NetNewsWire/Core/Sources/Core/MacroProcessor.swift

//
//  MacroProcessor.swift
//  RSCore
//
//  Created by Nate Weaver on 2020-01-01.
//  Copyright © 2020 Ranchero Software, LLC. All rights reserved.
//

import Foundation

enum MacroProcessorError: Error {
	case emptyMacroDelimiter
}

public class MacroProcessor {

	let template: String
	let substitutions: [String: String]
	let macroStart: String
	let macroEnd: String
	lazy var renderedText: String = processMacros()

	/// Parses a template string and replaces macros with specified values.
	///
	/// - Returns: A copy of `template` with defined macros replaced by their values.
	///   Macros with undefined values are left as-is.
	///
	/// - Parameters:
	///   - template: The template string to parse, with macros surrounded by `macroStart` and `macroEnd`.
	///   - substitutions: A dictionary mapping macro keys to their replacement values.
	///   - macroStart: A string denoting the beginning of a macro.
	///   - macroEnd: A string denoting the end of a macro.
	///
	/// - Throws: An error of type `MacroProcessorError`.

	public static func renderedText(withTemplate template: String, substitutions: [String: String], macroStart: String = "[[", macroEnd: String = "]]") throws -> String {
		let processor = try MacroProcessor(template: template, substitutions: substitutions, macroStart: macroStart, macroEnd: macroEnd)
		return processor.renderedText
	}

	init(template: String, substitutions: [String: String], macroStart: String = "[[", macroEnd: String = "]]") throws {
		if macroStart.isEmpty || macroEnd.isEmpty {
			throw MacroProcessorError.emptyMacroDelimiter
		}

		self.template = template
		self.substitutions = substitutions
		self.macroStart = macroStart
		self.macroEnd = macroEnd
	}

}

private extension MacroProcessor {

	func processMacros() -> String {
		var result = String()

		var index = template.startIndex

		while true {
			guard let macroStartRange = template[index...].range(of: macroStart) else {
				break
			}

			result.append(contentsOf: template[index..<macroStartRange.lowerBound])

			guard let macroEndRange = template[macroStartRange.upperBound...].range(of: macroEnd) else {
				index = macroStartRange.lowerBound
				break
			}

			let key = String(template[macroStartRange.upperBound..<macroEndRange.lowerBound])
			let replacement = substitutions[key] ?? "\(macroStart)\(key)\(macroEnd)"

			result.append(contentsOf: replacement)

			index = macroEndRange.upperBound
		}

		result.append(contentsOf: template[index...])

		return result
	}

}