Sapling/Sources/SaplingEditor/HybridMarkdownEditor.swift

import SwiftUI
import SaplingCore
import SaplingRenderer

@MainActor
public final class HybridMarkdownEditorViewModel: ObservableObject {
    @Published public var document: MarkdownDocument
    @Published public var activeLineIndex: Int

    public init(document: MarkdownDocument, activeLineIndex: Int = 0) {
        self.document = document
        self.activeLineIndex = activeLineIndex
    }

    public var lines: [String] {
        document.content.split(separator: "\n", omittingEmptySubsequences: false).map(String.init)
    }

    public func bindingForActiveLine() -> Binding<String> {
        Binding(
            get: { [weak self] in
                guard let self else { return "" }
                let lines = self.lines
                guard lines.indices.contains(self.activeLineIndex) else { return "" }
                return lines[self.activeLineIndex]
            },
            set: { [weak self] newValue in
                self?.replaceActiveLine(with: newValue)
            }
        )
    }

    public func activateLine(at index: Int) {
        activeLineIndex = index
    }

    private func replaceActiveLine(with newValue: String) {
        var updatedLines = lines
        if updatedLines.indices.contains(activeLineIndex) {
            updatedLines[activeLineIndex] = newValue
        } else {
            updatedLines.append(newValue)
            activeLineIndex = updatedLines.endIndex - 1
        }
        document.content = updatedLines.joined(separator: "\n")
    }
}

public struct HybridMarkdownEditor: View {
    @ObservedObject private var viewModel: HybridMarkdownEditorViewModel
    private let renderer: any MarkdownRendering

    public init(
        viewModel: HybridMarkdownEditorViewModel,
        renderer: any MarkdownRendering = MarkdownRenderer()
    ) {
        self.viewModel = viewModel
        self.renderer = renderer
    }

    public var body: some View {
        ScrollView {
            LazyVStack(alignment: .leading, spacing: 4) {
                ForEach(Array(viewModel.lines.enumerated()), id: \.offset) { index, line in
                    if index == viewModel.activeLineIndex {
                        TextEditor(text: viewModel.bindingForActiveLine())
                            .font(.system(.body, design: .monospaced))
                            .scrollContentBackground(.hidden)
                            .padding(.horizontal, 8)
                            .padding(.vertical, 4)
                            .frame(minHeight: 34)
                            .background(.quaternary.opacity(0.35), in: RoundedRectangle(cornerRadius: 6))
                    } else {
                        RenderedMarkdownLine(line: line, renderer: renderer)
                            .contentShape(Rectangle())
                            .onTapGesture {
                                viewModel.activateLine(at: index)
                            }
                    }
                }
            }
            .padding(20)
            .frame(maxWidth: .infinity, alignment: .leading)
        }
        .background(platformTextBackground)
    }
}

private struct RenderedMarkdownLine: View {
    let line: String
    let renderer: any MarkdownRendering

    var body: some View {
        let block = renderer.blocks(for: line).first ?? .blank(id: UUID())
        blockView(block)
            .frame(maxWidth: .infinity, alignment: .leading)
            .padding(.horizontal, 8)
            .padding(.vertical, 3)
    }

    @ViewBuilder
    private func blockView(_ block: RenderedMarkdownBlock) -> some View {
        switch block {
        case .heading(_, let level, let text):
            Text(text)
                .font(headingFont(level: level))
                .fontWeight(.semibold)
        case .paragraph(_, let text):
            Text(text)
                .font(.body)
        case .codeBlock(_, let language, let code):
            VStack(alignment: .leading, spacing: 6) {
                if let language {
                    Text(language.uppercased())
                        .font(.caption2)
                        .foregroundStyle(.secondary)
                }
                Text(code)
                    .font(.system(.body, design: .monospaced))
            }
            .padding(10)
            .background(.quaternary.opacity(0.45), in: RoundedRectangle(cornerRadius: 6))
        case .task(_, let checked, let text):
            HStack(spacing: 8) {
                Image(systemName: checked ? "checkmark.square.fill" : "square")
                    .foregroundStyle(checked ? .green : .secondary)
                Text(text)
            }
        case .image(_, let altText, let source):
            HStack(spacing: 8) {
                Image(systemName: "photo")
                VStack(alignment: .leading, spacing: 2) {
                    Text(altText.isEmpty ? "Image" : altText)
                    Text(source)
                        .font(.caption)
                        .foregroundStyle(.secondary)
                }
            }
        case .blank:
            Spacer(minLength: 20)
        }
    }

    private func headingFont(level: Int) -> Font {
        switch level {
        case 1: .largeTitle
        case 2: .title
        case 3: .title2
        default: .headline
        }
    }
}

private var platformTextBackground: Color {
    #if os(macOS)
    Color(nsColor: .textBackgroundColor)
    #else
    Color(uiColor: .systemBackground)
    #endif
}