CHGenerator/Sources/Generator/Templates/Template.swift

import Foundation

protocol Template {

    associatedtype Key where Key: RawRepresentable, Key.RawValue == String, Key: CaseIterable, Key: Hashable

    static var templateName: String { get }
    
    var raw: String { get }

    var results: GenerationResultsHandler { get }

    init(raw: String, results: GenerationResultsHandler)

}

extension Template {

    init(in folder: URL, results: GenerationResultsHandler) throws {
        let url = folder.appendingPathComponent(Self.templateName)
        try self.init(from: url, results: results)
    }

    init(from url: URL, results: GenerationResultsHandler) throws {
        do {
            let raw = try String(contentsOf: url)
            self.init(raw: raw, results: results)
        } catch {
            results.warning("Failed to load: \(error)", source: "Template \(url.lastPathComponent)")
           throw error
        }
    }

    @discardableResult
    func generate(_ content: [Key : String], to file: String, source: String) -> Bool {
        let content = generate(content).data(using: .utf8)!
        return results.writeIfChanged(content, file: file, source: source)
    }

    func generate(_ content: [Key : String], shouldIndent: Bool = false) -> String {
        var result = raw.components(separatedBy: "\n")

        Key.allCases.forEach { key in
            let newContent = content[key]?.withoutEmptyLines ?? ""
            let stringMarker = "<!--\(key.rawValue)-->"
            let indices = result.enumerated().filter { $0.element.contains(stringMarker) }
                .map { $0.offset }
            guard !indices.isEmpty else {
                return
            }
            for index in indices {
                let old = result[index].components(separatedBy: stringMarker)
                // Add indentation to all added lines
                let indentation = old.first!
                guard shouldIndent, indentation.trimmingCharacters(in: .whitespaces).isEmpty else {
                    // Prefix is not indentation, so just insert new content
                    result[index] = old.joined(separator: newContent)
                    continue
                }
                let indentedReplacements = newContent.indented(by: indentation)
                result[index] = old.joined(separator: indentedReplacements)
            }
        }
        return result.joined(separator: "\n").withoutEmptyLines
    }
}