import Foundation

/// Renders an ``OrgDocument`` to HTML by walking the tree.
///
/// The point of the prototype: this produces the same shape of output as the shipped
/// single-pass ``OrgRenderer``, but from a parsed tree rather than from source, so a second
/// renderer (see ``OrgAttributedStringRenderer``) can share the parse instead of re-deriving it.
///
/// Not `Sendable`: it holds a ``CodeHighlighter``, and a real one wraps a thread-confined
/// engine (Highlightr's JavaScriptCore context, say). Sending the renderer across isolation
/// domains would call that engine off its thread. Make one where you render.
public struct OrgHTMLTreeRenderer {
    /// The same options the shipped ``OrgRenderer`` takes, so this renderer is a drop-in for it.
    public var options: OrgRenderOptions
    public var highlighter: CodeHighlighter

    public init(options: OrgRenderOptions = .init(), highlighter: CodeHighlighter = PlainCodeHighlighter()) {
        self.options = options
        self.highlighter = highlighter
    }

    public func render(_ document: OrgDocument) -> String {
        var footnotes = OrgFootnotes(document: document)
        var html = metadataHeader(document)
        html += document.elements.map { element($0, &footnotes) }.joined()
        html += renderFootnoteSection(&footnotes)
        return html
    }

    /// The leading `#+TITLE`/`#+AUTHOR`/`#+DATE` block, when the caller wants one. orgo treats
    /// these as document metadata carried by a page template, so the corpus renders with this
    /// off; an app showing a README title turns it on.
    private func metadataHeader(_ document: OrgDocument) -> String {
        guard options.metadataHeader else { return "" }
        let title = document.keyword("title")
        let author = document.keyword("author")
        let date = document.keyword("date")
        guard title != nil || author != nil || date != nil else { return "" }

        var html = "<div class=\"org-metadata\">\n"
        if let title { html += "<h1 class=\"org-title\">" + escapeHTML(title) + "</h1>\n" }
        if let author { html += "<p class=\"org-author\">" + escapeHTML(author) + "</p>\n" }
        if let date { html += "<p class=\"org-date\">" + escapeHTML(date) + "</p>\n" }
        return html + "</div>\n"
    }

    // MARK: - URL resolution

    /// Link and image resolution is shared with the other renderers; see ``OrgURLResolver``.
    private var resolver: OrgURLResolver { OrgURLResolver(options: options) }

    // MARK: - Blocks

    private func element(_ element: OrgElement, _ notes: inout OrgFootnotes) -> String {
        switch element {
        case .heading(let heading):
            let level = min(6, max(1, heading.level + options.headingLevelOffset))
            var inner = ""
            if let todo = heading.todo {
                inner += #"<span class="\#(todo.lowercased()) \#(todo)">\#(todo)</span> "#
            }
            if let priority = heading.priority {
                inner += #"<span class="priority">[#\#(priority)]</span> "#
            }
            inner += renderInline(heading.title, &notes)
            for tag in heading.tags {
                inner += #" <span class="tag">\#(escapeHTML(tag))</span>"#
            }
            return "<h\(level)>\(inner)</h\(level)>\n"

        case .paragraph(let objects):
            return "<p>" + renderInline(objects, &notes) + "</p>\n"

        case .list(let list):
            return renderList(list, &notes)

        case .table(let table):
            return renderTable(table, &notes)

        case .srcBlock(let language, let code):
            let classAttribute = language.map { #" class="language-\#(escapeHTMLAttribute($0))""# } ?? ""
            let body = highlighter.highlightedHTML(code: code, language: language) ?? escapeHTML(code)
            return "<pre><code\(classAttribute)>\(body)</code></pre>\n"

        case .exampleBlock(let text):
            return "<pre>\(escapeHTML(text))</pre>\n"

        case .quoteBlock(let children):
            return "<blockquote>\n" + children.map { self.element($0, &notes) }.joined() + "</blockquote>\n"

        case .centerBlock(let children):
            return #"<div class="center">"# + "\n" + children.map { self.element($0, &notes) }.joined() + "</div>\n"

        case .verseBlock(let lines):
            let body = lines.map { renderInline($0, &notes) }.joined(separator: "<br>\n")
            return #"<p class="verse">"# + "\n" + body + "\n</p>\n"

        case .specialBlock(let name, let children):
            return #"<div class="\#(escapeHTMLAttribute(name))">"# + "\n"
                + children.map { self.element($0, &notes) }.joined() + "</div>\n"

        case .exportBlock(let backend, let raw):
            return backend == "html" ? raw + "\n" : ""

        case .figure(let figure):
            guard let tag = imageTag(figure, caption: figure.caption) else {
                return "<p>" + escapeHTML(figure.alt ?? figure.source) + "</p>\n"
            }
            // A bare image is a paragraph; a caption or explicit attributes promote it to a
            // <figure>, matching org's exporter.
            guard figure.caption != nil || !figure.attributes.isEmpty else {
                return "<p>" + tag + "</p>\n"
            }
            var html = "<figure>" + tag
            if let caption = figure.caption {
                notes.figureNumber += 1
                html += #"<figcaption><span class="figure-number">Figure \#(notes.figureNumber): </span>"#
                    + renderInline(caption, &notes) + "</figcaption>"
            }
            return html + "</figure>\n"

        case .captioned(let name, let caption, let content):
            let idAttribute = name.map { #" id="\#(escapeHTMLAttribute($0))""# } ?? ""
            var html = #"<figure class="org-block"\#(idAttribute)>"# + "\n"
            html += self.element(content, &notes)
            if let caption {
                html += "<figcaption>" + renderInline(caption, &notes) + "</figcaption>\n"
            }
            return html + "</figure>\n"

        case .horizontalRule:
            return "<hr>\n"

        case .footnoteDefinition:
            return ""  // collected and emitted in the notes section
        }
    }

    private func renderList(_ list: OrgList, _ notes: inout OrgFootnotes) -> String {
        if list.kind == .description {
            var html = "<dl>\n"
            for item in list.items {
                if let term = item.term {
                    html += "<dt>" + renderInline(term, &notes) + "</dt>\n"
                }
                if !item.content.isEmpty {
                    html += "<dd>" + renderItemContent(item, &notes) + "</dd>\n"
                }
            }
            return html + "</dl>\n"
        }

        let tag = list.kind == .ordered ? "ol" : "ul"
        var html = "<\(tag)>\n"
        for item in list.items {
            html += "<li>"
            if let checkbox = item.checkbox {
                switch checkbox {
                case .off: html += "<code>[&nbsp;]</code> "
                case .on: html += "<code>[X]</code> "
                case .partial: html += "<code>[-]</code> "
                }
            }
            html += renderItemContent(item, &notes)
            html += "</li>\n"
        }
        return html + "</\(tag)>\n"
    }

    /// A list item's body. A lone paragraph renders bare inside the `<li>`; two or more get
    /// their own `<p>`, matching org's exporter. Anything that is not a paragraph — a nested
    /// list, a source block, a table — renders as the block it is, in source order.
    private func renderItemContent(_ item: OrgListItem, _ notes: inout OrgFootnotes) -> String {
        let wrapParagraphs = item.paragraphs.count > 1
        var parts: [String] = []
        for child in item.content {
            if case .paragraph(let objects) = child {
                let text = renderInline(objects, &notes)
                parts.append(wrapParagraphs ? "<p>" + text + "</p>" : text)
            } else {
                parts.append(element(child, &notes))
            }
        }
        return parts.joined(separator: "\n")
    }

    private func renderTable(_ table: OrgTable, _ notes: inout OrgFootnotes) -> String {
        var html = "<table>\n"
        var wroteHeader = false
        var inBody = false
        let headerCount = table.headerRowCount

        for (index, row) in table.rows.enumerated() {
            switch row {
            case .rule:
                if wroteHeader, !inBody { html += "</thead>\n<tbody>\n"; inBody = true }
            case .cells(let cells):
                let isHeader = headerCount > 0 && index < headerCount
                if isHeader, !wroteHeader { html += "<thead>\n"; wroteHeader = true }
                if !isHeader, !inBody { html += "<tbody>\n"; inBody = true }
                html += "<tr>\n"
                for (column, cell) in cells.enumerated() {
                    let tag = isHeader ? "th" : "td"
                    let alignment = column < table.alignments.count ? table.alignments[column] : nil
                    let style = alignment.map { #" style="text-align: \#($0.rawValue);""# } ?? ""
                    html += "<\(tag)\(style)>" + renderInline(cell, &notes) + "</\(tag)>\n"
                }
                html += "</tr>\n"
            }
        }
        if inBody { html += "</tbody>\n" }
        return html + "</table>\n"
    }

    /// The `<img>` for a figure, or nil when the source cannot be resolved to a safe URL —
    /// in which case the caller falls back to text rather than pointing at something unsafe.
    private func imageTag(_ figure: OrgFigure, caption: [OrgObject]?) -> String? {
        guard let source = resolver.imageSource(figure.source) else { return nil }
        let alt = figure.alt ?? caption.map { plainText($0) } ?? ""
        var html = #"<img src="\#(source)" alt="\#(escapeHTMLAttribute(alt))""#
        for (key, value) in figure.attributes where key != "alt" {
            html += " \(escapeHTMLAttribute(key))=\"\(escapeHTMLAttribute(value))\""
        }
        return html + ">"
    }

    // MARK: - Inline

    func renderInline(_ objects: [OrgObject], _ notes: inout OrgFootnotes) -> String {
        var html = ""
        for object in objects {
            switch object {
            case .text(let text): html += escapeHTML(text)
            case .bold(let children): html += "<strong>" + renderInline(children, &notes) + "</strong>"
            case .italic(let children): html += "<em>" + renderInline(children, &notes) + "</em>"
            case .underline(let children): html += "<u>" + renderInline(children, &notes) + "</u>"
            case .strikeThrough(let children): html += "<del>" + renderInline(children, &notes) + "</del>"
            case .verbatim(let text), .code(let text): html += "<code>" + escapeHTML(text) + "</code>"
            case .superscript(let children): html += "<sup>" + renderInline(children, &notes) + "</sup>"
            case .lineBreak: html += "<br>"
            case .image(let figure):
                html += imageTag(figure, caption: nil) ?? escapeHTML(figure.alt ?? figure.source)
            case .timestamp(let stamp):
                let cssClass = stamp.active ? "timestamp" : "timestamp inactive"
                func time(_ machine: String, _ display: String) -> String {
                    #"<time class="\#(cssClass)" datetime="\#(machine)">\#(display)</time>"#
                }
                html += time(stamp.machineValue, stamp.displayValue)
                // A range is two <time> elements joined by an en-dash, as org exports it.
                if let end = stamp.end {
                    html += "&#8211;" + time(end.machineValue, end.displayValue)
                }
            case .footnoteRef(let label, let inline):
                let number = notes.number(for: label, inline: inline)
                html += ##"<sup class="footnote-ref"><a id="fnr-\##(number)" href="#fn-\##(number)">\##(number)</a></sup>"##
            case .link(let link):
                let text = link.description.map { renderInline($0, &notes) }
                    ?? escapeHTML(resolver.displayValue(link.target))
                // An unsafe or unresolvable target degrades to its text, never a bad anchor.
                if let href = resolver.href(for: link.target) {
                    html += #"<a href="\#(href)">\#(text)</a>"#
                } else {
                    html += text
                }
            }
        }
        return html
    }

    /// Inline objects reduced to plain text, for an `alt` attribute.
    func plainText(_ objects: [OrgObject]) -> String {
        objects.map { object in
            switch object {
            case .text(let text): return text
            case .verbatim(let text), .code(let text): return text
            case .bold(let c), .italic(let c), .underline(let c), .strikeThrough(let c), .superscript(let c):
                return plainText(c)
            case .link(let link): return link.description.map { plainText($0) } ?? resolver.displayValue(link.target)
            case .timestamp(let stamp): return stamp.displayValue
            case .image(let figure): return figure.alt ?? ""
            case .footnoteRef, .lineBreak: return ""
            }
        }.joined()
    }
}

// MARK: - Footnote section

private extension OrgHTMLTreeRenderer {
    /// The `<section class="footnotes">` org's exporter puts at the end of a document.
    func renderFootnoteSection(_ notes: inout OrgFootnotes) -> String {
        let collected = notes.notes
        guard !collected.isEmpty else { return "" }
        var html = "<section class=\"footnotes\" aria-label=\"Footnotes\">\n<hr>\n<ol>\n"
        for note in collected {
            let n = note.number
            let back = ##"<a class="footnote-back" href="#fnr-\##(n)" aria-label="Back to reference \##(n)">&#8617;</a>"##
            let body = renderInline(note.content, &notes)
            if note.isInline {
                html += "<li id=\"fn-\(n)\">\(body) \(back)</li>\n"
            } else {
                html += "<li id=\"fn-\(n)\"><p>\(body)</p>\n \(back)</li>\n"
            }
        }
        return html + "</ol>\n</section>\n"
    }
}