//! Pure state transition functions for pipeline and execution state machines.

use chrono::Utc;
use serde::{Deserialize, Serialize};

use super::{
    AgentName, ArchiveReason, BranchName, ExecutionState, GitSha, Stage, StoryId, TransitionError,
    stage_label,
};

// ── Pipeline events ─────────────────────────────────────────────────────────

/// Events that drive Stage transitions. Each variant carries the data needed
/// to construct the destination state, so the transition function can never
/// accidentally land in an underspecified state.
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum PipelineEvent {
    /// Dependencies met; promote from backlog.
    DepsMet,
    /// Coder starting gates.
    GatesStarted,
    /// Gates passed — ready to merge.
    GatesPassed {
        feature_branch: BranchName,
        commits_ahead: std::num::NonZeroU32,
    },
    /// Gates failed; retry.
    GatesFailed { reason: String },
    /// QA mode is "server" — skip QA, go straight to merge.
    QaSkipped {
        feature_branch: BranchName,
        commits_ahead: std::num::NonZeroU32,
    },
    /// Mergemaster squash succeeded.
    MergeSucceeded { merge_commit: GitSha },
    /// Mergemaster gave up after retry budget.
    MergeFailedFinal { reason: String },
    /// Story accepted (Done → Archived).
    Accepted,
    /// User blocked the story.
    Block { reason: String },
    /// User unblocked.
    Unblock,
    /// User abandoned.
    Abandon,
    /// Story superseded by another.
    Supersede { by: StoryId },
    /// Story put on review hold.
    ReviewHold { reason: String },
    /// Story rejected by QA or reviewer.
    Reject { reason: String },
    /// Story triaged from upcoming to backlog.
    Triage,
    /// Direct completion — item closed without going through the full pipeline
    /// (e.g. spike auto-merge, bug closure, manual acceptance).
    Close,
    /// Manual demotion back to backlog from an active stage.
    Demote,
    /// Freeze the story at its current stage (suspends pipeline and auto-assign).
    Freeze,
    /// Unfreeze the story, restoring it to the stage it was at when frozen.
    Unfreeze,
}

// ── Per-node execution events ───────────────────────────────────────────────

/// Events that drive per-node [`ExecutionState`] transitions (agent lifecycle).
#[derive(Debug, Clone, Serialize, Deserialize)]
pub enum ExecutionEvent {
    SpawnRequested { agent: AgentName },
    SpawnedSuccessfully,
    Heartbeat,
    HitRateLimit { resume_at: chrono::DateTime<Utc> },
    Exited { exit_code: i32 },
    Stopped,
    Reset,
}

// ── Label helper ────────────────────────────────────────────────────────────

/// Human-readable label for a `PipelineEvent` variant.
pub fn event_label(e: &PipelineEvent) -> &'static str {
    match e {
        PipelineEvent::DepsMet => "DepsMet",
        PipelineEvent::GatesStarted => "GatesStarted",
        PipelineEvent::GatesPassed { .. } => "GatesPassed",
        PipelineEvent::GatesFailed { .. } => "GatesFailed",
        PipelineEvent::QaSkipped { .. } => "QaSkipped",
        PipelineEvent::MergeSucceeded { .. } => "MergeSucceeded",
        PipelineEvent::MergeFailedFinal { .. } => "MergeFailedFinal",
        PipelineEvent::Accepted => "Accepted",
        PipelineEvent::Block { .. } => "Block",
        PipelineEvent::Unblock => "Unblock",
        PipelineEvent::Abandon => "Abandon",
        PipelineEvent::Supersede { .. } => "Supersede",
        PipelineEvent::ReviewHold { .. } => "ReviewHold",
        PipelineEvent::Reject { .. } => "Reject",
        PipelineEvent::Triage => "Triage",
        PipelineEvent::Close => "Close",
        PipelineEvent::Demote => "Demote",
        PipelineEvent::Freeze => "Freeze",
        PipelineEvent::Unfreeze => "Unfreeze",
    }
}

// ── The pipeline transition function ────────────────────────────────────────

/// Pure state transition. Takes the current Stage and an event, returns the
/// new Stage or a TransitionError. Side effects are dispatched separately
/// via the event bus.
pub fn transition(state: Stage, event: PipelineEvent) -> Result<Stage, TransitionError> {
    use PipelineEvent::*;
    use Stage::*;

    let sl = stage_label(&state);
    let el = event_label(&event);
    let invalid = || TransitionError::InvalidTransition {
        from_stage: sl.to_string(),
        event: el.to_string(),
    };

    let now = Utc::now();

    match (state, event) {
        // ── Triage: upcoming → backlog ──────────────────────────────────
        (Upcoming, Triage) => Ok(Backlog),

        // ── Forward path ────────────────────────────────────────────────
        (Backlog, DepsMet) => Ok(Coding),
        (Coding, GatesStarted) => Ok(Qa),
        (
            Coding,
            QaSkipped {
                feature_branch,
                commits_ahead,
            },
        ) => Ok(Merge {
            feature_branch,
            commits_ahead,
        }),
        (
            Qa,
            GatesPassed {
                feature_branch,
                commits_ahead,
            },
        ) => Ok(Merge {
            feature_branch,
            commits_ahead,
        }),
        (Qa, GatesFailed { .. }) => Ok(Coding),
        (Merge { .. }, MergeSucceeded { merge_commit }) => Ok(Done {
            merged_at: now,
            merge_commit,
        }),

        // ── Done → Archived(Completed) ──────────────────────────────────
        (Done { .. }, Accepted) => Ok(Archived {
            archived_at: now,
            reason: ArchiveReason::Completed,
        }),

        // ── Block: any active → Blocked ──────────────────────────────
        (Backlog, Block { reason })
        | (Coding, Block { reason })
        | (Qa, Block { reason })
        | (Merge { .. }, Block { reason }) => Ok(Blocked { reason }),

        (Backlog, ReviewHold { reason })
        | (Coding, ReviewHold { reason })
        | (Qa, ReviewHold { reason })
        | (Merge { .. }, ReviewHold { reason }) => Ok(Archived {
            archived_at: now,
            reason: ArchiveReason::ReviewHeld { reason },
        }),

        (Merge { .. }, MergeFailedFinal { reason }) => Ok(Archived {
            archived_at: now,
            reason: ArchiveReason::MergeFailed { reason },
        }),

        // ── Abandon / supersede from any active or done stage ───────────
        (Upcoming, Abandon)
        | (Backlog, Abandon)
        | (Coding, Abandon)
        | (Qa, Abandon)
        | (Merge { .. }, Abandon)
        | (Done { .. }, Abandon) => Ok(Archived {
            archived_at: now,
            reason: ArchiveReason::Abandoned,
        }),

        (Upcoming, Supersede { by })
        | (Backlog, Supersede { by })
        | (Coding, Supersede { by })
        | (Qa, Supersede { by })
        | (Merge { .. }, Supersede { by })
        | (Done { .. }, Supersede { by }) => Ok(Archived {
            archived_at: now,
            reason: ArchiveReason::Superseded { by },
        }),

        // ── Reject from any active stage or QA ──────────────────────────
        (Backlog, Reject { reason })
        | (Coding, Reject { reason })
        | (Qa, Reject { reason })
        | (Merge { .. }, Reject { reason }) => Ok(Archived {
            archived_at: now,
            reason: ArchiveReason::Rejected { reason },
        }),

        // ── Demote: send an active item back to backlog ────────────────
        (Coding, Demote) | (Qa, Demote) | (Merge { .. }, Demote) => Ok(Backlog),

        // ── Close: direct completion from any active stage ─────────────
        (Backlog, Close) | (Coding, Close) | (Qa, Close) | (Merge { .. }, Close) => Ok(Done {
            merged_at: now,
            merge_commit: GitSha("closed".to_string()),
        }),

        // ── Unblock: Blocked → Coding ─────────────────────────────────
        (Blocked { .. }, Unblock) => Ok(Coding),

        // ── Legacy unblock: Archived(Blocked|MergeFailed) → Backlog ──
        (
            Archived {
                reason: ArchiveReason::Blocked { .. },
                ..
            },
            Unblock,
        )
        | (
            Archived {
                reason: ArchiveReason::MergeFailed { .. },
                ..
            },
            Unblock,
        ) => Ok(Backlog),

        // ── Freeze: any active stage → Frozen(resume_to=current) ────────
        (stage @ (Upcoming | Backlog | Coding | Qa), Freeze) => Ok(Frozen {
            resume_to: Box::new(stage),
        }),
        (stage @ Merge { .. }, Freeze) => Ok(Frozen {
            resume_to: Box::new(stage),
        }),

        // ── Unfreeze: Frozen → resume_to ─────────────────────────────────
        (Frozen { resume_to }, Unfreeze) => Ok(*resume_to),

        // ── Everything else is invalid ──────────────────────────────────
        _ => Err(invalid()),
    }
}

// ── The execution state transition function ─────────────────────────────────

/// Pure execution-state transition. Takes the current ExecutionState and an
/// ExecutionEvent, returns the new ExecutionState or a TransitionError.
pub fn execution_transition(
    state: ExecutionState,
    event: ExecutionEvent,
) -> Result<ExecutionState, TransitionError> {
    use ExecutionEvent::*;
    use ExecutionState::*;

    let now = Utc::now();

    match (state, event) {
        (Idle, SpawnRequested { agent }) => Ok(Pending { agent, since: now }),

        (Pending { agent, .. }, SpawnedSuccessfully) => Ok(Running {
            agent,
            started_at: now,
            last_heartbeat: now,
        }),

        (
            Running {
                agent, started_at, ..
            },
            Heartbeat,
        ) => Ok(Running {
            agent,
            started_at,
            last_heartbeat: now,
        }),

        (Running { agent, .. }, HitRateLimit { resume_at })
        | (Pending { agent, .. }, HitRateLimit { resume_at }) => {
            Ok(RateLimited { agent, resume_at })
        }

        (RateLimited { agent, .. }, SpawnedSuccessfully) => Ok(Running {
            agent,
            started_at: now,
            last_heartbeat: now,
        }),

        (Running { agent, .. }, Exited { exit_code })
        | (Pending { agent, .. }, Exited { exit_code })
        | (RateLimited { agent, .. }, Exited { exit_code }) => Ok(Completed {
            agent,
            exit_code,
            completed_at: now,
        }),

        (_, Stopped) | (_, Reset) => Ok(Idle),

        _ => Err(TransitionError::InvalidTransition {
            from_stage: "ExecutionState".to_string(),
            event: "<exec event>".to_string(),
        }),
    }
}