/*
 * Utilities for paths and pathnames
 */
#include "cache.h"
#include "strbuf.h"
#include "string-list.h"
#include "dir.h"

static int get_st_mode_bits(const char *path, int *mode)
{
        struct stat st;
        if (lstat(path, &st) < 0)
                return -1;
        *mode = st.st_mode;
        return 0;
}

static char bad_path[] = "/bad-path/";

static struct strbuf *get_pathname(void)
{
        static struct strbuf pathname_array[4] = {
                STRBUF_INIT, STRBUF_INIT, STRBUF_INIT, STRBUF_INIT
        };
        static int index;
        struct strbuf *sb = &pathname_array[3 & ++index];
        strbuf_reset(sb);
        return sb;
}

static char *cleanup_path(char *path)
{
        /* Clean it up */
        if (!memcmp(path, "./", 2)) {
                path += 2;
                while (*path == '/')
                        path++;
        }
        return path;
}

static void strbuf_cleanup_path(struct strbuf *sb)
{
        char *path = cleanup_path(sb->buf);
        if (path > sb->buf)
                strbuf_remove(sb, 0, path - sb->buf);
}

char *mksnpath(char *buf, size_t n, const char *fmt, ...)
{
        va_list args;
        unsigned len;

        va_start(args, fmt);
        len = vsnprintf(buf, n, fmt, args);
        va_end(args);
        if (len >= n) {
                strlcpy(buf, bad_path, n);
                return buf;
        }
        return cleanup_path(buf);
}

static int dir_prefix(const char *buf, const char *dir)
{
        int len = strlen(dir);
        return !strncmp(buf, dir, len) &&
                (is_dir_sep(buf[len]) || buf[len] == '\0');
}

/* $buf =~ m|$dir/+$file| but without regex */
static int is_dir_file(const char *buf, const char *dir, const char *file)
{
        int len = strlen(dir);
        if (strncmp(buf, dir, len) || !is_dir_sep(buf[len]))
                return 0;
        while (is_dir_sep(buf[len]))
                len++;
        return !strcmp(buf + len, file);
}

static void replace_dir(struct strbuf *buf, int len, const char *newdir)
{
        int newlen = strlen(newdir);
        int need_sep = (buf->buf[len] && !is_dir_sep(buf->buf[len])) &&
                !is_dir_sep(newdir[newlen - 1]);
        if (need_sep)
                len--;   /* keep one char, to be replaced with '/'  */
        strbuf_splice(buf, 0, len, newdir, newlen);
        if (need_sep)
                buf->buf[newlen] = '/';
}

struct common_dir {
        /* Not considered garbage for report_linked_checkout_garbage */
        unsigned ignore_garbage:1;
        unsigned is_dir:1;
        /* Not common even though its parent is */
        unsigned exclude:1;
        const char *dirname;
};

static struct common_dir common_list[] = {
        { 0, 1, 0, "branches" },
        { 0, 1, 0, "hooks" },
        { 0, 1, 0, "info" },
        { 0, 0, 1, "info/sparse-checkout" },
        { 1, 1, 0, "logs" },
        { 1, 1, 1, "logs/HEAD" },
        { 0, 1, 1, "logs/refs/bisect" },
        { 0, 1, 0, "lost-found" },
        { 0, 1, 0, "objects" },
        { 0, 1, 0, "refs" },
        { 0, 1, 1, "refs/bisect" },
        { 0, 1, 0, "remotes" },
        { 0, 1, 0, "worktrees" },
        { 0, 1, 0, "rr-cache" },
        { 0, 1, 0, "svn" },
        { 0, 0, 0, "config" },
        { 1, 0, 0, "gc.pid" },
        { 0, 0, 0, "packed-refs" },
        { 0, 0, 0, "shallow" },
        { 0, 0, 0, NULL }
};

/*
 * A compressed trie.  A trie node consists of zero or more characters that
 * are common to all elements with this prefix, optionally followed by some
 * children.  If value is not NULL, the trie node is a terminal node.
 *
 * For example, consider the following set of strings:
 * abc
 * def
 * definite
 * definition
 *
 * The trie would look look like:
 * root: len = 0, children a and d non-NULL, value = NULL.
 *    a: len = 2, contents = bc, value = (data for "abc")
 *    d: len = 2, contents = ef, children i non-NULL, value = (data for "def")
 *       i: len = 3, contents = nit, children e and i non-NULL, value = NULL
 *           e: len = 0, children all NULL, value = (data for "definite")
 *           i: len = 2, contents = on, children all NULL,
 *              value = (data for "definition")
 */
struct trie {
        struct trie *children[256];
        int len;
        char *contents;
        void *value;
};

static struct trie *make_trie_node(const char *key, void *value)
{
        struct trie *new_node = xcalloc(1, sizeof(*new_node));
        new_node->len = strlen(key);
        if (new_node->len) {
                new_node->contents = xmalloc(new_node->len);
                memcpy(new_node->contents, key, new_node->len);
        }
        new_node->value = value;
        return new_node;
}

/*
 * Add a key/value pair to a trie.  The key is assumed to be \0-terminated.
 * If there was an existing value for this key, return it.
 */
static void *add_to_trie(struct trie *root, const char *key, void *value)
{
        struct trie *child;
        void *old;
        int i;

        if (!*key) {
                /* we have reached the end of the key */
                old = root->value;
                root->value = value;
                return old;
        }

        for (i = 0; i < root->len; i++) {
                if (root->contents[i] == key[i])
                        continue;

                /*
                 * Split this node: child will contain this node's
                 * existing children.
                 */
                child = malloc(sizeof(*child));
                memcpy(child->children, root->children, sizeof(root->children));

                child->len = root->len - i - 1;
                if (child->len) {
                        child->contents = xstrndup(root->contents + i + 1,
                                                   child->len);
                }
                child->value = root->value;
                root->value = NULL;
                root->len = i;

                memset(root->children, 0, sizeof(root->children));
                root->children[(unsigned char)root->contents[i]] = child;

                /* This is the newly-added child. */
                root->children[(unsigned char)key[i]] =
                        make_trie_node(key + i + 1, value);
                return NULL;
        }

        /* We have matched the entire compressed section */
        if (key[i]) {
                child = root->children[(unsigned char)key[root->len]];
                if (child) {
                        return add_to_trie(child, key + root->len + 1, value);
                } else {
                        child = make_trie_node(key + root->len + 1, value);
                        root->children[(unsigned char)key[root->len]] = child;
                        return NULL;
                }
        }

        old = root->value;
        root->value = value;
        return old;
}

typedef int (*match_fn)(const char *unmatched, void *data, void *baton);

/*
 * Search a trie for some key.  Find the longest /-or-\0-terminated
 * prefix of the key for which the trie contains a value.  Call fn
 * with the unmatched portion of the key and the found value, and
 * return its return value.  If there is no such prefix, return -1.
 *
 * The key is partially normalized: consecutive slashes are skipped.
 *
 * For example, consider the trie containing only [refs,
 * refs/worktree] (both with values).
 *
 * | key             | unmatched  | val from node | return value |
 * |-----------------|------------|---------------|--------------|
 * | a               | not called | n/a           | -1           |
 * | refs            | \0         | refs          | as per fn    |
 * | refs/           | /          | refs          | as per fn    |
 * | refs/w          | /w         | refs          | as per fn    |
 * | refs/worktree   | \0         | refs/worktree | as per fn    |
 * | refs/worktree/  | /          | refs/worktree | as per fn    |
 * | refs/worktree/a | /a         | refs/worktree | as per fn    |
 * |-----------------|------------|---------------|--------------|
 *
 */
static int trie_find(struct trie *root, const char *key, match_fn fn,
                     void *baton)
{
        int i;
        int result;
        struct trie *child;

        if (!*key) {
                /* we have reached the end of the key */
                if (root->value && !root->len)
                        return fn(key, root->value, baton);
                else
                        return -1;
        }

        for (i = 0; i < root->len; i++) {
                /* Partial path normalization: skip consecutive slashes. */
                if (key[i] == '/' && key[i+1] == '/') {
                        key++;
                        continue;
                }
                if (root->contents[i] != key[i])
                        return -1;
        }

        /* Matched the entire compressed section */
        key += i;
        if (!*key)
                /* End of key */
                return fn(key, root->value, baton);

        /* Partial path normalization: skip consecutive slashes */
        while (key[0] == '/' && key[1] == '/')
                key++;

        child = root->children[(unsigned char)*key];
        if (child)
                result = trie_find(child, key + 1, fn, baton);
        else
                result = -1;

        if (result >= 0 || (*key != '/' && *key != 0))
                return result;
        if (root->value)
                return fn(key, root->value, baton);
        else
                return -1;
}

static struct trie common_trie;
static int common_trie_done_setup;

static void init_common_trie(void)
{
        struct common_dir *p;

        if (common_trie_done_setup)
                return;

        for (p = common_list; p->dirname; p++)
                add_to_trie(&common_trie, p->dirname, p);

        common_trie_done_setup = 1;
}

/*
 * Helper function for update_common_dir: returns 1 if the dir
 * prefix is common.
 */
static int check_common(const char *unmatched, void *value, void *baton)
{
        struct common_dir *dir = value;

        if (!dir)
                return 0;

        if (dir->is_dir && (unmatched[0] == 0 || unmatched[0] == '/'))
                return !dir->exclude;

        if (!dir->is_dir && unmatched[0] == 0)
                return !dir->exclude;

        return 0;
}

static void update_common_dir(struct strbuf *buf, int git_dir_len,
                              const char *common_dir)
{
        char *base = buf->buf + git_dir_len;
        init_common_trie();
        if (!common_dir)
                common_dir = get_git_common_dir();
        if (trie_find(&common_trie, base, check_common, NULL) > 0)
                replace_dir(buf, git_dir_len, common_dir);
}

void report_linked_checkout_garbage(void)
{
        struct strbuf sb = STRBUF_INIT;
        const struct common_dir *p;
        int len;

        if (!git_common_dir_env)
                return;
        strbuf_addf(&sb, "%s/", get_git_dir());
        len = sb.len;
        for (p = common_list; p->dirname; p++) {
                const char *path = p->dirname;
                if (p->ignore_garbage)
                        continue;
                strbuf_setlen(&sb, len);
                strbuf_addstr(&sb, path);
                if (file_exists(sb.buf))
                        report_garbage(PACKDIR_FILE_GARBAGE, sb.buf);
        }
        strbuf_release(&sb);
}

static void adjust_git_path(struct strbuf *buf, int git_dir_len)
{
        const char *base = buf->buf + git_dir_len;
        if (git_graft_env && is_dir_file(base, "info", "grafts"))
                strbuf_splice(buf, 0, buf->len,
                              get_graft_file(), strlen(get_graft_file()));
        else if (git_index_env && !strcmp(base, "index"))
                strbuf_splice(buf, 0, buf->len,
                              get_index_file(), strlen(get_index_file()));
        else if (git_db_env && dir_prefix(base, "objects"))
                replace_dir(buf, git_dir_len + 7, get_object_directory());
        else if (git_hooks_path && dir_prefix(base, "hooks"))
                replace_dir(buf, git_dir_len + 5, git_hooks_path);
        else if (git_common_dir_env)
                update_common_dir(buf, git_dir_len, NULL);
}

static void do_git_path(struct strbuf *buf, const char *fmt, va_list args)
{
        int gitdir_len;
        strbuf_addstr(buf, get_git_dir());
        if (buf->len && !is_dir_sep(buf->buf[buf->len - 1]))
                strbuf_addch(buf, '/');
        gitdir_len = buf->len;
        strbuf_vaddf(buf, fmt, args);
        adjust_git_path(buf, gitdir_len);
        strbuf_cleanup_path(buf);
}

char *git_path_buf(struct strbuf *buf, const char *fmt, ...)
{
        va_list args;
        strbuf_reset(buf);
        va_start(args, fmt);
        do_git_path(buf, fmt, args);
        va_end(args);
        return buf->buf;
}

void strbuf_git_path(struct strbuf *sb, const char *fmt, ...)
{
        va_list args;
        va_start(args, fmt);
        do_git_path(sb, fmt, args);
        va_end(args);
}

const char *git_path(const char *fmt, ...)
{
        struct strbuf *pathname = get_pathname();
        va_list args;
        va_start(args, fmt);
        do_git_path(pathname, fmt, args);
        va_end(args);
        return pathname->buf;
}

char *git_pathdup(const char *fmt, ...)
{
        struct strbuf path = STRBUF_INIT;
        va_list args;
        va_start(args, fmt);
        do_git_path(&path, fmt, args);
        va_end(args);
        return strbuf_detach(&path, NULL);
}

char *mkpathdup(const char *fmt, ...)
{
        struct strbuf sb = STRBUF_INIT;
        va_list args;
        va_start(args, fmt);
        strbuf_vaddf(&sb, fmt, args);
        va_end(args);
        strbuf_cleanup_path(&sb);
        return strbuf_detach(&sb, NULL);
}

const char *mkpath(const char *fmt, ...)
{
        va_list args;
        struct strbuf *pathname = get_pathname();
        va_start(args, fmt);
        strbuf_vaddf(pathname, fmt, args);
        va_end(args);
        return cleanup_path(pathname->buf);
}

static void do_submodule_path(struct strbuf *buf, const char *path,
                              const char *fmt, va_list args)
{
        const char *git_dir;
        struct strbuf git_submodule_common_dir = STRBUF_INIT;
        struct strbuf git_submodule_dir = STRBUF_INIT;

        strbuf_addstr(buf, path);
        strbuf_complete(buf, '/');
        strbuf_addstr(buf, ".git");

        git_dir = read_gitfile(buf->buf);
        if (git_dir) {
                strbuf_reset(buf);
                strbuf_addstr(buf, git_dir);
        }
        strbuf_addch(buf, '/');
        strbuf_addstr(&git_submodule_dir, buf->buf);

        strbuf_vaddf(buf, fmt, args);

        if (get_common_dir_noenv(&git_submodule_common_dir, git_submodule_dir.buf))
                update_common_dir(buf, git_submodule_dir.len, git_submodule_common_dir.buf);

        strbuf_cleanup_path(buf);

        strbuf_release(&git_submodule_dir);
        strbuf_release(&git_submodule_common_dir);
}

char *git_pathdup_submodule(const char *path, const char *fmt, ...)
{
        va_list args;
        struct strbuf buf = STRBUF_INIT;
        va_start(args, fmt);
        do_submodule_path(&buf, path, fmt, args);
        va_end(args);
        return strbuf_detach(&buf, NULL);
}

void strbuf_git_path_submodule(struct strbuf *buf, const char *path,
                               const char *fmt, ...)
{
        va_list args;
        va_start(args, fmt);
        do_submodule_path(buf, path, fmt, args);
        va_end(args);
}

int validate_headref(const char *path)
{
        struct stat st;
        char *buf, buffer[256];
        unsigned char sha1[20];
        int fd;
        ssize_t len;

        if (lstat(path, &st) < 0)
                return -1;

        /* Make sure it is a "refs/.." symlink */
        if (S_ISLNK(st.st_mode)) {
                len = readlink(path, buffer, sizeof(buffer)-1);
                if (len >= 5 && !memcmp("refs/", buffer, 5))
                        return 0;
                return -1;
        }

        /*
         * Anything else, just open it and try to see if it is a symbolic ref.
         */
        fd = open(path, O_RDONLY);
        if (fd < 0)
                return -1;
        len = read_in_full(fd, buffer, sizeof(buffer)-1);
        close(fd);

        /*
         * Is it a symbolic ref?
         */
        if (len < 4)
                return -1;
        if (!memcmp("ref:", buffer, 4)) {
                buf = buffer + 4;
                len -= 4;
                while (len && isspace(*buf))
                        buf++, len--;
                if (len >= 5 && !memcmp("refs/", buf, 5))
                        return 0;
        }

        /*
         * Is this a detached HEAD?
         */
        if (!get_sha1_hex(buffer, sha1))
                return 0;

        return -1;
}

static struct passwd *getpw_str(const char *username, size_t len)
{
        struct passwd *pw;
        char *username_z = xmemdupz(username, len);
        pw = getpwnam(username_z);
        free(username_z);
        return pw;
}

/*
 * Return a string with ~ and ~user expanded via getpw*.  If buf != NULL,
 * then it is a newly allocated string. Returns NULL on getpw failure or
 * if path is NULL.
 */
char *expand_user_path(const char *path)
{
        struct strbuf user_path = STRBUF_INIT;
        const char *to_copy = path;

        if (path == NULL)
                goto return_null;
        if (path[0] == '~') {
                const char *first_slash = strchrnul(path, '/');
                const char *username = path + 1;
                size_t username_len = first_slash - username;
                if (username_len == 0) {
                        const char *home = getenv("HOME");
                        if (!home)
                                goto return_null;
                        strbuf_addstr(&user_path, home);
#ifdef GIT_WINDOWS_NATIVE
                        convert_slashes(user_path.buf);
#endif
                } else {
                        struct passwd *pw = getpw_str(username, username_len);
                        if (!pw)
                                goto return_null;
                        strbuf_addstr(&user_path, pw->pw_dir);
                }
                to_copy = first_slash;
        }
        strbuf_addstr(&user_path, to_copy);
        return strbuf_detach(&user_path, NULL);
return_null:
        strbuf_release(&user_path);
        return NULL;
}

/*
 * First, one directory to try is determined by the following algorithm.
 *
 * (0) If "strict" is given, the path is used as given and no DWIM is
 *     done. Otherwise:
 * (1) "~/path" to mean path under the running user's home directory;
 * (2) "~user/path" to mean path under named user's home directory;
 * (3) "relative/path" to mean cwd relative directory; or
 * (4) "/absolute/path" to mean absolute directory.
 *
 * Unless "strict" is given, we check "%s/.git", "%s", "%s.git/.git", "%s.git"
 * in this order. We select the first one that is a valid git repository, and
 * chdir() to it. If none match, or we fail to chdir, we return NULL.
 *
 * If all goes well, we return the directory we used to chdir() (but
 * before ~user is expanded), avoiding getcwd() resolving symbolic
 * links.  User relative paths are also returned as they are given,
 * except DWIM suffixing.
 */
const char *enter_repo(const char *path, int strict)
{
        static struct strbuf validated_path = STRBUF_INIT;
        static struct strbuf used_path = STRBUF_INIT;

        if (!path)
                return NULL;

        if (!strict) {
                static const char *suffix[] = {
                        "/.git", "", ".git/.git", ".git", NULL,
                };
                const char *gitfile;
                int len = strlen(path);
                int i;
                while ((1 < len) && (path[len-1] == '/'))
                        len--;

                /*
                 * We can handle arbitrary-sized buffers, but this remains as a
                 * sanity check on untrusted input.
                 */
                if (PATH_MAX <= len)
                        return NULL;

                strbuf_reset(&used_path);
                strbuf_reset(&validated_path);
                strbuf_add(&used_path, path, len);
                strbuf_add(&validated_path, path, len);

                if (used_path.buf[0] == '~') {
                        char *newpath = expand_user_path(used_path.buf);
                        if (!newpath)
                                return NULL;
                        strbuf_attach(&used_path, newpath, strlen(newpath),
                                      strlen(newpath));
                }
                for (i = 0; suffix[i]; i++) {
                        struct stat st;
                        size_t baselen = used_path.len;
                        strbuf_addstr(&used_path, suffix[i]);
                        if (!stat(used_path.buf, &st) &&
                            (S_ISREG(st.st_mode) ||
                            (S_ISDIR(st.st_mode) && is_git_directory(used_path.buf)))) {
                                strbuf_addstr(&validated_path, suffix[i]);
                                break;
                        }
                        strbuf_setlen(&used_path, baselen);
                }
                if (!suffix[i])
                        return NULL;
                gitfile = read_gitfile(used_path.buf);
                if (gitfile) {
                        strbuf_reset(&used_path);
                        strbuf_addstr(&used_path, gitfile);
                }
                if (chdir(used_path.buf))
                        return NULL;
                path = validated_path.buf;
        }
        else {
                const char *gitfile = read_gitfile(path);
                if (gitfile)
                        path = gitfile;
                if (chdir(path))
                        return NULL;
        }

        if (is_git_directory(".")) {
                set_git_dir(".");
                check_repository_format();
                return path;
        }

        return NULL;
}

static int calc_shared_perm(int mode)
{
        int tweak;

        if (shared_repository < 0)
                tweak = -shared_repository;
        else
                tweak = shared_repository;

        if (!(mode & S_IWUSR))
                tweak &= ~0222;
        if (mode & S_IXUSR)
                /* Copy read bits to execute bits */
                tweak |= (tweak & 0444) >> 2;
        if (shared_repository < 0)
                mode = (mode & ~0777) | tweak;
        else
                mode |= tweak;

        return mode;
}


int adjust_shared_perm(const char *path)
{
        int old_mode, new_mode;

        if (!shared_repository)
                return 0;
        if (get_st_mode_bits(path, &old_mode) < 0)
                return -1;

        new_mode = calc_shared_perm(old_mode);
        if (S_ISDIR(old_mode)) {
                /* Copy read bits to execute bits */
                new_mode |= (new_mode & 0444) >> 2;
                new_mode |= FORCE_DIR_SET_GID;
        }

        if (((old_mode ^ new_mode) & ~S_IFMT) &&
                        chmod(path, (new_mode & ~S_IFMT)) < 0)
                return -2;
        return 0;
}

void safe_create_dir(const char *dir, int share)
{
        if (mkdir(dir, 0777) < 0) {
                if (errno != EEXIST) {
                        perror(dir);
                        exit(1);
                }
        }
        else if (share && adjust_shared_perm(dir))
                die(_("Could not make %s writable by group"), dir);
}

static int have_same_root(const char *path1, const char *path2)
{
        int is_abs1, is_abs2;

        is_abs1 = is_absolute_path(path1);
        is_abs2 = is_absolute_path(path2);
        return (is_abs1 && is_abs2 && tolower(path1[0]) == tolower(path2[0])) ||
               (!is_abs1 && !is_abs2);
}

/*
 * Give path as relative to prefix.
 *
 * The strbuf may or may not be used, so do not assume it contains the
 * returned path.
 */
const char *relative_path(const char *in, const char *prefix,
                          struct strbuf *sb)
{
        int in_len = in ? strlen(in) : 0;
        int prefix_len = prefix ? strlen(prefix) : 0;
        int in_off = 0;
        int prefix_off = 0;
        int i = 0, j = 0;

        if (!in_len)
                return "./";
        else if (!prefix_len)
                return in;

        if (have_same_root(in, prefix))
                /* bypass dos_drive, for "c:" is identical to "C:" */
                i = j = has_dos_drive_prefix(in);
        else {
                return in;
        }

        while (i < prefix_len && j < in_len && prefix[i] == in[j]) {
                if (is_dir_sep(prefix[i])) {
                        while (is_dir_sep(prefix[i]))
                                i++;
                        while (is_dir_sep(in[j]))
                                j++;
                        prefix_off = i;
                        in_off = j;
                } else {
                        i++;
                        j++;
                }
        }

        if (
            /* "prefix" seems like prefix of "in" */
            i >= prefix_len &&
            /*
             * but "/foo" is not a prefix of "/foobar"
             * (i.e. prefix not end with '/')
             */
            prefix_off < prefix_len) {
                if (j >= in_len) {
                        /* in="/a/b", prefix="/a/b" */
                        in_off = in_len;
                } else if (is_dir_sep(in[j])) {
                        /* in="/a/b/c", prefix="/a/b" */
                        while (is_dir_sep(in[j]))
                                j++;
                        in_off = j;
                } else {
                        /* in="/a/bbb/c", prefix="/a/b" */
                        i = prefix_off;
                }
        } else if (
                   /* "in" is short than "prefix" */
                   j >= in_len &&
                   /* "in" not end with '/' */
                   in_off < in_len) {
                if (is_dir_sep(prefix[i])) {
                        /* in="/a/b", prefix="/a/b/c/" */
                        while (is_dir_sep(prefix[i]))
                                i++;
                        in_off = in_len;
                }
        }
        in += in_off;
        in_len -= in_off;

        if (i >= prefix_len) {
                if (!in_len)
                        return "./";
                else
                        return in;
        }

        strbuf_reset(sb);
        strbuf_grow(sb, in_len);

        while (i < prefix_len) {
                if (is_dir_sep(prefix[i])) {
                        strbuf_addstr(sb, "../");
                        while (is_dir_sep(prefix[i]))
                                i++;
                        continue;
                }
                i++;
        }
        if (!is_dir_sep(prefix[prefix_len - 1]))
                strbuf_addstr(sb, "../");

        strbuf_addstr(sb, in);

        return sb->buf;
}

/*
 * A simpler implementation of relative_path
 *
 * Get relative path by removing "prefix" from "in". This function
 * first appears in v1.5.6-1-g044bbbc, and makes git_dir shorter
 * to increase performance when traversing the path to work_tree.
 */
const char *remove_leading_path(const char *in, const char *prefix)
{
        static struct strbuf buf = STRBUF_INIT;
        int i = 0, j = 0;

        if (!prefix || !prefix[0])
                return in;
        while (prefix[i]) {
                if (is_dir_sep(prefix[i])) {
                        if (!is_dir_sep(in[j]))
                                return in;
                        while (is_dir_sep(prefix[i]))
                                i++;
                        while (is_dir_sep(in[j]))
                                j++;
                        continue;
                } else if (in[j] != prefix[i]) {
                        return in;
                }
                i++;
                j++;
        }
        if (
            /* "/foo" is a prefix of "/foo" */
            in[j] &&
            /* "/foo" is not a prefix of "/foobar" */
            !is_dir_sep(prefix[i-1]) && !is_dir_sep(in[j])
           )
                return in;
        while (is_dir_sep(in[j]))
                j++;

        strbuf_reset(&buf);
        if (!in[j])
                strbuf_addstr(&buf, ".");
        else
                strbuf_addstr(&buf, in + j);
        return buf.buf;
}

/*
 * It is okay if dst == src, but they should not overlap otherwise.
 *
 * Performs the following normalizations on src, storing the result in dst:
 * - Ensures that components are separated by '/' (Windows only)
 * - Squashes sequences of '/'.
 * - Removes "." components.
 * - Removes ".." components, and the components the precede them.
 * Returns failure (non-zero) if a ".." component appears as first path
 * component anytime during the normalization. Otherwise, returns success (0).
 *
 * Note that this function is purely textual.  It does not follow symlinks,
 * verify the existence of the path, or make any system calls.
 *
 * prefix_len != NULL is for a specific case of prefix_pathspec():
 * assume that src == dst and src[0..prefix_len-1] is already
 * normalized, any time "../" eats up to the prefix_len part,
 * prefix_len is reduced. In the end prefix_len is the remaining
 * prefix that has not been overridden by user pathspec.
 *
 * NEEDSWORK: This function doesn't perform normalization w.r.t. trailing '/'.
 * For everything but the root folder itself, the normalized path should not
 * end with a '/', then the callers need to be fixed up accordingly.
 *
 */
int normalize_path_copy_len(char *dst, const char *src, int *prefix_len)
{
        char *dst0;
        int i;

        for (i = has_dos_drive_prefix(src); i > 0; i--)
                *dst++ = *src++;
        dst0 = dst;

        if (is_dir_sep(*src)) {
                *dst++ = '/';
                while (is_dir_sep(*src))
                        src++;
        }

        for (;;) {
                char c = *src;

                /*
                 * A path component that begins with . could be
                 * special:
                 * (1) "." and ends   -- ignore and terminate.
                 * (2) "./"           -- ignore them, eat slash and continue.
                 * (3) ".." and ends  -- strip one and terminate.
                 * (4) "../"          -- strip one, eat slash and continue.
                 */
                if (c == '.') {
                        if (!src[1]) {
                                /* (1) */
                                src++;
                        } else if (is_dir_sep(src[1])) {
                                /* (2) */
                                src += 2;
                                while (is_dir_sep(*src))
                                        src++;
                                continue;
                        } else if (src[1] == '.') {
                                if (!src[2]) {
                                        /* (3) */
                                        src += 2;
                                        goto up_one;
                                } else if (is_dir_sep(src[2])) {
                                        /* (4) */
                                        src += 3;
                                        while (is_dir_sep(*src))
                                                src++;
                                        goto up_one;
                                }
                        }
                }

                /* copy up to the next '/', and eat all '/' */
                while ((c = *src++) != '\0' && !is_dir_sep(c))
                        *dst++ = c;
                if (is_dir_sep(c)) {
                        *dst++ = '/';
                        while (is_dir_sep(c))
                                c = *src++;
                        src--;
                } else if (!c)
                        break;
                continue;

        up_one:
                /*
                 * dst0..dst is prefix portion, and dst[-1] is '/';
                 * go up one level.
                 */
                dst--;  /* go to trailing '/' */
                if (dst <= dst0)
                        return -1;
                /* Windows: dst[-1] cannot be backslash anymore */
                while (dst0 < dst && dst[-1] != '/')
                        dst--;
                if (prefix_len && *prefix_len > dst - dst0)
                        *prefix_len = dst - dst0;
        }
        *dst = '\0';
        return 0;
}

int normalize_path_copy(char *dst, const char *src)
{
        return normalize_path_copy_len(dst, src, NULL);
}

/*
 * path = Canonical absolute path
 * prefixes = string_list containing normalized, absolute paths without
 * trailing slashes (except for the root directory, which is denoted by "/").
 *
 * Determines, for each path in prefixes, whether the "prefix"
 * is an ancestor directory of path.  Returns the length of the longest
 * ancestor directory, excluding any trailing slashes, or -1 if no prefix
 * is an ancestor.  (Note that this means 0 is returned if prefixes is
 * ["/"].) "/foo" is not considered an ancestor of "/foobar".  Directories
 * are not considered to be their own ancestors.  path must be in a
 * canonical form: empty components, or "." or ".." components are not
 * allowed.
 */
int longest_ancestor_length(const char *path, struct string_list *prefixes)
{
        int i, max_len = -1;

        if (!strcmp(path, "/"))
                return -1;

        for (i = 0; i < prefixes->nr; i++) {
                const char *ceil = prefixes->items[i].string;
                int len = strlen(ceil);

                if (len == 1 && ceil[0] == '/')
                        len = 0; /* root matches anything, with length 0 */
                else if (!strncmp(path, ceil, len) && path[len] == '/')
                        ; /* match of length len */
                else
                        continue; /* no match */

                if (len > max_len)
                        max_len = len;
        }

        return max_len;
}

/* strip arbitrary amount of directory separators at end of path */
static inline int chomp_trailing_dir_sep(const char *path, int len)
{
        while (len && is_dir_sep(path[len - 1]))
                len--;
        return len;
}

/*
 * If path ends with suffix (complete path components), returns the
 * part before suffix (sans trailing directory separators).
 * Otherwise returns NULL.
 */
char *strip_path_suffix(const char *path, const char *suffix)
{
        int path_len = strlen(path), suffix_len = strlen(suffix);

        while (suffix_len) {
                if (!path_len)
                        return NULL;

                if (is_dir_sep(path[path_len - 1])) {
                        if (!is_dir_sep(suffix[suffix_len - 1]))
                                return NULL;
                        path_len = chomp_trailing_dir_sep(path, path_len);
                        suffix_len = chomp_trailing_dir_sep(suffix, suffix_len);
                }
                else if (path[--path_len] != suffix[--suffix_len])
                        return NULL;
        }

        if (path_len && !is_dir_sep(path[path_len - 1]))
                return NULL;
        return xstrndup(path, chomp_trailing_dir_sep(path, path_len));
}

int daemon_avoid_alias(const char *p)
{
        int sl, ndot;

        /*
         * This resurrects the belts and suspenders paranoia check by HPA
         * done in <435560F7.4080006@zytor.com> thread, now enter_repo()
         * does not do getcwd() based path canonicalization.
         *
         * sl becomes true immediately after seeing '/' and continues to
         * be true as long as dots continue after that without intervening
         * non-dot character.
         */
        if (!p || (*p != '/' && *p != '~'))
                return -1;
        sl = 1; ndot = 0;
        p++;

        while (1) {
                char ch = *p++;
                if (sl) {
                        if (ch == '.')
                                ndot++;
                        else if (ch == '/') {
                                if (ndot < 3)
                                        /* reject //, /./ and /../ */
                                        return -1;
                                ndot = 0;
                        }
                        else if (ch == 0) {
                                if (0 < ndot && ndot < 3)
                                        /* reject /.$ and /..$ */
                                        return -1;
                                return 0;
                        }
                        else
                                sl = ndot = 0;
                }
                else if (ch == 0)
                        return 0;
                else if (ch == '/') {
                        sl = 1;
                        ndot = 0;
                }
        }
}

static int only_spaces_and_periods(const char *path, size_t len, size_t skip)
{
        if (len < skip)
                return 0;
        len -= skip;
        path += skip;
        while (len-- > 0) {
                char c = *(path++);
                if (c != ' ' && c != '.')
                        return 0;
        }
        return 1;
}

int is_ntfs_dotgit(const char *name)
{
        int len;

        for (len = 0; ; len++)
                if (!name[len] || name[len] == '\\' || is_dir_sep(name[len])) {
                        if (only_spaces_and_periods(name, len, 4) &&
                                        !strncasecmp(name, ".git", 4))
                                return 1;
                        if (only_spaces_and_periods(name, len, 5) &&
                                        !strncasecmp(name, "git~1", 5))
                                return 1;
                        if (name[len] != '\\')
                                return 0;
                        name += len + 1;
                        len = -1;
                }
}

char *xdg_config_home(const char *filename)
{
        const char *home, *config_home;

        assert(filename);
        config_home = getenv("XDG_CONFIG_HOME");
        if (config_home && *config_home)
                return mkpathdup("%s/git/%s", config_home, filename);

        home = getenv("HOME");
        if (home)
                return mkpathdup("%s/.config/git/%s", home, filename);
        return NULL;
}

GIT_PATH_FUNC(git_path_cherry_pick_head, "CHERRY_PICK_HEAD")
GIT_PATH_FUNC(git_path_revert_head, "REVERT_HEAD")
GIT_PATH_FUNC(git_path_squash_msg, "SQUASH_MSG")
GIT_PATH_FUNC(git_path_merge_msg, "MERGE_MSG")
GIT_PATH_FUNC(git_path_merge_rr, "MERGE_RR")
GIT_PATH_FUNC(git_path_merge_mode, "MERGE_MODE")
GIT_PATH_FUNC(git_path_merge_head, "MERGE_HEAD")
GIT_PATH_FUNC(git_path_fetch_head, "FETCH_HEAD")
GIT_PATH_FUNC(git_path_shallow, "shallow")