import Foundation
import SwiftGitX
import os

// MARK: - Failure

/// **Why a git operation didn't happen**, named and carrying libgit2's own message.
///
/// One type rather than a case per operation because everything that reaches a user goes through
/// the same two sentences — what was being attempted, and what the library said — and because the
/// operations that will join `initialize` here (commit, checkout, pull) all fail in exactly that
/// shape (06-history-undo.md ▸ Interaction with external writers: "An operation that fails
/// *cleanly* … surfaces as a one-shot banner failure naming the operation and the error").
public struct GitOperationFailure: Error, Sendable, Equatable, CustomStringConvertible {

    /// What was being attempted, in the user's words rather than libgit2's — "Adding git to this
    /// board", not `git_repository_init`.
    public let operation: String

    /// libgit2's message for the failure, verbatim. Kept rather than mapped: the messages are
    /// specific ("could not write to '…': Permission denied") in a way no re-phrasing of ours would
    /// be, and the alternative to showing it is a shrug.
    public let message: String

    public init(operation: String, message: String) {
        self.operation = operation
        self.message = message
    }

    public var description: String { "\(operation) failed: \(message)" }
}

// MARK: - GitRepository

/// **The board's repository, through the bundled libgit2** (06-history-undo.md ▸ Rules ▸ Opt-in
/// init: "Bundled libgit2 — no git install required").
///
/// SwiftGitX vendors libgit2 as an in-process library, so every call here runs inside the sandbox
/// with no `Process`, no `/usr/bin/git` and no sandbox extension — the shipped Release build behaves
/// identically on a machine that has never had the command-line tools installed.
///
/// ### Isolation
///
/// Every function is `nonisolated` and **opens its own `Repository`, confined to its own
/// synchronous scope**. `Repository` is `Sendable` (SwiftGitX marks it so to make handles
/// transferable), but the libgit2 handle underneath is not safe for concurrent use from several
/// threads at once, so no handle here is ever shared across an `await`, a `Task`, or a stored
/// property. `HistoryStore` — which is `@MainActor` — reaches these through `Task.detached`, so the
/// main actor never blocks on libgit2 and libgit2 never sees two threads at once.
///
/// This is the pathfinder's `GitSource` shape, kept because it was right, with the pathfinder's
/// *policy* deliberately left behind: nothing here auto-initializes anything, nothing seeds a
/// `.gitignore` (06 ▸ Repository hygiene, a later card), and nothing commits on its own schedule.
enum GitRepository {

    /// **The root commit's own subject** (06-history-undo.md ▸ Rules ▸ Abnormal repo states,
    /// settled): "whenever the app creates a repo's first commit … it commits the whole tree as
    /// *Initial board state*, never a folded diff-from-empty: there is no last-committed snapshot to
    /// diff against, and forty Adds would bury the event."
    static let initialCommitSubject = "Initial board state"

    /// The branch a board's first commit lands on.
    ///
    /// **Forced rather than inherited, deliberately.** libgit2's compiled-in initial-branch name
    /// comes from `init.defaultBranch` in whatever config layer it can find at
    /// `git_repository_init` time — which is non-deterministic across machines and simply
    /// unavailable in the sandbox (redirected, empty HOME). `Repository.create(at:)` has no
    /// initial-branch parameter, so this is applied by writing `.git/HEAD` directly: on a freshly
    /// created, unborn, non-bare repository that file is nothing but the plain-text symbolic ref, so
    /// writing it is exactly `git symbolic-ref HEAD refs/heads/main` before anything else touches
    /// the repo.
    ///
    /// DESIGN is silent on the name; `main` is git's own modern default and the pathfinder's choice.
    static let initialBranchName = "main"

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

    // MARK: Opt-in init

    /// **Add-git** (06-history-undo.md ▸ Rules ▸ Opt-in init): initializes a repository at
    /// `boardRoot` and immediately commits the whole tree as `Initial board state`.
    ///
    /// The commit is not deferred to any debounce — "init doesn't wait for the debounce; the board
    /// is protected from the moment git exists" — so the two halves are one operation and a failure
    /// in either is one failure.
    ///
    /// **It refuses a board that already has a `.git`.** The app "never mutates repo state it didn't
    /// create" (06), and `git_repository_init` over an existing repository is a re-initialization —
    /// harmless in the common case and precisely the kind of thing that rule exists to forbid. The
    /// caller (`HistoryStore.addGit`) has already established mode `none`; this is the check that
    /// makes it impossible rather than merely unlikely.
    ///
    /// Returns the branch the root commit landed on, which is the popover's display line.
    nonisolated static func create(at boardRoot: URL) -> Result<String, GitOperationFailure> {
        let operation = "Adding git to this board"

        guard !BoardGitMode.hasGitEntry(at: boardRoot) else {
            return .failure(GitOperationFailure(
                operation: operation,
                message: "this board already has a git repository"
            ))
        }

        let gitDirectory: URL
        do {
            let created = try Repository.create(at: boardRoot)
            gitDirectory = created.path
            // Before anything else touches the repo — see `initialBranchName`.
            try? "ref: refs/heads/\(initialBranchName)\n".write(
                to: gitDirectory.appendingPathComponent("HEAD"),
                atomically: true,
                encoding: .utf8
            )
        } catch {
            return .failure(GitOperationFailure(operation: operation, message: reason(error)))
        }

        // A **fresh handle** for the staging and the commit, so nothing reads HEAD through a
        // repository object that predates the symbolic ref just written: libgit2 caches refs per
        // repository, and the whole point of writing that file was to decide where the first commit
        // lands. The creating handle is dropped above.
        let repository: Repository
        do {
            repository = try Repository.open(at: boardRoot)
        } catch {
            return .failure(GitOperationFailure(operation: operation, message: reason(error)))
        }

        applyIdentity(to: repository, gitDirectory: gitDirectory)

        do {
            // An empty pathspec passed to `git_index_add_all` (via `add(paths:)`) matches every path
            // in the working tree — full `git add -A` semantics in one step, `.gitignore` respected
            // — which is what "commits the whole tree" means: the board's files, the agent guide,
            // strays and all (06 ▸ Commit messages: "the committer stages the whole board root").
            try repository.add(paths: [])
            _ = try repository.commit(message: initialCommitSubject)
        } catch {
            logger.error("initial commit failed at \(boardRoot.path, privacy: .public): \(reason(error), privacy: .public)")
            return .failure(GitOperationFailure(operation: operation, message: reason(error)))
        }

        return .success(branchName(at: boardRoot) ?? initialBranchName)
    }

    /// Gives the repository a commit identity **only when it has none** (06-history-undo.md
    /// ▸ Interaction with external writers ▸ "Where the user's git identity comes from").
    ///
    /// `GitIdentity` resolves what the identity *is*: repo-local config when present, the derived
    /// default otherwise. What this method adds is the mechanism — writing the resolved identity
    /// into the repository's own config so libgit2's default signature resolves to it.
    ///
    /// **That write is a mechanism, not a design decision, and it is the narrowest one available.**
    /// SwiftGitX 0.4.0's `commit(message:)` takes no signature (its `CommitOptions` leaves
    /// `author`/`committer` null, so libgit2 falls back to `git_signature_default`, which fails
    /// outright in a sandbox with no readable config). Every board this runs on is one the app
    /// created milliseconds earlier, whose config the app itself wrote, and the keys are only ever
    /// *added* — a config that already names an identity is left exactly as it was, which is the
    /// adopted-repo promise. The auto-commit card needs per-commit authorship anyway (foreign
    /// changes commit as `Lanework External`, `modified-by` windows as the agent), so it must reach
    /// a signature-capable commit path regardless; when it does, this materialization goes with it.
    private static func applyIdentity(to repository: Repository, gitDirectory: URL) {
        let configured = GitConfigFile.identity(inGitDirectory: gitDirectory)
        guard configured.name == nil || configured.email == nil else { return }

        let identity = GitIdentity.resolve(repoLocal: configured, derived: .derivedDefault())
        if configured.name == nil {
            try? repository.config.set("user.name", to: identity.name)
        }
        if configured.email == nil {
            try? repository.config.set("user.email", to: identity.email)
        }
    }

    // MARK: Reads

    /// The current branch's short name, or `nil` when there is no repository at `boardRoot` or
    /// libgit2 cannot open it — the popover's read-only branch line (03-board-ui.md ▸ Board
    /// popover), and nothing more: branch switching and creation are a later card.
    ///
    /// **An unborn HEAD answers with a name, not with `nil`** (06 ▸ Rules ▸ Abnormal repo states:
    /// "an unborn HEAD is normal git mode"). Every SwiftGitX HEAD accessor goes through
    /// `git_repository_head`, which refuses to resolve an unborn HEAD to a name and throws instead,
    /// so the only way to recover the branch a first commit *would* land on is to read `.git/HEAD`'s
    /// symbolic-ref target — the same plain text this file writes at init.
    nonisolated static func branchName(at boardRoot: URL) -> String? {
        guard BoardGitMode.hasGitEntry(at: boardRoot),
              let repository = try? Repository.open(at: boardRoot) else { return nil }

        if repository.isHEADUnborn {
            return unbornBranchName(gitDirectory: repository.path)
        }
        guard let head = try? repository.HEAD else { return nil }
        if repository.isHEADDetached {
            // Detached HEAD reports its branch `name` as the literal "HEAD", which labels nothing.
            // The short hash is what plain git shows in the same state. (The *posture* a detached
            // HEAD calls for — pausing the whole git surface honestly, 06 ▸ Abnormal repo states —
            // is the auto-commit card's; this is only the label.)
            return (head.target as? Commit)?.id.abbreviated ?? "HEAD"
        }
        return head.name
    }

    /// HEAD's commit, flattened to what a caller (and a test) can assert on: subject, author, and
    /// how many parents it has — a root commit having none is how "the root commit has its own
    /// subject" is checkable.
    nonisolated static func headCommit(at boardRoot: URL) -> CommitSummary? {
        guard BoardGitMode.hasGitEntry(at: boardRoot),
              let repository = try? Repository.open(at: boardRoot),
              !repository.isHEADUnborn,
              let head = try? repository.HEAD,
              let commit = head.target as? Commit else { return nil }

        return CommitSummary(
            oid: commit.id.hex,
            subject: commit.summary,
            authorName: commit.author.name,
            authorEmail: commit.author.email,
            parentCount: (try? commit.parents)?.count ?? 0
        )
    }

    /// Every file path in HEAD's tree, board-root-relative and sorted — what the repository actually
    /// tracks right now.
    nonisolated static func trackedPaths(at boardRoot: URL) -> [String] {
        guard BoardGitMode.hasGitEntry(at: boardRoot),
              let repository = try? Repository.open(at: boardRoot),
              !repository.isHEADUnborn,
              let head = try? repository.HEAD,
              let commit = head.target as? Commit else { return [] }
        return filePaths(of: commit, in: repository).sorted()
    }

    /// One commit, as much of it as anything outside this file needs.
    struct CommitSummary: Sendable, Equatable {
        let oid: String
        let subject: String
        let authorName: String
        let authorEmail: String
        let parentCount: Int
    }

    // MARK: Path history

    /// **When each path entered history** — the git half of the loader's earlier-occurrence-wins
    /// ladder (01-storage-format.md ▸ Fractal layout ▸ Rules: "on git boards, the path history
    /// already tracks outranks the newcomer"; `BoardLoader.IdentityHistoryRanker`).
    ///
    /// The answer is `git log --diff-filter=A`-shaped, walked here rather than shelled out: HEAD's
    /// **first-parent** ancestry oldest-first, with each commit's rank being its position in that
    /// walk. Paths present in the oldest commit reached rank 0 (its whole tree, since a root commit
    /// has no parent to diff against and a capped walk's base is "everything that already existed");
    /// every later commit contributes the paths its diff *adds*. Lower is earlier, which is exactly
    /// the ranker's contract, and a path never seen is absent — the `nil` the rule reads as
    /// "outranked by anything tracked".
    ///
    /// **Ranks are recorded for folders, not only files**, because the loader asks about *items*:
    /// a card is a folder, and what git tracks is the `index.md` inside it. Every directory prefix
    /// of an added file therefore takes that file's rank unless it already has an earlier one.
    ///
    /// Two honest limits. The walk is **capped** (`limit`), so a board with a longer history than
    /// that reads everything at its base as equally early — a tie the ladder resolves on birth date,
    /// exactly as it does without git. And **renames are not followed**: libgit2 reports a rename as
    /// an add plus a delete unless rename detection is run over the diff, so a card moved between
    /// lanes ranks at its move rather than at its birth (`--follow`'s job). Both degrade toward the
    /// no-history answer rather than toward a wrong one.
    nonisolated static func pathFirstAppearanceRanks(at boardRoot: URL, limit: Int = 512) -> [String: Int] {
        guard BoardGitMode.hasGitEntry(at: boardRoot),
              let repository = try? Repository.open(at: boardRoot),
              !repository.isHEADUnborn,
              let head = try? repository.HEAD,
              let tip = head.target as? Commit else { return [:] }

        var chain: [Commit] = []
        var current: Commit? = tip
        while let commit = current, chain.count < limit {
            chain.append(commit)
            current = (try? commit.parents)?.first
        }

        var ranks: [String: Int] = [:]
        for (rank, commit) in chain.reversed().enumerated() {
            if rank == 0 {
                for path in filePaths(of: commit, in: repository) {
                    record(path: path, rank: rank, into: &ranks)
                }
                continue
            }
            guard let diff = try? repository.diff(commit: commit) else { continue }
            for delta in diff.changes where delta.type == .added || delta.type == .renamed || delta.type == .copied {
                record(path: delta.newFile.path, rank: rank, into: &ranks)
            }
        }
        return ranks
    }

    /// Records `path` and every directory prefix above it at `rank`, keeping the earliest rank any
    /// of them has already earned.
    private static func record(path: String, rank: Int, into ranks: inout [String: Int]) {
        var components = path.split(separator: "/").map(String.init)
        while !components.isEmpty {
            let key = components.joined(separator: "/")
            if let existing = ranks[key] {
                ranks[key] = min(existing, rank)
            } else {
                ranks[key] = rank
            }
            components.removeLast()
        }
    }

    // MARK: - Private helpers

    /// Every blob path under `commit`'s tree, recursively.
    private static func filePaths(of commit: Commit, in repository: Repository) -> [String] {
        guard let tree = try? commit.tree else { return [] }
        var paths: [String] = []

        func walk(_ tree: Tree, prefix: String) {
            for entry in tree.entries {
                let path = prefix.isEmpty ? entry.name : prefix + "/" + entry.name
                if entry.type == .tree {
                    guard let subtree: Tree = try? repository.show(id: entry.id) else { continue }
                    walk(subtree, prefix: path)
                } else {
                    paths.append(path)
                }
            }
        }

        walk(tree, prefix: "")
        return paths
    }

    /// The unborn HEAD's symbolic target, parsed out of `.git/HEAD`'s plain text
    /// (`ref: refs/heads/main` → `main`).
    private static func unbornBranchName(gitDirectory: URL) -> String? {
        guard let contents = try? String(
            contentsOf: gitDirectory.appendingPathComponent("HEAD"),
            encoding: .utf8
        ) else { return nil }
        let trimmed = contents.trimmingCharacters(in: .whitespacesAndNewlines)
        let prefix = "ref: refs/heads/"
        guard trimmed.hasPrefix(prefix) else { return nil }
        let name = String(trimmed.dropFirst(prefix.count))
        return name.isEmpty ? nil : name
    }

    /// libgit2's own message for a SwiftGitX error — far more useful than the struct's synthesized
    /// description — falling back to the description for anything else.
    private static func reason(_ error: any Error) -> String {
        if let gitError = error as? SwiftGitXError { return gitError.message }
        return String(describing: error)
    }
}