Simple file reader in NCurses

#include <curses.h>
#include <stdlib.h>
#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>

#define MAX_LINE 128

#define MIN(a, b) ({ \
    __auto_type _a = (a); \
    __auto_type _b = (b); \
    _a < _b ? _a : _b; \
})

struct line {
    char text[MAX_LINE];
};

struct buffer {
    struct line *lines;
    int nlines;
};

int read_file(struct buffer *buf, const char *file)
{
    int fd;
    char b[1024];
    const char *p;
    struct line *lines, *newlines;
    int nlines;
    int idx;
    ssize_t r;

    if ((fd = open(file, O_RDONLY)) < 0)
        return -1;

    lines = malloc(sizeof(*lines) * 8);
    if (lines == NULL) {
        close(fd);
        return -1;
    }
    nlines = 0;
    idx = 0;
    while ((r = read(fd, b, sizeof(b))) > 0) {
        p = b;
        do {
            if (*p == '\n') {
                lines[nlines].text[idx] = '\0';
                nlines++;
                newlines = realloc(lines, sizeof(*lines) * (nlines + 1));
                if (newlines == NULL)
                    goto end;
                lines = newlines;
                idx = 0;
            } else {
                if (idx + 1 == MAX_LINE)
                    continue; /* cut off lines that are too long */
                lines[nlines].text[idx++] = *p;
            }
            p++;
        } while(--r);
    }

end:
    close(fd);
    lines[nlines].text[idx] = '\0';
    buf->lines = lines;
    buf->nlines = nlines + 1;
    return 0;
}

void draw_lines(WINDOW *win, int y, struct line *lines, int nlines, int from, int to)
{
    int begy, begx;
    int maxy, maxx;

    getbegyx(win, begy, begx);
    getmaxyx(win, maxy, maxx);
    maxx -= begx;
    maxy -= begy;
    if (from < 0)
        from = 0;
    if (to < 0 || to >= nlines)
        to = nlines - 1;
    if (maxy - y < to - from)
        to = maxy - y + from;
    for (; from <= to; from++, y++)
        mvwaddstr(win, y, 0, lines[from].text);
}

int main(int argc, char **argv)
{
    struct buffer buf;

    int width, height;
    WINDOW *win;
    int scroll;

    if (argc < 2) {
        fprintf(stderr, "usage: %s <file name>\n", argv[0]);
        return -1;
    }

    initscr();
    noecho();
    raw();
    keypad(stdscr, true);
    curs_set(0);
    start_color();
    refresh();

    if (read_file(&buf, argv[1]) < 0) {
        endwin();
        fprintf(stderr, "error reading file '%s'\n", argv[1]);
        return -1;
    }

    width = MIN(COLS, MAX_LINE);
    height = MIN(LINES, buf.nlines);
    win = newwin(height, width, 0, 0);
    if (win == NULL) {
        endwin();
        fprintf(stderr, "couldn't create window\n");
        return -1;
    }
    scrollok(win, true);
    scroll = 0;

    /* Display initial content */
    draw_lines(win, 0, buf.lines, buf.nlines, -1, -1);

    /* Main loop */
    while (1) {
        wrefresh(win);
        /* Show status bar */
        attr_set(A_REVERSE, 0, NULL);
        mvprintw(LINES - 1, 0, "%4d (%3d%%)", scroll + 1, 100 * scroll / (buf.nlines - height));
        hline(' ', COLS - getcurx(stdscr));

        const int c = getch();
        if (c == 0x03 || c == 'q')
            break;
        switch (c) {
        case 'j':
            if (scroll == buf.nlines - height)
                break;
            wscrl(win, 1);
            scroll++;
            draw_lines(win, height - 1, buf.lines, buf.nlines, scroll + height, scroll + height);
            break;
        case 'k':
            if (scroll == 0)
                break;
            wscrl(win, -1);
            scroll--;
            draw_lines(win, 0, buf.lines, buf.nlines, scroll, scroll);
            break;
        case 'g':
            werase(win);
            draw_lines(win, 0, buf.lines, buf.nlines, -1, -1);
            break;
        case 'G':
            werase(win);
            draw_lines(win, 0, buf.lines, buf.nlines, buf.nlines - height, -1);
            break;
        }
    }

    endwin();
    return 0;
}