import SwiftUI

/// TextEditor wrapped with a `@`-triggered autocomplete dropdown.
///
/// Detection heuristic: after every keystroke we scan the text for an
/// `@[\w.-]*` token that reaches the very end of the string. If one is
/// found, the dropdown opens filtered by the characters after the `@`.
/// SwiftUI's TextEditor does not expose the caret, so mid-text insertion
/// won't trigger the picker — users need to type the mention at the
/// current end of the body. This matches how most mobile clients work
/// in practice and avoids a UIKit bridge.
///
/// On selection the `@query` token is replaced with `@nick ` (trailing
/// space closes the mention), and the caller-owned `mentionMap` gets
/// `nick -> feedURL` so the rewrite on save can produce
/// `[[org-social:URL][nick]]`.
struct MentionAutocompleteField<Focus: Hashable & Sendable>: View {
    @Binding var text: String
    @Binding var mentionMap: [String: String]
    let candidates: [MentionCandidate]
    let placeholder: String
    var minHeight: CGFloat = 140

    // Callers that want to drive focus from outside (Compose does,
    // Edit no longer auto-focuses) pass a @FocusState binding. When
    // not needed, pass nil.
    var focusBinding: FocusState<Focus>.Binding?
    var focusedValue: Focus?

    @State private var showPicker = false
    @State private var filterQuery = ""

    var body: some View {
        VStack(alignment: .leading, spacing: 0) {
            ZStack(alignment: .topLeading) {
                editor

                if text.isEmpty {
                    Text(placeholder)
                        .foregroundStyle(.tertiary)
                        .padding(.top, 12)
                        .padding(.leading, 8)
                        .allowsHitTesting(false)
                }
            }

            if showPicker, !filteredCandidates.isEmpty {
                mentionDropdown
            }
        }
        .onChange(of: text) { _, newValue in
            updatePicker(for: newValue)
        }
    }

    @ViewBuilder
    private var editor: some View {
        if let binding = focusBinding, let value = focusedValue {
            TextEditor(text: $text)
                .frame(minHeight: minHeight)
                .focused(binding, equals: value)
                .padding(4)
        } else {
            TextEditor(text: $text)
                .frame(minHeight: minHeight)
                .padding(4)
        }
    }

    private var filteredCandidates: [MentionCandidate] {
        let q = filterQuery.lowercased()
        let base = q.isEmpty
            ? candidates
            : candidates.filter { $0.nick.lowercased().hasPrefix(q) }
        // Cap the visible dropdown so a big follow list stays usable.
        return Array(base.prefix(8))
    }

    private var mentionDropdown: some View {
        VStack(alignment: .leading, spacing: 0) {
            ForEach(filteredCandidates) { candidate in
                Button {
                    insert(candidate)
                } label: {
                    HStack(spacing: 10) {
                        AvatarView(url: candidate.avatarURL, nick: candidate.nick, size: 28)
                        VStack(alignment: .leading, spacing: 1) {
                            Text("@\(candidate.nick)").font(.subheadline.weight(.medium))
                            Text(candidate.feedURL.host ?? candidate.feedURL.absoluteString)
                                .font(.caption2)
                                .foregroundStyle(.secondary)
                                .lineLimit(1)
                                .truncationMode(.middle)
                        }
                        Spacer(minLength: 0)
                    }
                    .padding(.horizontal, 12).padding(.vertical, 8)
                    .contentShape(Rectangle())
                }
                .buttonStyle(.plain)
                if candidate.id != filteredCandidates.last?.id {
                    Divider().padding(.leading, 50)
                }
            }
        }
        .background(Color(.systemBackground), in: RoundedRectangle(cornerRadius: 12))
        .overlay(RoundedRectangle(cornerRadius: 12).strokeBorder(Color.secondary.opacity(0.2)))
        .padding(.top, 6)
        .shadow(color: Color.black.opacity(0.08), radius: 4, y: 2)
    }

    private func updatePicker(for text: String) {
        // Match a trailing @word. `\w` in Swift regex already includes
        // letters / digits / underscore; we widen with `.-` for common
        // nick punctuation.
        let pattern = #"@([A-Za-z0-9_.-]*)$"#
        guard let regex = try? NSRegularExpression(pattern: pattern) else {
            showPicker = false
            return
        }
        let nsrange = NSRange(text.startIndex..., in: text)
        if let match = regex.firstMatch(in: text, range: nsrange),
           let qr = Range(match.range(at: 1), in: text) {
            filterQuery = String(text[qr])
            showPicker = true
        } else {
            filterQuery = ""
            showPicker = false
        }
    }

    private func insert(_ candidate: MentionCandidate) {
        let pattern = #"@[A-Za-z0-9_.-]*$"#
        guard let regex = try? NSRegularExpression(pattern: pattern) else { return }
        let nsrange = NSRange(text.startIndex..., in: text)
        let replacement = "@\(candidate.nick) "
        if let match = regex.firstMatch(in: text, range: nsrange),
           let fr = Range(match.range, in: text) {
            text.replaceSubrange(fr, with: replacement)
        } else {
            text.append(replacement)
        }
        mentionMap[candidate.nick] = candidate.feedURL.absoluteString
        showPicker = false
        filterQuery = ""
    }
}