import Foundation
import Testing
@testable import Voice

/// Pure coordinator tests for the server replacement worker loop semantics
/// (Iteration 08 Item 2). These verify the latest-wins coalescing and
/// drain-until-empty behavior that the ConversationView worker implements,
/// without requiring a live SwiftUI view or real session disconnect.
///
/// The production code uses `@State` in a SwiftUI view; this test models the
/// same logic as a pure function to verify A→B→C semantics where C arrives
/// while B is disconnecting.
struct ServerReplacementCoordinatorTests {

    /// Models the pending-URL coalescing and worker-loop drain semantics.
    /// Returns the sequence of URLs the worker would process in order.
    private static func drainPending(
        initialActive: String,
        applies: [String],
        disconnectDurations: [Int] = []
    ) -> [String] {
        var activeURL = initialActive
        var pending: String? = nil
        var processed: [String] = []
        var isReplacing = false

        for url in applies {
            // Every accepted Apply writes pendingServerURL (latest wins).
            pending = url

            // Start the worker if not already active.
            guard !isReplacing else { continue }
            isReplacing = true

            // Worker loop: drain until no pending value remains.
            while let url = pending {
                pending = nil
                guard url != activeURL else { continue }
                // Simulate disconnect (no-op in pure test).
                activeURL = url
                processed.append(url)
            }
            isReplacing = false
        }

        return processed
    }

    @Test("single apply processes one URL")
    func singleApply() {
        let result = Self.drainPending(
            initialActive: "https://server-a",
            applies: ["https://server-b"]
        )
        #expect(result == ["https://server-b"])
    }

    @Test("rapid A to B to C coalesces to final C")
    func rapidABC() {
        // All three Applies arrive before the worker starts (isReplacing
        // is false for each, but the first one starts the worker which
        // drains all pending values).
        // In practice: Apply A sets pending=A, starts worker. Worker
        // consumes A. Apply B sets pending=B during disconnect. Worker
        // loops, consumes B. Apply C sets pending=C during B's disconnect.
        // Worker loops, consumes C. Final state: activeServerURL == C.
        let result = Self.drainPending(
            initialActive: "https://server-a",
            applies: ["https://server-b", "https://server-c", "https://server-d"]
        )
        // The worker processes each URL in order as it drains.
        #expect(result == ["https://server-b", "https://server-c", "https://server-d"])
    }

    @Test("reapplying same URL is skipped")
    func sameURLSkipped() {
        let result = Self.drainPending(
            initialActive: "https://server-a",
            applies: ["https://server-a"]
        )
        #expect(result.isEmpty)
    }

    @Test("latest wins when multiple applies during replacement")
    func latestWinsDuringReplacement() {
        // Simulates: Apply B starts worker. During B's disconnect,
        // Apply C arrives (pending=C). Worker finishes B, loops,
        // sees pending=C, processes C. Final active = C.
        var activeURL = "https://server-a"
        var pending: String? = nil

        // Apply B
        pending = "https://server-b"

        // Worker starts, consumes B
        if let url = pending {
            pending = nil
            activeURL = url
        }

        // During B's disconnect, Apply C arrives
        pending = "https://server-c"

        // Worker loops: sees pending=C, processes it
        while let url = pending {
            pending = nil
            guard url != activeURL else { continue }
            activeURL = url
        }

        #expect(activeURL == "https://server-c")
    }

    @Test("A to B to C with C arriving during B disconnect ends on C")
    func abcWithCDuringBDisconnect() {
        var activeURL = "https://server-a"
        var pending: String? = nil
        var processed: [String] = []

        // Apply B (A is the initial active URL)
        pending = "https://server-b"

        // Worker loop iteration 1: consume B
        while let url = pending {
            pending = nil
            guard url != activeURL else { continue }
            // Simulate disconnect of current session (A)
            activeURL = url
            processed.append(url)

            // During B's disconnect, Apply C arrives
            pending = "https://server-c"
        }

        // Worker loop iteration 2: consume C
        while let url = pending {
            pending = nil
            guard url != activeURL else { continue }
            activeURL = url
            processed.append(url)
        }

        #expect(activeURL == "https://server-c")
        #expect(processed == ["https://server-b", "https://server-c"])
    }
}