ReadViewSDK/Sources/RDReaderView/EPUBTextRendering/RDEPUBDTCoreTextRenderer.swift

import UIKit

#if canImport(DTCoreText)
import DTCoreText
#endif

/// DTCoreText 渲染器实现：将 EPUB 章节 HTML 转换为富文本（NSAttributedString）。
///
/// 这是 `RDEPUBTextRenderer` 协议的默认实现，位于渲染链路的第二步：
/// 章节 HTML → DTCoreText 渲染 → NSAttributedString → 提取 fragment 偏移量 → 统一字体/行距
///
/// DTCoreText 库负责将 HTML 解析为带有排版属性的富文本，
/// 渲染过程中会通过 `willFlushCallback` 回调对每个 DOM 元素做布局规范化（图片尺寸等）。
/// 当 DTCoreText 不可用时（条件编译失败），回退到纯文本渲染。
public struct RDEPUBDTCoreTextRenderer: RDEPUBTextRenderer {
    public init() {}

    /// DTCoreText 库是否在当前编译环境中可用
    public static var isAvailable: Bool {
#if canImport(DTCoreText)
        return true
#else
        return false
#endif
    }

    /// 渲染单个章节：HTML → NSAttributedString，同时提取 fragment 和语义标记。
    ///
    /// 渲染流程：
    /// 1. 将 HTML 字符串编码为 Data
    /// 2. 通过 DTCoreText 解析为富文本（失败则回退到纯文本）
    /// 3. 注入分页语义标记（${rd-sem-start/end} → 属性字典）
    /// 4. 提取 fragment 偏移量映射表
    /// 5. 规范化阅读属性（字体、行距、颜色统一）
    public func renderChapter(
        request: RDEPUBTextChapterRenderRequest
    ) throws -> RDEPUBRenderedChapterContent {
#if canImport(DTCoreText)
        let chapterContext = request.context
        guard let data = chapterContext.html.data(using: .utf8) else {
            throw RDEPUBTextRenderingError.htmlEncodingFailed
        }

        guard let rendered = makeAttributedString(from: data, request: request) else {
            return fallbackRenderedContent(request: request)
        }

        let attributedString = NSMutableAttributedString(attributedString: rendered)
        RDEPUBTextRendererSupport.applyPaginationSemantics(in: attributedString)
        let fragmentOffsets = RDEPUBTextRendererSupport.extractFragmentOffsets(from: attributedString)
        RDEPUBTextRendererSupport.normalizeReadingAttributes(in: attributedString, style: request.style)
        return RDEPUBRenderedChapterContent(
            attributedString: attributedString,
            fragmentOffsets: fragmentOffsets,
            resourceDiagnostics: chapterContext.resourceDiagnostics
        )
#else
        return fallbackRenderedContent(request: request)
#endif
    }

    /// 便捷方法：直接传入 HTML 字符串进行渲染（不含上下文信息）
    public func renderChapter(
        html: String,
        baseURL: URL?,
        style: RDEPUBTextRenderStyle
    ) throws -> RDEPUBRenderedChapterContent {
        let request = RDEPUBTextRendererSupport.makeChapterRenderRequest(
            href: "",
            title: "",
            rawHTML: html,
            baseURL: baseURL,
            style: style,
            resourceResolver: nil
        )
        return try renderChapter(request: request)
    }

    /// 回退渲染：当 DTCoreText 不可用时，将 HTML 源码当作纯文本处理
    private func fallbackRenderedContent(request: RDEPUBTextChapterRenderRequest) -> RDEPUBRenderedChapterContent {
        let attributedString = RDEPUBTextRendererSupport.fallbackAttributedString(for: request.context.html, style: request.style)
        RDEPUBTextRendererSupport.applyPaginationSemantics(in: attributedString)
        let fragmentOffsets = RDEPUBTextRendererSupport.extractFragmentOffsets(from: attributedString)
        RDEPUBTextRendererSupport.normalizeReadingAttributes(in: attributedString, style: request.style)
        return RDEPUBRenderedChapterContent(
            attributedString: attributedString,
            fragmentOffsets: fragmentOffsets,
            resourceDiagnostics: request.context.resourceDiagnostics
        )
    }

#if canImport(DTCoreText)
    /// 使用 DTCoreText 将 HTML Data 解析为富文本。
    ///
    /// 通过 `willFlushCallback` 回调，在每个 DOM 元素最终写入富文本前，
    /// 对图片附件做尺寸规范化、判断是否为脚注/封面等特殊元素。
    private func makeAttributedString(from data: Data, request: RDEPUBTextChapterRenderRequest) -> NSAttributedString? {
        let builder = DTHTMLAttributedStringBuilder(
            html: data,
            options: dtOptions(request: request),
            documentAttributes: nil
        )
        builder?.willFlushCallback = { element in
            guard let element else { return }
            RDEPUBTextRendererSupport.prepareHTMLElementForReaderRendering(
                element,
                style: request.style,
                maxImageSize: resolvedMaxImageSize(for: request)
            )
        }
        return builder?.generatedAttributedString()
    }

    /// 构建 DTCoreText 的解析选项字典，包括字体、行高、图片尺寸限制等。
    ///
    /// - 注意：行高倍率计算公式为 `(字体行高 + 行间距) / 字体行高`，
    ///   确保最终行距与用户设置的 style.lineSpacing 一致。
    private func dtOptions(request: RDEPUBTextChapterRenderRequest) -> [AnyHashable: Any] {
        let style = request.style
        let maxImageSize = resolvedMaxImageSize(for: request)
        var options: [AnyHashable: Any] = [
            NSTextSizeMultiplierDocumentOption: 1.0,
            DTDefaultFontFamily: style.font.familyName,
            DTDefaultFontName: style.font.fontName,
            DTDefaultFontSize: style.font.pointSize,
            DTDefaultLineHeightMultiplier: max((style.font.lineHeight + style.lineSpacing) / max(style.font.lineHeight, 1), 1),
            DTUseiOS6Attributes: true,
            DTMaxImageSize: NSValue(cgSize: maxImageSize)
        ]

        if let baseURL = request.context.baseURL {
            options[NSBaseURLDocumentOption] = baseURL
        }
        if let textColor = style.textColor {
            options[DTDefaultTextColor] = textColor
        }

        return options
    }

    private func resolvedMaxImageSize(for request: RDEPUBTextChapterRenderRequest) -> CGSize {
        if let pageSize = request.pageSize {
            let layoutConfig = request.layoutConfig ?? .default
            let contentRect = layoutConfig.contentRect(fallback: pageSize)
            let maxWidth = max(round(contentRect.width), 1)
            let maxHeight = max(round(contentRect.height * layoutConfig.imageMaxHeightRatio), 1)
            return CGSize(width: maxWidth, height: maxHeight)
        }

        let screenBounds = UIScreen.main.bounds.insetBy(dx: 20, dy: 28)
        return CGSize(width: max(round(screenBounds.width), 1), height: max(round(screenBounds.height * 0.85), 1))
    }
#endif
}