//! Mail-to-action translation (PLAN.md, Management mail): a post mail is
//! bare text or a block whose only field is Reply-To; a command mail opens a
//! §5.5 block. Pure — the caller owns admission, retention and error replies.

use smolboid_core::feed::MailLine;
use smolboid_core::rules::validate_text;
use smolboid_core::time;
use smolboid_core::{repost_body, reply_to_body};

use crate::state::PruneMode;

/// One fetched mail, opened and verified by the caller.
pub struct Incoming {
    pub sender: [u8; 32],
    pub epk: [u8; 32],
    pub sig: [u8; 64],
    pub time: i64,
    pub body: Vec<u8>,
}

pub enum Action {
    Post {
        timestamp: String,
        text: String,
        /// The Reply-To value the body carried, when the ordinary-send block
        /// form was used; bound in the feed's `smol:reply_to` metadata.
        reply_to: Option<String>,
        mail: MailLine,
    },
    Repost {
        timestamp: String,
        args: String,
        reply_to: Option<String>,
        mail: MailLine,
    },
    Commands(Vec<Command>),
    /// Nothing to do: recognized by the rules as benign, never published.
    Ignore,
}

pub enum Command {
    Nick(String),
    Delete(String),
    Prune(PruneMode),
}

/// The `Twt-` frontmatter fields a command mail may carry; unknown keys are
/// preserved upstream and never alter behaviour (SPEC.md §5.5).
const F_NICK: &str = "twt-nick";
const F_DELETE: &str = "twt-delete";
const F_REPOST: &str = "twt-repost";
const F_PRUNE: &str = "twt-prune";
const F_REPLY_TO: &str = "reply-to";

/// Fails closed: a block with unreconstructible fields is never published.
pub fn dispatch(incoming: &Incoming) -> Result<Action, String> {
    let text = std::str::from_utf8(&incoming.body).map_err(|_| "body is not UTF-8".to_owned())?;

    if !text.starts_with("---\n") {
        // The bare post form: the body is the twt, byte for byte.
        return post(incoming, text, None);
    }

    let (fields, rest) = fumi_core::message::parse_frontmatter(text);
    if fields.is_empty() {
        // An empty or malformed block: nothing reliable, nothing published.
        return Ok(Action::Ignore);
    }

    let reply_to = fields.get(F_REPLY_TO).cloned();
    let beyond_reply_to: Vec<&String> = fields
        .keys()
        .filter(|k| k.as_str() != F_REPLY_TO)
        .collect();

    // A post in the ordinary-send form: a block whose only field is Reply-To.
    if beyond_reply_to.is_empty() {
        if let Some(reply_to) = &reply_to {
            if text != reply_to_body(reply_to, &rest) {
                return Err("post mail is not in canonical form".to_owned());
            }
            return post(incoming, &rest, Some(reply_to.clone()));
        }
    }

    // A repost: the Twt-Repost field, optionally with the same Reply-To an
    // ordinary send carries; both must reconstruct byte-exactly.
    if fields.len() <= 2 {
        if let Some(args) = fields.get(F_REPOST) {
            let canonical_no = repost_body(args, None);
            let canonical_yes = repost_body(args, reply_to.as_deref());
            if text == canonical_no || text == canonical_yes {
                validate_text(args).map_err(|e| e.to_string())?;
                return Ok(Action::Repost {
                    timestamp: time::render(incoming.time),
                    args: args.clone(),
                    reply_to,
                    mail: MailLine {
                        sender: incoming.sender,
                        epk: incoming.epk,
                        sig: incoming.sig,
                    },
                });
            }
            return Err("repost mail is not in canonical form".to_owned());
        }
    }

    // A block with extra fields and no Twt field: the author plainly meant to
    // publish, and silence would hide the problem.
    if !fields.contains_key(F_NICK)
        && !fields.contains_key(F_DELETE)
        && !fields.contains_key(F_PRUNE)
        && !fields.contains_key(F_REPOST)
    {
        return Err(
            "post mail carries fields beyond Reply-To (Subject, headers or an Accept token); \
             resend the bare text"
                .to_owned(),
        );
    }

    let mut commands = Vec::new();
    if let Some(nick) = fields.get(F_NICK) {
        if !crate::feedstore::valid_username(nick) {
            return Err("nick must be 1-30 bytes of a-z, 0-9 or hyphen".to_owned());
        }
        commands.push(Command::Nick(nick.clone()));
    }
    if let Some(timestamp) = fields.get(F_DELETE) {
        if time::parse(timestamp).is_none() {
            return Err("Twt-Delete needs an RFC 3339 UTC timestamp".to_owned());
        }
        commands.push(Command::Delete(timestamp.clone()));
    }
    if let Some(mode) = fields.get(F_PRUNE) {
        commands.push(Command::Prune(match mode.as_str() {
            "fifo" => PruneMode::Fifo,
            "off" => PruneMode::Off,
            other => return Err(format!("unknown prune mode `{other}`, want fifo or off")),
        }));
    }
    // A mixed mail is a command mail; the body is ignored, never posted.
    if commands.is_empty() {
        Ok(Action::Ignore)
    } else {
        Ok(Action::Commands(commands))
    }
}

fn post(incoming: &Incoming, text: &str, reply_to: Option<String>) -> Result<Action, String> {
    validate_text(text).map_err(|e| e.to_string())?;
    Ok(Action::Post {
        timestamp: time::render(incoming.time),
        text: text.to_owned(),
        reply_to,
        mail: MailLine {
            sender: incoming.sender,
            epk: incoming.epk,
            sig: incoming.sig,
        },
    })
}

#[cfg(test)]
mod tests {
    use super::*;

    fn incoming(body: &str) -> Incoming {
        Incoming {
            sender: [1; 32],
            epk: [2; 32],
            sig: [3; 64],
            time: 1_760_000_000,
            body: body.as_bytes().to_vec(),
        }
    }

    #[test]
    fn plain_body_is_a_post() {
        let Action::Post { timestamp, text, reply_to, mail } = dispatch(&incoming("hello smolverse")).unwrap() else {
            panic!("expected a post");
        };
        assert_eq!(timestamp, "2025-10-09T08:53:20Z");
        assert_eq!(text, "hello smolverse");
        assert!(reply_to.is_none());
        assert_eq!(mail.sender, [1; 32]);
    }

    #[test]
    fn ordinary_send_form_is_a_post() {
        // What `fumi send --body x` emits: a block with only Reply-To.
        let body = "---\nReply-To: smol://tobias@smol.place/k\n---\nhello smolverse";
        let Action::Post { text, reply_to, .. } = dispatch(&incoming(body)).unwrap() else {
            panic!("expected a post");
        };
        assert_eq!(text, "hello smolverse");
        assert_eq!(reply_to.as_deref(), Some("smol://tobias@smol.place/k"));
    }

    #[test]
    fn dashed_body_is_still_a_post() {
        // A single line can never open a block, so nothing is escaped.
        let Action::Post { text, .. } = dispatch(&incoming("--- dashed opening")).unwrap() else {
            panic!("expected a post");
        };
        assert_eq!(text, "--- dashed opening");
    }

    #[test]
    fn post_rules_are_enforced_with_reasons() {
        assert!(dispatch(&incoming(&"a".repeat(161))).is_err());
        assert!(dispatch(&incoming("a\tb")).is_err());
        let bad = Incoming { body: vec![0xff, 0xfe], ..incoming("") };
        assert!(dispatch(&bad).is_err());
    }

    #[test]
    fn extra_fields_make_a_rejected_post() {
        // A Subject, or the Accept token of an accepted recipient.
        assert!(dispatch(&incoming("---\nReply-To: smol://a/b\nSubject: hi\n---\nbody\n")).is_err());
        assert!(dispatch(&incoming("---\nAccept: token\n---\nbody\n")).is_err());
    }

    #[test]
    fn canonical_repost_command_publishes() {
        let args = "@<dana https://smol.place/twtxt/dana.txt> #abc1234";
        let no_reply_to = format!("---\nTwt-Repost: {args}\n---\n");
        let Action::Repost { args: got, reply_to, mail, .. } =
            dispatch(&incoming(&no_reply_to)).unwrap() else {
                panic!("expected a repost");
            };
        assert_eq!(got, args);
        assert!(reply_to.is_none());
        assert_eq!(mail.sig, [3; 64]);

        // The ordinary-send form, Twt-Repost before Reply-To as fumi emits.
        let with_reply_to = format!("---\nTwt-Repost: {args}\nReply-To: smol://tobias@s/k\n---\n");
        let Action::Repost { args: got, reply_to, .. } =
            dispatch(&incoming(&with_reply_to)).unwrap() else {
                panic!("expected a repost");
            };
        assert_eq!(got, args);
        assert_eq!(reply_to.as_deref(), Some("smol://tobias@s/k"));
    }

    #[test]
    fn noncanonical_reposts_are_rejected() {
        let args = "@<d u> #abc";
        assert!(dispatch(&incoming(&format!("---\nTwt-Repost:  {args} \n---\n"))).is_err());
        assert!(dispatch(&incoming(&format!("---\nTwt-Repost: {args}\n---\nextra\n"))).is_err());
        assert!(dispatch(&incoming(&format!(
            "---\nTwt-Repost: {args}\nTwt-Nick: x\n---\n"
        )))
        .is_err());
        // Reply-To after the closing marker, wrong order relative to the
        // canonical forms.
        assert!(dispatch(&incoming(&format!(
            "---\nReply-To: smol://a/b\nTwt-Repost: {args}\n---\n"
        )))
        .is_err());
    }

    #[test]
    fn command_mails_apply_all_recognized_fields() {
        let body = "---\nTwt-Nick: toby\nTwt-Prune: fifo\nSubject: ignored\n---\nignored body\n";
        let Action::Commands(cmds) = dispatch(&incoming(body)).unwrap() else {
            panic!("expected commands");
        };
        assert_eq!(cmds.len(), 2);
        assert!(matches!(cmds[0], Command::Nick(ref n) if n == "toby"));
        assert!(matches!(cmds[1], Command::Prune(PruneMode::Fifo)));
    }

    #[test]
    fn command_values_are_validated() {
        assert!(dispatch(&incoming("---\nTwt-Nick: Bad Nick\n---\n")).is_err());
        assert!(dispatch(&incoming("---\nTwt-Delete: not-a-date\n---\n")).is_err());
        assert!(dispatch(&incoming("---\nTwt-Prune: sometimes\n---\n")).is_err());
    }

    #[test]
    fn odd_mail_shapes_fail_closed_to_ignore() {
        // Malformed block, empty escape block, only unknown fields with a
        // Twt- field absent — but pure unknown-field blocks are rejected
        // as posts, so the ignore set is the malformed and empty ones.
        assert!(matches!(dispatch(&incoming("---\nno colon here\n---\n")), Ok(Action::Ignore)));
        assert!(matches!(dispatch(&incoming("---\n---\n")), Ok(Action::Ignore)));
    }
}