using System;
using System.IO;
using System.Collections.Generic;
using Finn.AST;

namespace Finn;

class Program
{
	private static readonly Interpreter interpreter = new Interpreter();
	static bool hadError = false;
	static bool hadRuntimeError = true;

	static void Main(string[] args)
	{
		if (args.Length > 1)
		{
			Console.WriteLine("Usage: finn [script]");
			Environment.Exit(64);
		}
		else if (args.Length == 1)
		{
			runFile(args[0]);
		}
		else
		{
			runPrompt();
		}
	}

	static void runFile(string path)
	{
		var src = File.ReadAllText(path);
		run(src);

		if (hadError) Environment.Exit(65);
		if (hadRuntimeError) Environment.Exit(70);
	}

	static void run(string src)
	{
		var scanner = new Scanner(src);
		List<Token> tokens = scanner.scanTokens();
		Console.WriteLine("TOKENS\n=======");
		foreach (var token in tokens)
		{
			Console.WriteLine(token);
		}
		Parser parser = new Parser(tokens);
		Expr? expression = parser.parse();
		Console.WriteLine("\nAST\n=======");
		Console.WriteLine(expression);
		Console.WriteLine();

		if (hadError)
		{
			hadError = false;
			return;
		}

		interpreter.Interpret(expression!);
	}

	static void runPrompt()
	{
		var input = new BufferedStream(Console.OpenStandardInput());
		var reader = new StreamReader(input);

		while (true)
		{
			Console.Write("> ");
			var line = reader.ReadLine();
			if (line == null) break;
			run(line);
		}
	}

	public static void error(Position position, String message)
	{
		report(position, "", message);
	}

	public static void error(Token token, String message)
	{
		if (token.type == TokenType.EOF)
		{
			report(token.position, " at end", message);
		}
		else
		{
			report(token.position, " at '" + token.lexeme + "'", message);
		}
	}

	public static void runtimeError(RuntimeError err)
	{
		Console.Error.WriteLine($"{err.Message}\n[{err.Token.position}]");
		hadRuntimeError = true;
	}

	static void report(Position position, String where, String message)
	{
		Console.WriteLine($"[{position}] Error{where}: {message}");
		hadError = true;
	}
}

public enum TokenType
{
	LParen, RParen,
	LBrace, RBrace,
	LBracket, RBracket,
	Less, Greater,
	Pipe,
	Colon,
	Tick,
	Backtick,
	At,
	Comma,
	Semicolon,
	Period,
	Equal,
	Plus, Minus, Asterisk, Slash,
	PlusPlus,
	DoubleEqual,
	Bang, BangEqual,
	LessEqual, GreaterEqual,
	SingleArrow,
	DoubleArrow,
	If, Then, Else,
	When,
	Is,
	Let,
	And,
	In,
	With,
	Fn,
	Type,
	Alias,
	Recurse,
	Def,
	Blank,
	Identifier, QuotedIdentifier, Number, String,
	EOF
}

public record struct Position(int Offset, int Line, int Column)
{
	public override string ToString()
	{
		return $"{this.Line}:{this.Column}";
	}

	public static Position operator ++(Position p)
	{
		return p.Next();
	}

	public Position Next()
	{
		return this with { Offset = Offset + 1, Column = Column + 1 };
	}

	public Position NewLine()
	{
		return this with { Line = Line + 1, Column = 1 };
	}
}

public record Token(TokenType type, String lexeme, Object? literal, Position position);

class Scanner
{
	private readonly String source;
	private readonly List<Token> tokens = new List<Token>();
	private Position start = new(0, 1, 1);
	private Position current = new(0, 1, 1);

	private static readonly Dictionary<String, TokenType> keywords = new Dictionary<string, TokenType>()
	{
		{"def", TokenType.Def},
		{"if", TokenType.If},
		{"then", TokenType.Then},
		{"else", TokenType.Else},
		{"when", TokenType.When},
		{"is", TokenType.Is},
		{"let", TokenType.Let},
		{"and", TokenType.And},
		{"in", TokenType.In},
		{"with", TokenType.With},
		{"fn", TokenType.Fn},
		{"type", TokenType.Type},
		{"alias", TokenType.Alias},
		{"recurse", TokenType.Recurse},
		{"_", TokenType.Blank},
	};

	public Scanner(String source)
	{
		this.source = source;
	}

	private bool isAtEnd()
	{
		return current.Offset >= source.Length;
	}

	private char advance()
	{
		return source[(current++).Offset];
	}

	private void addToken(TokenType type)
	{
		addToken(type, null);
	}

	private string lexeme
	{
		get => source.Substring(start.Offset, current.Offset - start.Offset);
	}

	private void addToken(TokenType type, Object? literal)
	{
		tokens.Add(new Token(type, lexeme, literal, start));
	}

	private bool match(char expected)
	{
		if (isAtEnd()) return false;
		if (source[current.Offset] != expected) return false;

		current++;
		return true;
	}

	private bool match(Predicate<char> pred)
	{
		if (isAtEnd()) return false;
		if (!pred(source[current.Offset])) return false;

		current++;
		return true;
	}

	private char? peek()
	{
		if (isAtEnd()) return null;
		return source[current.Offset];
	}

	private string? _stringLiteral(string errorName)
	{
		var valueStart = current.Offset;
		while (peek() != '"' && !isAtEnd())
		{
			if (peek() == '\n')
				current = current.NewLine();
			advance();
		}

		if (isAtEnd())
		{
			Program.error(current, $"Unterminated {errorName}.");
			return null;
		}

		// The closing ".
		advance();

		// Trim the closing quote.
		return source.Substring(valueStart, current.Offset - valueStart - 1);
	}

	private void quotedIdentifier()
	{
		if (!match('"'))
		{
			Program.error(current, "Expect \" after @.");
		}
		var value = _stringLiteral("quoted identifier");
		if (value == null) return;
		addToken(TokenType.QuotedIdentifier, value);
	}

	private void stringLiteral()
	{
		var value = _stringLiteral("string");
		if (value == null) return;
		addToken(TokenType.String, value);
	}

	private bool isDigitOrSeparator(char c)
	{
		return Char.IsAsciiDigit(c) || c == '_';
	}

	private char? peekNext()
	{
		if (current.Offset + 1 >= source.Length) return null;
		return source[current.Offset + 1];
	}
	private void numberLiteral()
	{
		while (match(isDigitOrSeparator)) ;

		// Look for a fractional part.
		if (peek() == '.' && isDigitOrSeparator(peekNext() ?? '\0'))
		{
			match('.');
			while (match(isDigitOrSeparator)) ;
		}
		double value = Double.Parse(source.Substring(start.Offset, current.Offset - start.Offset));
		addToken(TokenType.Number, value);
	}

	private bool isIdentifierStartChar(char c)
	{
		return Char.IsAsciiLetter(c) || c == '_';
	}

	private bool isIdentifierChar(char c)
	{
		return isIdentifierStartChar(c) || Char.IsAsciiDigit(c);
	}

	private void identifier()
	{
		while (match(isIdentifierChar)) ;
		String text = source.Substring(start.Offset, current.Offset - start.Offset);
		TokenType type;
		var isKeyword = keywords.TryGetValue(text, out type);
		addToken(isKeyword ? type : TokenType.Identifier, text);
	}

	private void scanToken()
	{
		char c = advance();
		switch (c)
		{
			case '(': addToken(TokenType.LParen); break;
			case ')': addToken(TokenType.RParen); break;
			case '{': addToken(TokenType.LBrace); break;
			case '}': addToken(TokenType.RBrace); break;
			case '[': addToken(TokenType.LBracket); break;
			case ']': addToken(TokenType.RBracket); break;
			case ':': addToken(TokenType.Colon); break;
			case '\'': addToken(TokenType.Tick); break;
			case '`': addToken(TokenType.Backtick); break;
			case ',': addToken(TokenType.Comma); break;
			case ';': addToken(TokenType.Semicolon); break;
			case '.': addToken(TokenType.Period); break;
			case '*': addToken(TokenType.Asterisk); break;
			case '/': addToken(TokenType.Slash); break;
			case '|': addToken(TokenType.Pipe); break;
			case '-':
				addToken(match('>') ? TokenType.SingleArrow : TokenType.Minus);
				break;
			case '!':
				addToken(match('=') ? TokenType.BangEqual : TokenType.Bang);
				break;
			case '+':
				addToken(match('+') ? TokenType.PlusPlus : TokenType.Plus);
				break;
			case '=':
				addToken(match('=') ? TokenType.DoubleEqual :
					match('>') ? TokenType.DoubleArrow :
					TokenType.Equal);
				break;
			case '<':
				addToken(match('=') ? TokenType.LessEqual : TokenType.Less);
				break;
			case '>':
				addToken(match('=') ? TokenType.GreaterEqual : TokenType.Greater);
				break;
			case '@': quotedIdentifier(); break;
			case '"': stringLiteral(); break;
			case '#':
				while (peek() != '\n' && !isAtEnd()) advance();
				break;
			case ' ':
			case '\r':
			case '\t':
				break;
			case '\n':
				current = current.NewLine();
				break;
			default:
				if (Char.IsAsciiDigit(c))
				{
					numberLiteral();
				}
				else if (isIdentifierStartChar(c))
				{
					identifier();
				}
				else
				{
					Program.error(current, "Unexpected character.");
				}
				break;
		}
	}
	public List<Token> scanTokens()
	{
		while (!isAtEnd())
		{
			start = current;
			scanToken();
		}

		tokens.Add(new Token(TokenType.EOF, "", null, current));
		return tokens;
	}
}