import Foundation
import Observation

/// One merge request: `mr show`, the parsed diff, the review threads, and
/// every write the review cycle needs. Writes reload — the server's state
/// is the state.
@Observable
@MainActor
final class MRDetailViewModel {

    private(set) var state: LoadState<MRDetail> = .loading
    private(set) var diff: UnifiedDiff?
    private(set) var threads: [ReviewThread] = []
    /// A write failed; the server's sentence, shown until the next action.
    private(set) var actionError: String?
    private(set) var working = false

    private let client: GitbayClient
    let repoPath: String
    let number: Int64

    init(client: GitbayClient, repoPath: String, number: Int64) {
        self.client = client
        self.repoPath = repoPath
        self.number = number
    }

    private var ref: [String] { [repoPath, String(number)] }

    func load() async {
        do {
            let detail = try await client.read(["mr", "show"] + ref, as: MRDetail.self)
            state = .loaded(detail)
        } catch {
            state = .from(error)
            return
        }
        // The diff and threads are secondary: their failure leaves the
        // header usable rather than sinking the screen.
        async let diffText = try? client.readText(["mr", "diff"] + ref)
        async let threadList = try? client.readList(["mr", "threads"] + ref, of: ReviewThread.self)
        diff = (await diffText).map(UnifiedDiff.parse)
        threads = await threadList ?? []
    }

    var unresolvedCount: Int {
        threads.count { !$0.isResolved }
    }

    // MARK: - Writes

    enum Verdict: String, Sendable {
        case approve = "--approve"
        case requestChanges = "--request-changes"
    }

    func review(_ verdict: Verdict) async {
        await perform(["mr", "review"] + ref + [verdict.rawValue])
    }

    /// Both fields are always sent; an empty body clears it.
    func edit(title: String, body: String) async {
        await perform(
            ["mr", "edit"] + ref + ["--title", title, "--file", "-"],
            stdin: body
        )
    }

    /// Open milestones for the picker; fetched on first use.
    private(set) var availableMilestones: [Milestone]?

    func loadMilestones() async {
        guard availableMilestones == nil else { return }
        availableMilestones = (try? await client.readList(
            ["milestone", "list", repoPath, "--state", "open"], of: Milestone.self
        )) ?? []
    }

    /// nil clears it; the command spells that "none".
    func setMilestone(_ title: String?) async {
        await perform(["mr", "milestone"] + ref + [title ?? "none"])
    }

    func comment(_ text: String) async {
        // Long text travels in stdin, but the server only reads it when
        // argv says so: --file - is required, not implied.
        await perform(["mr", "comment"] + ref + ["--file", "-"], stdin: text)
    }

    func merge(strategy: String? = nil) async {
        var argv = ["mr", "merge"] + ref
        if let strategy { argv.append(contentsOf: ["--strategy", strategy]) }
        await perform(argv)
    }

    func close() async {
        await perform(["mr", "close"] + ref)
    }

    func setResolved(_ thread: ReviewThread, _ resolved: Bool) async {
        await perform(["mr", resolved ? "resolve" : "unresolve"] + ref + [String(thread.id)])
    }

    func reply(to thread: ReviewThread, _ text: String) async {
        await perform(
            ["mr", "diff-comment"] + ref + ["--reply", String(thread.id), "--file", "-"],
            stdin: text
        )
    }

    private func perform(_ argv: [String], stdin: String? = nil) async {
        working = true
        actionError = nil
        defer { working = false }
        do {
            try await client.run(argv, stdin: stdin)
            await load()
        } catch let error as GitbayError {
            // A refusal explains the rule (approvals missing, threads
            // unresolved, checks red). Surface it verbatim; never route
            // around it.
            actionError = error.userFacingMessage
        } catch {
            actionError = GitbayError.transport(error).userFacingMessage
        }
    }
}