// Message text as styled text. Inline markdown (bold, italics, inline code, links) goes through
// `AttributedString(markdown:)`. Block syntax is rewritten line by line first, since SwiftUI's Text
// runs full-parsed paragraphs together: headings turn bold, list markers turn into bullets, quote
// markers go, and fenced code keeps its lines in monospace.

import Foundation

enum Markdown {
    static func render(_ source: String) -> AttributedString {
        var result = AttributedString()
        var fenced = false
        var first = true
        for line in source.split(separator: "\n", omittingEmptySubsequences: false).map(String.init) {
            if line.trimmingCharacters(in: .whitespaces).hasPrefix("```") {
                fenced.toggle()
                continue
            }
            if !first { result += AttributedString("\n") }
            first = false
            if fenced {
                var code = AttributedString(line)
                code.inlinePresentationIntent = .code
                result += code
            } else {
                result += inline(block(line))
            }
        }
        return result
    }

    /// One line with its block syntax rewritten as inline markdown or plain text.
    static func block(_ line: String) -> String {
        if let heading = line.wholeMatch(of: /\s{0,3}#{1,6}\s+(.+?)[\s#]*/) {
            return "**\(heading.1)**"
        }
        if let item = line.wholeMatch(of: /(\s*)[-*+]\s+(.*)/) {
            return "\(item.1)• \(item.2)"
        }
        if let quote = line.wholeMatch(of: /\s{0,3}>\s?(.*)/) {
            return String(quote.1)
        }
        return line
    }

    /// Inline markdown, or the line as plain text when it does not parse.
    private static func inline(_ line: String) -> AttributedString {
        let options = AttributedString.MarkdownParsingOptions(interpretedSyntax: .inlineOnlyPreservingWhitespace)
        return (try? AttributedString(markdown: line, options: options)) ?? AttributedString(line)
    }
}