import SwiftUI

/// Read-only unified diff viewer for a single changed file.
///
/// Parses a Gitea unified diff patch into line hunks so added/deleted lines can
/// be colour-coded. Gitea only returns `patch` for small diffs; large files
/// fall back to a raw text view.
struct DiffView: View {
    let file: ChangedFile

    var body: some View {
        Group {
            if let patch = file.patch, !patch.isEmpty {
                ScrollView([.horizontal, .vertical]) {
                    LazyVStack(alignment: .leading, spacing: 0) {
                        ForEach(parse(patch)) { line in
                            diffLine(line)
                        }
                    }
                    .font(.caption.monospaced())
                }
            } else {
                ContentUnavailableView(
                    "Diff too large to preview",
                    systemImage: "doc.text",
                    description: Text("Gitea didn't return a patch for this file."))
            }
        }
        .navigationTitle(file.filename)
        .navigationBarTitleDisplayMode(.inline)
    }

    private func diffLine(_ line: DiffLine) -> some View {
        Text(line.text)
            .frame(maxWidth: .infinity, alignment: .leading)
            .padding(.horizontal, 8)
            .padding(.vertical, 1)
            .background(background(for: line.type))
            .foregroundColor(foreground(for: line.type))
    }

    private func background(for type: DiffLineType) -> Color {
        switch type {
        case .addition: return Color.green.opacity(0.15)
        case .deletion: return Color.red.opacity(0.15)
        case .hunk, .meta: return Color(.secondarySystemBackground)
        case .context: return .clear
        }
    }
    private func foreground(for type: DiffLineType) -> Color {
        switch type {
        case .addition: return .green
        case .deletion: return .red
        case .hunk: return .secondary
        case .meta, .context: return .primary
        }
    }

    private func parse(_ patch: String) -> [DiffLine] {
        patch.components(separatedBy: "\n").map { line in
            if line.hasPrefix("@@") { return DiffLine(text: line, type: .hunk) }
            if line.hasPrefix("diff ") || line.hasPrefix("index ")
                || line.hasPrefix("--- ") || line.hasPrefix("+++ ") {
                return DiffLine(text: line, type: .meta)
            }
            if line.hasPrefix("+") { return DiffLine(text: line, type: .addition) }
            if line.hasPrefix("-") { return DiffLine(text: line, type: .deletion) }
            return DiffLine(text: line, type: .context)
        }
    }
}

private enum DiffLineType { case context, addition, deletion, hunk, meta }

private struct DiffLine: Identifiable {
    let text: String
    let type: DiffLineType
    var id: UUID { UUID() }
}