- new applet diff. Rob Sullivan writes:

Here's my attempt at a mini diff applet - it's adapted from the code at
http://www.openbsd.org/cgi-bin/cvsweb/src/usr.bin/diff/, and only supports
unified diffs.

I've busyboxified everything to a reasonable degree, so I think the code is
suitable enough to be included, but there's still a fair bit of cleaning up
to be done.

5 files changed, 1331 insertions, 0 deletions

diff --git a/coreutils/Config.in b/coreutils/Config.in
index cd45cc077..62c5c327d 100644
--- a/coreutils/Config.in
+++ b/coreutils/Config.in
@@ -109,6 +109,38 @@ config CONFIG_DF
           df reports the amount of disk space used and available
           on filesystems.
 
+config CONFIG_DIFF
+        bool "diff"
+        default n
+        help
+          diff compares two files or directories and outputs the
+          differences between them in a form that can be given to
+          the patch command.
+
+config CONFIG_FEATURE_DIFF_BINARY
+        bool "  Enable checks for binary files"
+        default y
+        depends on CONFIG_DIFF
+        help
+          This option enables support for checking for binary files
+          before a comparison is carried out.
+
+config CONFIG_FEATURE_DIFF_DIR
+        bool "  Enable directory support"
+        default y
+        depends on CONFIG_DIFF
+        help
+          This option enables support for directory and subdirectory
+          comparison.
+
+config CONFIG_FEATURE_DIFF_MINIMAL
+        bool "  Enable -d option to find smaller sets of changes"
+        default n
+        depends on CONFIG_DIFF
+        help
+          Enabling this option allows the use of -d to make diff
+          try hard to find the smallest possible set of changes.
+
 config CONFIG_DIRNAME
         bool "dirname"
         default n
diff --git a/coreutils/Makefile.in b/coreutils/Makefile.in
index 2ab256c15..ff0e89b4d 100644
--- a/coreutils/Makefile.in
+++ b/coreutils/Makefile.in
@@ -25,6 +25,7 @@ COREUTILS-$(CONFIG_CUT)       += cut.o
 COREUTILS-$(CONFIG_DATE)      += date.o
 COREUTILS-$(CONFIG_DD)        += dd.o
 COREUTILS-$(CONFIG_DF)        += df.o
+COREUTILS-$(CONFIG_DIFF)      += diff.o
 COREUTILS-$(CONFIG_DIRNAME)   += dirname.o
 COREUTILS-$(CONFIG_DOS2UNIX)  += dos2unix.o
 COREUTILS-$(CONFIG_DU)        += du.o
diff --git a/coreutils/diff.c b/coreutils/diff.c
new file mode 100644
index 000000000..b2945dd87
--- /dev/null
+++ b/coreutils/diff.c
@@ -0,0 +1,1277 @@
+/* vi: set sw=4 ts=4: */
+/*
+ * Mini diff implementation for busybox, adapted from OpenBSD diff.
+ *
+ * Copyright (C) 2006 by Robert Sullivan <cogito.ergo.cogito@hotmail.com>
+ * 
+ * Licensed under GPLv2 or later, see file LICENSE in this tarball for details.
+ */
+
+/*
+ * Copyright (c) 2003 Todd C. Miller <Todd.Miller@courtesan.com>
+ *
+ * Permission to 
+ * use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ * 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.
+ */
+
+#include <time.h>
+#include <sys/types.h>
+#include <sys/param.h>
+#include <sys/stat.h>
+#include <ctype.h>
+#include <errno.h>
+#include <signal.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdarg.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/wait.h>
+#include <fcntl.h>
+#include <stddef.h>
+#include <paths.h>
+#include <dirent.h>
+#include "busybox.h"
+
+#define FSIZE_MAX 32768
+
+/*
+ * Output flags
+ */
+#define D_HEADER        1        /* Print a header/footer between files */
+#define D_EMPTY1        2        /* Treat first file as empty (/dev/null) */
+#define D_EMPTY2        4        /* Treat second file as empty (/dev/null) */
+
+/*
+ * Status values for print_status() and diffreg() return values
+ * Guide:
+ * D_SAME - files are the same
+ * D_DIFFER - files differ
+ * D_BINARY - binary files differ
+ * D_COMMON - subdirectory common to both dirs
+ * D_ONLY - file only exists in one dir
+ * D_MISMATCH1 - path1 a dir, path2 a file
+ * D_MISMATCH2 - path1 a file, path2 a dir
+ * D_ERROR - error occurred
+ * D_SKIPPED1 - skipped path1 as it is a special file
+ * D_SKIPPED2 - skipped path2 as it is a special file
+ */
+
+#define D_SAME  0
+#define D_DIFFER        1
+#define D_BINARY        (1<<1)
+#define D_COMMON        (1<<2)
+#define D_ONLY          (1<<3)
+#define D_MISMATCH1     (1<<4)
+#define D_MISMATCH2     (1<<5)
+#define D_ERROR         (1<<6)
+#define D_SKIPPED1      (1<<7)
+#define D_SKIPPED2      (1<<8)
+
+/* Command line options */
+static unsigned long cmd_flags;
+#define FLAG_a  1
+#define FLAG_b  (1<<1)
+#define FLAG_d  (1<<2)
+#define FLAG_i  (1<<3)
+#define FLAG_N  (1<<4)
+#define FLAG_q  (1<<5)
+#define FLAG_r  (1<<6)
+#define FLAG_s  (1<<7)
+#define FLAG_S  (1<<8)
+#define FLAG_t  (1<<9)
+#define FLAG_T  (1<<10)
+#define FLAG_U  (1<<11)
+#define FLAG_w  (1<<12)
+
+int context, status;
+char *start, *label[2];
+struct stat stb1, stb2;
+char **dl;
+int dl_count = 0;
+
+struct cand {
+        int x;
+        int y;
+        int pred;
+};
+
+struct line {
+        int serial;
+        int value;
+} *file[2];
+
+/*
+ * The following struct is used to record change information
+ * doing a "context" or "unified" diff.  (see routine "change" to
+ * understand the highly mnemonic field names)
+ */
+struct context_vec {
+        int a;                        /* start line in old file */
+        int b;                        /* end line in old file */
+        int c;                        /* start line in new file */
+        int d;                        /* end line in new file */
+};
+
+static int  *J;                 /* will be overlaid on class */
+static int  *class;             /* will be overlaid on file[0] */
+static int  *klist;             /* will be overlaid on file[0] after class */
+static int  *member;            /* will be overlaid on file[1] */
+static int   clen;
+static int   len[2];
+static int   pref, suff;        /* length of prefix and suffix */
+static int   slen[2];
+static int   anychange;
+static long *ixnew;             /* will be overlaid on file[1] */
+static long *ixold;             /* will be overlaid on klist */
+static struct cand *clist;      /* merely a free storage pot for candidates */
+static int   clistlen;          /* the length of clist */
+static struct line *sfile[2];   /* shortened by pruning common prefix/suffix */
+static struct context_vec *context_vec_start;
+static struct context_vec *context_vec_end;
+static struct context_vec *context_vec_ptr;
+
+static void output(char *, FILE *, char *, FILE *);
+static void check(char *, FILE *, char *, FILE *);
+static void uni_range(int, int);
+static void dump_unified_vec(FILE *, FILE *);
+static void prepare(int, FILE *, off_t);
+static void prune(void);
+static void equiv(struct line *, int, struct line *, int, int *);
+static void unravel(int);
+static void unsort(struct line *, int, int *);
+static void change(char *, FILE *, char *, FILE *, int, int, int, int);
+static void sort(struct line *, int);
+static void print_header(const char *, const char *);
+static int  asciifile(FILE *);
+static int  fetch(long *, int, int, FILE *, int, int);
+static int  newcand(int, int, int);
+static int  search(int *, int, int);
+static int  skipline(FILE *);
+static int  stone(int *, int, int *, int *);
+static int  readhash(FILE *);
+static int  files_differ(FILE *, FILE *, int);
+
+static int     diffreg(char *, char *, int);
+static void    print_only(const char *, size_t, const char *);
+static void    print_status(int, char *, char *, char *);
+
+#ifdef CONFIG_FEATURE_DIFF_DIR
+static int dir_strcmp(const void *p1, const void *p2) {
+        return strcmp(*(char * const *)p1, *(char * const *)p2);
+}
+
+/* This function adds a filename to dl, the directory listing. */
+
+static int add_to_dirlist (const char *filename, struct stat *statbuf, void *userdata) {
+        dl_count++;
+        dl = xrealloc(dl, dl_count * sizeof(char *));
+        dl[dl_count - 1] = bb_xstrdup(filename);
+        if (cmd_flags & FLAG_r) {
+                int *pp = (int *) userdata;
+                int path_len = *pp + 1;
+                dl[dl_count - 1] = &(dl[dl_count - 1])[path_len];
+        }
+        return TRUE;
+}
+
+/* This returns a sorted directory listing. */
+static char **get_dir(char *path) {
+
+        int i;
+
+        /* Reset dl_count - there's no need to free dl as bb_xrealloc does
+         * the job nicely. */   
+        dl_count = 0;
+
+        /* If -r has been set, then the recursive_action function will be
+         * used. Unfortunately, this outputs the root directory along with
+         * the recursed paths, so use void *userdata to specify the string
+         * length of the root directory. It can then be removed in
+         * add_to_dirlist. */
+
+        int path_len = strlen(path);
+        void *userdata = &path_len;
+        
+        /* Now fill dl with a listing. */
+        if (cmd_flags & FLAG_r)
+                recursive_action(path, TRUE, TRUE, FALSE, add_to_dirlist, NULL, userdata);
+        else {
+                DIR *dp;
+                struct dirent *ep;
+                if ((dp = opendir(path)) == NULL)
+                        bb_error_msg("Error reading directory");
+                while ((ep = readdir(dp))) {
+                        if ((!strcmp(ep->d_name, "..")) || (!strcmp(ep->d_name, ".")))
+                                continue;
+                        add_to_dirlist(ep->d_name, NULL, NULL);
+                }
+                closedir(dp);
+        }
+
+        /* Sort dl alphabetically. */
+        qsort(dl, dl_count, sizeof(char *), dir_strcmp);
+
+        /* Copy dl so that we can return it. */
+        char **retval = xmalloc(dl_count * sizeof(char *));
+        for (i = 0; i < dl_count; i++)
+                retval[i] = bb_xstrdup(dl[i]);
+
+        return retval;
+}
+
+static void do_diff (char *dir1, char *path1, char *dir2, char *path2) {
+        
+        int flags = D_HEADER;
+        int val;
+        
+        char *fullpath1 = bb_xasprintf("%s/%s", dir1, path1);
+        char *fullpath2 = bb_xasprintf("%s/%s", dir2, path2);
+
+        if (stat(fullpath1, &stb1) != 0) {
+                flags |= D_EMPTY1;
+                memset(&stb1, 0, sizeof(stb1));
+                fullpath1 = bb_xasprintf("%s/%s", dir1, path2);
+        }
+        if (stat(fullpath2, &stb2) != 0) {
+                flags |= D_EMPTY2;
+                memset(&stb2, 0, sizeof(stb2));
+                stb2.st_mode = stb1.st_mode;
+                fullpath2 = bb_xasprintf("%s/%s", dir2, path1);
+        }
+
+        if (stb1.st_mode == 0)
+                stb1.st_mode = stb2.st_mode;
+        
+        if (S_ISDIR(stb1.st_mode) && S_ISDIR(stb2.st_mode)) {
+                printf("Common subdirectories: %s and %s\n", fullpath1, fullpath2);
+                return;
+        }
+
+        if (!S_ISREG(stb1.st_mode) && !S_ISDIR(stb1.st_mode))
+                val = D_SKIPPED1;
+        else if (!S_ISREG(stb2.st_mode) && !S_ISDIR(stb2.st_mode))
+                val = D_SKIPPED2;
+        else
+                val = diffreg(fullpath1, fullpath2, flags);
+        
+        print_status(val, fullpath1, fullpath2, NULL);
+}
+
+static void diffdir (char *p1, char *p2) {
+        
+        char **dirlist1, **dirlist2;
+        char *dp1, *dp2;
+        int dirlist1_count, dirlist2_count;
+        int pos;
+
+        /* Check for trailing slashes. */
+        
+        if (p1[strlen(p1) - 1] == '/')
+                p1[strlen(p1) - 1] = '\0';
+        if (p2[strlen(p2) - 1] == '/')
+                p2[strlen(p2) - 1] = '\0';
+        
+        /* Get directory listings for p1 and p2. */
+        
+        dirlist1 = get_dir(p1);
+        dirlist1_count = dl_count;
+        dirlist1[dirlist1_count] = NULL;
+        dirlist2 = get_dir(p2);
+        dirlist2_count = dl_count;
+        dirlist2[dirlist2_count] = NULL;
+        
+        /* If -S was set, find the starting point. */
+        if (start) {
+                while (*dirlist1 != NULL && strcmp(*dirlist1, start) < 0)
+                        dirlist1++;
+                while (*dirlist2 != NULL && strcmp(*dirlist2, start) < 0)
+                        dirlist2++;
+                if ((*dirlist1 == NULL) || (*dirlist2 == NULL))
+                        bb_error_msg("Invalid argument to -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 (*dirlist1 != NULL || *dirlist2 != NULL) {
+                dp1 = *dirlist1;
+                dp2 = *dirlist2;
+                pos = dp1 == NULL ? 1 : dp2 == NULL ? -1 : strcmp(dp1, dp2);
+                if (pos == 0) {
+                        do_diff(p1, dp1, p2, dp2);
+                        dirlist1++;
+                        dirlist2++;
+                }
+                else if (pos < 0) {
+                        if (cmd_flags & FLAG_N)
+                                do_diff(p1, dp1, p2, NULL);
+                        else
+                                print_only(p1, strlen(p1) + 1, dp1);
+                        dirlist1++;
+                }
+                else {
+                        if (cmd_flags & FLAG_N)
+                                do_diff(p1, NULL, p2, dp2);
+                        else
+                                print_only(p2, strlen(p2) + 1, dp2);
+                        dirlist2++;
+                }
+        }
+}
+#endif
+
+static void
+print_only(const char *path, size_t dirlen, const char *entry)
+{
+        if (dirlen > 1)
+                dirlen--;
+        printf("Only in %.*s: %s\n", (int)dirlen, path, entry);
+}
+
+static void
+print_status(int val, char *path1, char *path2, char *entry)
+{
+        switch (val) {
+        case D_ONLY:
+                print_only(path1, strlen(path1), entry);
+                break;
+        case D_COMMON:
+                printf("Common subdirectories: %s%s and %s%s\n",
+                    path1, entry ? entry : "", path2, entry ? entry : "");
+                break;
+        case D_BINARY:
+                printf("Binary files %s%s and %s%s differ\n",
+                    path1, entry ? entry : "", path2, entry ? entry : "");
+                break;
+        case D_DIFFER:
+                if (cmd_flags & FLAG_q)
+                        printf("Files %s%s and %s%s differ\n",
+                            path1, entry ? entry : "",
+                            path2, entry ? entry : "");
+                break;
+        case D_SAME:
+                if (cmd_flags & FLAG_s)
+                        printf("Files %s%s and %s%s are identical\n",
+                            path1, entry ? entry : "",
+                            path2, entry ? entry : "");
+                break;
+        case D_MISMATCH1:
+                printf("File %s%s is a directory while file %s%s is a regular file\n",
+                    path1, entry ? entry : "", path2, entry ? entry : "");
+                break;
+        case D_MISMATCH2:
+                printf("File %s%s is a regular file while file %s%s is a directory\n",
+                    path1, entry ? entry : "", path2, entry ? entry : "");
+                break;
+        case D_SKIPPED1:
+                printf("File %s%s is not a regular file or directory and was skipped\n",
+                    path1, entry ? entry : "");
+                break;
+        case D_SKIPPED2:
+                printf("File %s%s is not a regular file or directory and was skipped\n",
+                    path2, entry ? entry : "");
+                break;
+        }
+}
+
+/*
+ *      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 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.
+ */
+
+static int diffreg(char *ofile1, char *ofile2, int flags)
+{
+        char *file1 = ofile1;
+        char *file2 = ofile2;
+        FILE *f1 = NULL;
+        FILE *f2 = NULL;
+        int rval = D_SAME;
+        int i;
+
+        anychange = 0;
+        context_vec_ptr = context_vec_start - 1;
+                
+        if (S_ISDIR(stb1.st_mode) != S_ISDIR(stb2.st_mode))
+                return (S_ISDIR(stb1.st_mode) ? D_MISMATCH1 : D_MISMATCH2);
+        if (strcmp(file1, "-") == 0 && strcmp(file2, "-") == 0)
+                goto closem;
+
+        if (flags & D_EMPTY1)
+                f1 = bb_xfopen(_PATH_DEVNULL, "r");
+        else {
+                if (strcmp(file1, "-") == 0)
+                        f1 = stdin;
+                else
+                        f1 = bb_xfopen(file1, "r");
+        }
+
+        if (flags & D_EMPTY2)
+                f2 = bb_xfopen(_PATH_DEVNULL, "r");
+        else {
+                if (strcmp(file2, "-") == 0)
+                        f2 = stdin;
+                else
+                        f2 = bb_xfopen(file2, "r");
+        }
+        
+        switch (files_differ(f1, f2, flags)) {
+        case 0:
+                goto closem;
+        case 1:
+                break;
+        default:
+                /* error */
+                status |= 2;
+                goto closem;
+        }
+
+        if (!asciifile(f1) || !asciifile(f2)) {
+                rval = D_BINARY;
+                status |= 1;
+                goto closem;
+        }
+
+        prepare(0, f1, stb1.st_size);
+        prepare(1, f2, stb2.st_size);
+        prune();
+        sort(sfile[0], slen[0]);
+        sort(sfile[1], slen[1]);
+
+        member = (int *)file[1];
+        equiv(sfile[0], slen[0], sfile[1], slen[1], member);
+        member = xrealloc(member, (slen[1] + 2) * sizeof(int));
+
+        class = (int *)file[0];
+        unsort(sfile[0], slen[0], class);
+        class = xrealloc(class, (slen[0] + 2) * sizeof(int));
+
+        klist = xmalloc((slen[0] + 2) * sizeof(int));
+        clen = 0;
+        clistlen = 100;
+        clist = xmalloc(clistlen * sizeof(struct cand));
+        i = stone(class, slen[0], member, klist);
+        free(member);
+        free(class);
+
+        J = xrealloc(J, (len[0] + 2) * sizeof(int));
+        unravel(klist[i]);
+        free(clist);
+        free(klist);
+
+        ixold = xrealloc(ixold, (len[0] + 2) * sizeof(long));
+        ixnew = xrealloc(ixnew, (len[1] + 2) * sizeof(long));
+        check(file1, f1, file2, f2);
+        output(file1, f1, file2, f2);
+
+closem:
+        if (anychange) {
+                status |= 1;
+                if (rval == D_SAME)
+                        rval = D_DIFFER;
+        }
+        if (f1 != NULL)
+                fclose(f1);
+        if (f2 != NULL)
+                fclose(f2);
+        if (file1 != ofile1)
+                free(file1);
+        if (file2 != ofile2)
+                free(file2);
+        return (rval);
+}
+
+/*
+ * Check to see if the given files differ.
+ * Returns 0 if they are the same, 1 if different, and -1 on error.
+ */
+static int
+files_differ(FILE *f1, FILE *f2, int flags)
+{
+        char buf1[BUFSIZ], buf2[BUFSIZ];
+        size_t i, j;
+
+        if ((flags & (D_EMPTY1|D_EMPTY2)) || stb1.st_size != stb2.st_size ||
+            (stb1.st_mode & S_IFMT) != (stb2.st_mode & S_IFMT))
+                return (1);
+        while(1) {
+                i = fread(buf1, 1, sizeof(buf1), f1);
+                j = fread(buf2, 1, sizeof(buf2), f2);
+                if (i != j)
+                        return (1);
+                if (i == 0 && j == 0) {
+                        if (ferror(f1) || ferror(f2))
+                                return (1);
+                        return (0);
+                }
+                if (memcmp(buf1, buf2, i) != 0)
+                        return (1);
+        }
+}
+
+static void
+prepare(int i, FILE *fd, off_t filesize)
+{
+        struct line *p;
+        int j, h;
+        size_t sz;
+
+        rewind(fd);
+
+        sz = (filesize <= FSIZE_MAX ? filesize : FSIZE_MAX) / 25;
+        if (sz < 100)
+                sz = 100;
+
+        p = xmalloc((sz + 3) * sizeof(struct line));
+        for (j = 0; (h = readhash(fd));) {
+                if (j == sz) {
+                        sz = sz * 3 / 2;
+                        p = xrealloc(p, (sz + 3) * sizeof(struct line));
+                }
+                p[++j].value = h;
+        }
+        len[i] = j;
+        file[i] = p;
+}
+
+static void
+prune(void)
+{
+        int i, j;
+
+        for (pref = 0; pref < len[0] && pref < len[1] &&
+            file[0][pref + 1].value == file[1][pref + 1].value;
+            pref++)
+                ;
+        for (suff = 0; suff < len[0] - pref && suff < len[1] - pref &&
+            file[0][len[0] - suff].value == file[1][len[1] - suff].value;
+            suff++)
+                ;
+        for (j = 0; j < 2; j++) {
+                sfile[j] = file[j] + pref;
+                slen[j] = len[j] - pref - suff;
+                for (i = 0; i <= slen[j]; i++)
+                        sfile[j][i].serial = i;
+        }
+}
+
+static void
+equiv(struct line *a, int n, struct line *b, int m, int *c)
+{
+        int i, j;
+
+        i = 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 int isqrt(int n) {
+        int y, x = 1;
+        if (n == 0) return(0);
+        
+        do {
+                y = x;
+                x = n / x;
+                x += y;
+                x /= 2;
+        } while ((x - y) > 1 || (x - y) < -1);
+
+        return (x);
+}
+
+static int
+stone(int *a, int n, int *b, int *c)
+{
+        int i, k, y, j, l;
+        int oldc, tc, oldl;
+        u_int numtries;
+#ifdef CONFIG_FEATURE_DIFF_MINIMAL
+        const u_int bound = (cmd_flags & FLAG_d) ? UINT_MAX : MAX(256, isqrt(n));
+#else
+        const u_int bound = MAX(256, isqrt(n));
+#endif
+        k = 0;
+        c[0] = newcand(0, 0, 0);
+        for (i = 1; i <= n; i++) {
+                j = a[i];
+                if (j == 0)
+                        continue;
+                y = -b[j];
+                oldl = 0;
+                oldc = c[0];
+                numtries = 0;
+                do {
+                        if (y <= clist[oldc].y)
+                                continue;
+                        l = search(c, k, y);
+                        if (l != oldl + 1)
+                                oldc = c[l - 1];
+                        if (l <= k) {
+                                if (clist[c[l]].y <= y)
+                                        continue;
+                                tc = c[l];
+                                c[l] = newcand(i, y, oldc);
+                                oldc = tc;
+                                oldl = l;
+                                numtries++;
+                        } else {
+                                c[l] = newcand(i, y, oldc);
+                                k++;
+                                break;
+                        }
+                } while ((y = b[++j]) > 0 && numtries < bound);
+        }
+        return (k);
+}
+
+static int
+newcand(int x, int y, int pred)
+{
+        struct cand *q;
+
+        if (clen == clistlen) {
+                clistlen = clistlen * 11 / 10;
+                clist = xrealloc(clist, clistlen * sizeof(struct cand));
+        }
+        q = clist + clen;
+        q->x = x;
+        q->y = y;
+        q->pred = pred;
+        return (clen++);
+}
+
+static int
+search(int *c, int k, int y)
+{
+        int i, j, l, t;
+
+        if (clist[c[k]].y < y)        /* quick look for typical case */
+                return (k + 1);
+        i = 0;
+        j = k + 1;
+        while (1) {
+                l = i + j;
+                if ((l >>= 1) <= i)
+                        break;
+                t = clist[c[l]].y;
+                if (t > y)
+                        j = l;
+                else if (t < y)
+                        i = l;
+                else
+                        return (l);
+        }
+        return (l + 1);
+}
+
+static void
+unravel(int p)
+{
+        struct cand *q;
+        int i;
+
+        for (i = 0; i <= len[0]; i++)
+                J[i] = i <= pref ? i :
+                    i > len[0] - suff ? i + len[1] - len[0] : 0;
+        for (q = clist + p; q->y != 0; q = clist + q->pred)
+                J[q->x + pref] = q->y + pref;
+}
+
+/*
+ * Check does double duty:
+ *  1.  ferret out any fortuitous correspondences due
+ *      to confounding by hashing (which result in "jackpot")
+ *  2.  collect random access indexes to the two files
+ */
+static void
+check(char *file1, FILE *f1, char *file2, FILE *f2)
+{
+        int i, j, jackpot, c, d;
+        long ctold, ctnew;
+
+        rewind(f1);
+        rewind(f2);
+        j = 1;
+        ixold[0] = ixnew[0] = 0;
+        jackpot = 0;
+        ctold = ctnew = 0;
+        for (i = 1; i <= len[0]; i++) {
+                if (J[i] == 0) {
+                        ixold[i] = ctold += skipline(f1);
+                        continue;
+                }
+                while (j < J[i]) {
+                        ixnew[j] = ctnew += skipline(f2);
+                        j++;
+                }
+                if ((cmd_flags & FLAG_b) || (cmd_flags & FLAG_w) || (cmd_flags & FLAG_i)) {
+                        while (1) {
+                                c = getc(f1);
+                                d = getc(f2);
+                                /*
+                                 * GNU diff ignores a missing newline
+                                 * in one file if bflag || wflag.
+                                 */
+                                if (((cmd_flags & FLAG_b) || (cmd_flags & FLAG_w)) &&
+                                    ((c == EOF && d == '\n') ||
+                                    (c == '\n' && d == EOF))) {
+                                        break;
+                                }
+                                ctold++;
+                                ctnew++;
+                                if ((cmd_flags & FLAG_b) && isspace(c) && isspace(d)) {
+                                        do {
+                                                if (c == '\n')
+                                                        break;
+                                                ctold++;
+                                        } while (isspace(c = getc(f1)));
+                                        do {
+                                                if (d == '\n')
+                                                        break;
+                                                ctnew++;
+                                        } while (isspace(d = getc(f2)));
+                                } else if (cmd_flags & FLAG_w) {
+                                        while (isspace(c) && c != '\n') {
+                                                c = getc(f1);
+                                                ctold++;
+                                        }
+                                        while (isspace(d) && d != '\n') {
+                                                d = getc(f2);
+                                                ctnew++;
+                                        }
+                                }
+                                if (c != d) {
+                                        jackpot++;
+                                        J[i] = 0;
+                                        if (c != '\n' && c != EOF)
+                                                ctold += skipline(f1);
+                                        if (d != '\n' && c != EOF)
+                                                ctnew += skipline(f2);
+                                        break;
+                                }
+                                if (c == '\n' || c == EOF)
+                                        break;
+                        }
+                } else {
+                        while (1) {
+                                ctold++;
+                                ctnew++;
+                                if ((c = getc(f1)) != (d = getc(f2))) {
+                                        J[i] = 0;
+                                        if (c != '\n' && c != EOF)
+                                                ctold += skipline(f1);
+                                        if (d != '\n' && c != EOF)
+                                                ctnew += skipline(f2);
+                                        break;
+                                }
+                                if (c == '\n' || c == EOF)
+                                        break;
+                        }
+                }
+                ixold[i] = ctold;
+                ixnew[j] = ctnew;
+                j++;
+        }
+        for (; j <= len[1]; j++)
+                ixnew[j] = ctnew += skipline(f2);
+}
+
+/* shellsort CACM #201 */
+static void
+sort(struct line *a, int n)
+{
+        struct line *ai, *aim, w;
+        int j, m = 0, k;
+
+        if (n == 0)
+                return;
+        for (j = 1; j <= n; j *= 2)
+                m = 2 * j - 1;
+        for (m /= 2; m != 0; m /= 2) {
+                k = n - m;
+                for (j = 1; j <= k; j++) {
+                        for (ai = &a[j]; ai > a; ai -= m) {
+                                aim = &ai[m];
+                                if (aim < ai)
+                                        break;        /* wraparound */
+                                if (aim->value > ai[0].value ||
+                                    (aim->value == ai[0].value &&
+                                        aim->serial > ai[0].serial))
+                                        break;
+                                w.value = ai[0].value;
+                                ai[0].value = aim->value;
+                                aim->value = w.value;
+                                w.serial = ai[0].serial;
+                                ai[0].serial = aim->serial;
+                                aim->serial = w.serial;
+                        }
+                }
+        }
+}
+
+static void
+unsort(struct line *f, int l, int *b)
+{
+        int *a, i;
+
+        a = xmalloc((l + 1) * sizeof(int));
+        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
+skipline(FILE *f)
+{
+        int i, c;
+
+        for (i = 1; (c = getc(f)) != '\n' && c != EOF; i++)
+                continue;
+        return (i);
+}
+
+static void
+output(char *file1, FILE *f1, char *file2, FILE *f2)
+{
+        int m, i0, i1, j0, j1;
+
+        rewind(f1);
+        rewind(f2);
+        m = len[0];
+        J[0] = 0;
+        J[m + 1] = len[1] + 1;
+        for (i0 = 1; i0 <= m; i0 = i1 + 1) {
+                        while (i0 <= m && J[i0] == J[i0 - 1] + 1)
+                                i0++;
+                        j0 = J[i0 - 1] + 1;
+                        i1 = i0 - 1;
+                        while (i1 < m && J[i1 + 1] == 0)
+                                i1++;
+                        j1 = J[i1 + 1] - 1;
+                        J[i1] = j1;
+                        change(file1, f1, file2, f2, i0, i1, j0, j1);
+        }
+        if (m == 0) {
+                change(file1, f1, file2, f2, 1, 0, 1, len[1]);
+        }
+        if (anychange != 0) {
+                dump_unified_vec(f1, f2);
+        }
+}
+
+static void uni_range(int a, int b)
+{
+        if (a < b)
+                printf("%d,%d", a, b - a + 1);
+        else if (a == b)
+                printf("%d", b);
+        else
+                printf("%d,0", b);
+}
+
+/*
+ * 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 then 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.
+ */
+static void
+change(char *file1, FILE *f1, char *file2, FILE *f2, int a, int b, int c, int d)
+{
+        static size_t max_context = 64;
+
+        if (a > b && c > d)     return;
+        if (cmd_flags & FLAG_q) return;
+        
+        /*
+        * Allocate change records as needed.
+        */
+                if (context_vec_ptr == context_vec_end - 1) {
+                        ptrdiff_t offset = context_vec_ptr - context_vec_start;
+                        max_context <<= 1;
+                        context_vec_start = xrealloc(context_vec_start,
+                            max_context * sizeof(struct context_vec));
+                        context_vec_end = context_vec_start + max_context;
+                        context_vec_ptr = context_vec_start + offset;
+                }
+                if (anychange == 0) {
+                        /*
+                         * Print the context/unidiff header first time through.
+                         */
+                        print_header(file1, file2);
+                        anychange = 1;
+                } else if (a > context_vec_ptr->b + (2 * context) + 1 &&
+                    c > context_vec_ptr->d + (2 * context) + 1) {
+                        /*
+                         * If this change is more than 'context' lines from the
+                         * previous change, dump the record and reset it.
+                         */
+                        dump_unified_vec(f1, f2);
+                }
+                context_vec_ptr++;
+                context_vec_ptr->a = a;
+                context_vec_ptr->b = b;
+                context_vec_ptr->c = c;
+                context_vec_ptr->d = d;
+                return;
+        
+}
+
+static int
+fetch(long *f, int a, int b, FILE *lb, int ch, int oldfile)
+{
+        int i, j, c, lastc, col, nc;
+
+        if (a > b)
+                return (0);
+        for (i = a; i <= b; i++) {
+                fseek(lb, f[i - 1], SEEK_SET);
+                nc = f[i] - f[i - 1];
+                if (ch != '\0') {
+                        putchar(ch);
+                        if (cmd_flags & FLAG_T)
+                                putchar('\t');
+                }
+                col = 0;
+                for (j = 0, lastc = '\0'; j < nc; j++, lastc = c) {
+                        if ((c = getc(lb)) == EOF) {
+                                puts("\n\\ No newline at end of file");
+                                return (0);
+                        }
+                        if (c == '\t' && (cmd_flags & FLAG_t)) {
+                                do {
+                                        putchar(' ');
+                                } while (++col & 7);
+                        } else {
+                                putchar(c);
+                                col++;
+                        }
+                }
+        }
+        return (0);
+}
+
+/*
+ * Hash function taken from Robert Sedgewick, Algorithms in C, 3d ed., p 578.
+ */
+static int
+readhash(FILE *f)
+{
+        int i, t, space;
+        int sum;
+
+        sum = 1;
+        space = 0;
+        if (!(cmd_flags & FLAG_b) && !(cmd_flags & FLAG_w)) {
+                if (FLAG_i)
+                        for (i = 0; (t = getc(f)) != '\n'; i++) {
+                                if (t == EOF) {
+                                        if (i == 0)
+                                                return (0);
+                                        break;
+                                }
+                                sum = sum * 127 + t;
+                        }
+                else
+                        for (i = 0; (t = getc(f)) != '\n'; i++) {
+                                if (t == EOF) {
+                                        if (i == 0)
+                                                return (0);
+                                        break;
+                                }
+                                sum = sum * 127 + t;
+                        }
+        } else {
+                for (i = 0;;) {
+                        switch (t = getc(f)) {
+                        case '\t':
+                        case '\r':
+                        case '\v':
+                        case '\f':
+                        case ' ':
+                                space++;
+                                continue;
+                        default:
+                                if (space && !(cmd_flags & FLAG_w)) {
+                                        i++;
+                                        space = 0;
+                                }
+                                sum = sum * 127 + t;
+                                i++;
+                                continue;
+                        case EOF:
+                                if (i == 0)
+                                        return (0);
+                                /* FALLTHROUGH */
+                        case '\n':
+                                break;
+                        }
+                        break;
+                }
+        }
+        /*
+         * There is a remote possibility that we end up with a zero sum.
+         * Zero is used as an EOF marker, so return 1 instead.
+         */
+        return (sum == 0 ? 1 : sum);
+}
+
+static int
+asciifile(FILE *f)
+{
+
+        if ((cmd_flags & FLAG_a) || f == NULL)
+                return (1);
+#ifdef CONFIG_FEATURE_DIFF_BINARY
+        unsigned char buf[BUFSIZ];
+        int i, cnt;
+        
+        rewind(f);
+        cnt = fread(buf, 1, sizeof(buf), f);
+        for (i = 0; i < cnt; i++)
+                if (!isprint(buf[i]) && !isspace(buf[i]))
+                        return (0);
+#endif
+        return (1);
+}
+
+/* dump accumulated "unified" diff changes */
+static void
+dump_unified_vec(FILE *f1, FILE *f2)
+{
+        struct context_vec *cvp = context_vec_start;
+        int lowa, upb, lowc, upd;
+        int a, b, c, d;
+        char ch;
+
+        if (context_vec_start > context_vec_ptr)
+                return;
+
+        b = d = 0;                /* gcc */
+        lowa = MAX(1, cvp->a - context);
+        upb = MIN(len[0], context_vec_ptr->b + context);
+        lowc = MAX(1, cvp->c - context);
+        upd = MIN(len[1], context_vec_ptr->d + context);
+
+        fputs("@@ -", stdout);
+        uni_range(lowa, upb);
+        fputs(" +", stdout);
+        uni_range(lowc, upd);
+        fputs(" @@", stdout);
+        putchar('\n');
+
+        /*
+         * Output changes in "unified" diff format--the old and new lines
+         * are printed together.
+         */
+        for (; cvp <= context_vec_ptr; cvp++) {
+                a = cvp->a;
+                b = cvp->b;
+                c = cvp->c;
+                d = cvp->d;
+
+                /*
+                 * c: both new and old changes
+                 * d: only changes in the old file
+                 * a: only changes in the new file
+                 */
+                if (a <= b && c <= d)
+                        ch = 'c';
+                else
+                        ch = (a <= b) ? 'd' : 'a';
+
+                switch (ch) {
+                case 'c':
+                        fetch(ixold, lowa, a - 1, f1, ' ', 0);
+                        fetch(ixold, a, b, f1, '-', 0);
+                        fetch(ixnew, c, d, f2, '+', 0);
+                        break;
+                case 'd':
+                        fetch(ixold, lowa, a - 1, f1, ' ', 0);
+                        fetch(ixold, a, b, f1, '-', 0);
+                        break;
+                case 'a':
+                        fetch(ixnew, lowc, c - 1, f2, ' ', 0);
+                        fetch(ixnew, c, d, f2, '+', 0);
+                        break;
+                }
+                lowa = b + 1;
+                lowc = d + 1;
+        }
+        fetch(ixnew, d + 1, upd, f2, ' ', 0);
+
+        context_vec_ptr = context_vec_start - 1;
+}
+
+static void
+print_header(const char *file1, const char *file2)
+{
+        if (label[0] != NULL)
+                printf("%s %s\n", "---",
+                    label[0]);
+        else
+                printf("%s %s\t%s", "---",
+                    file1, ctime(&stb1.st_mtime));
+        if (label[1] != NULL)
+                printf("%s %s\n", "+++",
+                    label[1]);
+        else
+                printf("%s %s\t%s", "+++",
+                    file2, ctime(&stb2.st_mtime));
+}
+
+extern int diff_main(int argc, char **argv) {
+        char *ep;
+        int gotstdin = 0;
+
+        char *U_opt;
+        cmd_flags = bb_getopt_ulflags(argc, argv, "abdiNqrsS:tTU:wu", &start, &U_opt);
+        
+        context = 3;    /* This is the default number of lines of context. */
+        if (cmd_flags & FLAG_U) {
+                context = strtol(U_opt, &ep, 10);
+                if (context == 0) {
+                        bb_error_msg("Invalid context length");
+                        bb_show_usage();
+                }
+        }
+        argc -= optind;
+        argv += optind;
+
+        /*
+         * Do sanity checks, fill in stb1 and stb2 and call the appropriate
+         * driver routine.  Both drivers use the contents of stb1 and stb2.
+         */
+        if (argc < 2) {
+                bb_error_msg("Missing filename");
+                bb_show_usage();
+        }
+        if (strcmp(argv[0], "-") == 0) {
+                fstat(STDIN_FILENO, &stb1);
+                gotstdin = 1;
+        } else if (stat(argv[0], &stb1) != 0)
+                bb_perror_msg_and_die("Couldn't stat %s", argv[0]);
+        if (strcmp(argv[1], "-") == 0) {
+                fstat(STDIN_FILENO, &stb2);
+                gotstdin = 1;
+        } else if (stat(argv[1], &stb2) != 0)
+                bb_perror_msg_and_die("Couldn't stat %s", argv[1]);
+        if (gotstdin && (S_ISDIR(stb1.st_mode) || S_ISDIR(stb2.st_mode)))
+                bb_error_msg_and_die("Can't compare - to a directory");
+        if (S_ISDIR(stb1.st_mode) && S_ISDIR(stb2.st_mode)) {
+#ifdef CONFIG_FEATURE_DIFF_DIR
+                diffdir(argv[0], argv[1]);
+#else
+                bb_error_msg_and_die("Directory comparison not supported");
+#endif
+        }
+        else {  
+                if (S_ISDIR(stb1.st_mode)) {
+                        argv[0] = concat_path_file(argv[0], argv[1]);
+                        if (stat(argv[0], &stb1) < 0)
+                                bb_perror_msg_and_die("Couldn't stat %s", argv[0]);
+                }
+                if (S_ISDIR(stb2.st_mode)) {
+                        argv[1] = concat_path_file(argv[1], argv[0]);
+                        if (stat(argv[1], &stb2) < 0)
+                                bb_perror_msg_and_die("Couldn't stat %s", argv[1]);
+                }
+                print_status(diffreg(argv[0], argv[1], 0), argv[0], argv[1], NULL);
+        }
+        exit(status);
+}
+
diff --git a/include/applets.h b/include/applets.h
index 0f239822c..f0c6f7a0e 100644
--- a/include/applets.h
+++ b/include/applets.h
@@ -86,6 +86,7 @@ USE_DELGROUP(APPLET(delgroup, delgroup_main, _BB_DIR_BIN, _BB_SUID_NEVER))
 USE_DELUSER(APPLET(deluser, deluser_main, _BB_DIR_BIN, _BB_SUID_NEVER))
 USE_DEVFSD(APPLET(devfsd, devfsd_main, _BB_DIR_SBIN, _BB_SUID_NEVER))
 USE_DF(APPLET(df, df_main, _BB_DIR_BIN, _BB_SUID_NEVER))
+USE_DIFF(APPLET(diff, diff_main, _BB_DIR_USR_BIN, _BB_SUID_NEVER))
 USE_DIRNAME(APPLET(dirname, dirname_main, _BB_DIR_USR_BIN, _BB_SUID_NEVER))
 USE_DMESG(APPLET(dmesg, dmesg_main, _BB_DIR_BIN, _BB_SUID_NEVER))
 USE_DNSD(APPLET(dnsd, dnsd_main, _BB_DIR_USR_SBIN, _BB_SUID_ALWAYS))
diff --git a/include/usage.h b/include/usage.h
index 0ac0284ce..2eb548bcf 100644
--- a/include/usage.h
+++ b/include/usage.h
@@ -453,6 +453,26 @@
         "Filesystem           1k-blocks      Used Available Use% Mounted on\n" \
         "/dev/sda3              8690864   8553540    137324  98% /\n"
 
+#define diff_trivial_usage \
+        "[-abdiNqrTstw] [-S FILE] [-U LINES] FILE1 FILE2"
+#define diff_full_usage \
+        "Compare files line by line and output the differences between them.\n" \
+        "This diff implementation only supports unified diffs.\n\n" \
+        "Options:\n" \
+        "\t-a\tTreat all files as text\n" \
+        "\t-b\tIgnore changes in the amount of whitespace\n" \
+        "\t-d\tTry hard to find a smaller set of changes\n" \
+        "\t-i\tIgnore case differences\n" \
+        "\t-N\tTreat absent files as empty\n" \
+        "\t-q\tOutput only whether files differ\n" \
+        "\t-r\tRecursively compare any subdirectories\n" \
+        "\t-S\tStart with FILE when comparing directories\n" \
+        "\t-T\tMake tabs line up by prefixing a tab when necessary\n" \
+        "\t-s\tReport when two files are the same\n" \
+        "\t-t\tExpand tabs to spaces in output\n" \
+        "\t-U\tOutput LINES lines of context\n" \
+        "\t-w\tIgnore all whitespace\n"
+
 #define dirname_trivial_usage \
         "FILENAME"
 #define dirname_full_usage \