import Foundation
@preconcurrency import Parsing

var yamlString = """
---
author: Michael Housh
categories:
  - HVAC
  - General
  - Programming
copy: true
draft: false
date: 2023-10-21
lastmod: 2023-10-21
image: banner.png
featuredImage: banner.png
tags:
  - HVAC
  - General
  - Programming
title: "You Should Learn Markdown"
"""[...].utf8

struct YamlString: Parser {

  var body: some Parser<Substring.UTF8View, String> {
    OneOf {
      Parse {
        "\"".utf8
        PrefixUpTo("\"".utf8)
        "\"".utf8
      }.map(.string)

      Prefix { $0 != UInt8(ascii: "\n") }.map(.string)
    }
  }
}

struct YamlObject: Parser {

  enum Value {
    case array([String])
    case boolean(Bool)
    case date(YamlDate.Output)
    case string(String)
  }

  var body: some Parser<Substring.UTF8View, [String: Value]> {
    "---".utf8
    Many(into: [String: Value]()) { (dict: inout [String: Value], pair: (String, Value)) in
      dict[pair.0] = pair.1
    } element: {
      PrefixUpTo(":".utf8).map(.string)
      ":".utf8
      OneOf {
        Parse {
          "\n".utf8
          YamlArray()
        }.map { Value.array($0) }

        Parse {
          Whitespace()
          Bool.parser().map { Value.boolean($0) }
        }

        Parse {
          Whitespace()
          YamlDate().map { Value.date($0) }
        }

        Parse {
          Whitespace()
          YamlString().map { Value.string($0) }
        }
      }
    } separator: {
      "\n".utf8
    }
  }
}

struct YamlArray: Parser {
  var body: some Parser<Substring.UTF8View, [String]> {
    Many(into: [String]()) { array, string in
      array.append(string)
    } element: {
      Whitespace(.horizontal)
      "- ".utf8
      Prefix { $0 != UInt8(ascii: "\n") }.map(.string)
    } separator: {
      "\n".utf8
    }
  }
}

struct YamlDate: Parser {
  struct Output {
    let year: Int
    let month: Int
    let day: Int
  }

  var body: some Parser<Substring.UTF8View, Output> {
    Parse(.memberwise(Output.init)) {
      Digits(4)
      "-".utf8
      Digits(2)
      "-".utf8
      Digits(2)
    }
  }
}

try print(YamlObject().parse(&yamlString))