/* vi: set sw=4 ts=4: */
/*
 * Mini diff implementation for busybox, adapted from OpenBSD diff.
 *
 * Copyright (C) 2010 by Matheus Izvekov <mizvekov@gmail.com>
 * Copyright (C) 2006 by Robert Sullivan <cogito.ergo.cogito@hotmail.com>
 * Copyright (c) 2003 Todd C. Miller <Todd.Miller@courtesan.com>
 *
 * Sponsored in part by the Defense Advanced Research Projects
 * Agency (DARPA) and Air Force Research Laboratory, Air Force
 * Materiel Command, USAF, under agreement number F39502-99-1-0512.
 *
 * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
 */

/*
 * The following code uses an algorithm due to Harold Stone,
 * which finds a pair of longest identical subsequences in
 * the two files.
 *
 * The major goal is to generate the match vector J.
 * J[i] is the index of the line in file1 corresponding
 * to line i in file0. J[i] = 0 if there is no
 * such line in file1.
 *
 * Lines are hashed so as to work in core. All potential
 * matches are located by sorting the lines of each file
 * on the hash (called "value"). In particular, this
 * collects the equivalence classes in file1 together.
 * Subroutine equiv replaces the value of each line in
 * file0 by the index of the first element of its
 * matching equivalence in (the reordered) file1.
 * To save space equiv squeezes file1 into a single
 * array member in which the equivalence classes
 * are simply concatenated, except that their first
 * members are flagged by changing sign.
 *
 * Next the indices that point into member are unsorted into
 * array class according to the original order of file0.
 *
 * The cleverness lies in routine stone. This marches
 * through the lines of file0, developing a vector klist
 * of "k-candidates". At step i a k-candidate is a matched
 * pair of lines x,y (x in file0, y in file1) such that
 * there is a common subsequence of length k
 * between the first i lines of file0 and the first y
 * lines of file1, but there is no such subsequence for
 * any smaller y. x is the earliest possible mate to y
 * that occurs in such a subsequence.
 *
 * Whenever any of the members of the equivalence class of
 * lines in file1 matable to a line in file0 has serial number
 * less than the y of some k-candidate, that k-candidate
 * with the smallest such y is replaced. The new
 * k-candidate is chained (via pred) to the current
 * k-1 candidate so that the actual subsequence can
 * be recovered. When a member has serial number greater
 * that the y of all k-candidates, the klist is extended.
 * At the end, the longest subsequence is pulled out
 * and placed in the array J by unravel
 *
 * With J in hand, the matches there recorded are
 * checked against reality to assure that no spurious
 * matches have crept in due to hashing. If they have,
 * they are broken, and "jackpot" is recorded--a harmless
 * matter except that a true match for a spuriously
 * mated line may now be unnecessarily reported as a change.
 *
 * Much of the complexity of the program comes simply
 * from trying to minimize core utilization and
 * maximize the range of doable problems by dynamically
 * allocating what is needed and reusing what is not.
 * The core requirements for problems larger than somewhat
 * are (in words) 2*length(file0) + length(file1) +
 * 3*(number of k-candidates installed), typically about
 * 6n words for files of length n.
 */

#include "libbb.h"

#if 0
//#define dbg_error_msg(...) bb_error_msg(__VA_ARGS__)
#else
#define dbg_error_msg(...) ((void)0)
#endif

enum {                  /* print_status() and diffreg() return values */
        STATUS_SAME,    /* files are the same */
        STATUS_DIFFER,  /* files differ */
        STATUS_BINARY,  /* binary files differ */
};

enum {                  /* Commandline flags */
        FLAG_a,
        FLAG_b,
        FLAG_d,
        FLAG_i,
        FLAG_L,         /* never used, handled by getopt32 */
        FLAG_N,
        FLAG_q,
        FLAG_r,
        FLAG_s,
        FLAG_S,         /* never used, handled by getopt32 */
        FLAG_t,
        FLAG_T,
        FLAG_U,         /* never used, handled by getopt32 */
        FLAG_w,
        FLAG_u,         /* ignored, this is the default */
        FLAG_p,         /* not implemented */
        FLAG_B,
        FLAG_E,         /* not implemented */
};
#define FLAG(x) (1 << FLAG_##x)

/* We cache file position to avoid excessive seeking */
typedef struct FILE_and_pos_t {
        FILE *ft_fp;
        off_t ft_pos;
} FILE_and_pos_t;

struct globals {
        smallint exit_status;
        int opt_U_context;
        char *label[2];
        struct stat stb[2];
};
#define G (*ptr_to_globals)
#define exit_status        (G.exit_status       )
#define opt_U_context      (G.opt_U_context     )
#define label              (G.label             )
#define stb                (G.stb               )
#define INIT_G() do { \
        SET_PTR_TO_GLOBALS(xzalloc(sizeof(G))); \
        opt_U_context = 3; \
} while (0)

typedef int token_t;

enum {
        /* Public */
        TOK_EMPTY = 1 << 9,  /* Line fully processed, you can proceed to the next */
        TOK_EOF   = 1 << 10, /* File ended */
        /* Private (Only to be used by read_token() */
        TOK_EOL   = 1 << 11, /* we saw EOL (sticky) */
        TOK_SPACE = 1 << 12, /* used -b code, means we are skipping spaces */
        SHIFT_EOF = (sizeof(token_t)*8 - 8) - 1,
        CHAR_MASK = 0x1ff,   /* 8th bit is used to distinguish EOF from 0xff */
};

/* Restores full EOF from one 8th bit: */
//#define TOK2CHAR(t) (((t) << SHIFT_EOF) >> SHIFT_EOF)
/* We don't really need the above, we only need to have EOF != any_real_char: */
#define TOK2CHAR(t) ((t) & CHAR_MASK)

static void seek_ft(FILE_and_pos_t *ft, off_t pos)
{
        if (ft->ft_pos != pos) {
                ft->ft_pos = pos;
                fseeko(ft->ft_fp, pos, SEEK_SET);
        }
}

/* Reads tokens from given fp, handling -b and -w flags
 * The user must reset tok every line start
 */
static int read_token(FILE_and_pos_t *ft, token_t tok)
{
        tok |= TOK_EMPTY;
        while (!(tok & TOK_EOL)) {
                bool is_space;
                int t;

                t = fgetc(ft->ft_fp);
                if (t != EOF)
                        ft->ft_pos++;
                is_space = (t == EOF || isspace(t));

                /* If t == EOF (-1), set both TOK_EOF and TOK_EOL */
                tok |= (t & (TOK_EOF + TOK_EOL));
                /* Only EOL? */
                if (t == '\n')
                        tok |= TOK_EOL;

                if (option_mask32 & FLAG(i)) /* Handcoded tolower() */
                        t = (t >= 'A' && t <= 'Z') ? t - ('A' - 'a') : t;

                if ((option_mask32 & FLAG(w)) && is_space)
                        continue;

                /* Trim char value to low 9 bits */
                t &= CHAR_MASK;

                if (option_mask32 & FLAG(b)) {
                        /* Was prev char whitespace? */
                        if (tok & TOK_SPACE) { /* yes */
                                if (is_space) /* this one too, ignore it */
                                        continue;
                                tok &= ~TOK_SPACE;
                        } else if (is_space) {
                                /* 1st whitespace char.
                                 * Set TOK_SPACE and replace char by ' ' */
                                t = TOK_SPACE + ' ';
                        }
                }
                /* Clear EMPTY */
                tok &= ~(TOK_EMPTY + CHAR_MASK);
                /* Assign char value (low 9 bits) and maybe set TOK_SPACE */
                tok |= t;
                break;
        }
#if 0
        bb_error_msg("fp:%p tok:%x '%c'%s%s%s%s", fp, tok, tok & 0xff
                , tok & TOK_EOF ? " EOF" : ""
                , tok & TOK_EOL ? " EOL" : ""
                , tok & TOK_EMPTY ? " EMPTY" : ""
                , tok & TOK_SPACE ? " SPACE" : ""
        );
#endif
        return tok;
}

struct cand {
        int x;
        int y;
        int pred;
};

static int search(const int *c, int k, int y, const struct cand *list)
{
        int i, j;

        if (list[c[k]].y < y)   /* quick look for typical case */
                return k + 1;

        for (i = 0, j = k + 1;;) {
                const int l = (i + j) >> 1;
                if (l > i) {
                        const int t = list[c[l]].y;
                        if (t > y)
                                j = l;
                        else if (t < y)
                                i = l;
                        else
                                return l;
                } else
                        return l + 1;
        }
}

static unsigned isqrt(unsigned n)
{
        unsigned x = 1;
        while (1) {
                const unsigned y = x;
                x = ((n / x) + x) >> 1;
                if (x <= (y + 1) && x >= (y - 1))
                        return x;
        }
}

static void stone(const int *a, int n, const int *b, int *J, int pref)
{
        const unsigned isq = isqrt(n);
        const unsigned bound =
                (option_mask32 & FLAG(d)) ? UINT_MAX : MAX(256, isq);
        int clen = 1;
        int clistlen = 100;
        int k = 0;
        struct cand *clist = xzalloc(clistlen * sizeof(clist[0]));
        struct cand cand;
        struct cand *q;
        int *klist = xzalloc((n + 2) * sizeof(klist[0]));
        /*clist[0] = (struct cand){0}; - xzalloc did it */
        /*klist[0] = 0; */

        for (cand.x = 1; cand.x <= n; cand.x++) {
                int j = a[cand.x], oldl = 0;
                unsigned numtries = 0;
                if (j == 0)
                        continue;
                cand.y = -b[j];
                cand.pred = klist[0];
                do {
                        int l, tc;
                        if (cand.y <= clist[cand.pred].y)
                                continue;
                        l = search(klist, k, cand.y, clist);
                        if (l != oldl + 1)
                                cand.pred = klist[l - 1];
                        if (l <= k && clist[klist[l]].y <= cand.y)
                                continue;
                        if (clen == clistlen) {
                                clistlen = clistlen * 11 / 10;
                                clist = xrealloc(clist, clistlen * sizeof(clist[0]));
                        }
                        clist[clen] = cand;
                        tc = klist[l];
                        klist[l] = clen++;
                        if (l <= k) {
                                cand.pred = tc;
                                oldl = l;
                                numtries++;
                        } else {
                                k++;
                                break;
                        }
                } while ((cand.y = b[++j]) > 0 && numtries < bound);
        }
        /* Unravel */
        for (q = clist + klist[k]; q->y; q = clist + q->pred)
                J[q->x + pref] = q->y + pref;
        free(klist);
        free(clist);
}

struct line {
        /* 'serial' is not used in the begining, so we reuse it
         * to store line offsets, thus reducing memory pressure
         */
        union {
                unsigned serial;
                off_t offset;
        };
        unsigned value;
};

static void equiv(struct line *a, int n, struct line *b, int m, int *c)
{
        int i = 1, j = 1;

        while (i <= n && j <= m) {
                if (a[i].value < b[j].value)
                        a[i++].value = 0;
                else if (a[i].value == b[j].value)
                        a[i++].value = j;
                else
                        j++;
        }
        while (i <= n)
                a[i++].value = 0;
        b[m + 1].value = 0;
        j = 0;
        while (++j <= m) {
                c[j] = -b[j].serial;
                while (b[j + 1].value == b[j].value) {
                        j++;
                        c[j] = b[j].serial;
                }
        }
        c[j] = -1;
}

static void unsort(const struct line *f, int l, int *b)
{
        int i;
        int *a = xmalloc((l + 1) * sizeof(a[0]));
        for (i = 1; i <= l; i++)
                a[f[i].serial] = f[i].value;
        for (i = 1; i <= l; i++)
                b[i] = a[i];
        free(a);
}

static int line_compar(const void *a, const void *b)
{
#define l0 ((const struct line*)a)
#define l1 ((const struct line*)b)
        int r = l0->value - l1->value;
        if (r)
                return r;
        return l0->serial - l1->serial;
#undef l0
#undef l1
}

static void fetch(FILE_and_pos_t *ft, const off_t *ix, int a, int b, int ch)
{
        int i, j, col;
        for (i = a; i <= b; i++) {
                seek_ft(ft, ix[i - 1]);
                putchar(ch);
                if (option_mask32 & FLAG(T))
                        putchar('\t');
                for (j = 0, col = 0; j < ix[i] - ix[i - 1]; j++) {
                        int c = fgetc(ft->ft_fp);
                        if (c == EOF) {
                                printf("\n\\ No newline at end of file\n");
                                return;
                        }
                        ft->ft_pos++;
                        if (c == '\t' && (option_mask32 & FLAG(t)))
                                do putchar(' '); while (++col & 7);
                        else {
                                putchar(c);
                                col++;
                        }
                }
        }
}

/* Creates the match vector J, where J[i] is the index
 * of the line in the new file corresponding to the line i
 * in the old file. Lines start at 1 instead of 0, that value
 * being used instead to denote no corresponding line.
 * This vector is dynamically allocated and must be freed by the caller.
 *
 * * fp is an input parameter, where fp[0] and fp[1] are the open
 *   old file and new file respectively.
 * * nlen is an output variable, where nlen[0] and nlen[1]
 *   gets the number of lines in the old and new file respectively.
 * * ix is an output variable, where ix[0] and ix[1] gets
 *   assigned dynamically allocated vectors of the offsets of the lines
 *   of the old and new file respectively. These must be freed by the caller.
 */
static NOINLINE int *create_J(FILE_and_pos_t ft[2], int nlen[2], off_t *ix[2])
{
        int *J, slen[2], *class, *member;
        struct line *nfile[2], *sfile[2];
        int pref = 0, suff = 0, i, j, delta;

        /* Lines of both files are hashed, and in the process
         * their offsets are stored in the array ix[fileno]
         * where fileno == 0 points to the old file, and
         * fileno == 1 points to the new one.
         */
        for (i = 0; i < 2; i++) {
                unsigned hash;
                token_t tok;
                size_t sz = 100;
                nfile[i] = xmalloc((sz + 3) * sizeof(nfile[i][0]));
                /* ft gets here without the correct position, cant use seek_ft */
                ft[i].ft_pos = 0;
                fseeko(ft[i].ft_fp, 0, SEEK_SET);

                nlen[i] = 0;
                /* We could zalloc nfile, but then zalloc starts showing in gprof at ~1% */
                nfile[i][0].offset = 0;
                goto start; /* saves code */
                while (1) {
                        tok = read_token(&ft[i], tok);
                        if (!(tok & TOK_EMPTY)) {
                                /* Hash algorithm taken from Robert Sedgewick, Algorithms in C, 3d ed., p 578. */
                                /*hash = hash * 128 - hash + TOK2CHAR(tok);
                                 * gcc insists on optimizing above to "hash * 127 + ...", thus... */
                                unsigned o = hash - TOK2CHAR(tok);
                                hash = hash * 128 - o; /* we want SPEED here */
                                continue;
                        }
                        if (nlen[i]++ == sz) {
                                sz = sz * 3 / 2;
                                nfile[i] = xrealloc(nfile[i], (sz + 3) * sizeof(nfile[i][0]));
                        }
                        /* line_compar needs hashes fit into positive int */
                        nfile[i][nlen[i]].value = hash & INT_MAX;
                        /* like ftello(ft[i].ft_fp) but faster (avoids lseek syscall) */
                        nfile[i][nlen[i]].offset = ft[i].ft_pos;
                        if (tok & TOK_EOF) {
                                /* EOF counts as a token, so we have to adjust it here */
                                nfile[i][nlen[i]].offset++;
                                break;
                        }
start:
                        hash = tok = 0;
                }
                /* Exclude lone EOF line from the end of the file, to make fetch()'s job easier */
                if (nfile[i][nlen[i]].offset - nfile[i][nlen[i] - 1].offset == 1)
                        nlen[i]--;
                /* Now we copy the line offsets into ix */
                ix[i] = xmalloc((nlen[i] + 2) * sizeof(ix[i][0]));
                for (j = 0; j < nlen[i] + 1; j++)
                        ix[i][j] = nfile[i][j].offset;
        }

        /* length of prefix and suffix is calculated */
        for (; pref < nlen[0] && pref < nlen[1] &&
               nfile[0][pref + 1].value == nfile[1][pref + 1].value;
               pref++);
        for (; suff < nlen[0] - pref && suff < nlen[1] - pref &&
               nfile[0][nlen[0] - suff].value == nfile[1][nlen[1] - suff].value;
               suff++);
        /* Arrays are pruned by the suffix and prefix lenght,
         * the result being sorted and stored in sfile[fileno],
         * and their sizes are stored in slen[fileno]
         */
        for (j = 0; j < 2; j++) {
                sfile[j] = nfile[j] + pref;
                slen[j] = nlen[j] - pref - suff;
                for (i = 0; i <= slen[j]; i++)
                        sfile[j][i].serial = i;
                qsort(sfile[j] + 1, slen[j], sizeof(*sfile[j]), line_compar);
        }
        /* nfile arrays are reused to reduce memory pressure
         * The #if zeroed out section performs the same task as the
         * one in the #else section.
         * Peak memory usage is higher, but one array copy is avoided
         * by not using unsort()
         */
#if 0
        member = xmalloc((slen[1] + 2) * sizeof(member[0]));
        equiv(sfile[0], slen[0], sfile[1], slen[1], member);
        free(nfile[1]);

        class = xmalloc((slen[0] + 1) * sizeof(class[0]));
        for (i = 1; i <= slen[0]; i++) /* Unsorting */
                class[sfile[0][i].serial] = sfile[0][i].value;
        free(nfile[0]);
#else
        member = (int *)nfile[1];
        equiv(sfile[0], slen[0], sfile[1], slen[1], member);
        member = xrealloc(member, (slen[1] + 2) * sizeof(member[0]));

        class = (int *)nfile[0];
        unsort(sfile[0], slen[0], (int *)nfile[0]);
        class = xrealloc(class, (slen[0] + 2) * sizeof(class[0]));
#endif
        J = xmalloc((nlen[0] + 2) * sizeof(J[0]));
        /* The elements of J which fall inside the prefix and suffix regions
         * are marked as unchanged, while the ones which fall outside
         * are initialized with 0 (no matches), so that function stone can
         * then assign them their right values
         */
        for (i = 0, delta = nlen[1] - nlen[0]; i <= nlen[0]; i++)
                J[i] = i <= pref            ?  i :
                       i > (nlen[0] - suff) ? (i + delta) : 0;
        /* Here the magic is performed */
        stone(class, slen[0], member, J, pref);
        J[nlen[0] + 1] = nlen[1] + 1;

        free(class);
        free(member);

        /* Both files are rescanned, in an effort to find any lines
         * which, due to limitations intrinsic to any hashing algorithm,
         * are different but ended up confounded as the same
         */
        for (i = 1; i <= nlen[0]; i++) {
                if (!J[i])
                        continue;

                seek_ft(&ft[0], ix[0][i - 1]);
                seek_ft(&ft[1], ix[1][J[i] - 1]);

                for (j = J[i]; i <= nlen[0] && J[i] == j; i++, j++) {
                        token_t tok0 = 0, tok1 = 0;
                        do {
                                tok0 = read_token(&ft[0], tok0);
                                tok1 = read_token(&ft[1], tok1);

                                if (((tok0 ^ tok1) & TOK_EMPTY) != 0 /* one is empty (not both) */
                                 || (!(tok0 & TOK_EMPTY) && TOK2CHAR(tok0) != TOK2CHAR(tok1))
                                ) {
                                        J[i] = 0; /* Break the correspondence */
                                }
                        } while (!(tok0 & tok1 & TOK_EMPTY));
                }
        }

        return J;
}

static bool diff(FILE* fp[2], char *file[2])
{
        int nlen[2];
        off_t *ix[2];
        FILE_and_pos_t ft[2];
        typedef struct { int a, b; } vec_t[2];
        vec_t *vec = NULL;
        int i = 1, j, k, idx = -1;
        bool anychange = false;
        int *J;

        ft[0].ft_fp = fp[0];
        ft[1].ft_fp = fp[1];
        /* note that ft[i].ft_pos is unintitalized, create_J()
         * must not assume otherwise */
        J = create_J(ft, nlen, ix);

        do {
                bool nonempty = false;

                while (1) {
                        vec_t v;

                        for (v[0].a = i; v[0].a <= nlen[0] && J[v[0].a] == J[v[0].a - 1] + 1; v[0].a++)
                                continue;
                        v[1].a = J[v[0].a - 1] + 1;

                        for (v[0].b = v[0].a - 1; v[0].b < nlen[0] && !J[v[0].b + 1]; v[0].b++)
                                continue;
                        v[1].b = J[v[0].b + 1] - 1;
                        /*
                         * Indicate that there is a difference between lines a and b of the 'from' file
                         * to get to lines c to d of the 'to' file. If a is greater than b then there
                         * are no lines in the 'from' file involved and this means that there were
                         * lines appended (beginning at b).  If c is greater than d then there are
                         * lines missing from the 'to' file.
                         */
                        if (v[0].a <= v[0].b || v[1].a <= v[1].b) {
                                /*
                                 * If this change is more than 'context' lines from the
                                 * previous change, dump the record and reset it.
                                 */
                                int ct = (2 * opt_U_context) + 1;
                                if (idx >= 0
                                 && v[0].a > vec[idx][0].b + ct
                                 && v[1].a > vec[idx][1].b + ct
                                ) {
                                        break;
                                }

                                for (j = 0; j < 2; j++)
                                        for (k = v[j].a; k < v[j].b; k++)
                                                nonempty |= (ix[j][k+1] - ix[j][k] != 1);

                                vec = xrealloc_vector(vec, 6, ++idx);
                                memcpy(vec[idx], v, sizeof(v));
                        }

                        i = v[0].b + 1;
                        if (i > nlen[0])
                                break;
                        J[v[0].b] = v[1].b;
                }
                if (idx < 0 || ((option_mask32 & FLAG(B)) && !nonempty))
                        goto cont;
                if (!(option_mask32 & FLAG(q))) {
                        int lowa;
                        vec_t span, *cvp = vec;

                        if (!anychange) {
                                /* Print the context/unidiff header first time through */
                                printf("--- %s\n", label[0] ? label[0] : file[0]);
                                printf("+++ %s\n", label[1] ? label[1] : file[1]);
                        }

                        printf("@@");
                        for (j = 0; j < 2; j++) {
                                int a = span[j].a = MAX(1, (*cvp)[j].a - opt_U_context);
                                int b = span[j].b = MIN(nlen[j], vec[idx][j].b + opt_U_context);

                                printf(" %c%d", j ? '+' : '-', MIN(a, b));
                                if (a == b)
                                        continue;
                                printf(",%d", (a < b) ? b - a + 1 : 0);
                        }
                        printf(" @@\n");
                        /*
                         * Output changes in "unified" diff format--the old and new lines
                         * are printed together.
                         */
                        for (lowa = span[0].a; ; lowa = (*cvp++)[0].b + 1) {
                                bool end = cvp > &vec[idx];
                                fetch(&ft[0], ix[0], lowa, end ? span[0].b : (*cvp)[0].a - 1, ' ');
                                if (end)
                                        break;
                                for (j = 0; j < 2; j++)
                                        fetch(&ft[j], ix[j], (*cvp)[j].a, (*cvp)[j].b, j ? '+' : '-');
                        }
                }
                anychange = true;
 cont:
                idx = -1;
        } while (i <= nlen[0]);

        free(vec);
        free(ix[0]);
        free(ix[1]);
        free(J);
        return anychange;
}

static int diffreg(char *file[2])
{
        FILE *fp[2] = { stdin, stdin };
        bool binary = false, differ = false;
        int status = STATUS_SAME, i;

        for (i = 0; i < 2; i++) {
                int fd = open_or_warn_stdin(file[i]);
                if (fd == -1)
                        goto out;
                /* Our diff implementation is using seek.
                 * When we meet non-seekable file, we must make a temp copy.
                 */
                if (lseek(fd, 0, SEEK_SET) == -1 && errno == ESPIPE) {
                        char name[] = "/tmp/difXXXXXX";
                        int fd_tmp = mkstemp(name);
                        if (fd_tmp < 0)
                                bb_perror_msg_and_die("mkstemp");
                        unlink(name);
                        if (bb_copyfd_eof(fd, fd_tmp) < 0)
                                xfunc_die();
                        if (fd) /* Prevents closing of stdin */
                                close(fd);
                        fd = fd_tmp;
                }
                fp[i] = fdopen(fd, "r");
        }

        while (1) {
                const size_t sz = COMMON_BUFSIZE / 2;
                char *const buf0 = bb_common_bufsiz1;
                char *const buf1 = buf0 + sz;
                int j, k;
                i = fread(buf0, 1, sz, fp[0]);
                j = fread(buf1, 1, sz, fp[1]);
                if (i != j) {
                        differ = true;
                        i = MIN(i, j);
                }
                if (i == 0)
                        break;
                for (k = 0; k < i; k++) {
                        if (!buf0[k] || !buf1[k])
                                binary = true;
                        if (buf0[k] != buf1[k])
                                differ = true;
                }
        }
        if (differ) {
                if (binary && !(option_mask32 & FLAG(a)))
                        status = STATUS_BINARY;
                else if (diff(fp, file))
                        status = STATUS_DIFFER;
        }
        if (status != STATUS_SAME)
                exit_status |= 1;
out:
        fclose_if_not_stdin(fp[0]);
        fclose_if_not_stdin(fp[1]);

        return status;
}

static void print_status(int status, char *path[2])
{
        switch (status) {
        case STATUS_BINARY:
        case STATUS_DIFFER:
                if ((option_mask32 & FLAG(q)) || status == STATUS_BINARY)
                        printf("Files %s and %s differ\n", path[0], path[1]);
                break;
        case STATUS_SAME:
                if (option_mask32 & FLAG(s))
                        printf("Files %s and %s are identical\n", path[0], path[1]);
                break;
        }
}

#if ENABLE_FEATURE_DIFF_DIR
struct dlist {
        size_t len;
        int s, e;
        char **dl;
};

/* This function adds a filename to dl, the directory listing. */
static int FAST_FUNC add_to_dirlist(const char *filename,
                struct stat *sb UNUSED_PARAM,
                void *userdata, int depth UNUSED_PARAM)
{
        struct dlist *const l = userdata;
        l->dl = xrealloc_vector(l->dl, 6, l->e);
        /* + 1 skips "/" after dirname */
        l->dl[l->e] = xstrdup(filename + l->len + 1);
        l->e++;
        return TRUE;
}

/* If recursion is not set, this function adds the directory
 * to the list and prevents recursive_action from recursing into it.
 */
static int FAST_FUNC skip_dir(const char *filename,
                struct stat *sb, void *userdata,
                int depth)
{
        if (!(option_mask32 & FLAG(r)) && depth) {
                add_to_dirlist(filename, sb, userdata, depth);
                return SKIP;
        }
        return TRUE;
}

static void diffdir(char *p[2], const char *s_start)
{
        struct dlist list[2];
        int i;

        memset(&list, 0, sizeof(list));
        for (i = 0; i < 2; i++) {
                /*list[i].s = list[i].e = 0; - memset did it */
                /*list[i].dl = NULL; */

                /* We need to trim root directory prefix.
                 * Using list.len to specify its length,
                 * add_to_dirlist will remove it. */
                list[i].len = strlen(p[i]);
                recursive_action(p[i], ACTION_RECURSE | ACTION_FOLLOWLINKS,
                                 add_to_dirlist, skip_dir, &list[i], 0);
                /* Sort dl alphabetically.
                 * GNU diff does this ignoring any number of trailing dots.
                 * We don't, so for us dotted files almost always are
                 * first on the list.
                 */
                qsort_string_vector(list[i].dl, list[i].e);
                /* If -S was set, find the starting point. */
                if (!s_start)
                        continue;
                while (list[i].s < list[i].e && strcmp(list[i].dl[list[i].s], s_start) < 0)
                        list[i].s++;
        }
        /* Now that both dirlist1 and dirlist2 contain sorted directory
         * listings, we can start to go through dirlist1. If both listings
         * contain the same file, then do a normal diff. Otherwise, behaviour
         * is determined by whether the -N flag is set. */
        while (1) {
                char *dp[2];
                int pos;
                int k;

                dp[0] = list[0].s < list[0].e ? list[0].dl[list[0].s] : NULL;
                dp[1] = list[1].s < list[1].e ? list[1].dl[list[1].s] : NULL;
                if (!dp[0] && !dp[1])
                        break;
                pos = !dp[0] ? 1 : (!dp[1] ? -1 : strcmp(dp[0], dp[1]));
                k = pos > 0;
                if (pos && !(option_mask32 & FLAG(N)))
                        printf("Only in %s: %s\n", p[k], dp[k]);
                else {
                        char *fullpath[2], *path[2]; /* if -N */

                        for (i = 0; i < 2; i++) {
                                if (pos == 0 || i == k) {
                                        path[i] = fullpath[i] = concat_path_file(p[i], dp[i]);
                                        stat(fullpath[i], &stb[i]);
                                } else {
                                        fullpath[i] = concat_path_file(p[i], dp[1 - i]);
                                        path[i] = (char *)bb_dev_null;
                                }
                        }
                        if (pos)
                                stat(fullpath[k], &stb[1 - k]);

                        if (S_ISDIR(stb[0].st_mode) && S_ISDIR(stb[1].st_mode))
                                printf("Common subdirectories: %s and %s\n", fullpath[0], fullpath[1]);
                        else if (!S_ISREG(stb[0].st_mode) && !S_ISDIR(stb[0].st_mode))
                                printf("File %s is not a regular file or directory and was skipped\n", fullpath[0]);
                        else if (!S_ISREG(stb[1].st_mode) && !S_ISDIR(stb[1].st_mode))
                                printf("File %s is not a regular file or directory and was skipped\n", fullpath[1]);
                        else if (S_ISDIR(stb[0].st_mode) != S_ISDIR(stb[1].st_mode)) {
                                if (S_ISDIR(stb[0].st_mode))
                                        printf("File %s is a %s while file %s is a %s\n", fullpath[0], "directory", fullpath[1], "regular file");
                                else
                                        printf("File %s is a %s while file %s is a %s\n", fullpath[0], "regular file", fullpath[1], "directory");
                        } else
                                print_status(diffreg(path), fullpath);

                        free(fullpath[0]);
                        free(fullpath[1]);
                }
                free(dp[k]);
                list[k].s++;
                if (pos == 0) {
                        free(dp[1 - k]);
                        list[1 - k].s++;
                }
        }
        if (ENABLE_FEATURE_CLEAN_UP) {
                free(list[0].dl);
                free(list[1].dl);
        }
}
#endif

#if ENABLE_FEATURE_DIFF_LONG_OPTIONS
static const char diff_longopts[] ALIGN1 =
        "ignore-case\0"              No_argument       "i"
        "ignore-tab-expansion\0"     No_argument       "E"
        "ignore-space-change\0"      No_argument       "b"
        "ignore-all-space\0"         No_argument       "w"
        "ignore-blank-lines\0"       No_argument       "B"
        "text\0"                     No_argument       "a"
        "unified\0"                  Required_argument "U"
        "label\0"                    Required_argument "L"
        "show-c-function\0"          No_argument       "p"
        "brief\0"                    No_argument       "q"
        "expand-tabs\0"              No_argument       "t"
        "initial-tab\0"              No_argument       "T"
        "recursive\0"                No_argument       "r"
        "new-file\0"                 No_argument       "N"
        "report-identical-files\0"   No_argument       "s"
        "starting-file\0"            Required_argument "S"
        "minimal\0"                  No_argument       "d"
        ;
#endif

int diff_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int diff_main(int argc UNUSED_PARAM, char **argv)
{
        int gotstdin = 0, i;
        char *file[2], *s_start = NULL;
        llist_t *L_arg = NULL;

        INIT_G();

        /* exactly 2 params; collect multiple -L <label>; -U N */
        opt_complementary = "=2:L::U+";
#if ENABLE_FEATURE_DIFF_LONG_OPTIONS
        applet_long_options = diff_longopts;
#endif
        getopt32(argv, "abdiL:NqrsS:tTU:wupBE",
                        &L_arg, &s_start, &opt_U_context);
        argv += optind;
        while (L_arg)
                label[!!label[0]] = llist_pop(&L_arg);
        xfunc_error_retval = 2;
        for (i = 0; i < 2; i++) {
                file[i] = argv[i];
                /* Compat: "diff file name_which_doesnt_exist" exits with 2 */
                if (LONE_DASH(file[i])) {
                        fstat(STDIN_FILENO, &stb[i]);
                        gotstdin++;
                } else
                        xstat(file[i], &stb[i]);
        }
        xfunc_error_retval = 1;
        if (gotstdin && (S_ISDIR(stb[0].st_mode) || S_ISDIR(stb[1].st_mode)))
                bb_error_msg_and_die("can't compare stdin to a directory");

        if (S_ISDIR(stb[0].st_mode) && S_ISDIR(stb[1].st_mode)) {
#if ENABLE_FEATURE_DIFF_DIR
                diffdir(file, s_start);
#else
                bb_error_msg_and_die("no support for directory comparison");
#endif
        } else {
                bool dirfile = S_ISDIR(stb[0].st_mode) || S_ISDIR(stb[1].st_mode);
                bool dir = S_ISDIR(stb[1].st_mode);
                if (dirfile) {
                        const char *slash = strrchr(file[!dir], '/');
                        file[dir] = concat_path_file(file[dir], slash ? slash + 1 : file[!dir]);
                        xstat(file[dir], &stb[dir]);
                }
                /* diffreg can get non-regular files here */
                print_status(gotstdin > 1 ? STATUS_SAME : diffreg(file), file);

                if (dirfile)
                        free(file[dir]);
        }

        return exit_status;
}