#include <stdio.h>
#include <ncurses.h>
#include <stdlib.h>
#include <signal.h>
#include <string.h>
#include <stdbool.h>
#include <unistd.h>

typedef struct Buffer_struct Buffer;
struct Buffer_struct {
	char* text;
	char* start;
	char* end;
	int size;
	int gap_size;
};

Buffer* new_buffer(int size) {
	Buffer* buffer = malloc(sizeof(Buffer));
	buffer->size = size;
	buffer->text = malloc(buffer->size);

	buffer->gap_size = size;
	buffer->start = buffer->text;
	buffer->end = buffer->start + buffer->gap_size;
	return buffer;
}

void buffer_grow(Buffer* buffer) {

	int old_size = buffer->size;  /* I am making use of the fact that, whenever this 
					function is called, the strlen of the string must 
					equal the size of the buffer (since the gap size is zero) */

	int start_offset = buffer->start - buffer->text;
	buffer->size += 10;
	buffer->text = realloc(buffer->text,buffer->size);
	buffer->start = buffer->text + start_offset;

	buffer->gap_size = 10;
	for (int i=0; i < (old_size - start_offset); i++) {
		*(buffer->start + i + buffer->gap_size) = *(buffer->start + i);
		*(buffer->start + i) = 0;
	}

	buffer->end = buffer->start + buffer->gap_size;

}

void buffer_insert(char ch, Buffer* buffer) {
	*(buffer->start) = ch;
	buffer->start++;
	buffer->gap_size--;
	if (buffer->gap_size == 0) {
		buffer_grow(buffer);
	}
}

void buffer_delete(Buffer* buffer) {
	if (buffer->start != buffer->text) {
		buffer->start--;
		buffer->gap_size++;
	}
}

void buffer_right(Buffer* buffer) {
	if (buffer->end != buffer->text + buffer->size) {
		char c = *(buffer->end);
		buffer->start++;
		buffer->end++;
		*(buffer->start -1) = c;
	}
}

void buffer_left(Buffer* buffer) {
	if (buffer->start != buffer->text) {
		char c = *(buffer->start - 1);
		buffer->start--;
		buffer->end--;
		*(buffer->end) = c;
		*(buffer->start) = 0;
	}
}

void init_curses() {
	initscr();
	noecho();
	keypad(stdscr,TRUE);
	cbreak();
}

void sigint_handler(int dummy) {
	endwin();
	exit(130);
}

int main(int argc, char** argv) {
	signal(SIGINT,sigint_handler);
	Buffer* buffer = new_buffer(10);

	if (argc == 2) {
		if (access(argv[1],F_OK) == 0) {  /* If the file exists */

			FILE* file = fopen(argv[1],"r");
			char c;
			while (c = fgetc(file) != EOF) {
				buffer_insert(c,buffer);
			}
	
		} else {
			printf("File does not exist.\n");
			return -10;
		}
	}

	init_curses();
	int ch;

	int y, x;
	while (true) {
		clear();

		int i=0;
		if (buffer->start != buffer->text) {  /* We don't want to print the string, if the 
							gap starts at the first index of the string */

	                while (i < buffer->size) {
	                        addch(*(buffer->text + i));
				i++;
	                        if ((buffer->start - buffer->text) == i) {  /* If we have encountered 
										the start of the gap */
					getyx(stdscr,y,x);
	                                i += buffer->gap_size;
				}
	                }

			move(y,x);

		}


		ch = getch();

		switch(ch) {
			case KEY_BACKSPACE:
				buffer_delete(buffer);
				break;

			case KEY_LEFT:
				buffer_left(buffer);
				break;

			case KEY_RIGHT:
				buffer_right(buffer);
				break;

			case 10:  /* Enter key */
				buffer_insert('\n',buffer);  /* Why handle this separately? 
								Because, by default, curses seems to send '\r\n', 
								which is technically two characters. I should 
								probably add some code to deal with this scenario 
								in the 'insert' method (instead of creating an 
								exception here), but that's a problem for another day. */
				break;

			default:
				buffer_insert(ch,buffer);

		}
		
		continue_while_loop:

	}
	endwin();
}