import Foundation

#if canImport(UIKit)
import UIKit
#elseif canImport(AppKit)
import AppKit
#endif

/// Builds an NSAttributedString from plain text with formatting ranges.
/// The resulting .string is character-for-character identical to the input text.
enum AttributedStringBuilder {
	struct FormattingRange {
		enum Style {
			case body
			case bold
			case italic
			case heading(level: Int)
			case paragraphStart
		}
		let range: NSRange
		let style: Style
	}

	/// Build an attributed string from plain text and formatting ranges.
	/// The plain text is used as-is — no characters are added or removed.
	static func build(text: String, ranges: [FormattingRange]) -> NSAttributedString {
		let result = NSMutableAttributedString(
			string: text,
			attributes: [
				.font: bodyFont(),
				.foregroundColor: textColor(),
			]
		)

		for range in ranges {
			switch range.style {
			case .body:
				break
			case .bold:
				result.addAttribute(.font, value: boldFont(), range: range.range)
			case .italic:
				result.addAttribute(.font, value: italicFont(), range: range.range)
			case .heading(let level):
				result.addAttribute(.font, value: headingFont(level: level), range: range.range)
				let paragraphStyle = NSMutableParagraphStyle()
				paragraphStyle.paragraphSpacingBefore = 16
				result.addAttribute(.paragraphStyle, value: paragraphStyle, range: range.range)
			case .paragraphStart:
				let paragraphStyle = NSMutableParagraphStyle()
				paragraphStyle.paragraphSpacingBefore = 8
				result.addAttribute(.paragraphStyle, value: paragraphStyle, range: range.range)
			}
		}

		return NSAttributedString(attributedString: result)
	}

	/// Build a plain attributed string with body font (no formatting ranges).
	static func buildPlain(text: String) -> NSAttributedString {
		NSAttributedString(
			string: text,
			attributes: [
				.font: bodyFont(),
				.foregroundColor: textColor(),
			]
		)
	}

	// MARK: - Platform fonts

	#if canImport(UIKit)
	private static func bodyFont() -> UIFont {
		.preferredFont(forTextStyle: .body)
	}

	private static func boldFont() -> UIFont {
		let descriptor = UIFont.preferredFont(forTextStyle: .body).fontDescriptor.withSymbolicTraits(.traitBold)!
		return UIFont(descriptor: descriptor, size: 0)
	}

	private static func italicFont() -> UIFont {
		let descriptor = UIFont.preferredFont(forTextStyle: .body).fontDescriptor.withSymbolicTraits(.traitItalic)!
		return UIFont(descriptor: descriptor, size: 0)
	}

	private static func headingFont(level: Int) -> UIFont {
		let style: UIFont.TextStyle = level <= 2 ? .title1 : .title3
		let descriptor = UIFont.preferredFont(forTextStyle: style).fontDescriptor.withSymbolicTraits(.traitBold)!
		return UIFont(descriptor: descriptor, size: 0)
	}

	private static func textColor() -> UIColor {
		.label
	}

	#elseif canImport(AppKit)
	private static func bodyFont() -> NSFont {
		.preferredFont(forTextStyle: .body)
	}

	private static func boldFont() -> NSFont {
		let body = NSFont.preferredFont(forTextStyle: .body)
		return NSFontManager.shared.convert(body, toHaveTrait: .boldFontMask)
	}

	private static func italicFont() -> NSFont {
		let body = NSFont.preferredFont(forTextStyle: .body)
		return NSFontManager.shared.convert(body, toHaveTrait: .italicFontMask)
	}

	private static func headingFont(level: Int) -> NSFont {
		let style: NSFont.TextStyle = level <= 2 ? .title1 : .title3
		let heading = NSFont.preferredFont(forTextStyle: style)
		return NSFontManager.shared.convert(heading, toHaveTrait: .boldFontMask)
	}

	private static func textColor() -> NSColor {
		.textColor
	}
	#endif
}