//! Which of a PR's reviews still stand: the rule `hive-forge pr
//! assign-reviewer` and the notification router's `request-reviewer` sink
//! share, so a request is skipped or made on the same grounds in both.
//!
//! The forge keeps a review's `state` (`APPROVED`, `REQUEST_CHANGES`, …) after
//! the head moves or the review is dismissed, so reading the state alone
//! reports a review that no longer applies as still good. [`latest_reviews`]
//! folds a PR's reviews to one verdict per reviewer, and
//! [`ReviewInfo::superseded`] says whether it still counts.
//!
//! Re-requesting a review from a reviewer whose review stands makes the forge
//! dismiss it, approval included. [`standing`] is the check a caller makes
//! before it requests.

/// One review as the forge lists it, reduced to what the rule reads. Every
/// field is optional because the forge's listing is.
#[derive(Clone, Copy, Debug, Default)]
pub struct Listed<'a> {
    pub login: Option<&'a str>,
    pub state: Option<&'a str>,
    /// The commit the review was submitted against.
    pub commit_id: Option<&'a str>,
    /// The forge's own `stale` flag.
    pub stale: Option<bool>,
    pub dismissed: Option<bool>,
}

/// One reviewer's latest verdict on a PR, plus the `stale` / `dismissed` bits.
///
/// `stale` is set by the forge when the PR head commit changed after this
/// review was submitted; `dismissed` is set when the review was explicitly
/// dismissed. Either way the review no longer applies to the current head even
/// though its `state` string still reads `APPROVED` / `REQUEST_CHANGES`.
///
/// ⚠️ **`stale` here is not the forge's flag alone.** That flag is eventually
/// consistent: seconds after a push it still reports the pre-push answer, so a
/// verdict against the previous head reads as current in exactly the window
/// where a push has just happened. [`latest_reviews`] therefore ORs it with a
/// direct comparison of the review's own `commit_id` against the PR head: the
/// flag is right *eventually*, the comparison is right *immediately*.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct ReviewInfo {
    pub login: String,
    pub state: String,
    pub stale: bool,
    pub dismissed: bool,
}

impl ReviewInfo {
    /// True when the review no longer applies to the current head: stale
    /// (head moved) or dismissed. Such a verdict neither blocks a merge nor
    /// counts as a fresh approval.
    #[must_use]
    pub fn superseded(&self) -> bool {
        self.stale || self.dismissed
    }
}

/// `login`'s review that still stands, if it has one. Its presence is the
/// reason not to request a review from `login` again.
#[must_use]
pub fn standing<'a>(reviews: &'a [ReviewInfo], login: &str) -> Option<&'a ReviewInfo> {
    reviews.iter().find(|r| r.login == login && !r.superseded())
}

/// Whether this review was submitted against a commit that is no longer the PR
/// head: the race-free half of the staleness check.
///
/// Fails **closed on unknowns**, i.e. "not stale": either side missing means
/// the head's move can't be shown, and guessing wrong in that direction costs
/// one redundant re-review, where the other direction would void every
/// verdict on the PR and invite a re-request that dismisses a real approval.
#[must_use]
pub fn reviewed_older_head(reviewed_sha: Option<&str>, head_sha: Option<&str>) -> bool {
    let (Some(head), Some(reviewed)) = (head_sha, reviewed_sha) else {
        return false;
    };
    // Both emptiness checks matter, and for the same reason: a blank string is
    // a value the forge sent, not a sha it has. Treating one as real would make
    // every review compare unequal and mark the whole PR stale.
    !head.is_empty() && !reviewed.is_empty() && reviewed != head
}

/// Latest non-comment review per reviewer. `reviews` come oldest first, so a
/// later verdict from the same reviewer supersedes an earlier one; `COMMENT`
/// and `PENDING` reviews carry no verdict and are skipped. `head_sha` is the
/// PR's current head, or `None` when it couldn't be read, which leaves the
/// forge's `stale` flag alone to decide.
#[must_use]
pub fn latest_reviews<'a>(
    reviews: impl IntoIterator<Item = Listed<'a>>,
    head_sha: Option<&str>,
) -> Vec<ReviewInfo> {
    let mut latest: Vec<ReviewInfo> = Vec::new();
    for r in reviews {
        let Some(login) = r.login else { continue };
        let verdict = r.state.unwrap_or("");
        if verdict == "COMMENT" || verdict == "PENDING" || verdict.is_empty() {
            continue;
        }
        let stale = r.stale.unwrap_or(false) || reviewed_older_head(r.commit_id, head_sha);
        let dismissed = r.dismissed.unwrap_or(false);
        if let Some(slot) = latest.iter_mut().find(|info| info.login == login) {
            verdict.clone_into(&mut slot.state);
            slot.stale = stale;
            slot.dismissed = dismissed;
        } else {
            latest.push(ReviewInfo {
                login: login.to_owned(),
                state: verdict.to_owned(),
                stale,
                dismissed,
            });
        }
    }
    latest
}

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

    fn review<'a>(login: &'a str, state: &'a str, commit: &'a str) -> Listed<'a> {
        Listed {
            login: Some(login),
            state: Some(state),
            commit_id: Some(commit),
            ..Listed::default()
        }
    }

    #[test]
    fn review_on_an_older_commit_is_stale() {
        // The forge still reports `stale: false` here in the seconds after a
        // push, so the comparison has to catch it.
        assert!(reviewed_older_head(Some("81292f14"), Some("f4c47088")));
    }

    #[test]
    fn review_on_the_current_head_is_not_stale() {
        assert!(!reviewed_older_head(Some("f4c47088"), Some("f4c47088")));
    }

    /// Every unknown fails *toward* keeping the verdict. Voiding every review
    /// on a forge that stopped reporting one of these would be a far louder
    /// wrong answer than one redundant re-review.
    #[test]
    fn unknown_commit_or_head_is_not_stale() {
        assert!(!reviewed_older_head(None, Some("f4c47088")));
        assert!(!reviewed_older_head(Some("81292f14"), None));
        assert!(!reviewed_older_head(Some(""), Some("f4c47088")));
        // Absent and blank have to behave the same on BOTH sides: a blank head
        // that counted as real would mark every review on the PR stale.
        assert!(!reviewed_older_head(Some("81292f14"), Some("")));
        assert!(!reviewed_older_head(Some(""), Some("")));
    }

    #[test]
    fn a_review_of_the_current_head_stands_and_one_of_an_older_head_does_not() {
        let reviews = latest_reviews(
            [
                review("review-classifier", "APPROVED", "new"),
                review("argus", "APPROVED", "old"),
            ],
            Some("new"),
        );
        assert_eq!(
            standing(&reviews, "review-classifier").map(|r| r.state.as_str()),
            Some("APPROVED")
        );
        // The forge's flag is still false for argus: only the commit says so.
        assert!(standing(&reviews, "argus").is_none());
        // Control: a reviewer who never reviewed has nothing standing.
        assert!(standing(&reviews, "iris").is_none());
    }

    #[test]
    fn a_dismissed_or_flagged_review_does_not_stand() {
        let reviews = latest_reviews(
            [
                Listed {
                    dismissed: Some(true),
                    ..review("a", "APPROVED", "new")
                },
                Listed {
                    stale: Some(true),
                    ..review("b", "REQUEST_CHANGES", "new")
                },
                review("c", "APPROVED", "new"),
            ],
            Some("new"),
        );
        assert!(standing(&reviews, "a").is_none());
        assert!(standing(&reviews, "b").is_none());
        assert!(standing(&reviews, "c").is_some(), "control");
    }

    #[test]
    fn the_latest_verdict_per_reviewer_wins_and_comments_carry_none() {
        let reviews = latest_reviews(
            [
                review("a", "REQUEST_CHANGES", "old"),
                review("a", "APPROVED", "new"),
                review("a", "COMMENT", "new"),
                review("b", "PENDING", "new"),
                Listed {
                    login: None,
                    ..review("x", "APPROVED", "new")
                },
            ],
            Some("new"),
        );
        assert_eq!(
            reviews,
            [ReviewInfo {
                login: "a".to_owned(),
                state: "APPROVED".to_owned(),
                stale: false,
                dismissed: false,
            }]
        );
    }

    /// With the head unread, the forge's flag decides alone, so a review of an
    /// unknown commit stands.
    #[test]
    fn an_unread_head_leaves_the_forges_flag_to_decide() {
        let reviews = latest_reviews([review("a", "APPROVED", "old")], None);
        assert!(standing(&reviews, "a").is_some());
    }
}