/**
 * The Lexer class handles tokenizing the input in various ways. Since our
 * parser expects us to be able to backtrack, the lexer allows lexing from any
 * given starting point.
 *
 * Its main exposed function is the `lex` function, which takes a position to
 * lex from and a type of token to lex. It defers to the appropriate `_innerLex`
 * function.
 *
 * The various `_innerLex` functions perform the actual lexing of different
 * kinds.
 */

var matchAt = require("match-at");

var ParseError = require("./ParseError");

// The main lexer class
function Lexer(input) {
    this._input = input;
}

// The resulting token returned from `lex`.
function Token(text, data, position) {
    this.text = text;
    this.data = data;
    this.position = position;
}

// "normal" types of tokens. These are tokens which can be matched by a simple
// regex
var mathNormals = [
    /[/|@.""`0-9a-zA-Z]/, // ords
    /[*+-]/, // bins
    /[=<>:]/, // rels
    /[,;]/, // punctuation
    /['\^_{}]/, // misc
    /[(\[]/, // opens
    /[)\]?!]/, // closes
    /~/, // spacing
    /&/, // horizontal alignment
    /\\\\/ // line break
];

// These are "normal" tokens like above, but should instead be parsed in text
// mode.
var textNormals = [
    /[a-zA-Z0-9`!@*()-=+\[\]'";:?\/.,]/, // ords
    /[{}]/, // grouping
    /~/, // spacing
    /&/, // horizontal alignment
    /\\\\/ // line break
];

// Regexes for matching whitespace
var whitespaceRegex = /\s*/;
var whitespaceConcatRegex = / +|\\  +/;

// This regex matches any other TeX function, which is a backslash followed by a
// word or a single symbol
var anyFunc = /\\(?:[a-zA-Z]+|.)/;

/**
 * This function lexes a single normal token. It takes a position, a list of
 * "normal" tokens to try, and whether it should completely ignore whitespace or
 * not.
 */
Lexer.prototype._innerLex = function(pos, normals, ignoreWhitespace) {
    var input = this._input;
    var whitespace;

    if (ignoreWhitespace) {
        // Get rid of whitespace.
        whitespace = matchAt(whitespaceRegex, input, pos)[0];
        pos += whitespace.length;
    } else {
        // Do the funky concatenation of whitespace that happens in text mode.
        whitespace = matchAt(whitespaceConcatRegex, input, pos);
        if (whitespace !== null) {
            return new Token(" ", null, pos + whitespace[0].length);
        }
    }

    // If there's no more input to parse, return an EOF token
    if (pos === input.length) {
        return new Token("EOF", null, pos);
    }

    var match;
    if ((match = matchAt(anyFunc, input, pos))) {
        // If we match a function token, return it
        return new Token(match[0], null, pos + match[0].length);
    } else {
        // Otherwise, we look through the normal token regexes and see if it's
        // one of them.
        for (var i = 0; i < normals.length; i++) {
            var normal = normals[i];

            if ((match = matchAt(normal, input, pos))) {
                // If it is, return it
                return new Token(
                    match[0], null, pos + match[0].length);
            }
        }
    }

    throw new ParseError(
            "Unexpected character: '" + input[pos] + "'",
            this, pos);
};

// A regex to match a CSS color (like #ffffff or BlueViolet)
var cssColor = /#[a-z0-9]+|[a-z]+/i;

/**
 * This function lexes a CSS color.
 */
Lexer.prototype._innerLexColor = function(pos) {
    var input = this._input;

    // Ignore whitespace
    var whitespace = matchAt(whitespaceRegex, input, pos)[0];
    pos += whitespace.length;

    var match;
    if ((match = matchAt(cssColor, input, pos))) {
        // If we look like a color, return a color
        return new Token(match[0], null, pos + match[0].length);
    } else {
        throw new ParseError("Invalid color", this, pos);
    }
};

// A regex to match a dimension. Dimensions look like
// "1.2em" or ".4pt" or "1 ex"
var sizeRegex = /(-?)\s*(\d+(?:\.\d*)?|\.\d+)\s*([a-z]{2})/;

/**
 * This function lexes a dimension.
 */
Lexer.prototype._innerLexSize = function(pos) {
    var input = this._input;

    // Ignore whitespace
    var whitespace = matchAt(whitespaceRegex, input, pos)[0];
    pos += whitespace.length;

    var match;
    if ((match = matchAt(sizeRegex, input, pos))) {
        var unit = match[3];
        // We only currently handle "em" and "ex" units
        if (unit !== "em" && unit !== "ex") {
            throw new ParseError("Invalid unit: '" + unit + "'", this, pos);
        }
        return new Token(match[0], {
                number: +(match[1] + match[2]),
                unit: unit
            }, pos + match[0].length);
    }

    throw new ParseError("Invalid size", this, pos);
};

/**
 * This function lexes a string of whitespace.
 */
Lexer.prototype._innerLexWhitespace = function(pos) {
    var input = this._input;

    var whitespace = matchAt(whitespaceRegex, input, pos)[0];
    pos += whitespace.length;

    return new Token(whitespace[0], null, pos);
};

/**
 * This function lexes a single token starting at `pos` and of the given mode.
 * Based on the mode, we defer to one of the `_innerLex` functions.
 */
Lexer.prototype.lex = function(pos, mode) {
    if (mode === "math") {
        return this._innerLex(pos, mathNormals, true);
    } else if (mode === "text") {
        return this._innerLex(pos, textNormals, false);
    } else if (mode === "color") {
        return this._innerLexColor(pos);
    } else if (mode === "size") {
        return this._innerLexSize(pos);
    } else if (mode === "whitespace") {
        return this._innerLexWhitespace(pos);
    }
};

module.exports = Lexer;