/*
 * "git difftool" builtin command
 *
 * This is a wrapper around the GIT_EXTERNAL_DIFF-compatible
 * git-difftool--helper script.
 *
 * This script exports GIT_EXTERNAL_DIFF and GIT_PAGER for use by git.
 * The GIT_DIFF* variables are exported for use by git-difftool--helper.
 *
 * Any arguments that are unknown to this script are forwarded to 'git diff'.
 *
 * Copyright (C) 2016 Johannes Schindelin
 */
#define USE_THE_INDEX_COMPATIBILITY_MACROS
#include "cache.h"
#include "config.h"
#include "builtin.h"
#include "run-command.h"
#include "exec-cmd.h"
#include "parse-options.h"
#include "argv-array.h"
#include "strbuf.h"
#include "lockfile.h"
#include "object-store.h"
#include "dir.h"

static int trust_exit_code;

static const char *const builtin_difftool_usage[] = {
        N_("git difftool [<options>] [<commit> [<commit>]] [--] [<path>...]"),
        NULL
};

static int difftool_config(const char *var, const char *value, void *cb)
{
        if (!strcmp(var, "difftool.trustexitcode")) {
                trust_exit_code = git_config_bool(var, value);
                return 0;
        }

        return git_default_config(var, value, cb);
}

static int print_tool_help(void)
{
        const char *argv[] = { "mergetool", "--tool-help=diff", NULL };
        return run_command_v_opt(argv, RUN_GIT_CMD);
}

static int parse_index_info(char *p, int *mode1, int *mode2,
                            struct object_id *oid1, struct object_id *oid2,
                            char *status)
{
        if (*p != ':')
                return error("expected ':', got '%c'", *p);
        *mode1 = (int)strtol(p + 1, &p, 8);
        if (*p != ' ')
                return error("expected ' ', got '%c'", *p);
        *mode2 = (int)strtol(p + 1, &p, 8);
        if (*p != ' ')
                return error("expected ' ', got '%c'", *p);
        if (parse_oid_hex(++p, oid1, (const char **)&p))
                return error("expected object ID, got '%s'", p);
        if (*p != ' ')
                return error("expected ' ', got '%c'", *p);
        if (parse_oid_hex(++p, oid2, (const char **)&p))
                return error("expected object ID, got '%s'", p);
        if (*p != ' ')
                return error("expected ' ', got '%c'", *p);
        *status = *++p;
        if (!*status)
                return error("missing status");
        if (p[1] && !isdigit(p[1]))
                return error("unexpected trailer: '%s'", p + 1);
        return 0;
}

/*
 * Remove any trailing slash from $workdir
 * before starting to avoid double slashes in symlink targets.
 */
static void add_path(struct strbuf *buf, size_t base_len, const char *path)
{
        strbuf_setlen(buf, base_len);
        if (buf->len && buf->buf[buf->len - 1] != '/')
                strbuf_addch(buf, '/');
        strbuf_addstr(buf, path);
}

/*
 * Determine whether we can simply reuse the file in the worktree.
 */
static int use_wt_file(const char *workdir, const char *name,
                       struct object_id *oid)
{
        struct strbuf buf = STRBUF_INIT;
        struct stat st;
        int use = 0;

        strbuf_addstr(&buf, workdir);
        add_path(&buf, buf.len, name);

        if (!lstat(buf.buf, &st) && !S_ISLNK(st.st_mode)) {
                struct object_id wt_oid;
                int fd = open(buf.buf, O_RDONLY);

                if (fd >= 0 &&
                    !index_fd(&the_index, &wt_oid, fd, &st, OBJ_BLOB, name, 0)) {
                        if (is_null_oid(oid)) {
                                oidcpy(oid, &wt_oid);
                                use = 1;
                        } else if (oideq(oid, &wt_oid))
                                use = 1;
                }
        }

        strbuf_release(&buf);

        return use;
}

struct working_tree_entry {
        struct hashmap_entry entry;
        char path[FLEX_ARRAY];
};

static int working_tree_entry_cmp(const void *unused_cmp_data,
                                  const void *entry,
                                  const void *entry_or_key,
                                  const void *unused_keydata)
{
        const struct working_tree_entry *a = entry;
        const struct working_tree_entry *b = entry_or_key;
        return strcmp(a->path, b->path);
}

/*
 * The `left` and `right` entries hold paths for the symlinks hashmap,
 * and a SHA-1 surrounded by brief text for submodules.
 */
struct pair_entry {
        struct hashmap_entry entry;
        char left[PATH_MAX], right[PATH_MAX];
        const char path[FLEX_ARRAY];
};

static int pair_cmp(const void *unused_cmp_data,
                    const void *entry,
                    const void *entry_or_key,
                    const void *unused_keydata)
{
        const struct pair_entry *a = entry;
        const struct pair_entry *b = entry_or_key;

        return strcmp(a->path, b->path);
}

static void add_left_or_right(struct hashmap *map, const char *path,
                              const char *content, int is_right)
{
        struct pair_entry *e, *existing;

        FLEX_ALLOC_STR(e, path, path);
        hashmap_entry_init(e, strhash(path));
        existing = hashmap_get(map, e, NULL);
        if (existing) {
                free(e);
                e = existing;
        } else {
                e->left[0] = e->right[0] = '\0';
                hashmap_add(map, e);
        }
        strlcpy(is_right ? e->right : e->left, content, PATH_MAX);
}

struct path_entry {
        struct hashmap_entry entry;
        char path[FLEX_ARRAY];
};

static int path_entry_cmp(const void *unused_cmp_data,
                          const void *entry,
                          const void *entry_or_key,
                          const void *key)
{
        const struct path_entry *a = entry;
        const struct path_entry *b = entry_or_key;

        return strcmp(a->path, key ? key : b->path);
}

static void changed_files(struct hashmap *result, const char *index_path,
                          const char *workdir)
{
        struct child_process update_index = CHILD_PROCESS_INIT;
        struct child_process diff_files = CHILD_PROCESS_INIT;
        struct strbuf index_env = STRBUF_INIT, buf = STRBUF_INIT;
        const char *git_dir = absolute_path(get_git_dir()), *env[] = {
                NULL, NULL
        };
        FILE *fp;

        strbuf_addf(&index_env, "GIT_INDEX_FILE=%s", index_path);
        env[0] = index_env.buf;

        argv_array_pushl(&update_index.args,
                         "--git-dir", git_dir, "--work-tree", workdir,
                         "update-index", "--really-refresh", "-q",
                         "--unmerged", NULL);
        update_index.no_stdin = 1;
        update_index.no_stdout = 1;
        update_index.no_stderr = 1;
        update_index.git_cmd = 1;
        update_index.use_shell = 0;
        update_index.clean_on_exit = 1;
        update_index.dir = workdir;
        update_index.env = env;
        /* Ignore any errors of update-index */
        run_command(&update_index);

        argv_array_pushl(&diff_files.args,
                         "--git-dir", git_dir, "--work-tree", workdir,
                         "diff-files", "--name-only", "-z", NULL);
        diff_files.no_stdin = 1;
        diff_files.git_cmd = 1;
        diff_files.use_shell = 0;
        diff_files.clean_on_exit = 1;
        diff_files.out = -1;
        diff_files.dir = workdir;
        diff_files.env = env;
        if (start_command(&diff_files))
                die("could not obtain raw diff");
        fp = xfdopen(diff_files.out, "r");
        while (!strbuf_getline_nul(&buf, fp)) {
                struct path_entry *entry;
                FLEX_ALLOC_STR(entry, path, buf.buf);
                hashmap_entry_init(entry, strhash(buf.buf));
                hashmap_add(result, entry);
        }
        fclose(fp);
        if (finish_command(&diff_files))
                die("diff-files did not exit properly");
        strbuf_release(&index_env);
        strbuf_release(&buf);
}

static NORETURN void exit_cleanup(const char *tmpdir, int exit_code)
{
        struct strbuf buf = STRBUF_INIT;
        strbuf_addstr(&buf, tmpdir);
        remove_dir_recursively(&buf, 0);
        if (exit_code)
                warning(_("failed: %d"), exit_code);
        exit(exit_code);
}

static int ensure_leading_directories(char *path)
{
        switch (safe_create_leading_directories(path)) {
                case SCLD_OK:
                case SCLD_EXISTS:
                        return 0;
                default:
                        return error(_("could not create leading directories "
                                       "of '%s'"), path);
        }
}

/*
 * Unconditional writing of a plain regular file is what
 * "git difftool --dir-diff" wants to do for symlinks.  We are preparing two
 * temporary directories to be fed to a Git-unaware tool that knows how to
 * show a diff of two directories (e.g. "diff -r A B").
 *
 * Because the tool is Git-unaware, if a symbolic link appears in either of
 * these temporary directories, it will try to dereference and show the
 * difference of the target of the symbolic link, which is not what we want,
 * as the goal of the dir-diff mode is to produce an output that is logically
 * equivalent to what "git diff" produces.
 *
 * Most importantly, we want to get textual comparison of the result of the
 * readlink(2).  get_symlink() provides that---it returns the contents of
 * the symlink that gets written to a regular file to force the external tool
 * to compare the readlink(2) result as text, even on a filesystem that is
 * capable of doing a symbolic link.
 */
static char *get_symlink(const struct object_id *oid, const char *path)
{
        char *data;
        if (is_null_oid(oid)) {
                /* The symlink is unknown to Git so read from the filesystem */
                struct strbuf link = STRBUF_INIT;
                if (has_symlinks) {
                        if (strbuf_readlink(&link, path, strlen(path)))
                                die(_("could not read symlink %s"), path);
                } else if (strbuf_read_file(&link, path, 128))
                        die(_("could not read symlink file %s"), path);

                data = strbuf_detach(&link, NULL);
        } else {
                enum object_type type;
                unsigned long size;
                data = read_object_file(oid, &type, &size);
                if (!data)
                        die(_("could not read object %s for symlink %s"),
                                oid_to_hex(oid), path);
        }

        return data;
}

static int checkout_path(unsigned mode, struct object_id *oid,
                         const char *path, const struct checkout *state)
{
        struct cache_entry *ce;
        int ret;

        ce = make_transient_cache_entry(mode, oid, path, 0);
        ret = checkout_entry(ce, state, NULL, NULL);

        discard_cache_entry(ce);
        return ret;
}

static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
                        int argc, const char **argv)
{
        char tmpdir[PATH_MAX];
        struct strbuf info = STRBUF_INIT, lpath = STRBUF_INIT;
        struct strbuf rpath = STRBUF_INIT, buf = STRBUF_INIT;
        struct strbuf ldir = STRBUF_INIT, rdir = STRBUF_INIT;
        struct strbuf wtdir = STRBUF_INIT;
        char *lbase_dir, *rbase_dir;
        size_t ldir_len, rdir_len, wtdir_len;
        const char *workdir, *tmp;
        int ret = 0, i;
        FILE *fp;
        struct hashmap working_tree_dups, submodules, symlinks2;
        struct hashmap_iter iter;
        struct pair_entry *entry;
        struct index_state wtindex;
        struct checkout lstate, rstate;
        int rc, flags = RUN_GIT_CMD, err = 0;
        struct child_process child = CHILD_PROCESS_INIT;
        const char *helper_argv[] = { "difftool--helper", NULL, NULL, NULL };
        struct hashmap wt_modified, tmp_modified;
        int indices_loaded = 0;

        workdir = get_git_work_tree();

        /* Setup temp directories */
        tmp = getenv("TMPDIR");
        xsnprintf(tmpdir, sizeof(tmpdir), "%s/git-difftool.XXXXXX", tmp ? tmp : "/tmp");
        if (!mkdtemp(tmpdir))
                return error("could not create '%s'", tmpdir);
        strbuf_addf(&ldir, "%s/left/", tmpdir);
        strbuf_addf(&rdir, "%s/right/", tmpdir);
        strbuf_addstr(&wtdir, workdir);
        if (!wtdir.len || !is_dir_sep(wtdir.buf[wtdir.len - 1]))
                strbuf_addch(&wtdir, '/');
        mkdir(ldir.buf, 0700);
        mkdir(rdir.buf, 0700);

        memset(&wtindex, 0, sizeof(wtindex));

        memset(&lstate, 0, sizeof(lstate));
        lstate.base_dir = lbase_dir = xstrdup(ldir.buf);
        lstate.base_dir_len = ldir.len;
        lstate.force = 1;
        memset(&rstate, 0, sizeof(rstate));
        rstate.base_dir = rbase_dir = xstrdup(rdir.buf);
        rstate.base_dir_len = rdir.len;
        rstate.force = 1;

        ldir_len = ldir.len;
        rdir_len = rdir.len;
        wtdir_len = wtdir.len;

        hashmap_init(&working_tree_dups, working_tree_entry_cmp, NULL, 0);
        hashmap_init(&submodules, pair_cmp, NULL, 0);
        hashmap_init(&symlinks2, pair_cmp, NULL, 0);

        child.no_stdin = 1;
        child.git_cmd = 1;
        child.use_shell = 0;
        child.clean_on_exit = 1;
        child.dir = prefix;
        child.out = -1;
        argv_array_pushl(&child.args, "diff", "--raw", "--no-abbrev", "-z",
                         NULL);
        for (i = 0; i < argc; i++)
                argv_array_push(&child.args, argv[i]);
        if (start_command(&child))
                die("could not obtain raw diff");
        fp = xfdopen(child.out, "r");

        /* Build index info for left and right sides of the diff */
        i = 0;
        while (!strbuf_getline_nul(&info, fp)) {
                int lmode, rmode;
                struct object_id loid, roid;
                char status;
                const char *src_path, *dst_path;

                if (starts_with(info.buf, "::"))
                        die(N_("combined diff formats('-c' and '--cc') are "
                               "not supported in\n"
                               "directory diff mode('-d' and '--dir-diff')."));

                if (parse_index_info(info.buf, &lmode, &rmode, &loid, &roid,
                                     &status))
                        break;
                if (strbuf_getline_nul(&lpath, fp))
                        break;
                src_path = lpath.buf;

                i++;
                if (status != 'C' && status != 'R') {
                        dst_path = src_path;
                } else {
                        if (strbuf_getline_nul(&rpath, fp))
                                break;
                        dst_path = rpath.buf;
                }

                if (S_ISGITLINK(lmode) || S_ISGITLINK(rmode)) {
                        strbuf_reset(&buf);
                        strbuf_addf(&buf, "Subproject commit %s",
                                    oid_to_hex(&loid));
                        add_left_or_right(&submodules, src_path, buf.buf, 0);
                        strbuf_reset(&buf);
                        strbuf_addf(&buf, "Subproject commit %s",
                                    oid_to_hex(&roid));
                        if (oideq(&loid, &roid))
                                strbuf_addstr(&buf, "-dirty");
                        add_left_or_right(&submodules, dst_path, buf.buf, 1);
                        continue;
                }

                if (S_ISLNK(lmode)) {
                        char *content = get_symlink(&loid, src_path);
                        add_left_or_right(&symlinks2, src_path, content, 0);
                        free(content);
                }

                if (S_ISLNK(rmode)) {
                        char *content = get_symlink(&roid, dst_path);
                        add_left_or_right(&symlinks2, dst_path, content, 1);
                        free(content);
                }

                if (lmode && status != 'C') {
                        if (checkout_path(lmode, &loid, src_path, &lstate)) {
                                ret = error("could not write '%s'", src_path);
                                goto finish;
                        }
                }

                if (rmode && !S_ISLNK(rmode)) {
                        struct working_tree_entry *entry;

                        /* Avoid duplicate working_tree entries */
                        FLEX_ALLOC_STR(entry, path, dst_path);
                        hashmap_entry_init(entry, strhash(dst_path));
                        if (hashmap_get(&working_tree_dups, entry, NULL)) {
                                free(entry);
                                continue;
                        }
                        hashmap_add(&working_tree_dups, entry);

                        if (!use_wt_file(workdir, dst_path, &roid)) {
                                if (checkout_path(rmode, &roid, dst_path,
                                                  &rstate)) {
                                        ret = error("could not write '%s'",
                                                    dst_path);
                                        goto finish;
                                }
                        } else if (!is_null_oid(&roid)) {
                                /*
                                 * Changes in the working tree need special
                                 * treatment since they are not part of the
                                 * index.
                                 */
                                struct cache_entry *ce2 =
                                        make_cache_entry(&wtindex, rmode, &roid,
                                                         dst_path, 0, 0);

                                add_index_entry(&wtindex, ce2,
                                                ADD_CACHE_JUST_APPEND);

                                add_path(&rdir, rdir_len, dst_path);
                                if (ensure_leading_directories(rdir.buf)) {
                                        ret = error("could not create "
                                                    "directory for '%s'",
                                                    dst_path);
                                        goto finish;
                                }
                                add_path(&wtdir, wtdir_len, dst_path);
                                if (symlinks) {
                                        if (symlink(wtdir.buf, rdir.buf)) {
                                                ret = error_errno("could not symlink '%s' to '%s'", wtdir.buf, rdir.buf);
                                                goto finish;
                                        }
                                } else {
                                        struct stat st;
                                        if (stat(wtdir.buf, &st))
                                                st.st_mode = 0644;
                                        if (copy_file(rdir.buf, wtdir.buf,
                                                      st.st_mode)) {
                                                ret = error("could not copy '%s' to '%s'", wtdir.buf, rdir.buf);
                                                goto finish;
                                        }
                                }
                        }
                }
        }

        fclose(fp);
        fp = NULL;
        if (finish_command(&child)) {
                ret = error("error occurred running diff --raw");
                goto finish;
        }

        if (!i)
                goto finish;

        /*
         * Changes to submodules require special treatment.This loop writes a
         * temporary file to both the left and right directories to show the
         * change in the recorded SHA1 for the submodule.
         */
        hashmap_iter_init(&submodules, &iter);
        while ((entry = hashmap_iter_next(&iter))) {
                if (*entry->left) {
                        add_path(&ldir, ldir_len, entry->path);
                        ensure_leading_directories(ldir.buf);
                        write_file(ldir.buf, "%s", entry->left);
                }
                if (*entry->right) {
                        add_path(&rdir, rdir_len, entry->path);
                        ensure_leading_directories(rdir.buf);
                        write_file(rdir.buf, "%s", entry->right);
                }
        }

        /*
         * Symbolic links require special treatment.The standard "git diff"
         * shows only the link itself, not the contents of the link target.
         * This loop replicates that behavior.
         */
        hashmap_iter_init(&symlinks2, &iter);
        while ((entry = hashmap_iter_next(&iter))) {
                if (*entry->left) {
                        add_path(&ldir, ldir_len, entry->path);
                        ensure_leading_directories(ldir.buf);
                        write_file(ldir.buf, "%s", entry->left);
                }
                if (*entry->right) {
                        add_path(&rdir, rdir_len, entry->path);
                        ensure_leading_directories(rdir.buf);
                        write_file(rdir.buf, "%s", entry->right);
                }
        }

        strbuf_release(&buf);

        strbuf_setlen(&ldir, ldir_len);
        helper_argv[1] = ldir.buf;
        strbuf_setlen(&rdir, rdir_len);
        helper_argv[2] = rdir.buf;

        if (extcmd) {
                helper_argv[0] = extcmd;
                flags = 0;
        } else
                setenv("GIT_DIFFTOOL_DIRDIFF", "true", 1);
        rc = run_command_v_opt(helper_argv, flags);

        /*
         * If the diff includes working copy files and those
         * files were modified during the diff, then the changes
         * should be copied back to the working tree.
         * Do not copy back files when symlinks are used and the
         * external tool did not replace the original link with a file.
         *
         * These hashes are loaded lazily since they aren't needed
         * in the common case of --symlinks and the difftool updating
         * files through the symlink.
         */
        hashmap_init(&wt_modified, path_entry_cmp, NULL, wtindex.cache_nr);
        hashmap_init(&tmp_modified, path_entry_cmp, NULL, wtindex.cache_nr);

        for (i = 0; i < wtindex.cache_nr; i++) {
                struct hashmap_entry dummy;
                const char *name = wtindex.cache[i]->name;
                struct stat st;

                add_path(&rdir, rdir_len, name);
                if (lstat(rdir.buf, &st))
                        continue;

                if ((symlinks && S_ISLNK(st.st_mode)) || !S_ISREG(st.st_mode))
                        continue;

                if (!indices_loaded) {
                        struct lock_file lock = LOCK_INIT;
                        strbuf_reset(&buf);
                        strbuf_addf(&buf, "%s/wtindex", tmpdir);
                        if (hold_lock_file_for_update(&lock, buf.buf, 0) < 0 ||
                            write_locked_index(&wtindex, &lock, COMMIT_LOCK)) {
                                ret = error("could not write %s", buf.buf);
                                goto finish;
                        }
                        changed_files(&wt_modified, buf.buf, workdir);
                        strbuf_setlen(&rdir, rdir_len);
                        changed_files(&tmp_modified, buf.buf, rdir.buf);
                        add_path(&rdir, rdir_len, name);
                        indices_loaded = 1;
                }

                hashmap_entry_init(&dummy, strhash(name));
                if (hashmap_get(&tmp_modified, &dummy, name)) {
                        add_path(&wtdir, wtdir_len, name);
                        if (hashmap_get(&wt_modified, &dummy, name)) {
                                warning(_("both files modified: '%s' and '%s'."),
                                        wtdir.buf, rdir.buf);
                                warning(_("working tree file has been left."));
                                warning("%s", "");
                                err = 1;
                        } else if (unlink(wtdir.buf) ||
                                   copy_file(wtdir.buf, rdir.buf, st.st_mode))
                                warning_errno(_("could not copy '%s' to '%s'"),
                                              rdir.buf, wtdir.buf);
                }
        }

        if (err) {
                warning(_("temporary files exist in '%s'."), tmpdir);
                warning(_("you may want to cleanup or recover these."));
                exit(1);
        } else
                exit_cleanup(tmpdir, rc);

finish:
        if (fp)
                fclose(fp);

        free(lbase_dir);
        free(rbase_dir);
        strbuf_release(&ldir);
        strbuf_release(&rdir);
        strbuf_release(&wtdir);
        strbuf_release(&buf);

        return ret;
}

static int run_file_diff(int prompt, const char *prefix,
                         int argc, const char **argv)
{
        struct argv_array args = ARGV_ARRAY_INIT;
        const char *env[] = {
                "GIT_PAGER=", "GIT_EXTERNAL_DIFF=git-difftool--helper", NULL,
                NULL
        };
        int ret = 0, i;

        if (prompt > 0)
                env[2] = "GIT_DIFFTOOL_PROMPT=true";
        else if (!prompt)
                env[2] = "GIT_DIFFTOOL_NO_PROMPT=true";


        argv_array_push(&args, "diff");
        for (i = 0; i < argc; i++)
                argv_array_push(&args, argv[i]);
        ret = run_command_v_opt_cd_env(args.argv, RUN_GIT_CMD, prefix, env);
        exit(ret);
}

int cmd_difftool(int argc, const char **argv, const char *prefix)
{
        int use_gui_tool = 0, dir_diff = 0, prompt = -1, symlinks = 0,
            tool_help = 0, no_index = 0;
        static char *difftool_cmd = NULL, *extcmd = NULL;
        struct option builtin_difftool_options[] = {
                OPT_BOOL('g', "gui", &use_gui_tool,
                         N_("use `diff.guitool` instead of `diff.tool`")),
                OPT_BOOL('d', "dir-diff", &dir_diff,
                         N_("perform a full-directory diff")),
                OPT_SET_INT_F('y', "no-prompt", &prompt,
                        N_("do not prompt before launching a diff tool"),
                        0, PARSE_OPT_NONEG),
                OPT_SET_INT_F(0, "prompt", &prompt, NULL,
                        1, PARSE_OPT_NONEG | PARSE_OPT_HIDDEN),
                OPT_BOOL(0, "symlinks", &symlinks,
                         N_("use symlinks in dir-diff mode")),
                OPT_STRING('t', "tool", &difftool_cmd, N_("tool"),
                           N_("use the specified diff tool")),
                OPT_BOOL(0, "tool-help", &tool_help,
                         N_("print a list of diff tools that may be used with "
                            "`--tool`")),
                OPT_BOOL(0, "trust-exit-code", &trust_exit_code,
                         N_("make 'git-difftool' exit when an invoked diff "
                            "tool returns a non - zero exit code")),
                OPT_STRING('x', "extcmd", &extcmd, N_("command"),
                           N_("specify a custom command for viewing diffs")),
                OPT_ARGUMENT("no-index", &no_index, N_("passed to `diff`")),
                OPT_END()
        };

        git_config(difftool_config, NULL);
        symlinks = has_symlinks;

        argc = parse_options(argc, argv, prefix, builtin_difftool_options,
                             builtin_difftool_usage, PARSE_OPT_KEEP_UNKNOWN |
                             PARSE_OPT_KEEP_DASHDASH);

        if (tool_help)
                return print_tool_help();

        if (!no_index && !startup_info->have_repository)
                die(_("difftool requires worktree or --no-index"));

        if (!no_index){
                setup_work_tree();
                setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
                setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);
        } else if (dir_diff)
                die(_("--dir-diff is incompatible with --no-index"));

        if (use_gui_tool + !!difftool_cmd + !!extcmd > 1)
                die(_("--gui, --tool and --extcmd are mutually exclusive"));

        if (use_gui_tool)
                setenv("GIT_MERGETOOL_GUI", "true", 1);
        else if (difftool_cmd) {
                if (*difftool_cmd)
                        setenv("GIT_DIFF_TOOL", difftool_cmd, 1);
                else
                        die(_("no <tool> given for --tool=<tool>"));
        }

        if (extcmd) {
                if (*extcmd)
                        setenv("GIT_DIFFTOOL_EXTCMD", extcmd, 1);
                else
                        die(_("no <cmd> given for --extcmd=<cmd>"));
        }

        setenv("GIT_DIFFTOOL_TRUST_EXIT_CODE",
               trust_exit_code ? "true" : "false", 1);

        /*
         * In directory diff mode, 'git-difftool--helper' is called once
         * to compare the a / b directories. In file diff mode, 'git diff'
         * will invoke a separate instance of 'git-difftool--helper' for
         * each file that changed.
         */
        if (dir_diff)
                return run_dir_diff(extcmd, symlinks, prefix, argc, argv);
        return run_file_diff(prompt, prefix, argc, argv);
}