/* vi: set sw=4 ts=4: */
/*
 * Mini less implementation for busybox
 *
 * Copyright (C) 2005 by Rob Sullivan <cogito.ergo.cogito@gmail.com>
 *
 * Licensed under the GPL v2 or later, see the file LICENSE in this tarball.
 */

/*
 * TODO:
 * - Add more regular expression support - search modifiers, certain matches, etc.
 * - Add more complex bracket searching - currently, nested brackets are
 *   not considered.
 * - Add support for "F" as an input. This causes less to act in
 *   a similar way to tail -f.
 * - Allow horizontal scrolling.
 *
 * Notes:
 * - the inp file pointer is used so that keyboard input works after
 *   redirected input has been read from stdin
 */

#include <sched.h>      /* sched_yield() */

#include "libbb.h"
#if ENABLE_FEATURE_LESS_REGEXP
#include "xregex.h"
#endif

/* The escape codes for highlighted and normal text */
#define HIGHLIGHT   "\033[7m"
#define NORMAL      "\033[0m"
/* The escape code to home and clear to the end of screen */
#define CLEAR       "\033[H\033[J"
/* The escape code to clear to the end of line */
#define CLEAR_2_EOL "\033[K"

enum {
/* Absolute max of lines eaten */
        MAXLINES = CONFIG_FEATURE_LESS_MAXLINES,
/* This many "after the end" lines we will show (at max) */
        TILDES = 1,
};

/* Command line options */
enum {
        FLAG_E = 1 << 0,
        FLAG_M = 1 << 1,
        FLAG_m = 1 << 2,
        FLAG_N = 1 << 3,
        FLAG_TILDE = 1 << 4,
        FLAG_I = 1 << 5,
        FLAG_S = (1 << 6) * ENABLE_FEATURE_LESS_DASHCMD,
/* hijack command line options variable for internal state vars */
        LESS_STATE_MATCH_BACKWARDS = 1 << 15,
};

#if !ENABLE_FEATURE_LESS_REGEXP
enum { pattern_valid = 0 };
#endif

struct globals {
        int cur_fline; /* signed */
        int kbd_fd;  /* fd to get input from */
        int less_gets_pos;
/* last position in last line, taking into account tabs */
        size_t last_line_pos;
        unsigned max_fline;
        unsigned max_lineno; /* this one tracks linewrap */
        unsigned max_displayed_line;
        unsigned width;
#if ENABLE_FEATURE_LESS_WINCH
        unsigned winch_counter;
#endif
        ssize_t eof_error; /* eof if 0, error if < 0 */
        ssize_t readpos;
        ssize_t readeof; /* must be signed */
        const char **buffer;
        const char **flines;
        const char *empty_line_marker;
        unsigned num_files;
        unsigned current_file;
        char *filename;
        char **files;
#if ENABLE_FEATURE_LESS_MARKS
        unsigned num_marks;
        unsigned mark_lines[15][2];
#endif
#if ENABLE_FEATURE_LESS_REGEXP
        unsigned *match_lines;
        int match_pos; /* signed! */
        int wanted_match; /* signed! */
        int num_matches;
        regex_t pattern;
        smallint pattern_valid;
#endif
        smallint terminated;
        struct termios term_orig, term_less;
        char kbd_input[KEYCODE_BUFFER_SIZE];
};
#define G (*ptr_to_globals)
#define cur_fline           (G.cur_fline         )
#define kbd_fd              (G.kbd_fd            )
#define less_gets_pos       (G.less_gets_pos     )
#define last_line_pos       (G.last_line_pos     )
#define max_fline           (G.max_fline         )
#define max_lineno          (G.max_lineno        )
#define max_displayed_line  (G.max_displayed_line)
#define width               (G.width             )
#define winch_counter       (G.winch_counter     )
/* This one is 100% not cached by compiler on read access */
#define WINCH_COUNTER (*(volatile unsigned *)&winch_counter)
#define eof_error           (G.eof_error         )
#define readpos             (G.readpos           )
#define readeof             (G.readeof           )
#define buffer              (G.buffer            )
#define flines              (G.flines            )
#define empty_line_marker   (G.empty_line_marker )
#define num_files           (G.num_files         )
#define current_file        (G.current_file      )
#define filename            (G.filename          )
#define files               (G.files             )
#define num_marks           (G.num_marks         )
#define mark_lines          (G.mark_lines        )
#if ENABLE_FEATURE_LESS_REGEXP
#define match_lines         (G.match_lines       )
#define match_pos           (G.match_pos         )
#define num_matches         (G.num_matches       )
#define wanted_match        (G.wanted_match      )
#define pattern             (G.pattern           )
#define pattern_valid       (G.pattern_valid     )
#endif
#define terminated          (G.terminated        )
#define term_orig           (G.term_orig         )
#define term_less           (G.term_less         )
#define kbd_input           (G.kbd_input         )
#define INIT_G() do { \
        SET_PTR_TO_GLOBALS(xzalloc(sizeof(G))); \
        less_gets_pos = -1; \
        empty_line_marker = "~"; \
        num_files = 1; \
        current_file = 1; \
        eof_error = 1; \
        terminated = 1; \
        IF_FEATURE_LESS_REGEXP(wanted_match = -1;) \
} while (0)

/* flines[] are lines read from stdin, each in malloc'ed buffer.
 * Line numbers are stored as uint32_t prepended to each line.
 * Pointer is adjusted so that flines[i] points directly past
 * line number. Accesor: */
#define MEMPTR(p) ((char*)(p) - 4)
#define LINENO(p) (*(uint32_t*)((p) - 4))


/* Reset terminal input to normal */
static void set_tty_cooked(void)
{
        fflush_all();
        tcsetattr(kbd_fd, TCSANOW, &term_orig);
}

/* Move the cursor to a position (x,y), where (0,0) is the
   top-left corner of the console */
static void move_cursor(int line, int row)
{
        printf("\033[%u;%uH", line, row);
}

static void clear_line(void)
{
        printf("\033[%u;0H" CLEAR_2_EOL, max_displayed_line + 2);
}

static void print_hilite(const char *str)
{
        printf(HIGHLIGHT"%s"NORMAL, str);
}

static void print_statusline(const char *str)
{
        clear_line();
        printf(HIGHLIGHT"%.*s"NORMAL, width - 1, str);
}

/* Exit the program gracefully */
static void less_exit(int code)
{
        set_tty_cooked();
        clear_line();
        if (code < 0)
                kill_myself_with_sig(- code); /* does not return */
        exit(code);
}

#if (ENABLE_FEATURE_LESS_DASHCMD && ENABLE_FEATURE_LESS_LINENUMS) \
 || ENABLE_FEATURE_LESS_WINCH
static void re_wrap(void)
{
        int w = width;
        int new_line_pos;
        int src_idx;
        int dst_idx;
        int new_cur_fline = 0;
        uint32_t lineno;
        char linebuf[w + 1];
        const char **old_flines = flines;
        const char *s;
        char **new_flines = NULL;
        char *d;

        if (option_mask32 & FLAG_N)
                w -= 8;

        src_idx = 0;
        dst_idx = 0;
        s = old_flines[0];
        lineno = LINENO(s);
        d = linebuf;
        new_line_pos = 0;
        while (1) {
                *d = *s;
                if (*d != '\0') {
                        new_line_pos++;
                        if (*d == '\t') /* tab */
                                new_line_pos += 7;
                        s++;
                        d++;
                        if (new_line_pos >= w) {
                                int sz;
                                /* new line is full, create next one */
                                *d = '\0';
 next_new:
                                sz = (d - linebuf) + 1; /* + 1: NUL */
                                d = ((char*)xmalloc(sz + 4)) + 4;
                                LINENO(d) = lineno;
                                memcpy(d, linebuf, sz);
                                new_flines = xrealloc_vector(new_flines, 8, dst_idx);
                                new_flines[dst_idx] = d;
                                dst_idx++;
                                if (new_line_pos < w) {
                                        /* if we came here thru "goto next_new" */
                                        if (src_idx > max_fline)
                                                break;
                                        lineno = LINENO(s);
                                }
                                d = linebuf;
                                new_line_pos = 0;
                        }
                        continue;
                }
                /* *d == NUL: old line ended, go to next old one */
                free(MEMPTR(old_flines[src_idx]));
                /* btw, convert cur_fline... */
                if (cur_fline == src_idx)
                        new_cur_fline = dst_idx;
                src_idx++;
                /* no more lines? finish last new line (and exit the loop) */
                if (src_idx > max_fline)
                        goto next_new;
                s = old_flines[src_idx];
                if (lineno != LINENO(s)) {
                        /* this is not a continuation line!
                         * create next _new_ line too */
                        goto next_new;
                }
        }

        free(old_flines);
        flines = (const char **)new_flines;

        max_fline = dst_idx - 1;
        last_line_pos = new_line_pos;
        cur_fline = new_cur_fline;
        /* max_lineno is screen-size independent */
#if ENABLE_FEATURE_LESS_REGEXP
        pattern_valid = 0;
#endif
}
#endif

#if ENABLE_FEATURE_LESS_REGEXP
static void fill_match_lines(unsigned pos);
#else
#define fill_match_lines(pos) ((void)0)
#endif

/* Devilishly complex routine.
 *
 * Has to deal with EOF and EPIPE on input,
 * with line wrapping, with last line not ending in '\n'
 * (possibly not ending YET!), with backspace and tabs.
 * It reads input again if last time we got an EOF (thus supporting
 * growing files) or EPIPE (watching output of slow process like make).
 *
 * Variables used:
 * flines[] - array of lines already read. Linewrap may cause
 *      one source file line to occupy several flines[n].
 * flines[max_fline] - last line, possibly incomplete.
 * terminated - 1 if flines[max_fline] is 'terminated'
 *      (if there was '\n' [which isn't stored itself, we just remember
 *      that it was seen])
 * max_lineno - last line's number, this one doesn't increment
 *      on line wrap, only on "real" new lines.
 * readbuf[0..readeof-1] - small preliminary buffer.
 * readbuf[readpos] - next character to add to current line.
 * last_line_pos - screen line position of next char to be read
 *      (takes into account tabs and backspaces)
 * eof_error - < 0 error, == 0 EOF, > 0 not EOF/error
 */
static void read_lines(void)
{
#define readbuf bb_common_bufsiz1
        char *current_line, *p;
        int w = width;
        char last_terminated = terminated;
#if ENABLE_FEATURE_LESS_REGEXP
        unsigned old_max_fline = max_fline;
        time_t last_time = 0;
        unsigned seconds_p1 = 3; /* seconds_to_loop + 1 */
#endif

        if (option_mask32 & FLAG_N)
                w -= 8;

 IF_FEATURE_LESS_REGEXP(again0:)

        p = current_line = ((char*)xmalloc(w + 4)) + 4;
        max_fline += last_terminated;
        if (!last_terminated) {
                const char *cp = flines[max_fline];
                strcpy(p, cp);
                p += strlen(current_line);
                free(MEMPTR(flines[max_fline]));
                /* last_line_pos is still valid from previous read_lines() */
        } else {
                last_line_pos = 0;
        }

        while (1) { /* read lines until we reach cur_fline or wanted_match */
                *p = '\0';
                terminated = 0;
                while (1) { /* read chars until we have a line */
                        char c;
                        /* if no unprocessed chars left, eat more */
                        if (readpos >= readeof) {
                                ndelay_on(0);
                                eof_error = safe_read(STDIN_FILENO, readbuf, sizeof(readbuf));
                                ndelay_off(0);
                                readpos = 0;
                                readeof = eof_error;
                                if (eof_error <= 0)
                                        goto reached_eof;
                        }
                        c = readbuf[readpos];
                        /* backspace? [needed for manpages] */
                        /* <tab><bs> is (a) insane and */
                        /* (b) harder to do correctly, so we refuse to do it */
                        if (c == '\x8' && last_line_pos && p[-1] != '\t') {
                                readpos++; /* eat it */
                                last_line_pos--;
                        /* was buggy (p could end up <= current_line)... */
                                *--p = '\0';
                                continue;
                        }
                        {
                                size_t new_last_line_pos = last_line_pos + 1;
                                if (c == '\t') {
                                        new_last_line_pos += 7;
                                        new_last_line_pos &= (~7);
                                }
                                if ((int)new_last_line_pos >= w)
                                        break;
                                last_line_pos = new_last_line_pos;
                        }
                        /* ok, we will eat this char */
                        readpos++;
                        if (c == '\n') {
                                terminated = 1;
                                last_line_pos = 0;
                                break;
                        }
                        /* NUL is substituted by '\n'! */
                        if (c == '\0') c = '\n';
                        *p++ = c;
                        *p = '\0';
                } /* end of "read chars until we have a line" loop */
                /* Corner case: linewrap with only "" wrapping to next line */
                /* Looks ugly on screen, so we do not store this empty line */
                if (!last_terminated && !current_line[0]) {
                        last_terminated = 1;
                        max_lineno++;
                        continue;
                }
 reached_eof:
                last_terminated = terminated;
                flines = xrealloc_vector(flines, 8, max_fline);

                flines[max_fline] = (char*)xrealloc(MEMPTR(current_line), strlen(current_line) + 1 + 4) + 4;
                LINENO(flines[max_fline]) = max_lineno;
                if (terminated)
                        max_lineno++;

                if (max_fline >= MAXLINES) {
                        eof_error = 0; /* Pretend we saw EOF */
                        break;
                }
                if (!(option_mask32 & FLAG_S)
                  ? (max_fline > cur_fline + max_displayed_line)
                  : (max_fline >= cur_fline
                     && max_lineno > LINENO(flines[cur_fline]) + max_displayed_line)
                ) {
#if !ENABLE_FEATURE_LESS_REGEXP
                        break;
#else
                        if (wanted_match >= num_matches) { /* goto_match called us */
                                fill_match_lines(old_max_fline);
                                old_max_fline = max_fline;
                        }
                        if (wanted_match < num_matches)
                                break;
#endif
                }
                if (eof_error <= 0) {
                        if (eof_error < 0) {
                                if (errno == EAGAIN) {
                                        /* not yet eof or error, reset flag (or else
                                         * we will hog CPU - select() will return
                                         * immediately */
                                        eof_error = 1;
                                } else {
                                        print_statusline(bb_msg_read_error);
                                }
                        }
#if !ENABLE_FEATURE_LESS_REGEXP
                        break;
#else
                        if (wanted_match < num_matches) {
                                break;
                        } else { /* goto_match called us */
                                time_t t = time(NULL);
                                if (t != last_time) {
                                        last_time = t;
                                        if (--seconds_p1 == 0)
                                                break;
                                }
                                sched_yield();
                                goto again0; /* go loop again (max 2 seconds) */
                        }
#endif
                }
                max_fline++;
                current_line = ((char*)xmalloc(w + 4)) + 4;
                p = current_line;
                last_line_pos = 0;
        } /* end of "read lines until we reach cur_fline" loop */
        fill_match_lines(old_max_fline);
#if ENABLE_FEATURE_LESS_REGEXP
        /* prevent us from being stuck in search for a match */
        wanted_match = -1;
#endif
#undef readbuf
}

#if ENABLE_FEATURE_LESS_FLAGS
/* Interestingly, writing calc_percent as a function saves around 32 bytes
 * on my build. */
static int calc_percent(void)
{
        unsigned p = (100 * (cur_fline+max_displayed_line+1) + max_fline/2) / (max_fline+1);
        return p <= 100 ? p : 100;
}

/* Print a status line if -M was specified */
static void m_status_print(void)
{
        int percentage;

        if (less_gets_pos >= 0) /* don't touch statusline while input is done! */
                return;

        clear_line();
        printf(HIGHLIGHT"%s", filename);
        if (num_files > 1)
                printf(" (file %i of %i)", current_file, num_files);
        printf(" lines %i-%i/%i ",
                        cur_fline + 1, cur_fline + max_displayed_line + 1,
                        max_fline + 1);
        if (cur_fline >= (int)(max_fline - max_displayed_line)) {
                printf("(END)"NORMAL);
                if (num_files > 1 && current_file != num_files)
                        printf(HIGHLIGHT" - next: %s"NORMAL, files[current_file]);
                return;
        }
        percentage = calc_percent();
        printf("%i%%"NORMAL, percentage);
}
#endif

/* Print the status line */
static void status_print(void)
{
        const char *p;

        if (less_gets_pos >= 0) /* don't touch statusline while input is done! */
                return;

        /* Change the status if flags have been set */
#if ENABLE_FEATURE_LESS_FLAGS
        if (option_mask32 & (FLAG_M|FLAG_m)) {
                m_status_print();
                return;
        }
        /* No flags set */
#endif

        clear_line();
        if (cur_fline && cur_fline < (int)(max_fline - max_displayed_line)) {
                bb_putchar(':');
                return;
        }
        p = "(END)";
        if (!cur_fline)
                p = filename;
        if (num_files > 1) {
                printf(HIGHLIGHT"%s (file %i of %i)"NORMAL,
                                p, current_file, num_files);
                return;
        }
        print_hilite(p);
}

static void cap_cur_fline(int nlines)
{
        int diff;
        if (cur_fline < 0)
                cur_fline = 0;
        if (cur_fline + max_displayed_line > max_fline + TILDES) {
                cur_fline -= nlines;
                if (cur_fline < 0)
                        cur_fline = 0;
                diff = max_fline - (cur_fline + max_displayed_line) + TILDES;
                /* As the number of lines requested was too large, we just move
                 * to the end of the file */
                if (diff > 0)
                        cur_fline += diff;
        }
}

static const char controls[] ALIGN1 =
        /* NUL: never encountered; TAB: not converted */
        /**/"\x01\x02\x03\x04\x05\x06\x07\x08"  "\x0a\x0b\x0c\x0d\x0e\x0f"
        "\x10\x11\x12\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f"
        "\x7f\x9b"; /* DEL and infamous Meta-ESC :( */
static const char ctrlconv[] ALIGN1 =
        /* why 40 instead of 4a below? - it is a replacement for '\n'.
         * '\n' is a former NUL - we subst it with @, not J */
        "\x40\x41\x42\x43\x44\x45\x46\x47\x48\x49\x40\x4b\x4c\x4d\x4e\x4f"
        "\x50\x51\x52\x53\x54\x55\x56\x57\x58\x59\x5a\x5b\x5c\x5d\x5e\x5f";

static void lineno_str(char *nbuf9, const char *line)
{
        nbuf9[0] = '\0';
        if (option_mask32 & FLAG_N) {
                const char *fmt;
                unsigned n;

                if (line == empty_line_marker) {
                        memset(nbuf9, ' ', 8);
                        nbuf9[8] = '\0';
                        return;
                }
                /* Width of 7 preserves tab spacing in the text */
                fmt = "%7u ";
                n = LINENO(line) + 1;
                if (n > 9999999) {
                        n %= 10000000;
                        fmt = "%07u ";
                }
                sprintf(nbuf9, fmt, n);
        }
}


#if ENABLE_FEATURE_LESS_REGEXP
static void print_found(const char *line)
{
        int match_status;
        int eflags;
        char *growline;
        regmatch_t match_structs;

        char buf[width];
        char nbuf9[9];
        const char *str = line;
        char *p = buf;
        size_t n;

        while (*str) {
                n = strcspn(str, controls);
                if (n) {
                        if (!str[n]) break;
                        memcpy(p, str, n);
                        p += n;
                        str += n;
                }
                n = strspn(str, controls);
                memset(p, '.', n);
                p += n;
                str += n;
        }
        strcpy(p, str);

        /* buf[] holds quarantined version of str */

        /* Each part of the line that matches has the HIGHLIGHT
           and NORMAL escape sequences placed around it.
           NB: we regex against line, but insert text
           from quarantined copy (buf[]) */
        str = buf;
        growline = NULL;
        eflags = 0;
        goto start;

        while (match_status == 0) {
                char *new = xasprintf("%s%.*s"HIGHLIGHT"%.*s"NORMAL,
                                growline ? growline : "",
                                match_structs.rm_so, str,
                                match_structs.rm_eo - match_structs.rm_so,
                                                str + match_structs.rm_so);
                free(growline);
                growline = new;
                str += match_structs.rm_eo;
                line += match_structs.rm_eo;
                eflags = REG_NOTBOL;
 start:
                /* Most of the time doesn't find the regex, optimize for that */
                match_status = regexec(&pattern, line, 1, &match_structs, eflags);
                /* if even "" matches, treat it as "not a match" */
                if (match_structs.rm_so >= match_structs.rm_eo)
                        match_status = 1;
        }

        lineno_str(nbuf9, line);
        if (!growline) {
                printf(CLEAR_2_EOL"%s%s\n", nbuf9, str);
                return;
        }
        printf(CLEAR_2_EOL"%s%s%s\n", nbuf9, growline, str);
        free(growline);
}
#else
void print_found(const char *line);
#endif

static void print_ascii(const char *str)
{
        char buf[width];
        char nbuf9[9];
        char *p;
        size_t n;

        lineno_str(nbuf9, str);
        printf(CLEAR_2_EOL"%s", nbuf9);

        while (*str) {
                n = strcspn(str, controls);
                if (n) {
                        if (!str[n]) break;
                        printf("%.*s", (int) n, str);
                        str += n;
                }
                n = strspn(str, controls);
                p = buf;
                do {
                        if (*str == 0x7f)
                                *p++ = '?';
                        else if (*str == (char)0x9b)
                        /* VT100's CSI, aka Meta-ESC. Who's inventor? */
                        /* I want to know who committed this sin */
                                *p++ = '{';
                        else
                                *p++ = ctrlconv[(unsigned char)*str];
                        str++;
                } while (--n);
                *p = '\0';
                print_hilite(buf);
        }
        puts(str);
}

/* Print the buffer */
static void buffer_print(void)
{
        unsigned i;

        move_cursor(0, 0);
        for (i = 0; i <= max_displayed_line; i++)
                if (pattern_valid)
                        print_found(buffer[i]);
                else
                        print_ascii(buffer[i]);
        status_print();
}

static void buffer_fill_and_print(void)
{
        unsigned i;
#if ENABLE_FEATURE_LESS_DASHCMD
        int fpos = cur_fline;

        if (option_mask32 & FLAG_S) {
                /* Go back to the beginning of this line */
                while (fpos && LINENO(flines[fpos]) == LINENO(flines[fpos-1]))
                        fpos--;
        }

        i = 0;
        while (i <= max_displayed_line && fpos <= max_fline) {
                int lineno = LINENO(flines[fpos]);
                buffer[i] = flines[fpos];
                i++;
                do {
                        fpos++;
                } while ((fpos <= max_fline)
                      && (option_mask32 & FLAG_S)
                      && lineno == LINENO(flines[fpos])
                );
        }
#else
        for (i = 0; i <= max_displayed_line && cur_fline + i <= max_fline; i++) {
                buffer[i] = flines[cur_fline + i];
        }
#endif
        for (; i <= max_displayed_line; i++) {
                buffer[i] = empty_line_marker;
        }
        buffer_print();
}

/* Move the buffer up and down in the file in order to scroll */
static void buffer_down(int nlines)
{
        cur_fline += nlines;
        read_lines();
        cap_cur_fline(nlines);
        buffer_fill_and_print();
}

static void buffer_up(int nlines)
{
        cur_fline -= nlines;
        if (cur_fline < 0) cur_fline = 0;
        read_lines();
        buffer_fill_and_print();
}

static void buffer_line(int linenum)
{
        if (linenum < 0)
                linenum = 0;
        cur_fline = linenum;
        read_lines();
        if (linenum + max_displayed_line > max_fline)
                linenum = max_fline - max_displayed_line + TILDES;
        if (linenum < 0)
                linenum = 0;
        cur_fline = linenum;
        buffer_fill_and_print();
}

static void open_file_and_read_lines(void)
{
        if (filename) {
                xmove_fd(xopen(filename, O_RDONLY), STDIN_FILENO);
        } else {
                /* "less" with no arguments in argv[] */
                /* For status line only */
                filename = xstrdup(bb_msg_standard_input);
        }
        readpos = 0;
        readeof = 0;
        last_line_pos = 0;
        terminated = 1;
        read_lines();
}

/* Reinitialize everything for a new file - free the memory and start over */
static void reinitialize(void)
{
        unsigned i;

        if (flines) {
                for (i = 0; i <= max_fline; i++)
                        free(MEMPTR(flines[i]));
                free(flines);
                flines = NULL;
        }

        max_fline = -1;
        cur_fline = 0;
        max_lineno = 0;
        open_file_and_read_lines();
        buffer_fill_and_print();
}

static int getch_nowait(void)
{
        int rd;
        struct pollfd pfd[2];

        pfd[0].fd = STDIN_FILENO;
        pfd[0].events = POLLIN;
        pfd[1].fd = kbd_fd;
        pfd[1].events = POLLIN;
 again:
        tcsetattr(kbd_fd, TCSANOW, &term_less);
        /* NB: select/poll returns whenever read will not block. Therefore:
         * if eof is reached, select/poll will return immediately
         * because read will immediately return 0 bytes.
         * Even if select/poll says that input is available, read CAN block
         * (switch fd into O_NONBLOCK'ed mode to avoid it)
         */
        rd = 1;
        /* Are we interested in stdin? */
//TODO: reuse code for determining this
        if (!(option_mask32 & FLAG_S)
           ? !(max_fline > cur_fline + max_displayed_line)
           : !(max_fline >= cur_fline
               && max_lineno > LINENO(flines[cur_fline]) + max_displayed_line)
        ) {
                if (eof_error > 0) /* did NOT reach eof yet */
                        rd = 0; /* yes, we are interested in stdin */
        }
        /* Position cursor if line input is done */
        if (less_gets_pos >= 0)
                move_cursor(max_displayed_line + 2, less_gets_pos + 1);
        fflush_all();

        if (kbd_input[0] == 0) { /* if nothing is buffered */
#if ENABLE_FEATURE_LESS_WINCH
                while (1) {
                        int r;
                        /* NB: SIGWINCH interrupts poll() */
                        r = poll(pfd + rd, 2 - rd, -1);
                        if (/*r < 0 && errno == EINTR &&*/ winch_counter)
                                return '\\'; /* anything which has no defined function */
                        if (r) break;
                }
#else
                safe_poll(pfd + rd, 2 - rd, -1);
#endif
        }

        /* We have kbd_fd in O_NONBLOCK mode, read inside read_key()
         * would not block even if there is no input available */
        rd = read_key(kbd_fd, kbd_input, /*timeout off:*/ -2);
        if (rd == -1) {
                if (errno == EAGAIN) {
                        /* No keyboard input available. Since poll() did return,
                         * we should have input on stdin */
                        read_lines();
                        buffer_fill_and_print();
                        goto again;
                }
                /* EOF/error (ssh session got killed etc) */
                less_exit(0);
        }
        set_tty_cooked();
        return rd;
}

/* Grab a character from input without requiring the return key.
 * May return KEYCODE_xxx values.
 * Note that this function works best with raw input. */
static int less_getch(int pos)
{
        int i;

 again:
        less_gets_pos = pos;
        i = getch_nowait();
        less_gets_pos = -1;

        /* Discard Ctrl-something chars */
        if (i >= 0 && i < ' ' && i != 0x0d && i != 8)
                goto again;
        return i;
}

static char* less_gets(int sz)
{
        int c;
        unsigned i = 0;
        char *result = xzalloc(1);

        while (1) {
                c = '\0';
                less_gets_pos = sz + i;
                c = getch_nowait();
                if (c == 0x0d) {
                        result[i] = '\0';
                        less_gets_pos = -1;
                        return result;
                }
                if (c == 0x7f)
                        c = 8;
                if (c == 8 && i) {
                        printf("\x8 \x8");
                        i--;
                }
                if (c < ' ') /* filters out KEYCODE_xxx too (<0) */
                        continue;
                if (i >= width - sz - 1)
                        continue; /* len limit */
                bb_putchar(c);
                result[i++] = c;
                result = xrealloc(result, i+1);
        }
}

static void examine_file(void)
{
        char *new_fname;

        print_statusline("Examine: ");
        new_fname = less_gets(sizeof("Examine: ") - 1);
        if (!new_fname[0]) {
                status_print();
 err:
                free(new_fname);
                return;
        }
        if (access(new_fname, R_OK) != 0) {
                print_statusline("Cannot read this file");
                goto err;
        }
        free(filename);
        filename = new_fname;
        /* files start by = argv. why we assume that argv is infinitely long??
        files[num_files] = filename;
        current_file = num_files + 1;
        num_files++; */
        files[0] = filename;
        num_files = current_file = 1;
        reinitialize();
}

/* This function changes the file currently being paged. direction can be one of the following:
 * -1: go back one file
 *  0: go to the first file
 *  1: go forward one file */
static void change_file(int direction)
{
        if (current_file != ((direction > 0) ? num_files : 1)) {
                current_file = direction ? current_file + direction : 1;
                free(filename);
                filename = xstrdup(files[current_file - 1]);
                reinitialize();
        } else {
                print_statusline(direction > 0 ? "No next file" : "No previous file");
        }
}

static void remove_current_file(void)
{
        unsigned i;

        if (num_files < 2)
                return;

        if (current_file != 1) {
                change_file(-1);
                for (i = 3; i <= num_files; i++)
                        files[i - 2] = files[i - 1];
                num_files--;
        } else {
                change_file(1);
                for (i = 2; i <= num_files; i++)
                        files[i - 2] = files[i - 1];
                num_files--;
                current_file--;
        }
}

static void colon_process(void)
{
        int keypress;

        /* Clear the current line and print a prompt */
        print_statusline(" :");

        keypress = less_getch(2);
        switch (keypress) {
        case 'd':
                remove_current_file();
                break;
        case 'e':
                examine_file();

                break;
#if ENABLE_FEATURE_LESS_FLAGS
        case 'f':
                m_status_print();
                break;
#endif
        case 'n':
                change_file(1);
                break;
        case 'p':
                change_file(-1);
                break;
        case 'q':
                less_exit(EXIT_SUCCESS);
                break;
        case 'x':
                change_file(0);
                break;
        }
}

#if ENABLE_FEATURE_LESS_REGEXP
static void normalize_match_pos(int match)
{
        if (match >= num_matches)
                match = num_matches - 1;
        if (match < 0)
                match = 0;
        match_pos = match;
}

static void goto_match(int match)
{
        if (!pattern_valid)
                return;
        if (match < 0)
                match = 0;
        /* Try to find next match if eof isn't reached yet */
        if (match >= num_matches && eof_error > 0) {
                wanted_match = match; /* "I want to read until I see N'th match" */
                read_lines();
        }
        if (num_matches) {
                normalize_match_pos(match);
                buffer_line(match_lines[match_pos]);
        } else {
                print_statusline("No matches found");
        }
}

static void fill_match_lines(unsigned pos)
{
        if (!pattern_valid)
                return;
        /* Run the regex on each line of the current file */
        while (pos <= max_fline) {
                /* If this line matches */
                if (regexec(&pattern, flines[pos], 0, NULL, 0) == 0
                /* and we didn't match it last time */
                 && !(num_matches && match_lines[num_matches-1] == pos)
                ) {
                        match_lines = xrealloc_vector(match_lines, 4, num_matches);
                        match_lines[num_matches++] = pos;
                }
                pos++;
        }
}

static void regex_process(void)
{
        char *uncomp_regex, *err;

        /* Reset variables */
        free(match_lines);
        match_lines = NULL;
        match_pos = 0;
        num_matches = 0;
        if (pattern_valid) {
                regfree(&pattern);
                pattern_valid = 0;
        }

        /* Get the uncompiled regular expression from the user */
        clear_line();
        bb_putchar((option_mask32 & LESS_STATE_MATCH_BACKWARDS) ? '?' : '/');
        uncomp_regex = less_gets(1);
        if (!uncomp_regex[0]) {
                free(uncomp_regex);
                buffer_print();
                return;
        }

        /* Compile the regex and check for errors */
        err = regcomp_or_errmsg(&pattern, uncomp_regex,
                                (option_mask32 & FLAG_I) ? REG_ICASE : 0);
        free(uncomp_regex);
        if (err) {
                print_statusline(err);
                free(err);
                return;
        }

        pattern_valid = 1;
        match_pos = 0;
        fill_match_lines(0);
        while (match_pos < num_matches) {
                if ((int)match_lines[match_pos] > cur_fline)
                        break;
                match_pos++;
        }
        if (option_mask32 & LESS_STATE_MATCH_BACKWARDS)
                match_pos--;

        /* It's possible that no matches are found yet.
         * goto_match() will read input looking for match,
         * if needed */
        goto_match(match_pos);
}
#endif

static void number_process(int first_digit)
{
        unsigned i;
        int num;
        int keypress;
        char num_input[sizeof(int)*4]; /* more than enough */

        num_input[0] = first_digit;

        /* Clear the current line, print a prompt, and then print the digit */
        clear_line();
        printf(":%c", first_digit);

        /* Receive input until a letter is given */
        i = 1;
        while (i < sizeof(num_input)-1) {
                keypress = less_getch(i + 1);
                if ((unsigned)keypress > 255 || !isdigit(num_input[i]))
                        break;
                num_input[i] = keypress;
                bb_putchar(keypress);
                i++;
        }

        num_input[i] = '\0';
        num = bb_strtou(num_input, NULL, 10);
        /* on format error, num == -1 */
        if (num < 1 || num > MAXLINES) {
                buffer_print();
                return;
        }

        /* We now know the number and the letter entered, so we process them */
        switch (keypress) {
        case KEYCODE_DOWN: case 'z': case 'd': case 'e': case ' ': case '\015':
                buffer_down(num);
                break;
        case KEYCODE_UP: case 'b': case 'w': case 'y': case 'u':
                buffer_up(num);
                break;
        case 'g': case '<': case 'G': case '>':
                cur_fline = num + max_displayed_line;
                read_lines();
                buffer_line(num - 1);
                break;
        case 'p': case '%':
                num = num * (max_fline / 100); /* + max_fline / 2; */
                cur_fline = num + max_displayed_line;
                read_lines();
                buffer_line(num);
                break;
#if ENABLE_FEATURE_LESS_REGEXP
        case 'n':
                goto_match(match_pos + num);
                break;
        case '/':
                option_mask32 &= ~LESS_STATE_MATCH_BACKWARDS;
                regex_process();
                break;
        case '?':
                option_mask32 |= LESS_STATE_MATCH_BACKWARDS;
                regex_process();
                break;
#endif
        }
}

#if ENABLE_FEATURE_LESS_DASHCMD
static void flag_change(void)
{
        int keypress;

        clear_line();
        bb_putchar('-');
        keypress = less_getch(1);

        switch (keypress) {
        case 'M':
                option_mask32 ^= FLAG_M;
                break;
        case 'm':
                option_mask32 ^= FLAG_m;
                break;
        case 'E':
                option_mask32 ^= FLAG_E;
                break;
        case '~':
                option_mask32 ^= FLAG_TILDE;
                break;
        case 'S':
                option_mask32 ^= FLAG_S;
                buffer_fill_and_print();
                break;
#if ENABLE_FEATURE_LESS_LINENUMS
        case 'N':
                option_mask32 ^= FLAG_N;
                re_wrap();
                buffer_fill_and_print();
                break;
#endif
        }
}

#ifdef BLOAT
static void show_flag_status(void)
{
        int keypress;
        int flag_val;

        clear_line();
        bb_putchar('_');
        keypress = less_getch(1);

        switch (keypress) {
        case 'M':
                flag_val = option_mask32 & FLAG_M;
                break;
        case 'm':
                flag_val = option_mask32 & FLAG_m;
                break;
        case '~':
                flag_val = option_mask32 & FLAG_TILDE;
                break;
        case 'N':
                flag_val = option_mask32 & FLAG_N;
                break;
        case 'E':
                flag_val = option_mask32 & FLAG_E;
                break;
        default:
                flag_val = 0;
                break;
        }

        clear_line();
        printf(HIGHLIGHT"The status of the flag is: %u"NORMAL, flag_val != 0);
}
#endif

#endif /* ENABLE_FEATURE_LESS_DASHCMD */

static void save_input_to_file(void)
{
        const char *msg = "";
        char *current_line;
        unsigned i;
        FILE *fp;

        print_statusline("Log file: ");
        current_line = less_gets(sizeof("Log file: ")-1);
        if (current_line[0]) {
                fp = fopen_for_write(current_line);
                if (!fp) {
                        msg = "Error opening log file";
                        goto ret;
                }
                for (i = 0; i <= max_fline; i++)
                        fprintf(fp, "%s\n", flines[i]);
                fclose(fp);
                msg = "Done";
        }
 ret:
        print_statusline(msg);
        free(current_line);
}

#if ENABLE_FEATURE_LESS_MARKS
static void add_mark(void)
{
        int letter;

        print_statusline("Mark: ");
        letter = less_getch(sizeof("Mark: ") - 1);

        if (isalpha(letter)) {
                /* If we exceed 15 marks, start overwriting previous ones */
                if (num_marks == 14)
                        num_marks = 0;

                mark_lines[num_marks][0] = letter;
                mark_lines[num_marks][1] = cur_fline;
                num_marks++;
        } else {
                print_statusline("Invalid mark letter");
        }
}

static void goto_mark(void)
{
        int letter;
        int i;

        print_statusline("Go to mark: ");
        letter = less_getch(sizeof("Go to mark: ") - 1);
        clear_line();

        if (isalpha(letter)) {
                for (i = 0; i <= num_marks; i++)
                        if (letter == mark_lines[i][0]) {
                                buffer_line(mark_lines[i][1]);
                                break;
                        }
                if (num_marks == 14 && letter != mark_lines[14][0])
                        print_statusline("Mark not set");
        } else
                print_statusline("Invalid mark letter");
}
#endif

#if ENABLE_FEATURE_LESS_BRACKETS
static char opp_bracket(char bracket)
{
        switch (bracket) {
                case '{': case '[': /* '}' == '{' + 2. Same for '[' */
                        bracket++;
                case '(':           /* ')' == '(' + 1 */
                        bracket++;
                        break;
                case '}': case ']':
                        bracket--;
                case ')':
                        bracket--;
                        break;
        };
        return bracket;
}

static void match_right_bracket(char bracket)
{
        unsigned i;

        if (strchr(flines[cur_fline], bracket) == NULL) {
                print_statusline("No bracket in top line");
                return;
        }
        bracket = opp_bracket(bracket);
        for (i = cur_fline + 1; i < max_fline; i++) {
                if (strchr(flines[i], bracket) != NULL) {
                        buffer_line(i);
                        return;
                }
        }
        print_statusline("No matching bracket found");
}

static void match_left_bracket(char bracket)
{
        int i;

        if (strchr(flines[cur_fline + max_displayed_line], bracket) == NULL) {
                print_statusline("No bracket in bottom line");
                return;
        }

        bracket = opp_bracket(bracket);
        for (i = cur_fline + max_displayed_line; i >= 0; i--) {
                if (strchr(flines[i], bracket) != NULL) {
                        buffer_line(i);
                        return;
                }
        }
        print_statusline("No matching bracket found");
}
#endif  /* FEATURE_LESS_BRACKETS */

static void keypress_process(int keypress)
{
        switch (keypress) {
        case KEYCODE_DOWN: case 'e': case 'j': case 0x0d:
                buffer_down(1);
                break;
        case KEYCODE_UP: case 'y': case 'k':
                buffer_up(1);
                break;
        case KEYCODE_PAGEDOWN: case ' ': case 'z': case 'f':
                buffer_down(max_displayed_line + 1);
                break;
        case KEYCODE_PAGEUP: case 'w': case 'b':
                buffer_up(max_displayed_line + 1);
                break;
        case 'd':
                buffer_down((max_displayed_line + 1) / 2);
                break;
        case 'u':
                buffer_up((max_displayed_line + 1) / 2);
                break;
        case KEYCODE_HOME: case 'g': case 'p': case '<': case '%':
                buffer_line(0);
                break;
        case KEYCODE_END: case 'G': case '>':
                cur_fline = MAXLINES;
                read_lines();
                buffer_line(cur_fline);
                break;
        case 'q': case 'Q':
                less_exit(EXIT_SUCCESS);
                break;
#if ENABLE_FEATURE_LESS_MARKS
        case 'm':
                add_mark();
                buffer_print();
                break;
        case '\'':
                goto_mark();
                buffer_print();
                break;
#endif
        case 'r': case 'R':
                buffer_print();
                break;
        /*case 'R':
                full_repaint();
                break;*/
        case 's':
                save_input_to_file();
                break;
        case 'E':
                examine_file();
                break;
#if ENABLE_FEATURE_LESS_FLAGS
        case '=':
                m_status_print();
                break;
#endif
#if ENABLE_FEATURE_LESS_REGEXP
        case '/':
                option_mask32 &= ~LESS_STATE_MATCH_BACKWARDS;
                regex_process();
                break;
        case 'n':
                goto_match(match_pos + 1);
                break;
        case 'N':
                goto_match(match_pos - 1);
                break;
        case '?':
                option_mask32 |= LESS_STATE_MATCH_BACKWARDS;
                regex_process();
                break;
#endif
#if ENABLE_FEATURE_LESS_DASHCMD
        case '-':
                flag_change();
                buffer_print();
                break;
#ifdef BLOAT
        case '_':
                show_flag_status();
                break;
#endif
#endif
#if ENABLE_FEATURE_LESS_BRACKETS
        case '{': case '(': case '[':
                match_right_bracket(keypress);
                break;
        case '}': case ')': case ']':
                match_left_bracket(keypress);
                break;
#endif
        case ':':
                colon_process();
                break;
        }

        if (isdigit(keypress))
                number_process(keypress);
}

static void sig_catcher(int sig)
{
        less_exit(- sig);
}

#if ENABLE_FEATURE_LESS_WINCH
static void sigwinch_handler(int sig UNUSED_PARAM)
{
        winch_counter++;
}
#endif

int less_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int less_main(int argc, char **argv)
{
        int keypress;

        INIT_G();

        /* TODO: -x: do not interpret backspace, -xx: tab also */
        /* -xxx: newline also */
        /* -w N: assume width N (-xxx -w 32: hex viewer of sorts) */
        getopt32(argv, "EMmN~I" IF_FEATURE_LESS_DASHCMD("S"));
        argc -= optind;
        argv += optind;
        num_files = argc;
        files = argv;

        /* Another popular pager, most, detects when stdout
         * is not a tty and turns into cat. This makes sense. */
        if (!isatty(STDOUT_FILENO))
                return bb_cat(argv);

        if (!num_files) {
                if (isatty(STDIN_FILENO)) {
                        /* Just "less"? No args and no redirection? */
                        bb_error_msg("missing filename");
                        bb_show_usage();
                }
        } else {
                filename = xstrdup(files[0]);
        }

        if (option_mask32 & FLAG_TILDE)
                empty_line_marker = "";

        kbd_fd = open(CURRENT_TTY, O_RDONLY);
        if (kbd_fd < 0)
                return bb_cat(argv);
        ndelay_on(kbd_fd);

        tcgetattr(kbd_fd, &term_orig);
        term_less = term_orig;
        term_less.c_lflag &= ~(ICANON | ECHO);
        term_less.c_iflag &= ~(IXON | ICRNL);
        /*term_less.c_oflag &= ~ONLCR;*/
        term_less.c_cc[VMIN] = 1;
        term_less.c_cc[VTIME] = 0;

        get_terminal_width_height(kbd_fd, &width, &max_displayed_line);
        /* 20: two tabstops + 4 */
        if (width < 20 || max_displayed_line < 3)
                return bb_cat(argv);
        max_displayed_line -= 2;

        /* We want to restore term_orig on exit */
        bb_signals(BB_FATAL_SIGS, sig_catcher);
#if ENABLE_FEATURE_LESS_WINCH
        signal(SIGWINCH, sigwinch_handler);
#endif

        buffer = xmalloc((max_displayed_line+1) * sizeof(char *));
        reinitialize();
        while (1) {
#if ENABLE_FEATURE_LESS_WINCH
                while (WINCH_COUNTER) {
 again:
                        winch_counter--;
                        get_terminal_width_height(kbd_fd, &width, &max_displayed_line);
                        /* 20: two tabstops + 4 */
                        if (width < 20)
                                width = 20;
                        if (max_displayed_line < 3)
                                max_displayed_line = 3;
                        max_displayed_line -= 2;
                        free(buffer);
                        buffer = xmalloc((max_displayed_line+1) * sizeof(char *));
                        /* Avoid re-wrap and/or redraw if we already know
                         * we need to do it again. These ops are expensive */
                        if (WINCH_COUNTER)
                                goto again;
                        re_wrap();
                        if (WINCH_COUNTER)
                                goto again;
                        buffer_fill_and_print();
                        /* This took some time. Loop back and check,
                         * were there another SIGWINCH? */
                }
#endif
                keypress = less_getch(-1); /* -1: do not position cursor */
                keypress_process(keypress);
        }
}

/*
Help text of less version 418 is below.
If you are implementing something, keeping
key and/or command line switch compatibility is a good idea:


                   SUMMARY OF LESS COMMANDS

      Commands marked with * may be preceded by a number, N.
      Notes in parentheses indicate the behavior if N is given.
  h  H                 Display this help.
  q  :q  Q  :Q  ZZ     Exit.
 ---------------------------------------------------------------------------
                           MOVING
  e  ^E  j  ^N  CR  *  Forward  one line   (or N lines).
  y  ^Y  k  ^K  ^P  *  Backward one line   (or N lines).
  f  ^F  ^V  SPACE  *  Forward  one window (or N lines).
  b  ^B  ESC-v      *  Backward one window (or N lines).
  z                 *  Forward  one window (and set window to N).
  w                 *  Backward one window (and set window to N).
  ESC-SPACE         *  Forward  one window, but don't stop at end-of-file.
  d  ^D             *  Forward  one half-window (and set half-window to N).
  u  ^U             *  Backward one half-window (and set half-window to N).
  ESC-)  RightArrow *  Left  one half screen width (or N positions).
  ESC-(  LeftArrow  *  Right one half screen width (or N positions).
  F                    Forward forever; like "tail -f".
  r  ^R  ^L            Repaint screen.
  R                    Repaint screen, discarding buffered input.
        ---------------------------------------------------
        Default "window" is the screen height.
        Default "half-window" is half of the screen height.
 ---------------------------------------------------------------------------
                          SEARCHING
  /pattern          *  Search forward for (N-th) matching line.
  ?pattern          *  Search backward for (N-th) matching line.
  n                 *  Repeat previous search (for N-th occurrence).
  N                 *  Repeat previous search in reverse direction.
  ESC-n             *  Repeat previous search, spanning files.
  ESC-N             *  Repeat previous search, reverse dir. & spanning files.
  ESC-u                Undo (toggle) search highlighting.
        ---------------------------------------------------
        Search patterns may be modified by one or more of:
        ^N or !  Search for NON-matching lines.
        ^E or *  Search multiple files (pass thru END OF FILE).
        ^F or @  Start search at FIRST file (for /) or last file (for ?).
        ^K       Highlight matches, but don't move (KEEP position).
        ^R       Don't use REGULAR EXPRESSIONS.
 ---------------------------------------------------------------------------
                           JUMPING
  g  <  ESC-<       *  Go to first line in file (or line N).
  G  >  ESC->       *  Go to last line in file (or line N).
  p  %              *  Go to beginning of file (or N percent into file).
  t                 *  Go to the (N-th) next tag.
  T                 *  Go to the (N-th) previous tag.
  {  (  [           *  Find close bracket } ) ].
  }  )  ]           *  Find open bracket { ( [.
  ESC-^F <c1> <c2>  *  Find close bracket <c2>.
  ESC-^B <c1> <c2>  *  Find open bracket <c1>
        ---------------------------------------------------
        Each "find close bracket" command goes forward to the close bracket
          matching the (N-th) open bracket in the top line.
        Each "find open bracket" command goes backward to the open bracket
          matching the (N-th) close bracket in the bottom line.
  m<letter>            Mark the current position with <letter>.
  '<letter>            Go to a previously marked position.
  ''                   Go to the previous position.
  ^X^X                 Same as '.
        ---------------------------------------------------
        A mark is any upper-case or lower-case letter.
        Certain marks are predefined:
             ^  means  beginning of the file
             $  means  end of the file
 ---------------------------------------------------------------------------
                        CHANGING FILES
  :e [file]            Examine a new file.
  ^X^V                 Same as :e.
  :n                *  Examine the (N-th) next file from the command line.
  :p                *  Examine the (N-th) previous file from the command line.
  :x                *  Examine the first (or N-th) file from the command line.
  :d                   Delete the current file from the command line list.
  =  ^G  :f            Print current file name.
 ---------------------------------------------------------------------------
                    MISCELLANEOUS COMMANDS
  -<flag>              Toggle a command line option [see OPTIONS below].
  --<name>             Toggle a command line option, by name.
  _<flag>              Display the setting of a command line option.
  __<name>             Display the setting of an option, by name.
  +cmd                 Execute the less cmd each time a new file is examined.
  !command             Execute the shell command with $SHELL.
  |Xcommand            Pipe file between current pos & mark X to shell command.
  v                    Edit the current file with $VISUAL or $EDITOR.
  V                    Print version number of "less".
 ---------------------------------------------------------------------------
                           OPTIONS
        Most options may be changed either on the command line,
        or from within less by using the - or -- command.
        Options may be given in one of two forms: either a single
        character preceded by a -, or a name preceeded by --.
  -?  ........  --help
                  Display help (from command line).
  -a  ........  --search-skip-screen
                  Forward search skips current screen.
  -b [N]  ....  --buffers=[N]
                  Number of buffers.
  -B  ........  --auto-buffers
                  Don't automatically allocate buffers for pipes.
  -c  ........  --clear-screen
                  Repaint by clearing rather than scrolling.
  -d  ........  --dumb
                  Dumb terminal.
  -D [xn.n]  .  --color=xn.n
                  Set screen colors. (MS-DOS only)
  -e  -E  ....  --quit-at-eof  --QUIT-AT-EOF
                  Quit at end of file.
  -f  ........  --force
                  Force open non-regular files.
  -F  ........  --quit-if-one-screen
                  Quit if entire file fits on first screen.
  -g  ........  --hilite-search
                  Highlight only last match for searches.
  -G  ........  --HILITE-SEARCH
                  Don't highlight any matches for searches.
  -h [N]  ....  --max-back-scroll=[N]
                  Backward scroll limit.
  -i  ........  --ignore-case
                  Ignore case in searches that do not contain uppercase.
  -I  ........  --IGNORE-CASE
                  Ignore case in all searches.
  -j [N]  ....  --jump-target=[N]
                  Screen position of target lines.
  -J  ........  --status-column
                  Display a status column at left edge of screen.
  -k [file]  .  --lesskey-file=[file]
                  Use a lesskey file.
  -L  ........  --no-lessopen
                  Ignore the LESSOPEN environment variable.
  -m  -M  ....  --long-prompt  --LONG-PROMPT
                  Set prompt style.
  -n  -N  ....  --line-numbers  --LINE-NUMBERS
                  Don't use line numbers.
  -o [file]  .  --log-file=[file]
                  Copy to log file (standard input only).
  -O [file]  .  --LOG-FILE=[file]
                  Copy to log file (unconditionally overwrite).
  -p [pattern]  --pattern=[pattern]
                  Start at pattern (from command line).
  -P [prompt]   --prompt=[prompt]
                  Define new prompt.
  -q  -Q  ....  --quiet  --QUIET  --silent --SILENT
                  Quiet the terminal bell.
  -r  -R  ....  --raw-control-chars  --RAW-CONTROL-CHARS
                  Output "raw" control characters.
  -s  ........  --squeeze-blank-lines
                  Squeeze multiple blank lines.
  -S  ........  --chop-long-lines
                  Chop long lines.
  -t [tag]  ..  --tag=[tag]
                  Find a tag.
  -T [tagsfile] --tag-file=[tagsfile]
                  Use an alternate tags file.
  -u  -U  ....  --underline-special  --UNDERLINE-SPECIAL
                  Change handling of backspaces.
  -V  ........  --version
                  Display the version number of "less".
  -w  ........  --hilite-unread
                  Highlight first new line after forward-screen.
  -W  ........  --HILITE-UNREAD
                  Highlight first new line after any forward movement.
  -x [N[,...]]  --tabs=[N[,...]]
                  Set tab stops.
  -X  ........  --no-init
                  Don't use termcap init/deinit strings.
                --no-keypad
                  Don't use termcap keypad init/deinit strings.
  -y [N]  ....  --max-forw-scroll=[N]
                  Forward scroll limit.
  -z [N]  ....  --window=[N]
                  Set size of window.
  -" [c[c]]  .  --quotes=[c[c]]
                  Set shell quote characters.
  -~  ........  --tilde
                  Don't display tildes after end of file.
  -# [N]  ....  --shift=[N]
                  Horizontal scroll amount (0 = one half screen width)

 ---------------------------------------------------------------------------
                          LINE EDITING
        These keys can be used to edit text being entered
        on the "command line" at the bottom of the screen.
 RightArrow                       ESC-l     Move cursor right one character.
 LeftArrow                        ESC-h     Move cursor left one character.
 CNTL-RightArrow  ESC-RightArrow  ESC-w     Move cursor right one word.
 CNTL-LeftArrow   ESC-LeftArrow   ESC-b     Move cursor left one word.
 HOME                             ESC-0     Move cursor to start of line.
 END                              ESC-$     Move cursor to end of line.
 BACKSPACE                                  Delete char to left of cursor.
 DELETE                           ESC-x     Delete char under cursor.
 CNTL-BACKSPACE   ESC-BACKSPACE             Delete word to left of cursor.
 CNTL-DELETE      ESC-DELETE      ESC-X     Delete word under cursor.
 CNTL-U           ESC (MS-DOS only)         Delete entire line.
 UpArrow                          ESC-k     Retrieve previous command line.
 DownArrow                        ESC-j     Retrieve next command line.
 TAB                                        Complete filename & cycle.
 SHIFT-TAB                        ESC-TAB   Complete filename & reverse cycle.
 CNTL-L                                     Complete filename, list all.
*/