Gemini/GeminiFormat/TableOfContents.swift

//
//  TableOfContents.swift
//  GeminiFormat
//
//  Created by Shadowfacts on 12/19/20.
//

import Foundation

public struct TableOfContents {
    public let entries: [Entry]
    
    public init(document: Document) {
        self.entries = TableOfContents.entries(lines: document.lines)
    }
    
    private static func entries(lines: [Document.Line]) -> [Entry] {
        var topLevelEntries = [Entry]()
        
        var currentEntries = [Entry]()
        
        var index = 0
        while index < lines.count {
            defer { index += 1 }
            
            let line = lines[index]
            guard case let .heading(_, level: level) = line else {
                continue
            }
            
            let newEntry = Entry(line: line, lineIndex: index)
            
            while !currentEntries.isEmpty && level <= currentEntries.last!.level {
                currentEntries.removeLast()
            }
            
            if let last = currentEntries.last {
                last.children.append(newEntry)
                currentEntries.append(newEntry)
            } else {
                topLevelEntries.append(newEntry)
                currentEntries.append(newEntry)
            }
            
        }
        
        return topLevelEntries
    }
    
}

public extension TableOfContents {
    class Entry: Equatable {
        public let line: Document.Line
        let level: Document.HeadingLevel
        public let lineIndex: Int
        public fileprivate(set) var children: [Entry]
        
        init(line: Document.Line, lineIndex: Int) {
            guard case let .heading(_, level: level) = line else { fatalError() }
            self.line = line
            self.level = level
            self.lineIndex = lineIndex
            self.children = []
        }
        
        public static func ==(lhs: Entry, rhs: Entry) -> Bool {
            return lhs.line == rhs.line && lhs.lineIndex == rhs.lineIndex && lhs.children == rhs.children
        }
    }
}