group-actor/src/group_handler/handle_mention.rs

use std::collections::HashSet;
use std::time::Duration;

use elefren::{FediClient, SearchType, StatusBuilder};
use elefren::entities::account::Account;
use elefren::entities::prelude::Status;
use elefren::status_builder::Visibility;

use crate::command::{RE_NOBOT_TAG, StatusCommand};
use crate::error::GroupError;
use crate::group_handler::GroupHandle;
use crate::store::data::GroupConfig;
use crate::utils::{LogError, normalize_acct};
use crate::utils;

pub struct ProcessMention<'a> {
    status: Status,
    group_account: &'a Account,
    config: &'a mut GroupConfig,
    client: &'a mut FediClient,
    group_acct: String,
    status_acct: String,
    status_user_id: String,
    can_write: bool,
    is_admin: bool,
    replies: Vec<String>,
    announcements: Vec<String>,
    do_boost_prev_post: bool,
    want_markdown: bool,
}

impl<'a> ProcessMention<'a> {
    async fn lookup_acct_id(&self, acct: &str, followed: bool) -> Result<Option<String>, GroupError> {
        debug!("Looking up user ID by acct: {}", acct);

        match tokio::time::timeout(Duration::from_secs(5), self.client.search_v2(
            acct,
            !followed,
            Some(SearchType::Accounts),
            Some(1),
            followed,
        )).await {
            Err(_) => {
                warn!("Account lookup timeout!");
                Err(GroupError::ApiTimeout)
            }
            Ok(Err(e)) => {
                // Elefren error
                Err(e.into())
            }
            Ok(Ok(res)) => {
                for item in res.accounts {
                    let acct_normalized = normalize_acct(&item.acct, &self.group_acct)?;
                    if acct_normalized == acct {
                        debug!("Search done, account found: {}", item.acct);
                        return Ok(Some(item.id));
                    } else {
                        warn!("Found wrong account: {}", item.acct);
                    }
                }

                debug!("Search done, nothing found");
                Ok(None)
            }
        }
    }

    fn append_admin_list_to_reply(&mut self) {
        let mut admins = self.config.get_admins().collect::<Vec<_>>();
        admins.sort();
        for a in admins {
            self.replies.push(format!("- {}", a));
        }
    }

    fn append_member_list_to_reply(&mut self) {
        let admins = self.config.get_admins().collect::<HashSet<_>>();
        let mut members = self.config.get_members().collect::<Vec<_>>();
        members.extend(admins.iter());
        members.sort();
        members.dedup();
        for m in members {
            self.replies.push(if admins.contains(&m) {
                format!("- {} [admin]", m)
            } else {
                format!("- {}", m)
            });
        }
    }

    async fn follow_user_by_id(&self, id: &str) -> Result<(), GroupError> {
        debug!("Trying to follow user #{}", id);
        self.client.follow(id).await?;
        Ok(())
    }

    async fn unfollow_user_by_id(&self, id: &str) -> Result<(), GroupError> {
        debug!("Trying to unfollow user #{}", id);
        self.client.unfollow(id).await?;
        Ok(())
    }

    pub(crate) async fn run(gh: &'a mut GroupHandle, status: Status) -> Result<(), GroupError> {
        let group_acct = gh.config.get_acct().to_string();
        let status_acct = normalize_acct(&status.account.acct, &group_acct)?.to_string();

        if gh.config.is_banned(&status_acct) {
            warn!("Status author {} is banned!", status_acct);
            return Ok(());
        }

        let pm = Self {
            group_account: &gh.group_account,
            status_user_id: status.account.id.to_string(),
            client: &mut gh.client,
            can_write: gh.config.can_write(&status_acct),
            is_admin: gh.config.is_admin(&status_acct),
            replies: vec![],
            announcements: vec![],
            do_boost_prev_post: false,
            want_markdown: false,
            group_acct,
            status_acct,
            status,
            config: &mut gh.config,
        };

        pm.handle().await
    }

    async fn reblog_status(&self) {
        self.client.reblog(&self.status.id)
            .await
            .log_error("Failed to reblog status")
    }

    fn add_reply(&mut self, line: impl AsRef<str>) {
        self.replies.push(line.as_ref().trim_matches(' ').to_string())
    }

    fn add_announcement<'t>(&mut self, line: impl AsRef<str>) {
        self.announcements.push(line.as_ref().trim_matches(' ').to_string())
    }

    async fn handle(mut self) -> Result<(), GroupError> {
        let commands = crate::command::parse_slash_commands(&self.status.content);

        if commands.is_empty() {
            self.handle_post_with_no_commands().await;
        } else {
            if commands.contains(&StatusCommand::Ignore) {
                debug!("Notif ignored because of ignore command");
                return Ok(());
            }

            for cmd in commands {
                match cmd {
                    StatusCommand::Undo => {
                        self.cmd_undo().await
                            .log_error("Error handling undo cmd");
                    }
                    StatusCommand::Ignore => {
                        unreachable!(); // Handled above
                    }
                    StatusCommand::Announce(a) => {
                        self.cmd_announce(a).await;
                    }
                    StatusCommand::Boost => {
                        self.cmd_boost().await;
                    }
                    StatusCommand::BanUser(u) => {
                        self.cmd_ban_user(&u).await
                            .log_error("Error handling ban-user cmd");
                    }
                    StatusCommand::UnbanUser(u) => {
                        self.cmd_unban_user(&u).await
                            .log_error("Error handling unban-user cmd");
                    }
                    StatusCommand::OptOut => {
                        self.cmd_optout().await;
                    }
                    StatusCommand::OptIn => {
                        self.cmd_optin().await;
                    }
                    StatusCommand::BanServer(s) => {
                        self.cmd_ban_server(&s).await;
                    }
                    StatusCommand::UnbanServer(s) => {
                        self.cmd_unban_server(&s).await;
                    }
                    StatusCommand::AddMember(u) => {
                        self.cmd_add_member(&u).await
                            .log_error("Error handling add-member cmd");
                    }
                    StatusCommand::RemoveMember(u) => {
                        self.cmd_remove_member(&u).await
                            .log_error("Error handling remove-member cmd");
                    }
                    StatusCommand::AddTag(tag) => {
                        self.cmd_add_tag(tag).await;
                    }
                    StatusCommand::RemoveTag(tag) => {
                        self.cmd_remove_tag(tag).await;
                    }
                    StatusCommand::GrantAdmin(u) => {
                        self.cmd_grant_admin(&u).await
                            .log_error("Error handling grant-admin cmd");
                    }
                    StatusCommand::RemoveAdmin(u) => {
                        self.cmd_revoke_member(&u).await
                            .log_error("Error handling grant-admin cmd");
                    }
                    StatusCommand::OpenGroup => {
                        self.cmd_open_group().await;
                    }
                    StatusCommand::CloseGroup => {
                        self.cmd_close_group().await;
                    }
                    StatusCommand::Help => {
                        self.cmd_help().await;
                    }
                    StatusCommand::ListMembers => {
                        self.cmd_list_members().await;
                    }
                    StatusCommand::ListTags => {
                        self.cmd_list_tags().await;
                    }
                    StatusCommand::Leave => {
                        self.cmd_leave().await;
                    }
                    StatusCommand::Join => {
                        self.cmd_join().await;
                    }
                    StatusCommand::Ping => {
                        self.cmd_ping().await;
                    }
                }
            }
        }

        if self.do_boost_prev_post {
            if let (Some(prev_acct_id), Some(prev_status_id)) = (self.status.in_reply_to_account_id.as_ref(), self.status.in_reply_to_id.as_ref()) {
                match self.id_to_acct_check_boostable(prev_acct_id).await {
                    Ok(_acct) => {
                        self.client
                            .reblog(prev_status_id)
                            .await
                            .log_error("Failed to boost");
                    }
                    Err(e) => {
                        warn!("Can't reblog: {}", e);
                    }
                }
            }
        }

        if !self.replies.is_empty() {
            let mut msg = self.replies.join("\n");
            debug!("r={}", msg);

            if self.want_markdown {
                apply_trailing_hashtag_pleroma_bug_workaround(&mut msg);
            }

            let mention = format!("@{user}", user = self.status_acct);
            self.send_reply_multipart(mention, msg).await?;
        }

        if !self.announcements.is_empty() {
            let mut msg = self.announcements.join("\n");
            debug!("a={}", msg);

            if self.want_markdown {
                apply_trailing_hashtag_pleroma_bug_workaround(&mut msg);
            }

            let msg = format!("**📢 Group announcement**\n{msg}", msg = msg);
            self.send_announcement_multipart(&msg).await?;
        }

        Ok(())
    }

    async fn send_reply_multipart(&self, mention : String, msg : String) -> Result<(), GroupError> {
        let parts = smart_split(&msg, Some(mention), self.config.get_character_limit());
        for p in parts {
            if let Ok(post) = StatusBuilder::new()
                .status(p)
                .content_type(if self.want_markdown {
                    "text/markdown"
                } else {
                    "text/plain"
                })
                .visibility(Visibility::Direct)
                .build()
            {
                self.client.new_status(post).await?;
            }

            // Sleep a bit to avoid throttling
            tokio::time::sleep(Duration::from_secs(1)).await;
        }

        Ok(())
    }

    async fn send_announcement_multipart(&self, msg : &str) -> Result<(), GroupError> {
        let parts = smart_split(msg, None, self.config.get_character_limit());

        for p in parts {
            let post = StatusBuilder::new()
                .status(p)
                .content_type("text/markdown")
                .visibility(Visibility::Public)
                .build()
                .expect("error build status");

            self.client.new_status(post).await?;

            // Sleep a bit to avoid throttling
            tokio::time::sleep(Duration::from_secs(1)).await;
        }

        Ok(())
    }

    async fn handle_post_with_no_commands(&mut self) {
        debug!("No commands in post");
        if self.status.in_reply_to_id.is_none() {
            if self.can_write {
                // Someone tagged the group in OP, boost it.
                info!("Boosting OP mention");
                // tokio::time::sleep(DELAY_BEFORE_ACTION).await;
                self.reblog_status().await;
            } else {
                warn!("User @{} can't post to group!", self.status_acct);
            }
            // Otherwise, don't react
        } else {
            debug!("Not OP, ignore mention");
        }
    }

    async fn cmd_announce(&mut self, msg: String) {
        info!("Sending PSA");
        self.add_announcement(msg);
    }

    async fn cmd_boost(&mut self) {
        if self.can_write {
            self.do_boost_prev_post = self.status.in_reply_to_id.is_some();
        } else {
            warn!("User @{} can't share to group!", self.status_acct);
        }
    }

    async fn cmd_optout(&mut self) {
        if self.is_admin {
            self.add_reply("Group admins can't opt-out.");
        } else if self.config.is_member(&self.status_acct) {
            self.add_reply("Group members can't opt-out. You have to leave first.");
        } else {
            self.config.set_optout(&self.status_acct, true);
            self.add_reply("Your posts will no longer be shared to the group.");
        }
    }

    async fn cmd_optin(&mut self) {
        if self.is_admin {
            self.add_reply("Opt-in has no effect for admins.");
        } else if self.config.is_member(&self.status_acct) {
            self.add_reply("Opt-in has no effect for members.");
        } else {
            self.config.set_optout(&self.status_acct, false);
            self.add_reply("Your posts can now be shared to the group.");
        }
    }

    async fn cmd_undo(&mut self) -> Result<(), GroupError> {
        if let (Some(ref parent_account_id), Some(ref parent_status_id)) = (&self.status.in_reply_to_account_id, &self.status.in_reply_to_id) {
            if parent_account_id == &self.group_account.id {
                // This is a post sent by the group user, likely an announcement.
                // Undo here means delete it.
                if self.is_admin {
                    info!("Deleting group post #{}", parent_status_id);
                    self.client.delete_status(parent_status_id).await?;
                } else {
                    warn!("Only admin can delete posts made by the group user");
                }
            } else {
                if self.is_admin || parent_account_id == &self.status_user_id {
                    info!("Un-reblogging post #{}", parent_status_id);
                    // User unboosting own post boosted by accident, or admin doing it
                    self.client.unreblog(parent_status_id).await?;
                } else {
                    warn!("Only the author and admins can undo reblogs");
                    // XXX this means when someone /b's someone else's post to a group,
                    //  they then can't reverse that (only admin or the post's author can)
                }
            }
        }

        Ok(())
    }

    async fn cmd_ban_user(&mut self, user: &str) -> Result<(), GroupError> {
        let u = normalize_acct(user, &self.group_acct)?;
        if self.is_admin {
            if !self.config.is_banned(&u) {
                match self.config.ban_user(&u, true) {
                    Ok(_) => {
                        self.add_reply(format!("User {} banned from group!", u));
                        self.unfollow_by_acct(&u).await
                            .log_error("Failed to unfollow banned user");
                    }
                    Err(e) => {
                        self.add_reply(format!("Failed to ban user {}: {}", u, e));
                    }
                }
            } else {
                self.add_reply(format!("No action, user {} is already banned", u));
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
        Ok(())
    }

    async fn cmd_unban_user(&mut self, user: &str) -> Result<(), GroupError> {
        let u = normalize_acct(user, &self.group_acct)?;
        if self.is_admin {
            if self.config.is_banned(&u) {
                match self.config.ban_user(&u, false) {
                    Ok(_) => {
                        self.add_reply(format!("User {} un-banned!", u));
                        // no announcement here
                    }
                    Err(_) => {
                        unreachable!()
                    }
                }
            } else {
                self.add_reply(format!("No action, user {} is not banned", u));
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
        Ok(())
    }

    async fn cmd_ban_server(&mut self, s: &str) {
        if self.is_admin {
            if !self.config.is_server_banned(s) {
                match self.config.ban_server(s, true) {
                    Ok(_) => {
                        self.add_reply(format!("Server {} banned from group!", s));
                    }
                    Err(e) => {
                        self.add_reply(format!("Failed to ban server {}: {}", s, e));
                    }
                }
            } else {
                self.add_reply(format!("No action, server {} already banned", s));
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
    }

    async fn cmd_unban_server(&mut self, s: &str) {
        if self.is_admin {
            if self.config.is_server_banned(s) {
                match self.config.ban_server(s, false) {
                    Ok(_) => {
                        self.add_reply(format!("Server {} un-banned!", s));
                    }
                    Err(e) => {
                        self.add_reply(format!("Unexpected error occured: {}", e));
                    }
                }
            } else {
                self.add_reply(format!("No action, server {} is not banned", s));
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
    }

    async fn cmd_add_member(&mut self, user: &str) -> Result<(), GroupError> {
        let u = normalize_acct(user, &self.group_acct)?;
        if self.is_admin {
            // Allow even if the user is already a member - that will trigger re-follow
            match self.config.set_member(&u, true) {
                Ok(_) => {
                    self.add_reply(format!("User {} added to the group!", u));
                    self.follow_by_acct(&u)
                        .await.log_error("Failed to follow");
                }
                Err(e) => {
                    self.add_reply(format!("Failed to add user {} to group: {}", u, e));
                }
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
        Ok(())
    }

    async fn cmd_remove_member(&mut self, user: &str) -> Result<(), GroupError> {
        let u = normalize_acct(user, &self.group_acct)?;
        if self.is_admin {
            match self.config.set_member(&u, false) {
                Ok(_) => {
                    self.add_reply(format!("User {} removed from the group.", u));
                    self.unfollow_by_acct(&u).await
                        .log_error("Failed to unfollow removed user");
                }
                Err(e) => {
                    self.add_reply(format!("Unexpected error occured: {}", e));
                }
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
        Ok(())
    }

    async fn cmd_add_tag(&mut self, tag: String) {
        if self.is_admin {
            if !self.config.is_tag_followed(&tag) {
                self.config.add_tag(&tag);
                self.add_reply(format!("Tag \"{}\" added to the group!", tag));
            } else {
                self.add_reply(format!("No action, \"{}\" is already a group tag", tag));
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
    }

    async fn cmd_remove_tag(&mut self, tag: String) {
        if self.is_admin {
            if self.config.is_tag_followed(&tag) {
                self.config.remove_tag(&tag);
                self.add_reply(format!("Tag \"{}\" removed from the group!", tag));
            } else {
                self.add_reply(format!("No action, \"{}\" is not a group tag", tag));
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
    }

    async fn cmd_grant_admin(&mut self, user: &str) -> Result<(), GroupError> {
        let u = normalize_acct(user, &self.group_acct)?;
        if self.is_admin {
            if !self.config.is_admin(&u) {
                match self.config.set_admin(&u, true) {
                    Ok(_) => {
                        // try to make the config a little more sane, admins should be members
                        let _ = self.config.set_member(&u, true);

                        self.add_reply(format!("User {} is now a group admin!", u));
                    }
                    Err(e) => {
                        self.add_reply(format!(
                            "Failed to make user {} a group admin: {}",
                            u, e
                        ));
                    }
                }
            } else {
                self.add_reply(format!("No action, \"{}\" is admin already", u));
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
        Ok(())
    }

    async fn cmd_revoke_member(&mut self, user: &str) -> Result<(), GroupError> {
        let u = normalize_acct(user, &self.group_acct)?;
        if self.is_admin {
            if self.config.is_admin(&u) {
                match self.config.set_admin(&u, false) {
                    Ok(_) => {
                        self.add_reply(format!("User {} is no longer a group admin!", u));
                    }
                    Err(e) => {
                        self.add_reply(format!("Failed to revoke {}'s group admin: {}", u, e));
                    }
                }
            } else {
                self.add_reply(format!("No action, user {} is not admin", u));
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
        Ok(())
    }

    async fn cmd_open_group(&mut self) {
        if self.is_admin {
            if self.config.is_member_only() {
                self.config.set_member_only(false);
                self.add_reply("Group changed to open-access");
            } else {
                self.add_reply("No action, group is open-access already");
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
    }

    async fn cmd_close_group(&mut self) {
        if self.is_admin {
            if !self.config.is_member_only() {
                self.config.set_member_only(true);
                self.add_reply("Group changed to member-only");
            } else {
                self.add_reply("No action, group is member-only already");
            }
        } else {
            warn!("Ignore cmd, user not admin");
        }
    }

    async fn cmd_help(&mut self) {
        self.want_markdown = true;

        let membership_line = if self.is_admin {
            "*You are an admin.*"
        } else if self.config.is_member(&self.status_acct) {
            "*You are a member.*"
        } else if self.config.is_member_only() {
            "*You are not a member, ask one of the admins to add you.*"
        } else {
            "*You are not a member, follow or use /join to join the group.*"
        };

        if self.config.is_member_only() {
            self.add_reply(format!("This is a member-only group. {}", membership_line));
        } else {
            self.add_reply(format!("This is a public-access group. {}", membership_line));
        }

        self.add_reply("\
            To share a post, @ the group user or use a group hashtag.\n\
            \n\
            **Supported commands:**\n\
            `/boost`, `/b` - boost the replied-to post into the group\n\
            `/ignore`, `/i` - make the group ignore the post\n\
            `/tags` - show group hashtags\n\
            `/join` - (re-)join the group\n\
            `/leave` - leave the group\n\
            `/optout` - forbid sharing of your posts");

        if self.is_admin {
            self.add_reply("`/members`, `/who` - show group members / admins");
            // undo is listed as an admin command
        } else {
            self.add_reply("`/admins` - show group admins");
            self.add_reply("`/undo` - un-boost your post (use in a reply)");
        }

        // XXX when used on instance with small character limit, this won't fit!

        if self.is_admin {
            self.add_reply("\n\
                **Admin commands:**\n\
                `/ping` - check the group works\n\
                `/add user` - add a member (user@domain)\n\
                `/remove user` - remove a member\n\
                `/add #hashtag` - add a group hashtag\n\
                `/remove #hashtag` - remove a group hashtag\n\
                `/undo` - un-boost a replied-to post, delete an announcement\n\
                `/ban x` - ban a user or server\n\
                `/unban x` - lift a ban\n\
                `/admin user` - grant admin rights\n\
                `/deadmin user` - revoke admin rights\n\
                `/closegroup` - make member-only\n\
                `/opengroup` - make public-access\n\
                `/announce x` - make a public announcement");
        }
    }

    async fn cmd_list_members(&mut self) {
        self.want_markdown = true;
        if self.is_admin {
            self.add_reply("Group members:");
            self.append_member_list_to_reply();
        } else {
            self.add_reply("Group admins:");
            self.append_admin_list_to_reply();
        }
    }

    async fn cmd_list_tags(&mut self) {
        self.add_reply("Group tags:");
        self.want_markdown = true;
        let mut tags = self.config.get_tags().collect::<Vec<_>>();
        tags.sort();
        for t in tags {
            self.replies.push(format!("- {}", t).to_string());
        }
    }

    async fn cmd_leave(&mut self) {
        if self.config.is_member_or_admin(&self.status_acct) {
            // admin can leave but that's a bad idea
            let _ = self.config.set_member(&self.status_acct, false);
            self.add_reply("You're no longer a group member. Unfollow the group user to stop receiving group messages.");
        }

        self.unfollow_user_by_id(&self.status_user_id).await
            .log_error("Failed to unfollow");
    }

    async fn cmd_join(&mut self) {
        if self.config.is_member_or_admin(&self.status_acct) {
            debug!("Already member or admin, try to follow-back again");
            // Already a member, so let's try to follow the user
            // again, maybe first time it failed
            self.follow_user_by_id(&self.status_user_id).await
                .log_error("Failed to follow");
        } else {
            // Not a member yet
            if self.config.is_member_only() {
                // No you can't
                self.add_reply("\
                    Sorry, this group is closed to new sign-ups.\n\
                    Please ask one of the group admins to add you:");

                self.append_admin_list_to_reply();
            } else {
                // Open access, try to follow back
                self.follow_user_by_id(&self.status_user_id).await
                    .log_error("Failed to follow");

                // This only fails if the user is banned, but that is filtered above
                let _ = self.config.set_member(&self.status_acct, true);
                self.add_reply("\
                    Welcome to the group! The group user will now follow you to complete the sign-up. \
                    Make sure you follow back to receive shared posts!\n\n\
                    Use /help for more info.");
            }
        }
    }

    async fn cmd_ping(&mut self) {
        self.add_reply(format!("pong, this is fedigroups service v{}", env!("CARGO_PKG_VERSION")));
    }

    async fn unfollow_by_acct(&self, acct: &str) -> Result<(), GroupError> {
        // Try to unfollow
        if let Ok(Some(id)) = self.lookup_acct_id(acct, true).await {
            self.unfollow_user_by_id(&id).await?;
        }
        Ok(())
    }

    async fn follow_by_acct(&self, acct: &str) -> Result<(), GroupError> {
        // Try to unfollow
        if let Ok(Some(id)) = self.lookup_acct_id(acct, false).await {
            self.follow_user_by_id(&id).await?;
        }
        Ok(())
    }

    /// Convert ID to account, checking if the user is boostable
    async fn id_to_acct_check_boostable(&self, id: &str) -> Result<String, GroupError> {
        // Try to unfollow
        let account = self.client.get_account(id).await?;
        let bio = utils::strip_html(&account.note);
        if RE_NOBOT_TAG.is_match(&bio) {
            // #nobot
            Err(GroupError::UserOptedOut)
        } else {
            let normalized = normalize_acct(&account.acct, &self.group_acct)?;
            if self.config.is_banned(&normalized) {
                return Err(GroupError::UserIsBanned);
            } else if self.config.is_optout(&normalized) {
                return Err(GroupError::UserOptedOut);
            } else {
                Ok(normalized)
            }
        }
    }
}

fn apply_trailing_hashtag_pleroma_bug_workaround(msg: &mut String) {
    if crate::command::RE_HASHTAG_TRIGGERING_PLEROMA_BUG.is_match(&msg) {
        // if a status ends with a hashtag, pleroma will fuck it up
        debug!("Adding \" .\" to fix pleroma hashtag eating bug!");
        msg.push_str(" .");
    }
}

fn smart_split(msg : &str, prefix: Option<String>, limit: usize) -> Vec<String> {
    let prefix = prefix.unwrap_or_default();

    if msg.len() + prefix.len() < limit {
        return vec![format!("{}{}", prefix, msg)];
    }

    let mut parts_to_send = vec![];
    let mut this_piece = prefix.clone();
    for l in msg.split("\n") {
        if this_piece.len() + l.len() == limit {
            this_piece.push_str(l);
            parts_to_send.push(std::mem::take(&mut this_piece).trim().to_owned());
            this_piece.push_str(&prefix);
        } else if this_piece.len() + l.len() > limit {
            let trimmed = this_piece.trim();
            if !trimmed.is_empty() {
                parts_to_send.push(trimmed.to_owned());
            }

            // start new piece
            this_piece = format!("{}{}", prefix, l);

            while this_piece.len() > limit {
                let to_send = if let Some(last_space) = (&this_piece[..limit]).rfind(' ') {
                    let mut p = this_piece.split_off(last_space);
                    std::mem::swap(&mut p, &mut this_piece);
                    p
                } else {
                    let mut p = this_piece.split_off(limit);
                    std::mem::swap(&mut p, &mut this_piece);
                    p
                };
                parts_to_send.push(to_send);
                this_piece = format!("{}{}", prefix, this_piece);
            }
            this_piece.push('\n');
        } else {
            this_piece.push_str(l);
            this_piece.push('\n');
        }
    }

    parts_to_send
}

#[cfg(test)]
mod test {
    #[test]
    fn test_smart_split1() {
        let to_split = "a234567890\nb234567890\nc234567890\nd234\n67890\ne234567890\n";

        let parts = super::smart_split(to_split, None, 10);
        assert_eq!(vec![
            "a234567890".to_string(),
            "b234567890".to_string(),
            "c234567890".to_string(),
            "d234\n67890".to_string(),
            "e234567890".to_string(),
        ], parts);
    }

    #[test]
    fn test_smart_split2() {
        let to_split = "foo\nbar\nbaz";

        let parts = super::smart_split(to_split, None, 1000);
        assert_eq!(vec![
            "foo\nbar\nbaz".to_string(),
        ], parts);
    }

    #[test]
    fn test_smart_split3() {
        let to_split = "foo\nbar\nbaz";
        let parts = super::smart_split(to_split, Some("PREFIX".to_string()), 1000);
        assert_eq!(vec![
            "PREFIXfoo\nbar\nbaz".to_string(),
        ], parts);
    }

    #[test]
    fn test_smart_split4() {
        let to_split = "1234\n56\n7";
        let parts = super::smart_split(to_split, Some("PREFIX".to_string()), 10);
        assert_eq!(vec![
            "PREFIX1234".to_string(),
            "PREFIX56\n7".to_string(),
        ], parts);
    }
}