        item->text[length] = character;
        item->text[length + 1] = '\0';
    }
}

static void run(struct List* list, const char* path)
{
    if (enter_raw_mode() != 0)
        exit(EXIT_FAILURE);

    enum Mode mode = MODE_NORMAL;
    int cursor = 0;
    int scroll = 0;

    for (;;)
    {
        int visible = content_rows();

        if (cursor >= list->count)
            cursor = list->count - 1;

        if (cursor < 0)
            cursor = 0;

        if (cursor < scroll)
            scroll = cursor;

        if (cursor >= scroll + visible)
            scroll = cursor - visible + 1;

        render(list, cursor, scroll, mode);

        char character;

        if (read(STDIN_FILENO, &character, 1) != 1)
            break;

        switch (mode)
        {
            case MODE_NORMAL:
                if (normal_key(list, path, &cursor, &mode, character) == 0)
                    return;

                break;

            case MODE_INSERT:
                insert_key(list, path, &cursor, &mode, character);
                break;
        }
    }
}

int main(void)
{
    char path[4096];
    todo_path(path, sizeof path);

    struct List* list = malloc(sizeof *list);

    if (list == NULL)
    {
        (void) fprintf(stderr, "%s: out of memory\n", TODO_PROGRAM);
        return EXIT_FAILURE;
    }

    load(list, path);
    run(list, path);
    free(list);

    return EXIT_SUCCESS;
}