//! PR-based config-flow merge primitives — the forge-side mechanics
//! hive-c0re's deploy apply node (`actions::run_deploy_apply`) orchestrates to
//! land an operator-approved config PR. Part of the operator trust
//! boundary; moved verbatim from the `forge` module root.

use anyhow::Context;
use forgejo_api::structs::{MergePullRequestOption, MergePullRequestOptionDo, StateType};

use super::{CONFIG_ORG, api, core_token, forge_git_url};

// ---------------------------------------------------------------------------
// PR-based config-flow merge primitives (part of the
// dashboard-approve-driven config-change flow).
//
// The dashboard-approve-driven flow has hive-c0re verify an operator-approved
// config PR, then land it: fast-forward-merge the verified sha into the
// protected default branch via the forge merge API (= the merge). These fns are
// the forge-side mechanics the c0re deploy apply node (`run_deploy_apply`)
// orchestrates; the orchestration fetches the verified sha into the agent's
// applied repo (for the eval-verify) before calling `merge_config_pr_ff`. The
// core token is sourced internally (`core_token`), never passed in. `repo` is
// the agent's editable forge config repo in `owner/name` form (e.g.
// `agent-configs/<agent>`).
// ---------------------------------------------------------------------------

/// Typed failure for the merge primitives so the c0re approve-handler can
/// `match` recoverable drift (surface a re-review message) against a hard
/// failure (fail the approval). `HeadDrift` carries the observed sha; `Other`
/// is everything else (a raced non-ff `main`, transport, API, unexpected) and
/// is not auto-retried.
#[derive(Debug)]
pub enum ForgeMergeError {
    /// The PR head moved off `expected` (now at `actual`) since the handler's
    /// pre-merge `pr_head_sha` re-read — caught by the `head_commit_id` pin on
    /// the merge call, which refuses to merge a head that isn't the reviewed sha.
    HeadDrift { expected: String, actual: String },
    /// Transport / API / unexpected failure — hard-fail, no auto-retry. Covers
    /// a raced non-fast-forwardable `main` (the `fast-forward-only` merge is
    /// refused by the forge) as well.
    Other(anyhow::Error),
}

impl std::fmt::Display for ForgeMergeError {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Self::HeadDrift { expected, actual } => {
                write!(f, "PR head drifted: expected {expected}, found {actual}")
            }
            Self::Other(e) => write!(f, "{e}"),
        }
    }
}

impl std::error::Error for ForgeMergeError {}

impl From<anyhow::Error> for ForgeMergeError {
    fn from(e: anyhow::Error) -> Self {
        Self::Other(e)
    }
}

/// Agent name from an `owner/name` forge repo string (the trailing segment).
fn repo_agent_name(repo: &str) -> &str {
    repo.rsplit('/').next().unwrap_or(repo)
}

/// Resolve a PR's head sha via `git ls-remote <repo> refs/pull/<pr>/head`
/// (Forgejo exposes PR heads there). Pure read, no mutation — the handler's
/// primary drift gate (compare against the approved sha), and
/// `merge_config_pr_ff` uses it to classify a merge-API rejection as head-drift.
///
/// # Errors
/// `Other` on transport failure or an empty/missing ref.
pub async fn pr_head_sha(repo: &str, pr: u64) -> Result<String, ForgeMergeError> {
    let token = core_token()
        .ok_or_else(|| ForgeMergeError::Other(anyhow::anyhow!("forge core token absent")))?;
    let url = forge_git_url(&token, repo);
    let refspec = format!("refs/pull/{pr}/head");
    let out = crate::lifecycle::git_command()
        .args(["ls-remote", &url, &refspec])
        .output()
        .await
        .context("git ls-remote PR head")?;
    if !out.status.success() {
        return Err(ForgeMergeError::Other(anyhow::anyhow!(
            "git ls-remote {repo} {refspec} failed ({}): {}",
            out.status,
            String::from_utf8_lossy(&out.stderr).trim()
        )));
    }
    let stdout = String::from_utf8_lossy(&out.stdout);
    let sha = stdout
        .split_whitespace()
        .next()
        .filter(|s| !s.is_empty())
        .ok_or_else(|| {
            ForgeMergeError::Other(anyhow::anyhow!(
                "no head ref for PR #{pr} in {repo} (ls-remote empty)"
            ))
        })?;
    Ok(sha.to_string())
}

/// Check whether PR `pr` on `repo` is still open.  Returns `Ok(true)` if
/// open, `Ok(false)` if closed or merged, or an error on transport failure.
///
/// Called at submission time to give an early, actionable error rather than
/// queuing an approval card that will fail later in the approve handler.
///
/// # Errors
/// `Other` on transport failure or a missing/malformed PR response.
pub async fn pr_is_open(repo: &str, pr: u64) -> Result<bool, ForgeMergeError> {
    let token = core_token()
        .ok_or_else(|| ForgeMergeError::Other(anyhow::anyhow!("forge core token absent")))?;
    let (owner, name) = repo.split_once('/').ok_or_else(|| {
        ForgeMergeError::Other(anyhow::anyhow!("forge repo `{repo}` is not owner/name"))
    })?;
    let index = i64::try_from(pr)
        .map_err(|_| ForgeMergeError::Other(anyhow::anyhow!("PR index {pr} overflows i64")))?;
    let client = api(&token).map_err(ForgeMergeError::Other)?;
    let pull = client
        .repo_get_pull_request(owner, name, index)
        .await
        .map_err(|e| ForgeMergeError::Other(anyhow::Error::from(e).context("GET pull request")))?;
    Ok(pull.state == Some(StateType::Open))
}

/// Full `owner/name` path of an agent's config repo on the forge — the
/// `agent-configs` org mirror that the PR-merge flow reads + fast-forwards.
pub fn config_repo(agent: &str) -> String {
    format!("{CONFIG_ORG}/{agent}")
}

/// Fetch PR #`pr`'s head into the agent's applied repo via
/// `refs/pull/<pr>/head` (which Forgejo always serves — a bare-sha fetch can
/// be refused by uploadpack policy). This makes the reviewed head an object
/// in the applied repo so the `git_update_ref(main, …)` in the deploy tail and
/// the eval-verify both resolve it locally before the irreversible merge.
///
/// # Errors
/// `Other` on transport failure or a non-zero git exit.
pub async fn fetch_pr_head_into_applied(repo: &str, pr: u64) -> Result<(), ForgeMergeError> {
    let token = core_token()
        .ok_or_else(|| ForgeMergeError::Other(anyhow::anyhow!("forge core token absent")))?;
    let url = forge_git_url(&token, repo);
    let applied = crate::paths::applied_dir(repo_agent_name(repo));
    let refspec = format!("refs/pull/{pr}/head");
    let out = crate::lifecycle::git_command()
        .current_dir(&applied)
        .args(["fetch", "--no-tags", &url, &refspec])
        .output()
        .await
        .context("git fetch PR head into applied")?;
    if !out.status.success() {
        return Err(ForgeMergeError::Other(anyhow::anyhow!(
            "git fetch {repo} {refspec} into applied failed ({}): {}",
            out.status,
            String::from_utf8_lossy(&out.stderr).trim()
        )));
    }
    Ok(())
}

/// Fast-forward-merge PR `pr` on `repo` to `sha` via the Forgejo merge API —
/// THE merge in the config-PR flow. Uses `Do=fast-forward-only` so `main` only
/// ever advances by fast-forward (never a merge commit; a raced non-ff `main`
/// is refused by the forge → `Other`), and pins `head_commit_id = sha` so the
/// forge atomically refuses to merge a head that isn't the reviewed sha —
/// closing the check-then-merge race without ever pushing the protected branch.
/// `core` needs only the repo's merge whitelist, never push access to `main`.
/// The single call fast-forwards `main` to exactly `sha` and marks the PR
/// merged.
///
/// # Errors
/// `HeadDrift` if the live PR head no longer matches `sha` (the pin rejected
/// it); `Other` for a raced non-ff `main`, transport, or API failure.
pub async fn merge_config_pr_ff(repo: &str, pr: u64, sha: &str) -> Result<(), ForgeMergeError> {
    let token = core_token()
        .ok_or_else(|| ForgeMergeError::Other(anyhow::anyhow!("forge core token absent")))?;
    let (owner, name) = repo.split_once('/').ok_or_else(|| {
        ForgeMergeError::Other(anyhow::anyhow!("forge repo `{repo}` is not owner/name"))
    })?;
    let index = i64::try_from(pr)
        .map_err(|_| ForgeMergeError::Other(anyhow::anyhow!("PR index {pr} overflows i64")))?;
    let body = MergePullRequestOption {
        r#do: MergePullRequestOptionDo::FastForwardOnly,
        merge_commit_id: None,
        merge_message_field: None,
        merge_title_field: None,
        delete_branch_after_merge: None,
        force_merge: None,
        head_commit_id: Some(sha.to_owned()),
        merge_when_checks_succeed: None,
    };
    let client = api(&token).map_err(ForgeMergeError::Other)?;
    match client
        .repo_merge_pull_request(owner, name, index, body)
        .await
    {
        Ok(()) => Ok(()),
        Err(e) => {
            // Classify: a live head that no longer matches `sha` is drift (the
            // `head_commit_id` pin rejected it); everything else — a raced
            // non-ff `main`, transport, an API error — is a hard failure.
            // Best-effort, since the handler's pre-merge re-read is the primary
            // gate; a transport failure that also breaks this read falls through
            // to `Other`.
            match pr_head_sha(repo, pr).await {
                Ok(actual) if actual != sha => Err(ForgeMergeError::HeadDrift {
                    expected: sha.to_string(),
                    actual,
                }),
                _ => Err(ForgeMergeError::Other(
                    anyhow::Error::from(e).context(format!("ff-merge PR #{pr} in {repo} to {sha}")),
                )),
            }
        }
    }
}

/// Post a comment to PR (= issue) `pr` on `repo` as the core forge user.
/// PRs are issues in Forgejo, so the PR number is the issue index. Used to
/// surface a failed config-approval deploy's build log back onto the PR so
/// the manager sees why it was rejected without leaving the forge. `repo`
/// is `owner/name`.
///
/// # Errors
/// `Other` on absent core token, malformed repo, or transport/API failure.
pub async fn post_pr_comment(repo: &str, pr: u64, body: &str) -> Result<(), ForgeMergeError> {
    let token = core_token()
        .ok_or_else(|| ForgeMergeError::Other(anyhow::anyhow!("forge core token absent")))?;
    let (owner, name) = repo.split_once('/').ok_or_else(|| {
        ForgeMergeError::Other(anyhow::anyhow!("forge repo `{repo}` is not owner/name"))
    })?;
    let index = i64::try_from(pr)
        .map_err(|_| ForgeMergeError::Other(anyhow::anyhow!("PR index {pr} overflows i64")))?;
    let client = api(&token).map_err(ForgeMergeError::Other)?;
    client
        .issue_create_comment(
            owner,
            name,
            index,
            forgejo_api::structs::CreateIssueCommentOption {
                body: body.to_owned(),
                updated_at: None,
            },
        )
        .await
        .map_err(|e| ForgeMergeError::Other(anyhow::Error::from(e).context("post PR comment")))?;
    Ok(())
}

#[cfg(test)]
mod tests {
    use super::repo_agent_name;
    use crate::forge::git_url_with_base;

    #[test]
    fn repo_agent_name_takes_trailing_segment() {
        assert_eq!(repo_agent_name("agent-configs/atlas"), "atlas");
        assert_eq!(repo_agent_name("atlas"), "atlas");
        assert_eq!(repo_agent_name("a/b/c"), "c");
    }

    #[test]
    fn forge_git_url_shape() {
        // Tests the pure half: credentials go between scheme and
        // authority. Deliberately not via `forge_git_url`, which reads
        // HIVE_FORGE_URL — setting that here would race every other
        // test in this binary, and there is no fallback to lean on any
        // more (a guessed base is the bug this issue removes).
        let url = git_url_with_base("http://forge.example.test", "tok", "a/iris");
        assert_eq!(url, "http://core:tok@forge.example.test/a/iris.git");
    }

    #[test]
    fn forge_git_url_preserves_https() {
        // The scheme is carried through rather than assumed: a swarm
        // whose forge is behind TLS must not be downgraded to http.
        let url = git_url_with_base("https://forge.example.test", "tok", "a/iris");
        assert!(
            url.starts_with("https://core:tok@"),
            "https must survive: {url}"
        );
    }
}