import SwiftUI
import Charts

/// Trends — the Favourites price-history chart.
///
/// Plots one line per favourited station for the active fuel across the
/// selected range (7/30/90 days), fed by the GitHub price mirror
/// (`FuelHistoryStore`). Default shows absolute prices; "vs cheapest" rebases
/// each day to the cheapest favourite (0 baseline, signed pence above it) —
/// the same delta pattern the list rows already use. Missing days are gaps,
/// never fabricated. No widget in v1 (one-kind rule).
struct TrendsView: View {
    /// All favourites (fuel-scoped entries) — the sheet derives the active
    /// fuel's stations and which fuels have favourites.
    let favourites: [FavouriteEntry]
    let selectedFuel: FuelType
    let priceDisplayStyle: PriceDisplayStyle

    @Environment(\.dismiss) private var dismiss

    @State private var fuel: FuelType = .e10
    @State private var rangeDays: Int = 30
    @State private var mode: TrendsMode = .price
    @State private var series: [StationHistory] = []
    @State private var isLoading = false
    @State private var loadFailed = false
    @State private var firstSnapshot: String?

    enum TrendsMode: String, CaseIterable, Identifiable {
        case price
        case vsCheapest
        var id: String { rawValue }
    }

    /// Fuels that currently have at least one favourite — only these tabs show.
    private var availableFuels: [FuelType] {
        FuelType.allCases.filter { fuel in
            favourites.contains { $0.fuel == fuel }
        }
    }

    /// The active fuel, with the same fallback as the Favourites tab.
    private var activeFuel: FuelType {
        availableFuels.contains(fuel) ? fuel : (availableFuels.first ?? .e10)
    }

    /// Stations favourited for the active fuel, in the user's stored order —
    /// the chart keeps this order so line colours are stable.
    private var orderedStations: [(id: String, name: String)] {
        favourites.filter { $0.fuel == activeFuel }.map { ($0.station.id, $0.station.name) }
    }

    private var displaySeries: [StationHistory] {
        mode == .price ? series : FuelHistoryStore.deltaSeries(series)
    }

    /// A line needs at least two points to draw; anything less is the
    /// "building up" state, not a broken chart.
    private var hasEnoughData: Bool {
        series.contains { $0.points.count >= 2 }
    }

    private var hasAnyData: Bool {
        series.contains { !$0.points.isEmpty }
    }

    private func seriesColor(_ index: Int) -> Color {
        let palette: [Color] = [.blue, .orange, .purple, .pink, .teal, .indigo, .brown, .green]
        return palette[index % palette.count]
    }

    /// X-axis tick density — a tick per day for short ranges, monthly for the
    /// 90-day view so labels never collide.
    private var xStride: Int {
        switch rangeDays {
        case ...14: return 1
        case 15...60: return 7
        default: return 30
        }
    }

    private func load() async {
        isLoading = true
        loadFailed = false
        defer { isLoading = false }
        // The pointer is a non-fatal hint for the empty state; history fetch
        // failures surface as the retry state.
        firstSnapshot = await FuelHistoryStore.fetchLatest()?.availableFrom
        let fetched = await FuelHistoryStore.fetchHistory(
            stations: orderedStations,
            fuel: activeFuel,
            days: rangeDays
        )
        if fetched.allSatisfy({ $0.points.isEmpty }), !orderedStations.isEmpty {
            // All days missing — either the mirror is unreachable (retry) or
            // genuinely empty (the building-up state). Distinguish by a quick
            // pointer probe already done above.
            loadFailed = firstSnapshot == nil
        }
        series = fetched
    }

    private func yLabel(_ pence: Double) -> String {
        switch mode {
        case .price:
            return FuelStore.priceText(pence, style: priceDisplayStyle)
        case .vsCheapest:
            return String(format: "%.1fp", pence)
        }
    }

    var body: some View {
        NavigationStack {
            VStack(spacing: 14) {
                FuelTypeSegmentedPicker(selection: $fuel, fuels: availableFuels)

                HStack(spacing: 10) {
                    Picker("Range", selection: $rangeDays) {
                        ForEach(FuelHistoryStore.rangeOptions, id: \.self) { days in
                            Text("\(days) days").tag(days)
                        }
                    }
                    .pickerStyle(.segmented)

                    Picker("Mode", selection: $mode) {
                        Text("Price").tag(TrendsMode.price)
                        Text("vs cheapest").tag(TrendsMode.vsCheapest)
                    }
                    .pickerStyle(.segmented)
                }

                chartArea
            }
            .padding()
            .navigationTitle("Trends")
            .navigationBarTitleDisplayMode(.inline)
            .toolbar {
                ToolbarItem(placement: .confirmationAction) {
                    Button("Done") { dismiss() }
                }
            }
            .task(id: "\(activeFuel.rawValue)-\(rangeDays)") {
                await load()
            }
        }
    }

    @ViewBuilder
    private var chartArea: some View {
        // Every state anchors to the TOP of the chart slot — the same spot the
        // chart occupies when data exists. The empty/loading/error states must
        // not float or centre in the sheet, or the layout jumps between states.
        Group {
            if isLoading {
                VStack(spacing: 12) {
                    ProgressView("Fetching price history…")
                }
                .padding(.top, 24)
            } else if loadFailed {
                VStack(spacing: 10) {
                    Image(systemName: "wifi.exclamationmark")
                        .font(.system(size: 32))
                        .foregroundStyle(.secondary)
                    Text("Couldn't load price history")
                        .font(.headline)
                    Button("Retry") { Task { await load() } }
                        .buttonStyle(.bordered)
                }
                .padding(.top, 24)
            } else if !hasAnyData {
                emptyState
            } else if !hasEnoughData {
                emptyState // single point — nothing to draw yet
            } else {
                VStack(spacing: 12) {
                    chart
                    legend
                }
            }
        }
        .frame(maxWidth: .infinity, maxHeight: .infinity, alignment: .top)
    }

    private var emptyState: some View {
        VStack(spacing: 10) {
            Image(systemName: "chart.xyaxis.line")
                .font(.system(size: 32))
                .foregroundStyle(.secondary)
            Text("No price history yet")
                .font(.headline)
            if let firstSnapshot {
                Text("First snapshot \(firstSnapshot) — a few days are needed to draw a trend.")
                    .font(.caption)
                    .foregroundStyle(.secondary)
                    .multilineTextAlignment(.center)
            } else {
                Text("Prices are recorded each day FuelBoard's relay runs — check back in a few days.")
                    .font(.caption)
                    .foregroundStyle(.secondary)
                    .multilineTextAlignment(.center)
            }
        }
        .frame(maxWidth: .infinity)
        .padding(.vertical, 24)
    }

    private var chart: some View {
        // NOTE: each LineMark MUST carry an explicit `series:` — without it
        // Swift Charts merges every station's points into ONE polyline
        // (points connect across stations, so only the first station's line
        // is recognisable). The outer ForEach keeps one chart with N series;
        // per-mark foregroundStyle then colours each series from the palette.
        Chart {
            ForEach(displaySeries) { history in
                ForEach(history.points) { point in
                    LineMark(
                        x: .value("Date", point.date),
                        y: .value("Price", point.pence),
                        series: .value("Station", history.name)
                    )
                    .foregroundStyle(seriesColor(index(of: history.stationID)))
                }
            }
        }
        .chartXAxis {
            AxisMarks(values: .stride(by: .day, count: xStride)) { _ in
                AxisGridLine()
                AxisTick()
                AxisValueLabel(format: .dateTime.month().day())
            }
        }
        .chartYAxis {
            AxisMarks { value in
                AxisGridLine()
                AxisValueLabel {
                    if let pence = value.as(Double.self) {
                        Text(yLabel(pence))
                    }
                }
            }
        }
        .frame(height: 260)
    }

    private func index(of stationID: String) -> Int {
        orderedStations.firstIndex(where: { $0.id == stationID }) ?? 0
    }

    private var legend: some View {
        VStack(alignment: .leading, spacing: 4) {
            ForEach(Array(displaySeries.enumerated()), id: \.element.stationID) { index, history in
                HStack(spacing: 8) {
                    Circle()
                        .fill(seriesColor(index))
                        .frame(width: 8, height: 8)
                    Text(history.name)
                        .font(.caption)
                        .lineLimit(1)
                    Spacer()
                }
            }
        }
        .padding(.horizontal, 4)
    }
}