import SwiftUI

/// The card's read-only view — `BoardRoute.card`'s destination.
///
/// Holds board-relative IDs only, never a `Card` value: every body evaluation re-reads the card
/// from `session.snapshot`, so a write this screen's own edit sheet makes (or one that lands from
/// elsewhere while it's open) is reflected the moment the session's reload lands, and a card
/// trashed on another device is noticed rather than shown stale.
///
/// **Read-only, by design.** There is no draft here and nothing this screen ever writes — editing
/// lives entirely in `CardEditScreen`, presented full-screen from the Edit button. That split is
/// the whole point of the transactional model: a screen that only ever renders `session.snapshot`
/// cannot itself go stale relative to disk, and there is no auto-save timing to reason about
/// because there is no save at all on this side of the Edit button.
struct CardDetailScreen: View {
    let boardRoot: URL
    let laneID: ItemID
    let cardID: ItemID

    @Environment(BoardIndexStore.self) private var index
    @Environment(\.dismiss) private var dismiss

    @State private var isPresentingEdit = false

    private var session: BoardSession { index.session(forBoardAt: boardRoot) }

    private var card: Card? {
        session.snapshot?.lanes.first { $0.id == laneID }?.cards.first { $0.id == cardID }
    }

    var body: some View {
        content
            .navigationTitle("Card")
            .navigationBarTitleDisplayMode(.inline)
            .toolbar {
                if card != nil {
                    ToolbarItem(placement: .primaryAction) {
                        Button("Edit") { isPresentingEdit = true }
                    }
                }
            }
            .task { session.open() }
            .fullScreenCover(isPresented: $isPresentingEdit) {
                CardEditScreen(boardRoot: boardRoot, laneID: laneID, cardID: cardID)
            }
    }

    @ViewBuilder
    private var content: some View {
        if let card {
            ScrollView {
                VStack(alignment: .leading, spacing: 20) {
                    if session.lastError != nil {
                        Label("Showing the last saved version — a recent update didn't load.", systemImage: "exclamationmark.triangle")
                            .font(.footnote)
                            .foregroundStyle(.secondary)
                    }

                    titleBlock(for: card)
                    bodyBlock(for: card)
                    detailsFooter(for: card)
                }
                .padding()
                .frame(maxWidth: .infinity, alignment: .leading)
            }
        } else if case .ready = session.phase {
            ContentUnavailableView("Card Removed", systemImage: "trash", description: Text("This card was deleted."))
                .task { dismiss() }
        } else {
            ProgressView("Opening card")
        }
    }

    /// The title, its icon when the card has one, and a subtle background tint lifted from the
    /// card's own `background` swatch — passive display only, no swatches to tap here. An
    /// unrecognized colour name (the same leniency `ItemIconView`/`CardPalette` document for
    /// themselves) just skips the tint rather than guessing at one.
    @ViewBuilder
    private func titleBlock(for card: Card) -> some View {
        HStack(alignment: .firstTextBaseline, spacing: 10) {
            ItemIconView(icon: card.icon.value, iconColor: card.iconColor.value)
            titleText(for: card)
        }
        .padding()
        .frame(maxWidth: .infinity, alignment: .leading)
        .background(
            (tintColor(for: card)?.opacity(0.15) ?? Color.clear),
            in: RoundedRectangle(cornerRadius: 12)
        )
    }

    @ViewBuilder
    private func titleText(for card: Card) -> some View {
        if let title = card.title.value, !title.isEmpty {
            Text(title)
                .font(.title.bold())
        } else {
            Text("Untitled Card")
                .font(.title.bold())
                .foregroundStyle(.secondary)
        }
    }

    private func tintColor(for card: Card) -> Color? {
        card.background.value.flatMap { CardPalette.color(named: $0, in: CardPalette.backgrounds) }
    }

    /// The body, rendered as Markdown — see `MarkdownBlocks` for why this is a per-paragraph
    /// `AttributedString(markdown:)` pass rather than a full block parser.
    @ViewBuilder
    private func bodyBlock(for card: Card) -> some View {
        let paragraphs = MarkdownBlocks.paragraphs(in: card.body)
        if paragraphs.isEmpty {
            Text("No description")
                .foregroundStyle(.secondary)
        } else {
            VStack(alignment: .leading, spacing: 12) {
                ForEach(Array(paragraphs.enumerated()), id: \.offset) { _, paragraph in
                    Text(MarkdownBlocks.rendered(paragraph))
                }
            }
        }
    }

    @ViewBuilder
    private func detailsFooter(for card: Card) -> some View {
        VStack(alignment: .leading, spacing: 4) {
            Text("Created \(card.created.value.map(Self.formatted) ?? "—")")
            Text("Modified \(card.modified.value.map(Self.formatted) ?? "—")")
        }
        .font(.footnote)
        .foregroundStyle(.secondary)
        .padding(.top, 8)
    }

    private static func formatted(_ date: Date) -> String {
        date.formatted(date: .abbreviated, time: .shortened)
    }
}

/// Dependency-free Markdown rendering for a card body: no `swift-markdown` block parser here, just
/// `Foundation`'s own `AttributedString(markdown:options:)` run once per paragraph.
///
/// **Why per-paragraph.** `AttributedString`'s built-in parser has no "render this as a sequence of
/// blocks" mode short of `.full`, which produces a block tree this view would still have to walk —
/// no simpler than doing the walk ourselves, and it collapses the very newlines a card body relies
/// on to separate paragraphs. Splitting on blank lines first and parsing each paragraph with
/// `.inlineOnlyPreservingWhitespace` gets both halves of what a card body needs cheaply: inline
/// emphasis/links/code render as styled runs, and the newlines *inside* a paragraph (soft line
/// breaks) survive as literal whitespace instead of being folded into a single space.
enum MarkdownBlocks {

    /// Splits `body` on runs of one or more blank lines — Markdown's own paragraph boundary — and
    /// trims each block, so a body with trailing blank lines or extra spacing between paragraphs
    /// doesn't render phantom empty blocks.
    static func paragraphs(in body: String) -> [String] {
        body
            .components(separatedBy: "\n\n")
            .map { $0.trimmingCharacters(in: .whitespacesAndNewlines) }
            .filter { !$0.isEmpty }
    }

    /// One paragraph, inline-parsed. Malformed Markdown (an unterminated code span, say) falls back
    /// to the raw paragraph text rather than dropping it — the same "never lose the author's text"
    /// posture `CardPalette.color(named:in:)` takes for an unrecognized colour name.
    static func rendered(_ paragraph: String) -> AttributedString {
        let options = AttributedString.MarkdownParsingOptions(interpretedSyntax: .inlineOnlyPreservingWhitespace)
        return (try? AttributedString(markdown: paragraph, options: options)) ?? AttributedString(paragraph)
    }
}