import Foundation
import libgit2
import os

// MARK: - Outcomes

/// **Why a housekeeping pass did nothing** — every one of these is a shrug, never a failure.
///
/// "Safe libgit2 housekeeping (repacking loose objects) may run periodically, but it rewrites
/// nothing" (06-history-undo.md ▸ Repository hygiene). Nothing here reaches a user, nothing here is
/// retried, and nothing here is worth a banner: maintenance that does not happen costs the board a
/// slightly larger `.git` and nothing else, so every uncertainty resolves to *not now*.
public enum GitHousekeepingSkip: String, Sendable, Equatable, CaseIterable {

    /// No repository at the board root, or libgit2 could not open the one that is there.
    case noRepository

    /// The repository is in a state the app does not write in (`GitRepositoryPause`) — a merge, a
    /// rebase, a detached HEAD. The commit engine holds for these; so does this, for the simpler
    /// reason that optional work has no business running beside somebody else's operation.
    case held

    /// `index.lock` is held right now — another writer is mid-operation.
    case indexLocked

    /// Every loose object libgit2 refused to read, so there was nothing to pack. A pass that inserts
    /// nothing writes no pack and deletes nothing.
    case nothingToPack

    /// The pack could not be written, or the written pack could not be re-opened for verification.
    /// **Nothing is deleted on this path** — the loose objects stay exactly where they were.
    case packFailed
}

/// What one repack actually did, in numbers a test can assert on.
public struct GitHousekeepingRepack: Sendable, Equatable {

    /// How many loose object files the pass found before it started.
    public let looseBefore: Int

    /// How many of them libgit2 accepted into the packbuilder.
    public let inserted: Int

    /// How many loose files were deleted — which is exactly how many were **proved** to be readable
    /// out of the newly written pack, one by one, before anything was removed.
    public let packedAway: Int

    /// The pack's name (`pack-<name>.pack` / `.idx` under `.git/objects/pack/`).
    public let packName: String

    public init(looseBefore: Int, inserted: Int, packedAway: Int, packName: String) {
        self.looseBefore = looseBefore
        self.inserted = inserted
        self.packedAway = packedAway
        self.packName = packName
    }
}

/// How a housekeeping pass ended.
public enum GitHousekeepingOutcome: Sendable, Equatable {
    case repacked(GitHousekeepingRepack)

    /// The repository has fewer loose objects than the threshold — the ordinary answer, and the one
    /// almost every board gives almost every time it opens.
    case belowThreshold(loose: Int)

    case skipped(GitHousekeepingSkip)
}

// MARK: - GitHousekeeping

/// **Periodic safe housekeeping** (06-history-undo.md ▸ Repository hygiene: "The app may run safe
/// libgit2 housekeeping (repacking loose objects) periodically — it rewrites nothing").
///
/// ### What it does, and the line it does not cross
///
/// libgit2 does no automatic maintenance of its own (14-git-operations.md ▸ A2 → 06), so a board
/// that commits every settled change accumulates loose objects forever. This packs them: the same
/// objects, byte for byte, moved from one storage form into another. **No commit, no ref, no
/// reachable content changes** — the object graph after a pass is the graph before it, and `git log`,
/// `git show` and every blob in every tree answer identically.
///
/// The whole class of destructive maintenance is **out**, permanently: nothing here prunes, expires a
/// reflog, drops an unreachable object, or rewrites a commit. "Deleting never forgets" and "repo
/// growth is accepted" are the design's stances (06), and a compaction that made a board smaller by
/// forgetting something would contradict both. Unreachable loose objects are packed like any other —
/// they stay readable by oid, which is what never-forget means at the object layer.
///
/// ### Why deleting a loose file is safe
///
/// Every deletion is *provably redundant* before it happens, and the proof is not a chain of
/// reasoning about the packbuilder — it is a read:
///
/// 1. The loose set is enumerated from the filesystem (`.git/objects/<xx>/<38 hex>`), so the pass
///    knows exactly which files it is considering and never touches anything else under `.git`.
/// 2. Each oid is inserted into a `git_packbuilder`, which is then written into
///    `.git/objects/pack/`. Writing a pack is purely **additive**: it creates two new files and
///    changes nothing that exists.
/// 3. The written `.idx` is re-opened as a standalone one-pack object database — no loose backend,
///    no repository, nothing that could answer from the very files about to be deleted — and each
///    oid is looked up in it. **A loose file is deleted only when that lookup says the object is in
///    the new pack.** Anything the lookup does not confirm is left exactly where it is, forever.
///
/// A failure at any point returns without deleting anything, so the worst outcome of a broken pass
/// is a stray pack file that costs disk and changes no answer.
///
/// ### What it deliberately does not do
///
/// **It never touches an existing pack** — not to delete one, not to consolidate several into one.
/// A repository maintained only by this accumulates roughly one pack per threshold's worth of
/// objects, forever, and that is the accepted cost: consolidating means rewriting storage the app did
/// not write, on a schedule nobody asked for, with a failure mode (a half-repacked object database)
/// far worse than the disk it would save. 06's stance is "repo growth is accepted", and `git gc` in a
/// terminal remains exactly as available as it always was for a user who wants more than this.
///
/// **It never narrows to reachability.** Every loose object is packed, reachable or not: an object
/// no ref can reach is still an object the repository can answer for by oid, and dropping those would
/// be the app deciding what history is allowed to remember (06 ▸ Deleting never forgets).
///
/// ### Concurrency
///
/// The pass is additive-then-provably-redundant, which is what makes a concurrent commit harmless:
/// objects a commit writes while this runs are not in the enumerated set, so they are never
/// considered, and objects this deletes are readable from the pack the same odb refresh that misses
/// the loose file will find. That is the same race `git repack -d` has always had, and the same
/// resolution. The scheduler above (`GitHousekeeper`) additionally declines to start while a flush is
/// in flight, and the pass itself declines under any pause or held lock — belt and braces over an
/// operation that is already safe rather than the thing that makes it safe.
///
/// ### Isolation
///
/// `GitRepository`'s rule, unchanged: every function is `nonisolated`, opens its own handles, and
/// frees them in the same synchronous scope. No handle crosses an `await`, a `Task`, or a stored
/// property.
enum GitHousekeeping {

    /// **When a repository has enough loose objects to be worth packing** — git's own `gc.auto`
    /// default, 6700.
    ///
    /// DESIGN names no number ("periodically" is all 06 says), so the number is borrowed from the
    /// tool whose reason for having one is identical: git picked 6700 as roughly where loose-object
    /// lookup and directory-scan costs start to matter, and a Lanework board's `.git` is an ordinary
    /// repository with ordinary objects in it. Borrowing it also means a board the user has been
    /// running `git gc` on by hand never sees a second opinion about when packing is due.
    ///
    /// Injectable at every level above (`GitHousekeeper.threshold`) so a test can spend three objects
    /// instead of six thousand seven hundred.
    static let defaultLooseObjectThreshold = 6700

    private static let logger = Logger(subsystem: "dev.rzen.indie.Kanban", category: "git")

    /// libgit2's global state, brought up exactly once per process — `GitCommitOperation.startUp`'s
    /// rule and its reason (a pass can run when no `Repository` is alive).
    private static let startUp: Bool = {
        git_libgit2_init() >= 0
    }()

    // MARK: - The pass

    /// **Runs one housekeeping pass**, or explains why it didn't.
    ///
    /// Synchronous and expected to be called from a detached low-priority task — packing is real CPU
    /// and real IO, and it is the least urgent work the app does.
    nonisolated static func run(
        at boardRoot: URL,
        threshold: Int = defaultLooseObjectThreshold
    ) -> GitHousekeepingOutcome {
        _ = startUp

        // Two of the three facts every flush checks, asked in the same words
        // (`GitCommitOperation.reading`) so the engine's vocabulary for "not now" and this one cannot
        // drift — the third, an unborn HEAD, is nothing to this: a repository with no commits has no
        // loose objects worth packing and is below any threshold anyway. Housekeeping reads the two it
        // does take more strictly than the committer does: the committer *holds* and retries, this
        // simply does not happen this time.
        let reading = GitCommitOperation.reading(at: boardRoot)
        if reading.pause != nil { return .skipped(.held) }
        if reading.isIndexLocked { return .skipped(.indexLocked) }

        guard BoardGitMode.hasGitEntry(at: boardRoot) else { return .skipped(.noRepository) }
        var repository: OpaquePointer?
        guard git_repository_open(&repository, boardRoot.path) == 0, let repository else {
            return .skipped(.noRepository)
        }
        defer { git_repository_free(repository) }

        let objectsDirectory = objectsDirectory(of: repository)
        let loose = looseObjects(in: objectsDirectory)
        guard loose.count >= threshold else { return .belowThreshold(loose: loose.count) }

        return repack(loose, in: repository, objectsDirectory: objectsDirectory)
    }

    /// **How many loose objects the repository has right now** — the gate's own reading, exposed
    /// because it is also the only honest way to assert that a pass reduced the count.
    ///
    /// `0` for a board with no repository, which is the same shrug every read in `GitRepository`
    /// gives one.
    nonisolated static func looseObjectCount(at boardRoot: URL) -> Int {
        _ = startUp
        guard BoardGitMode.hasGitEntry(at: boardRoot) else { return 0 }
        var repository: OpaquePointer?
        guard git_repository_open(&repository, boardRoot.path) == 0, let repository else { return 0 }
        defer { git_repository_free(repository) }
        return looseObjects(in: objectsDirectory(of: repository)).count
    }

    // MARK: - Repacking

    private static func repack(
        _ loose: [LooseObject],
        in repository: OpaquePointer,
        objectsDirectory: URL
    ) -> GitHousekeepingOutcome {
        var builder: OpaquePointer?
        guard git_packbuilder_new(&builder, repository) == 0, let builder else {
            return .skipped(.packFailed)
        }
        defer { git_packbuilder_free(builder) }

        // Inserted one oid at a time — never `insert_recur`, never `insert_walk`. The set that goes
        // into the pack is exactly the set enumerated off disk, so "packed" and "considered for
        // deletion" are the same list by construction, and reachability never enters into it.
        var inserted: [LooseObject] = []
        for object in loose {
            var oid = git_oid()
            guard git_oid_fromstr(&oid, object.hex) == 0 else { continue }
            // A loose object libgit2 cannot read (a truncated write, a corrupt file) is skipped
            // rather than fatal — and, never having entered the pack, is never a deletion candidate.
            guard git_packbuilder_insert(builder, &oid, nil) == 0 else { continue }
            inserted.append(object)
        }
        guard !inserted.isEmpty else { return .skipped(.nothingToPack) }

        // `nil` for the path: libgit2 resolves the repository's own objects/pack directory, which is
        // one fewer assumption than spelling it here. The name comes back afterwards, and the `.idx`
        // beside it is what the verification reads.
        guard git_packbuilder_write(builder, nil, 0, nil, nil) == 0,
              let namePointer = git_packbuilder_name(builder) else {
            return .skipped(.packFailed)
        }
        let packName = String(cString: namePointer)

        let indexFile = objectsDirectory
            .appendingPathComponent("pack", isDirectory: true)
            .appendingPathComponent("pack-\(packName).idx")
        guard FileManager.default.fileExists(atPath: indexFile.path) else {
            // libgit2 said it wrote the pack and the index is not where its own naming says it is.
            // Nothing is deleted on a fact that surprising.
            return .skipped(.packFailed)
        }

        guard let verifier = OnePackDatabase(indexFile: indexFile) else { return .skipped(.packFailed) }
        defer { verifier.close() }

        var packedAway = 0
        for object in inserted {
            // **The proof, read rather than reasoned**: the object is in the pack file just written,
            // answered by a database that has nothing else in it — no loose backend, no repository,
            // no alternates. A `false` here (or an oid that will not even parse) leaves the loose
            // file alone, permanently.
            guard verifier.contains(object.hex) else { continue }
            guard (try? FileManager.default.removeItem(at: object.url)) != nil else { continue }
            packedAway += 1
        }

        logger.debug("housekeeping packed \(packedAway, privacy: .public) of \(loose.count, privacy: .public) loose objects")
        return .repacked(GitHousekeepingRepack(
            looseBefore: loose.count,
            inserted: inserted.count,
            packedAway: packedAway,
            packName: packName
        ))
    }

    // MARK: - The loose set

    /// One loose object: its full hex oid, and the file it lives in.
    private struct LooseObject {
        let hex: String
        let url: URL
    }

    /// **Every loose object file under `objects/`**, found by reading the fanout directories.
    ///
    /// ### Why the filesystem rather than `git_odb_foreach`
    ///
    /// Because `git_odb_foreach` enumerates the *whole* database — packed objects included — and a
    /// pass that fed already-packed objects back into a new pack would rewrite the entire repository
    /// into a fresh pack on every run while leaving the old ones in place (nothing here deletes a
    /// pack, ever). Growth, not hygiene. The loose set is a directory listing by definition, and
    /// reading it directly is both the exact answer and the cheap one — 256 `readdir`s at background
    /// priority — and it yields the *file* to delete, which an oid alone does not.
    ///
    /// **Strictly shaped, so nothing else can be caught by it**: a two-hex-character directory
    /// containing thirty-eight-hex-character names. `objects/info`, `objects/pack`, an indexer's
    /// temp file, an alternates file and anything a user has parked down there all fail the shape and
    /// are invisible to this. (Thirty-eight is SHA-1's remainder; a SHA-256 repository would simply
    /// present no loose objects to this pass, which is the safe way for it to be wrong.)
    private static func looseObjects(in objectsDirectory: URL) -> [LooseObject] {
        let manager = FileManager.default
        guard let fanouts = try? manager.contentsOfDirectory(atPath: objectsDirectory.path) else {
            return []
        }

        var found: [LooseObject] = []
        for fanout in fanouts where isHex(fanout, count: 2) {
            let directory = objectsDirectory.appendingPathComponent(fanout, isDirectory: true)
            guard let names = try? manager.contentsOfDirectory(atPath: directory.path) else { continue }
            for name in names where isHex(name, count: 38) {
                found.append(LooseObject(
                    hex: fanout + name,
                    url: directory.appendingPathComponent(name)
                ))
            }
        }
        return found
    }

    private static func isHex(_ string: String, count: Int) -> Bool {
        guard string.count == count else { return false }
        return string.allSatisfy { $0.isHexDigit && !$0.isUppercase }
    }

    private static func objectsDirectory(of repository: OpaquePointer) -> URL {
        let gitDirectory = URL(
            fileURLWithPath: git_repository_path(repository).map { String(cString: $0) } ?? "",
            isDirectory: true
        )
        return gitDirectory.appendingPathComponent("objects", isDirectory: true)
    }

    // MARK: - The verifier

    /// **A database containing exactly one pack file and nothing else** — the deletion proof.
    ///
    /// It is deliberately not the repository's odb: that one answers from the loose objects too, so
    /// "the object exists" would be true of every candidate whether or not the pack ever received
    /// it. With one backend and no alternates, a positive answer can only have come from the pack
    /// that was just written.
    private struct OnePackDatabase {
        private let database: OpaquePointer

        init?(indexFile: URL) {
            var database: OpaquePointer?
            guard git_odb_new(&database) == 0, let database else { return nil }

            var backend: UnsafeMutablePointer<git_odb_backend>?
            guard git_odb_backend_one_pack(&backend, indexFile.path) == 0, let backend else {
                git_odb_free(database)
                return nil
            }
            // The odb takes ownership on success and frees the backend with itself; on failure it
            // does not, and the backend's own `free` is the only way to give it back.
            guard git_odb_add_backend(database, backend, 1) == 0 else {
                backend.pointee.free?(backend)
                git_odb_free(database)
                return nil
            }
            self.database = database
        }

        func contains(_ hex: String) -> Bool {
            var oid = git_oid()
            guard git_oid_fromstr(&oid, hex) == 0 else { return false }
            return git_odb_exists(database, &oid) == 1
        }

        func close() {
            git_odb_free(database)
        }
    }
}

// MARK: - GitHousekeeper

/// **When a housekeeping pass runs** (06-history-undo.md ▸ Repository hygiene) — one per git-mode
/// board session, scheduled at board open and never again.
///
/// ### Structurally unreachable off Pro
///
/// One of these exists per `HistoryStore` in mode `git`, and a `HistoryStore` exists only under Pro
/// (`HistoryStore.compose` is the tier gate) — `GitAutoCommitter`'s rule, for its reason. The free
/// tier has no housekeeper to disable and no `.git` to pack (12-editions.md ▸ The free tier and
/// `.git`, which `InertGitTests` pins against real bytes).
///
/// ### Off the open path, on purpose
///
/// Board open is where 02-architecture.md's hang-avoidance doctrine is strictest, and packing is the
/// single most expensive thing the git layer can do. So the pass is armed with a delay rather than
/// run, the delay outlasts the committer's launch catch-up (`GitAutoCommitter.debounceInterval`), the
/// work itself runs `Task.detached(priority: .background)`, and the main actor only ever holds the
/// verdict.
///
/// ### Once, and never retried
///
/// A pass that declines — a commit in flight, a paused repository, a held lock — is simply not run;
/// nothing re-arms and nothing is surfaced. Loose objects only accumulate, so the next board open
/// finds a threshold that is still crossed and tries again then. That is the whole retry policy, and
/// it is the right one for work whose failure costs the user nothing.
@MainActor
@Observable
public final class GitHousekeeper {

    /// The board whose repository this maintains.
    public let boardRoot: URL

    /// How many loose objects it takes to be worth a pass. See
    /// `GitHousekeeping.defaultLooseObjectThreshold`; settable so a test need not make 6700 objects.
    @ObservationIgnored
    public var threshold = GitHousekeeping.defaultLooseObjectThreshold

    /// How long after board open the pass is attempted.
    ///
    /// Comfortably past `GitAutoCommitter.debounceInterval` (two seconds), so the launch catch-up
    /// commit has come and gone before maintenance considers starting — the cheapest possible way to
    /// keep the two out of each other's way, and settable for the reason every other interval in this
    /// layer is: a test must not have to spend it.
    @ObservationIgnored
    public var delay: Duration = .seconds(8)

    /// **Whether a commit is in flight right now** — asked on the main actor at the moment of
    /// dispatch, and answered by the board's own committer (`HistoryStore.activateAutoCommit` wires
    /// it).
    ///
    /// `nil` where no committer exists, which reads as "no", and is the honest answer for a
    /// housekeeper with no engine beside it.
    @ObservationIgnored
    public var isCommitInFlight: (@MainActor () -> Bool)?

    /// How the last pass ended, or `nil` if none has run. Observable state for tests and for nothing
    /// else — housekeeping has no surface, by design.
    public private(set) var lastOutcome: GitHousekeepingOutcome?

    @ObservationIgnored
    private var pending: Task<Void, Never>?

    private static let logger = Logger(subsystem: "dev.rzen.indie.Kanban", category: "git")

    init(boardRoot: URL) {
        self.boardRoot = boardRoot
    }

    /// **Arms the one pass this session gets.** Idempotent: a second call while one is armed does
    /// nothing, so a board opened twice into the same store does not queue two.
    public func schedule() {
        guard pending == nil else { return }
        let delay = delay
        pending = Task { [weak self] in
            try? await Task.sleep(for: delay)
            guard !Task.isCancelled, let self else { return }
            self.pending = nil
            await self.runNow()
        }
    }

    /// Cancels an armed pass — the session's teardown, so a closed board's maintenance cannot fire
    /// against a store that has gone.
    public func cancel() {
        pending?.cancel()
        pending = nil
    }

    /// Runs a pass immediately, off the main actor. The scheduled body, and a test's way in.
    public func runNow() async {
        // **Never beside a commit.** The pass is safe next to one — it only ever adds a pack and
        // deletes files it has proved redundant — but "safe" is not "worth it", and optional work
        // that waits for the next board open costs nothing.
        if isCommitInFlight?() == true {
            Self.logger.debug("housekeeping skipped: a commit is in flight")
            return
        }
        let root = boardRoot
        let threshold = threshold
        lastOutcome = await Task.detached(priority: .background) {
            GitHousekeeping.run(at: root, threshold: threshold)
        }.value
    }
}