import SwiftUI

/// The parsed unified diff, file by file, hunk by hunk. Loads its own
/// text so the screen is reachable by value from anywhere.
struct DiffView: View {

    private let client: GitbayClient
    private let repo: String
    private let number: Int64
    @State private var state: LoadState<UnifiedDiff> = .loading

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

    var body: some View {
        ZStack {
            Color.clear
            if let diff = state.value, !diff.files.isEmpty {
                List {
                    ForEach(diff.files) { file in
                        FileDiffSection(file: file)
                    }
                }
                .listStyle(.plain)
            } else if case .loaded = state {
                ContentUnavailableView {
                    Label("No changes", systemImage: "plus.forwardslash.minus")
                }
            }
        }
        .overlay { LoadStateOverlay(state: state) }
        .navigationTitle("Diff")
        .navigationBarTitleDisplayMode(.inline)
        .task {
            do {
                let text = try await client.readText(["mr", "diff", repo, String(number)])
                state = .loaded(UnifiedDiff.parse(text))
            } catch {
                state = .from(error)
            }
        }
    }
}

private struct FileDiffSection: View {

    let file: UnifiedDiff.File
    @State private var collapsed = false

    var body: some View {
        Section {
            if !collapsed {
                if file.isBinary {
                    Text("Binary file")
                        .font(.caption)
                        .foregroundStyle(.secondary)
                } else {
                    ForEach(file.hunks) { hunk in
                        HunkView(hunk: hunk)
                    }
                }
            }
        } header: {
            Button {
                collapsed.toggle()
            } label: {
                HStack(spacing: 6) {
                    Image(systemName: collapsed ? "chevron.right" : "chevron.down")
                        .font(.caption2)
                    Text(file.displayPath)
                        .font(.caption.monospaced().weight(.semibold))
                        .lineLimit(1)
                        .truncationMode(.head)
                    if file.isNew {
                        Text("new").font(.caption2).foregroundStyle(.green)
                    }
                    if file.isDeleted {
                        Text("deleted").font(.caption2).foregroundStyle(.red)
                    }
                    Spacer()
                    Text("+\(file.additions)").foregroundStyle(.green).font(.caption2)
                    Text("−\(file.deletions)").foregroundStyle(.red).font(.caption2)
                }
            }
            .buttonStyle(.plain)
            .textCase(nil)
        }
    }
}

private struct HunkView: View {

    let hunk: UnifiedDiff.Hunk

    var body: some View {
        ScrollView(.horizontal) {
            VStack(alignment: .leading, spacing: 0) {
                Text(hunk.header)
                    .font(.caption2.monospaced())
                    .foregroundStyle(.blue)
                    .padding(.vertical, 2)
                ForEach(hunk.lines) { line in
                    LineView(line: line)
                }
            }
        }
        .listRowInsets(EdgeInsets(top: 2, leading: 8, bottom: 2, trailing: 8))
    }
}

private struct LineView: View {

    let line: UnifiedDiff.Line

    var body: some View {
        HStack(spacing: 0) {
            Text(line.oldNumber.map(String.init) ?? "")
                .frame(width: 34, alignment: .trailing)
                .foregroundStyle(.tertiary)
            Text(line.newNumber.map(String.init) ?? "")
                .frame(width: 34, alignment: .trailing)
                .foregroundStyle(.tertiary)
            Text(marker)
                .frame(width: 14)
                .foregroundStyle(markerColor)
            Text(line.text.isEmpty ? " " : line.text)
                .foregroundStyle(textColor)
        }
        .font(.caption2.monospaced())
        .background(background)
    }

    private var marker: String {
        switch line.kind {
        case .addition: "+"
        case .deletion: "−"
        case .context: " "
        }
    }

    private var markerColor: Color {
        switch line.kind {
        case .addition: .green
        case .deletion: .red
        case .context: .clear
        }
    }

    private var textColor: Color {
        .primary
    }

    private var background: Color {
        switch line.kind {
        case .addition: .green.opacity(0.12)
        case .deletion: .red.opacity(0.12)
        case .context: .clear
        }
    }
}