const std = @import("std");
const tokens = @import("tokens.zig");

const Allocator = std.mem.Allocator;
const Token = tokens.Token;
const TokenType = tokens.TokenType;

pub const ScannerError = error{
    Unexpected,
    Unterminated,
};

fn isDigit(char: u8) bool {
    return char >= '0' and char <= '9';
}

fn isAlpha(c: u8) bool {
    return (c >= 'a' and c <= 'z') or
        (c >= 'A' and c <= 'Z') or
        c == '_';
}

fn isAlphaNumeric(char: u8) bool {
    return isAlpha(char) or isDigit(char);
}

const keywords = [][]const u8{
    "break",
    "const",
    "continue",
    "defer",
    "else",
    "enum",
    "fn",
    "for",
    "go",
    "goto",
    "if",
    "import",
    "in",
    "interface",
    "match",
    "module",
    "mut",
    "or",
    "return",
    "struct",
    "type",
};

const keyword_ttypes = []TokenType{
    .Break,
    .Const,
    .Continue,
    .Defer,
    .Else,
    .Enum,
    .Fn,
    .For,
    .Go,
    .Goto,
    .If,
    .Import,
    .In,
    .Interface,
    .Match,
    .Module,
    .Mut,
    .Or,
    .Return,
    .Struct,
    .Type,
};

fn getKeyword(keyword: []const u8) ?TokenType {
    for (keywords) |kw, idx| {
        if (std.mem.eql(u8, keyword, kw)) {
            return keyword_ttypes[idx];
        }
    }

    return null;
}

/// Scanner for vlang tokens.
pub const Scanner = struct {
    allocator: *Allocator,
    source: []u8,

    start: usize = 0,
    current: usize = 0,
    line: usize = 1,

    pub fn init(allocator: *Allocator, source: []u8) Scanner {
        return Scanner{ .allocator = allocator, .source = source };
    }

    fn isAtEnd(self: *Scanner) bool {
        return self.current >= self.source.len;
    }

    fn advance(self: *Scanner) u8 {
        self.current += 1;
        return self.source[self.current - 1];
    }

    fn currentLexeme(self: *Scanner) []const u8 {
        return self.source[self.start..self.current];
    }

    fn makeToken(self: *Scanner, ttype: TokenType) Token {
        return Token{
            .ttype = ttype,
            .lexeme = self.currentLexeme(),
            .line = self.line,
        };
    }

    fn makeTokenAdvance(self: *Scanner, ttype: TokenType) Token {
        var tok = self.makeToken(ttype);
        self.current += 1;
        return tok;
    }

    /// Check if the next character matches what is expected.
    fn match(self: *Scanner, expected: u8) bool {
        if (self.isAtEnd()) return false;
        if (self.source[self.current] != expected) return false;

        self.current += 1;
        return true;
    }

    /// Add a SimpleToken of type_match if the next character is
    /// `expected`. Adds a SimpleToken of type_nomatch when it is not.
    fn makeMatchToken(
        self: *Scanner,
        expected: u8,
        type_match: TokenType,
        type_nomatch: TokenType,
    ) Token {
        if (self.match(expected)) {
            return self.makeToken(type_match);
        } else {
            return self.makeToken(type_nomatch);
        }
    }

    fn peek(self: *Scanner) u8 {
        if (self.isAtEnd()) return 0;
        return self.source[self.current];
    }

    fn peekNext(self: *Scanner) u8 {
        if (self.current + 1 >= self.source.len) return 0;
        return self.source[self.current + 1];
    }

    pub fn nextToken(self: *Scanner) !Token {
        self.start = self.current;

        if (self.isAtEnd()) return self.makeToken(TokenType.EOF);

        var c = self.advance();

        var token = switch (c) {
            '(' => self.makeToken(.LeftParen),
            ')' => self.makeToken(.RightParen),
            '{' => self.makeToken(.LeftBrace),
            '}' => self.makeToken(.RightBrace),
            '[' => self.makeToken(.LeftSquare),
            ']' => self.makeToken(.RightSquare),
            '.' => self.makeToken(.Dot),
            ';' => self.makeToken(.Semicolon),
            ',' => self.makeToken(.Comma),
            ':' => self.makeToken(.Colon),
            '&' => self.makeToken(.Ampersand),
            '|' => self.makeToken(.Pipe),
            '?' => self.makeToken(.QuestionMark),
            '$' => self.makeToken(.DollarSign),

            '!' => self.makeMatchToken('=', .BangEqual, .Bang),
            '=' => self.makeMatchToken('=', .EqualEqual, .Equal),

            // there can be three tokens from a <
            //  - <, which is LessThan
            //  - <=, which is LessEqual
            //  - <<, which is LeftDoubleChevron
            '<' => blk: {
                if (self.match('=')) {
                    break :blk self.makeToken(.LessEqual);
                } else if (self.match('<')) {
                    break :blk self.makeToken(.LeftDoubleChevron);
                } else {
                    break :blk self.makeToken(.Less);
                }
            },
            '>' => self.makeMatchToken('=', .GreaterEqual, .Greater),

            else => return ScannerError.Unexpected,
        };

        return token;
    }
};