/*
 * Builtin "git am"
 *
 * Based on git-am.sh by Junio C Hamano.
 */
#include "cache.h"
#include "builtin.h"
#include "exec_cmd.h"
#include "parse-options.h"
#include "dir.h"
#include "run-command.h"
#include "quote.h"
#include "lockfile.h"
#include "cache-tree.h"
#include "refs.h"
#include "commit.h"
#include "diff.h"
#include "diffcore.h"
#include "unpack-trees.h"
#include "branch.h"

/**
 * Returns 1 if the file is empty or does not exist, 0 otherwise.
 */
static int is_empty_file(const char *filename)
{
        struct stat st;

        if (stat(filename, &st) < 0) {
                if (errno == ENOENT)
                        return 1;
                die_errno(_("could not stat %s"), filename);
        }

        return !st.st_size;
}

/**
 * Like strbuf_getline(), but treats both '\n' and "\r\n" as line terminators.
 */
static int strbuf_getline_crlf(struct strbuf *sb, FILE *fp)
{
        if (strbuf_getwholeline(sb, fp, '\n'))
                return EOF;
        if (sb->buf[sb->len - 1] == '\n') {
                strbuf_setlen(sb, sb->len - 1);
                if (sb->len > 0 && sb->buf[sb->len - 1] == '\r')
                        strbuf_setlen(sb, sb->len - 1);
        }
        return 0;
}

/**
 * Returns the length of the first line of msg.
 */
static int linelen(const char *msg)
{
        return strchrnul(msg, '\n') - msg;
}

enum patch_format {
        PATCH_FORMAT_UNKNOWN = 0,
        PATCH_FORMAT_MBOX
};

struct am_state {
        /* state directory path */
        char *dir;

        /* current and last patch numbers, 1-indexed */
        int cur;
        int last;

        /* commit metadata and message */
        char *author_name;
        char *author_email;
        char *author_date;
        char *msg;
        size_t msg_len;

        /* number of digits in patch filename */
        int prec;
};

/**
 * Initializes am_state with the default values. The state directory is set to
 * dir.
 */
static void am_state_init(struct am_state *state, const char *dir)
{
        memset(state, 0, sizeof(*state));

        assert(dir);
        state->dir = xstrdup(dir);

        state->prec = 4;
}

/**
 * Releases memory allocated by an am_state.
 */
static void am_state_release(struct am_state *state)
{
        free(state->dir);
        free(state->author_name);
        free(state->author_email);
        free(state->author_date);
        free(state->msg);
}

/**
 * Returns path relative to the am_state directory.
 */
static inline const char *am_path(const struct am_state *state, const char *path)
{
        return mkpath("%s/%s", state->dir, path);
}

/**
 * Returns 1 if there is an am session in progress, 0 otherwise.
 */
static int am_in_progress(const struct am_state *state)
{
        struct stat st;

        if (lstat(state->dir, &st) < 0 || !S_ISDIR(st.st_mode))
                return 0;
        if (lstat(am_path(state, "last"), &st) || !S_ISREG(st.st_mode))
                return 0;
        if (lstat(am_path(state, "next"), &st) || !S_ISREG(st.st_mode))
                return 0;
        return 1;
}

/**
 * Reads the contents of `file` in the `state` directory into `sb`. Returns the
 * number of bytes read on success, -1 if the file does not exist. If `trim` is
 * set, trailing whitespace will be removed.
 */
static int read_state_file(struct strbuf *sb, const struct am_state *state,
                        const char *file, int trim)
{
        strbuf_reset(sb);

        if (strbuf_read_file(sb, am_path(state, file), 0) >= 0) {
                if (trim)
                        strbuf_trim(sb);

                return sb->len;
        }

        if (errno == ENOENT)
                return -1;

        die_errno(_("could not read '%s'"), am_path(state, file));
}

/**
 * Reads a KEY=VALUE shell variable assignment from `fp`, returning the VALUE
 * as a newly-allocated string. VALUE must be a quoted string, and the KEY must
 * match `key`. Returns NULL on failure.
 *
 * This is used by read_author_script() to read the GIT_AUTHOR_* variables from
 * the author-script.
 */
static char *read_shell_var(FILE *fp, const char *key)
{
        struct strbuf sb = STRBUF_INIT;
        const char *str;

        if (strbuf_getline(&sb, fp, '\n'))
                goto fail;

        if (!skip_prefix(sb.buf, key, &str))
                goto fail;

        if (!skip_prefix(str, "=", &str))
                goto fail;

        strbuf_remove(&sb, 0, str - sb.buf);

        str = sq_dequote(sb.buf);
        if (!str)
                goto fail;

        return strbuf_detach(&sb, NULL);

fail:
        strbuf_release(&sb);
        return NULL;
}

/**
 * Reads and parses the state directory's "author-script" file, and sets
 * state->author_name, state->author_email and state->author_date accordingly.
 * Returns 0 on success, -1 if the file could not be parsed.
 *
 * The author script is of the format:
 *
 *      GIT_AUTHOR_NAME='$author_name'
 *      GIT_AUTHOR_EMAIL='$author_email'
 *      GIT_AUTHOR_DATE='$author_date'
 *
 * where $author_name, $author_email and $author_date are quoted. We are strict
 * with our parsing, as the file was meant to be eval'd in the old git-am.sh
 * script, and thus if the file differs from what this function expects, it is
 * better to bail out than to do something that the user does not expect.
 */
static int read_author_script(struct am_state *state)
{
        const char *filename = am_path(state, "author-script");
        FILE *fp;

        assert(!state->author_name);
        assert(!state->author_email);
        assert(!state->author_date);

        fp = fopen(filename, "r");
        if (!fp) {
                if (errno == ENOENT)
                        return 0;
                die_errno(_("could not open '%s' for reading"), filename);
        }

        state->author_name = read_shell_var(fp, "GIT_AUTHOR_NAME");
        if (!state->author_name) {
                fclose(fp);
                return -1;
        }

        state->author_email = read_shell_var(fp, "GIT_AUTHOR_EMAIL");
        if (!state->author_email) {
                fclose(fp);
                return -1;
        }

        state->author_date = read_shell_var(fp, "GIT_AUTHOR_DATE");
        if (!state->author_date) {
                fclose(fp);
                return -1;
        }

        if (fgetc(fp) != EOF) {
                fclose(fp);
                return -1;
        }

        fclose(fp);
        return 0;
}

/**
 * Saves state->author_name, state->author_email and state->author_date in the
 * state directory's "author-script" file.
 */
static void write_author_script(const struct am_state *state)
{
        struct strbuf sb = STRBUF_INIT;

        strbuf_addstr(&sb, "GIT_AUTHOR_NAME=");
        sq_quote_buf(&sb, state->author_name);
        strbuf_addch(&sb, '\n');

        strbuf_addstr(&sb, "GIT_AUTHOR_EMAIL=");
        sq_quote_buf(&sb, state->author_email);
        strbuf_addch(&sb, '\n');

        strbuf_addstr(&sb, "GIT_AUTHOR_DATE=");
        sq_quote_buf(&sb, state->author_date);
        strbuf_addch(&sb, '\n');

        write_file(am_path(state, "author-script"), 1, "%s", sb.buf);

        strbuf_release(&sb);
}

/**
 * Reads the commit message from the state directory's "final-commit" file,
 * setting state->msg to its contents and state->msg_len to the length of its
 * contents in bytes.
 *
 * Returns 0 on success, -1 if the file does not exist.
 */
static int read_commit_msg(struct am_state *state)
{
        struct strbuf sb = STRBUF_INIT;

        assert(!state->msg);

        if (read_state_file(&sb, state, "final-commit", 0) < 0) {
                strbuf_release(&sb);
                return -1;
        }

        state->msg = strbuf_detach(&sb, &state->msg_len);
        return 0;
}

/**
 * Saves state->msg in the state directory's "final-commit" file.
 */
static void write_commit_msg(const struct am_state *state)
{
        int fd;
        const char *filename = am_path(state, "final-commit");

        fd = xopen(filename, O_WRONLY | O_CREAT, 0666);
        if (write_in_full(fd, state->msg, state->msg_len) < 0)
                die_errno(_("could not write to %s"), filename);
        close(fd);
}

/**
 * Loads state from disk.
 */
static void am_load(struct am_state *state)
{
        struct strbuf sb = STRBUF_INIT;

        if (read_state_file(&sb, state, "next", 1) < 0)
                die("BUG: state file 'next' does not exist");
        state->cur = strtol(sb.buf, NULL, 10);

        if (read_state_file(&sb, state, "last", 1) < 0)
                die("BUG: state file 'last' does not exist");
        state->last = strtol(sb.buf, NULL, 10);

        if (read_author_script(state) < 0)
                die(_("could not parse author script"));

        read_commit_msg(state);

        strbuf_release(&sb);
}

/**
 * Removes the am_state directory, forcefully terminating the current am
 * session.
 */
static void am_destroy(const struct am_state *state)
{
        struct strbuf sb = STRBUF_INIT;

        strbuf_addstr(&sb, state->dir);
        remove_dir_recursively(&sb, 0);
        strbuf_release(&sb);
}

/**
 * Determines if the file looks like a piece of RFC2822 mail by grabbing all
 * non-indented lines and checking if they look like they begin with valid
 * header field names.
 *
 * Returns 1 if the file looks like a piece of mail, 0 otherwise.
 */
static int is_mail(FILE *fp)
{
        const char *header_regex = "^[!-9;-~]+:";
        struct strbuf sb = STRBUF_INIT;
        regex_t regex;
        int ret = 1;

        if (fseek(fp, 0L, SEEK_SET))
                die_errno(_("fseek failed"));

        if (regcomp(&regex, header_regex, REG_NOSUB | REG_EXTENDED))
                die("invalid pattern: %s", header_regex);

        while (!strbuf_getline_crlf(&sb, fp)) {
                if (!sb.len)
                        break; /* End of header */

                /* Ignore indented folded lines */
                if (*sb.buf == '\t' || *sb.buf == ' ')
                        continue;

                /* It's a header if it matches header_regex */
                if (regexec(&regex, sb.buf, 0, NULL, 0)) {
                        ret = 0;
                        goto done;
                }
        }

done:
        regfree(&regex);
        strbuf_release(&sb);
        return ret;
}

/**
 * Attempts to detect the patch_format of the patches contained in `paths`,
 * returning the PATCH_FORMAT_* enum value. Returns PATCH_FORMAT_UNKNOWN if
 * detection fails.
 */
static int detect_patch_format(const char **paths)
{
        enum patch_format ret = PATCH_FORMAT_UNKNOWN;
        struct strbuf l1 = STRBUF_INIT;
        FILE *fp;

        /*
         * We default to mbox format if input is from stdin and for directories
         */
        if (!*paths || !strcmp(*paths, "-") || is_directory(*paths))
                return PATCH_FORMAT_MBOX;

        /*
         * Otherwise, check the first few lines of the first patch, starting
         * from the first non-blank line, to try to detect its format.
         */

        fp = xfopen(*paths, "r");

        while (!strbuf_getline_crlf(&l1, fp)) {
                if (l1.len)
                        break;
        }

        if (starts_with(l1.buf, "From ") || starts_with(l1.buf, "From: ")) {
                ret = PATCH_FORMAT_MBOX;
                goto done;
        }

        if (l1.len && is_mail(fp)) {
                ret = PATCH_FORMAT_MBOX;
                goto done;
        }

done:
        fclose(fp);
        strbuf_release(&l1);
        return ret;
}

/**
 * Splits out individual email patches from `paths`, where each path is either
 * a mbox file or a Maildir. Returns 0 on success, -1 on failure.
 */
static int split_mail_mbox(struct am_state *state, const char **paths)
{
        struct child_process cp = CHILD_PROCESS_INIT;
        struct strbuf last = STRBUF_INIT;

        cp.git_cmd = 1;
        argv_array_push(&cp.args, "mailsplit");
        argv_array_pushf(&cp.args, "-d%d", state->prec);
        argv_array_pushf(&cp.args, "-o%s", state->dir);
        argv_array_push(&cp.args, "-b");
        argv_array_push(&cp.args, "--");
        argv_array_pushv(&cp.args, paths);

        if (capture_command(&cp, &last, 8))
                return -1;

        state->cur = 1;
        state->last = strtol(last.buf, NULL, 10);

        return 0;
}

/**
 * Splits a list of files/directories into individual email patches. Each path
 * in `paths` must be a file/directory that is formatted according to
 * `patch_format`.
 *
 * Once split out, the individual email patches will be stored in the state
 * directory, with each patch's filename being its index, padded to state->prec
 * digits.
 *
 * state->cur will be set to the index of the first mail, and state->last will
 * be set to the index of the last mail.
 *
 * Returns 0 on success, -1 on failure.
 */
static int split_mail(struct am_state *state, enum patch_format patch_format,
                        const char **paths)
{
        switch (patch_format) {
        case PATCH_FORMAT_MBOX:
                return split_mail_mbox(state, paths);
        default:
                die("BUG: invalid patch_format");
        }
        return -1;
}

/**
 * Setup a new am session for applying patches
 */
static void am_setup(struct am_state *state, enum patch_format patch_format,
                        const char **paths)
{
        if (!patch_format)
                patch_format = detect_patch_format(paths);

        if (!patch_format) {
                fprintf_ln(stderr, _("Patch format detection failed."));
                exit(128);
        }

        if (mkdir(state->dir, 0777) < 0 && errno != EEXIST)
                die_errno(_("failed to create directory '%s'"), state->dir);

        if (split_mail(state, patch_format, paths) < 0) {
                am_destroy(state);
                die(_("Failed to split patches."));
        }

        /*
         * NOTE: Since the "next" and "last" files determine if an am_state
         * session is in progress, they should be written last.
         */

        write_file(am_path(state, "next"), 1, "%d", state->cur);

        write_file(am_path(state, "last"), 1, "%d", state->last);
}

/**
 * Increments the patch pointer, and cleans am_state for the application of the
 * next patch.
 */
static void am_next(struct am_state *state)
{
        free(state->author_name);
        state->author_name = NULL;

        free(state->author_email);
        state->author_email = NULL;

        free(state->author_date);
        state->author_date = NULL;

        free(state->msg);
        state->msg = NULL;
        state->msg_len = 0;

        unlink(am_path(state, "author-script"));
        unlink(am_path(state, "final-commit"));

        state->cur++;
        write_file(am_path(state, "next"), 1, "%d", state->cur);
}

/**
 * Returns the filename of the current patch email.
 */
static const char *msgnum(const struct am_state *state)
{
        static struct strbuf sb = STRBUF_INIT;

        strbuf_reset(&sb);
        strbuf_addf(&sb, "%0*d", state->prec, state->cur);

        return sb.buf;
}

/**
 * Refresh and write index.
 */
static void refresh_and_write_cache(void)
{
        struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));

        hold_locked_index(lock_file, 1);
        refresh_cache(REFRESH_QUIET);
        if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
                die(_("unable to write index file"));
}

/**
 * Returns 1 if the index differs from HEAD, 0 otherwise. When on an unborn
 * branch, returns 1 if there are entries in the index, 0 otherwise. If an
 * strbuf is provided, the space-separated list of files that differ will be
 * appended to it.
 */
static int index_has_changes(struct strbuf *sb)
{
        unsigned char head[GIT_SHA1_RAWSZ];
        int i;

        if (!get_sha1_tree("HEAD", head)) {
                struct diff_options opt;

                diff_setup(&opt);
                DIFF_OPT_SET(&opt, EXIT_WITH_STATUS);
                if (!sb)
                        DIFF_OPT_SET(&opt, QUICK);
                do_diff_cache(head, &opt);
                diffcore_std(&opt);
                for (i = 0; sb && i < diff_queued_diff.nr; i++) {
                        if (i)
                                strbuf_addch(sb, ' ');
                        strbuf_addstr(sb, diff_queued_diff.queue[i]->two->path);
                }
                diff_flush(&opt);
                return DIFF_OPT_TST(&opt, HAS_CHANGES) != 0;
        } else {
                for (i = 0; sb && i < active_nr; i++) {
                        if (i)
                                strbuf_addch(sb, ' ');
                        strbuf_addstr(sb, active_cache[i]->name);
                }
                return !!active_nr;
        }
}

/**
 * Parses `mail` using git-mailinfo, extracting its patch and authorship info.
 * state->msg will be set to the patch message. state->author_name,
 * state->author_email and state->author_date will be set to the patch author's
 * name, email and date respectively. The patch body will be written to the
 * state directory's "patch" file.
 *
 * Returns 1 if the patch should be skipped, 0 otherwise.
 */
static int parse_mail(struct am_state *state, const char *mail)
{
        FILE *fp;
        struct child_process cp = CHILD_PROCESS_INIT;
        struct strbuf sb = STRBUF_INIT;
        struct strbuf msg = STRBUF_INIT;
        struct strbuf author_name = STRBUF_INIT;
        struct strbuf author_date = STRBUF_INIT;
        struct strbuf author_email = STRBUF_INIT;
        int ret = 0;

        cp.git_cmd = 1;
        cp.in = xopen(mail, O_RDONLY, 0);
        cp.out = xopen(am_path(state, "info"), O_WRONLY | O_CREAT, 0777);

        argv_array_push(&cp.args, "mailinfo");
        argv_array_push(&cp.args, am_path(state, "msg"));
        argv_array_push(&cp.args, am_path(state, "patch"));

        if (run_command(&cp) < 0)
                die("could not parse patch");

        close(cp.in);
        close(cp.out);

        /* Extract message and author information */
        fp = xfopen(am_path(state, "info"), "r");
        while (!strbuf_getline(&sb, fp, '\n')) {
                const char *x;

                if (skip_prefix(sb.buf, "Subject: ", &x)) {
                        if (msg.len)
                                strbuf_addch(&msg, '\n');
                        strbuf_addstr(&msg, x);
                } else if (skip_prefix(sb.buf, "Author: ", &x))
                        strbuf_addstr(&author_name, x);
                else if (skip_prefix(sb.buf, "Email: ", &x))
                        strbuf_addstr(&author_email, x);
                else if (skip_prefix(sb.buf, "Date: ", &x))
                        strbuf_addstr(&author_date, x);
        }
        fclose(fp);

        /* Skip pine's internal folder data */
        if (!strcmp(author_name.buf, "Mail System Internal Data")) {
                ret = 1;
                goto finish;
        }

        if (is_empty_file(am_path(state, "patch"))) {
                printf_ln(_("Patch is empty. Was it split wrong?"));
                exit(128);
        }

        strbuf_addstr(&msg, "\n\n");
        if (strbuf_read_file(&msg, am_path(state, "msg"), 0) < 0)
                die_errno(_("could not read '%s'"), am_path(state, "msg"));
        stripspace(&msg, 0);

        assert(!state->author_name);
        state->author_name = strbuf_detach(&author_name, NULL);

        assert(!state->author_email);
        state->author_email = strbuf_detach(&author_email, NULL);

        assert(!state->author_date);
        state->author_date = strbuf_detach(&author_date, NULL);

        assert(!state->msg);
        state->msg = strbuf_detach(&msg, &state->msg_len);

finish:
        strbuf_release(&msg);
        strbuf_release(&author_date);
        strbuf_release(&author_email);
        strbuf_release(&author_name);
        strbuf_release(&sb);
        return ret;
}

/**
 * Applies current patch with git-apply. Returns 0 on success, -1 otherwise.
 */
static int run_apply(const struct am_state *state)
{
        struct child_process cp = CHILD_PROCESS_INIT;

        cp.git_cmd = 1;

        argv_array_push(&cp.args, "apply");
        argv_array_push(&cp.args, "--index");
        argv_array_push(&cp.args, am_path(state, "patch"));

        if (run_command(&cp))
                return -1;

        /* Reload index as git-apply will have modified it. */
        discard_cache();
        read_cache();

        return 0;
}

/**
 * Commits the current index with state->msg as the commit message and
 * state->author_name, state->author_email and state->author_date as the author
 * information.
 */
static void do_commit(const struct am_state *state)
{
        unsigned char tree[GIT_SHA1_RAWSZ], parent[GIT_SHA1_RAWSZ],
                      commit[GIT_SHA1_RAWSZ];
        unsigned char *ptr;
        struct commit_list *parents = NULL;
        const char *reflog_msg, *author;
        struct strbuf sb = STRBUF_INIT;

        if (write_cache_as_tree(tree, 0, NULL))
                die(_("git write-tree failed to write a tree"));

        if (!get_sha1_commit("HEAD", parent)) {
                ptr = parent;
                commit_list_insert(lookup_commit(parent), &parents);
        } else {
                ptr = NULL;
                fprintf_ln(stderr, _("applying to an empty history"));
        }

        author = fmt_ident(state->author_name, state->author_email,
                        state->author_date, IDENT_STRICT);

        if (commit_tree(state->msg, state->msg_len, tree, parents, commit,
                                author, NULL))
                die(_("failed to write commit object"));

        reflog_msg = getenv("GIT_REFLOG_ACTION");
        if (!reflog_msg)
                reflog_msg = "am";

        strbuf_addf(&sb, "%s: %.*s", reflog_msg, linelen(state->msg),
                        state->msg);

        update_ref(sb.buf, "HEAD", commit, ptr, 0, UPDATE_REFS_DIE_ON_ERR);

        strbuf_release(&sb);
}

/**
 * Validates the am_state for resuming -- the "msg" and authorship fields must
 * be filled up.
 */
static void validate_resume_state(const struct am_state *state)
{
        if (!state->msg)
                die(_("cannot resume: %s does not exist."),
                        am_path(state, "final-commit"));

        if (!state->author_name || !state->author_email || !state->author_date)
                die(_("cannot resume: %s does not exist."),
                        am_path(state, "author-script"));
}

/**
 * Applies all queued mail.
 *
 * If `resume` is true, we are "resuming". The "msg" and authorship fields, as
 * well as the state directory's "patch" file is used as-is for applying the
 * patch and committing it.
 */
static void am_run(struct am_state *state, int resume)
{
        const char *argv_gc_auto[] = {"gc", "--auto", NULL};
        struct strbuf sb = STRBUF_INIT;

        refresh_and_write_cache();

        if (index_has_changes(&sb))
                die(_("Dirty index: cannot apply patches (dirty: %s)"), sb.buf);

        strbuf_release(&sb);

        while (state->cur <= state->last) {
                const char *mail = am_path(state, msgnum(state));

                if (!file_exists(mail))
                        goto next;

                if (resume) {
                        validate_resume_state(state);
                        resume = 0;
                } else {
                        if (parse_mail(state, mail))
                                goto next; /* mail should be skipped */

                        write_author_script(state);
                        write_commit_msg(state);
                }

                printf_ln(_("Applying: %.*s"), linelen(state->msg), state->msg);

                if (run_apply(state) < 0) {
                        int advice_amworkdir = 1;

                        printf_ln(_("Patch failed at %s %.*s"), msgnum(state),
                                linelen(state->msg), state->msg);

                        git_config_get_bool("advice.amworkdir", &advice_amworkdir);

                        if (advice_amworkdir)
                                printf_ln(_("The copy of the patch that failed is found in: %s"),
                                                am_path(state, "patch"));

                        exit(128);
                }

                do_commit(state);

next:
                am_next(state);
        }

        am_destroy(state);
        run_command_v_opt(argv_gc_auto, RUN_GIT_CMD);
}

/**
 * Resume the current am session after patch application failure. The user did
 * all the hard work, and we do not have to do any patch application. Just
 * trust and commit what the user has in the index and working tree.
 */
static void am_resolve(struct am_state *state)
{
        validate_resume_state(state);

        printf_ln(_("Applying: %.*s"), linelen(state->msg), state->msg);

        if (!index_has_changes(NULL)) {
                printf_ln(_("No changes - did you forget to use 'git add'?\n"
                        "If there is nothing left to stage, chances are that something else\n"
                        "already introduced the same changes; you might want to skip this patch."));
                exit(128);
        }

        if (unmerged_cache()) {
                printf_ln(_("You still have unmerged paths in your index.\n"
                        "Did you forget to use 'git add'?"));
                exit(128);
        }

        do_commit(state);

        am_next(state);
        am_run(state, 0);
}

/**
 * Performs a checkout fast-forward from `head` to `remote`. If `reset` is
 * true, any unmerged entries will be discarded. Returns 0 on success, -1 on
 * failure.
 */
static int fast_forward_to(struct tree *head, struct tree *remote, int reset)
{
        struct lock_file *lock_file;
        struct unpack_trees_options opts;
        struct tree_desc t[2];

        if (parse_tree(head) || parse_tree(remote))
                return -1;

        lock_file = xcalloc(1, sizeof(struct lock_file));
        hold_locked_index(lock_file, 1);

        refresh_cache(REFRESH_QUIET);

        memset(&opts, 0, sizeof(opts));
        opts.head_idx = 1;
        opts.src_index = &the_index;
        opts.dst_index = &the_index;
        opts.update = 1;
        opts.merge = 1;
        opts.reset = reset;
        opts.fn = twoway_merge;
        init_tree_desc(&t[0], head->buffer, head->size);
        init_tree_desc(&t[1], remote->buffer, remote->size);

        if (unpack_trees(2, t, &opts)) {
                rollback_lock_file(lock_file);
                return -1;
        }

        if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
                die(_("unable to write new index file"));

        return 0;
}

/**
 * Clean the index without touching entries that are not modified between
 * `head` and `remote`.
 */
static int clean_index(const unsigned char *head, const unsigned char *remote)
{
        struct lock_file *lock_file;
        struct tree *head_tree, *remote_tree, *index_tree;
        unsigned char index[GIT_SHA1_RAWSZ];
        struct pathspec pathspec;

        head_tree = parse_tree_indirect(head);
        if (!head_tree)
                return error(_("Could not parse object '%s'."), sha1_to_hex(head));

        remote_tree = parse_tree_indirect(remote);
        if (!remote_tree)
                return error(_("Could not parse object '%s'."), sha1_to_hex(remote));

        read_cache_unmerged();

        if (fast_forward_to(head_tree, head_tree, 1))
                return -1;

        if (write_cache_as_tree(index, 0, NULL))
                return -1;

        index_tree = parse_tree_indirect(index);
        if (!index_tree)
                return error(_("Could not parse object '%s'."), sha1_to_hex(index));

        if (fast_forward_to(index_tree, remote_tree, 0))
                return -1;

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

        lock_file = xcalloc(1, sizeof(struct lock_file));
        hold_locked_index(lock_file, 1);

        if (read_tree(remote_tree, 0, &pathspec)) {
                rollback_lock_file(lock_file);
                return -1;
        }

        if (write_locked_index(&the_index, lock_file, COMMIT_LOCK))
                die(_("unable to write new index file"));

        remove_branch_state();

        return 0;
}

/**
 * Resume the current am session by skipping the current patch.
 */
static void am_skip(struct am_state *state)
{
        unsigned char head[GIT_SHA1_RAWSZ];

        if (get_sha1("HEAD", head))
                hashcpy(head, EMPTY_TREE_SHA1_BIN);

        if (clean_index(head, head))
                die(_("failed to clean index"));

        am_next(state);
        am_run(state, 0);
}

/**
 * parse_options() callback that validates and sets opt->value to the
 * PATCH_FORMAT_* enum value corresponding to `arg`.
 */
static int parse_opt_patchformat(const struct option *opt, const char *arg, int unset)
{
        int *opt_value = opt->value;

        if (!strcmp(arg, "mbox"))
                *opt_value = PATCH_FORMAT_MBOX;
        else
                return error(_("Invalid value for --patch-format: %s"), arg);
        return 0;
}

enum resume_mode {
        RESUME_FALSE = 0,
        RESUME_APPLY,
        RESUME_RESOLVED,
        RESUME_SKIP
};

int cmd_am(int argc, const char **argv, const char *prefix)
{
        struct am_state state;
        int patch_format = PATCH_FORMAT_UNKNOWN;
        enum resume_mode resume = RESUME_FALSE;

        const char * const usage[] = {
                N_("git am [options] [(<mbox>|<Maildir>)...]"),
                N_("git am [options] (--continue | --skip)"),
                NULL
        };

        struct option options[] = {
                OPT_CALLBACK(0, "patch-format", &patch_format, N_("format"),
                        N_("format the patch(es) are in"),
                        parse_opt_patchformat),
                OPT_CMDMODE(0, "continue", &resume,
                        N_("continue applying patches after resolving a conflict"),
                        RESUME_RESOLVED),
                OPT_CMDMODE('r', "resolved", &resume,
                        N_("synonyms for --continue"),
                        RESUME_RESOLVED),
                OPT_CMDMODE(0, "skip", &resume,
                        N_("skip the current patch"),
                        RESUME_SKIP),
                OPT_END()
        };

        /*
         * NEEDSWORK: Once all the features of git-am.sh have been
         * re-implemented in builtin/am.c, this preamble can be removed.
         */
        if (!getenv("_GIT_USE_BUILTIN_AM")) {
                const char *path = mkpath("%s/git-am", git_exec_path());

                if (sane_execvp(path, (char **)argv) < 0)
                        die_errno("could not exec %s", path);
        } else {
                prefix = setup_git_directory();
                trace_repo_setup(prefix);
                setup_work_tree();
        }

        git_config(git_default_config, NULL);

        am_state_init(&state, git_path("rebase-apply"));

        argc = parse_options(argc, argv, prefix, options, usage, 0);

        if (read_index_preload(&the_index, NULL) < 0)
                die(_("failed to read the index"));

        if (am_in_progress(&state)) {
                if (resume == RESUME_FALSE)
                        resume = RESUME_APPLY;

                am_load(&state);
        } else {
                struct argv_array paths = ARGV_ARRAY_INIT;
                int i;

                if (resume)
                        die(_("Resolve operation not in progress, we are not resuming."));

                for (i = 0; i < argc; i++) {
                        if (is_absolute_path(argv[i]) || !prefix)
                                argv_array_push(&paths, argv[i]);
                        else
                                argv_array_push(&paths, mkpath("%s/%s", prefix, argv[i]));
                }

                am_setup(&state, patch_format, paths.argv);

                argv_array_clear(&paths);
        }

        switch (resume) {
        case RESUME_FALSE:
                am_run(&state, 0);
                break;
        case RESUME_APPLY:
                am_run(&state, 1);
                break;
        case RESUME_RESOLVED:
                am_resolve(&state);
                break;
        case RESUME_SKIP:
                am_skip(&state);
                break;
        default:
                die("BUG: invalid resume value");
        }

        am_state_release(&state);

        return 0;
}