sprintf.js

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var sprintf = (function() {
    function get_type(variable) {
        return Object.prototype.toString.call(variable).slice(8, -1).toLowerCase();
    }
    function str_repeat(input, multiplier) {
        for (var output = []; multiplier > 0; output[--multiplier] = input) {/* do nothing */}
        return output.join('');
    }

    var str_format = function() {
        if (!str_format.cache.hasOwnProperty(arguments[0])) {
            str_format.cache[arguments[0]] = str_format.parse(arguments[0]);
        }
        return str_format.format.call(null, str_format.cache[arguments[0]], arguments);
    };

    str_format.format = function(parse_tree, argv) {
        var cursor = 1, tree_length = parse_tree.length, node_type = '', arg, output = [], i, k, match, pad, pad_character, pad_length;
        for (i = 0; i < tree_length; i++) {
            node_type = get_type(parse_tree[i]);
            if (node_type === 'string') {
                output.push(parse_tree[i]);
            }
            else if (node_type === 'array') {
                match = parse_tree[i]; // convenience purposes only
                if (match[2]) { // keyword argument
                    arg = argv[cursor];
                    for (k = 0; k < match[2].length; k++) {
                        if (!arg.hasOwnProperty(match[2][k])) {
                            throw(sprintf('[sprintf] property "%s" does not exist', match[2][k]));
                        }
                        arg = arg[match[2][k]];
                    }
                }
                else if (match[1]) { // positional argument (explicit)
                    arg = argv[match[1]];
                }
                else { // positional argument (implicit)
                    arg = argv[cursor++];
                }

                if (/[^s]/.test(match[8]) && (get_type(arg) != 'number')) {
                    throw(sprintf('[sprintf] expecting number but found %s', get_type(arg)));
                }
                switch (match[8]) {
                    case 'b': arg = arg.toString(2); break;
                    case 'c': arg = String.fromCharCode(arg); break;
                    case 'd': arg = parseInt(arg, 10); break;
                    case 'e': arg = match[7] ? arg.toExponential(match[7]) : arg.toExponential(); break;
                    case 'f': arg = match[7] ? parseFloat(arg).toFixed(match[7]) : parseFloat(arg); break;
                    case 'o': arg = arg.toString(8); break;
                    case 's': arg = ((arg = String(arg)) && match[7] ? arg.substring(0, match[7]) : arg); break;
                    case 'u': arg = Math.abs(arg); break;
                    case 'x': arg = arg.toString(16); break;
                    case 'X': arg = arg.toString(16).toUpperCase(); break;
                }
                arg = (/[def]/.test(match[8]) && match[3] && arg >= 0 ? '+'+ arg : arg);
                pad_character = match[4] ? match[4] == '0' ? '0' : match[4].charAt(1) : ' ';
                pad_length = match[6] - String(arg).length;
                pad = match[6] ? str_repeat(pad_character, pad_length) : '';
                output.push(match[5] ? arg + pad : pad + arg);
            }
        }
        return output.join('');
    };

    str_format.cache = {};

    str_format.parse = function(fmt) {
        var _fmt = fmt, match = [], parse_tree = [], arg_names = 0;
        while (_fmt) {
            if ((match = /^[^\x25]+/.exec(_fmt)) !== null) {
                parse_tree.push(match[0]);
            }
            else if ((match = /^\x25{2}/.exec(_fmt)) !== null) {
                parse_tree.push('%');
            }
            else if ((match = /^\x25(?:([1-9]\d*)\$|\(([^\)]+)\))?(\+)?(0|'[^$])?(-)?(\d+)?(?:\.(\d+))?([b-fosuxX])/.exec(_fmt)) !== null) {
                if (match[2]) {
                    arg_names |= 1;
                    var field_list = [], replacement_field = match[2], field_match = [];
                    if ((field_match = /^([a-z_][a-z_\d]*)/i.exec(replacement_field)) !== null) {
                        field_list.push(field_match[1]);
                        while ((replacement_field = replacement_field.substring(field_match[0].length)) !== '') {
                            if ((field_match = /^\.([a-z_][a-z_\d]*)/i.exec(replacement_field)) !== null) {
                                field_list.push(field_match[1]);
                            }
                            else if ((field_match = /^\[(\d+)\]/.exec(replacement_field)) !== null) {
                                field_list.push(field_match[1]);
                            }
                            else {
                                throw('[sprintf] huh?');
                            }
                        }
                    }
                    else {
                        throw('[sprintf] huh?');
                    }
                    match[2] = field_list;
                }
                else {
                    arg_names |= 2;
                }
                if (arg_names === 3) {
                    throw('[sprintf] mixing positional and named placeholders is not (yet) supported');
                }
                parse_tree.push(match);
            }
            else {
                throw('[sprintf] huh?');
            }
            _fmt = _fmt.substring(match[0].length);
        }
        return parse_tree;
    };

    return str_format;
})();

var vsprintf = function(fmt, argv) {
    argv.unshift(fmt);
    return sprintf.apply(null, argv);
};