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

const Token = tokens.Token;
const TokenType = tokens.TokenType;

const Allocator = std.mem.Allocator;

pub const TokenError = 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);
}

pub const Scanner = struct {
    source: []const u8,

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

    allocator: *Allocator,

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

    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];
    }

    pub 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,
        };
    }

    /// 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.isAtEnd()) return 0;
        return self.source[self.current - 1];
    }

    fn skipWhitespace(self: *Scanner) void {
        while (true) {
            var c = self.peek();
            switch (c) {
                ' ', '\r', '\t' => blk: {
                    _ = self.advance();
                },
                '\n' => blk: {
                    self.line += 1;
                    _ = self.advance();
                },
                else => return,
            }
        }
    }

    fn doString(self: *Scanner) !Token {
        // consume entire string
        while (self.peek() != '"' and !self.isAtEnd()) {
            if (self.peek() == '\n') self.line += 1;
            _ = self.advance();
        }

        // unterminated string.
        if (self.isAtEnd()) {
            return TokenError.Unterminated;
        }

        // the closing ".
        _ = self.advance();

        // trim the surrounding quotes.
        return self.makeToken(.STRING);
    }

    /// Consume a number
    fn doNumber(self: *Scanner) Token {
        while (isDigit(self.peek())) {
            _ = self.advance();
        }

        // check if its a number like 12.34, where the '.' character
        // exists and the one next to it is a digit.
        if (self.peek() == '.' and isDigit(self.peekNext())) {
            _ = self.advance();

            while (isDigit(self.peek())) {
                _ = self.advance();
            }
        }

        return self.makeToken(.NUMBER);
    }

    pub fn scanToken(self: *Scanner) !?Token {
        self.skipWhitespace();
        self.start = self.current;
        if (self.isAtEnd()) return self.makeToken(TokenType.EOF);

        var c = self.advance();
        if (isDigit(c)) return self.doNumber();

        var token = switch (c) {
            '(' => self.makeToken(.LEFT_PAREN),
            ')' => self.makeToken(.RIGHT_PAREN),
            '{' => self.makeToken(.LEFT_BRACE),
            '}' => self.makeToken(.RIGHT_BRACE),
            ',' => self.makeToken(.COMMA),
            '.' => self.makeToken(.DOT),
            '-' => self.makeToken(.MINUS),
            '+' => self.makeToken(.PLUS),
            ';' => self.makeToken(.SEMICOLON),
            '*' => self.makeToken(.STAR),

            '!' => self.makeMatchToken('=', .BANG_EQUAL, .BANG),
            '=' => self.makeMatchToken('=', .EQUAL_EQUAL, .EQUAL),
            '<' => self.makeMatchToken('=', .LESS_EQUAL, .LESS),
            '>' => self.makeMatchToken('=', .GREATER_EQUAL, .GREATER),

            '/' => blk: {
                if (self.peekNext() == '/') {
                    while (self.peek() != '\n' and !self.isAtEnd()) {
                        _ = self.advance();
                    }
                    break :blk null;
                } else {
                    break :blk self.makeToken(.SLASH);
                }
            },

            '"' => try self.doString(),

            else => return TokenError.Unexpected,
        };

        return token;
    }
};