skyview.astronomiemuseum.de/public/js/number_to_german_words.js

(function (global) {
    const ONES = ['', 'eins', 'zwei', 'drei', 'vier', 'fuenf', 'sechs', 'sieben', 'acht', 'neun'];
    const ONES_COMPOUND = ['', 'ein', 'zwei', 'drei', 'vier', 'fuenf', 'sechs', 'sieben', 'acht', 'neun'];
    const TEENS = ['zehn', 'elf', 'zwoelf', 'dreizehn', 'vierzehn', 'fuenfzehn', 'sechzehn', 'siebzehn', 'achtzehn', 'neunzehn'];
    const TENS = ['', '', 'zwanzig', 'dreissig', 'vierzig', 'fuenfzig', 'sechzig', 'siebzig', 'achtzig', 'neunzig'];
    const DIGIT_WORDS = ['null', 'eins', 'zwei', 'drei', 'vier', 'fuenf', 'sechs', 'sieben', 'acht', 'neun'];
    const SCALES = [
        { singular: '', plural: '' },
        { singular: 'Tausend', plural: 'Tausend' },
        { singular: 'Million', plural: 'Millionen' },
        { singular: 'Milliarde', plural: 'Milliarden' },
        { singular: 'Billion', plural: 'Billionen' },
        { singular: 'Billiarde', plural: 'Billiarden' },
        { singular: 'Trillion', plural: 'Trillionen' },
        { singular: 'Trilliarde', plural: 'Trilliarden' },
        { singular: 'Quadrillion', plural: 'Quadrillionen' }
    ];

    function chunkToWords(value) {
        if (value === 0) {
            return '';
        }

        if (value === 1) {
            return 'eins';
        }

        let text = '';
        const hundreds = Math.floor(value / 100);
        const remainder = value % 100;

        if (hundreds > 0) {
            text += (hundreds === 1 ? 'ein' : ONES_COMPOUND[hundreds]) + 'hundert';
        }

        if (remainder > 0) {
            if (remainder < 10) {
                text += ONES[remainder];
            } else if (remainder < 20) {
                text += TEENS[remainder - 10];
            } else {
                const ones = remainder % 10;
                const tens = Math.floor(remainder / 10);
                if (ones > 0) {
                    text += ONES_COMPOUND[ones] + 'und';
                }
                text += TENS[tens];
            }
        }

        return text;
    }

    function normalizeGermanNumberString(value) {
        if (typeof value === 'number') {
            if (!Number.isFinite(value)) {
                return '';
            }

            return String(value).replace('.', ',');
        }

        if (typeof value !== 'string') {
            return '';
        }

        return value.trim().replace(/\s+/g, '');
    }

    function splitNormalizedNumber(value) {
        const normalized = normalizeGermanNumberString(value);
        if (normalized === '') {
            return null;
        }

        const sign = normalized.startsWith('-') ? '-' : '';
        const unsigned = sign ? normalized.slice(1) : normalized;
        const cleaned = unsigned.replace(/\./g, '');
        const parts = cleaned.split(',');

        if (parts.length > 2) {
            return null;
        }

        const integerPart = (parts[0] || '0').replace(/^0+(?=\d)/, '') || '0';
        const fractionalPart = parts[1] || '';

        if (!/^\d+$/.test(integerPart) || (fractionalPart !== '' && !/^\d+$/.test(fractionalPart))) {
            return null;
        }

        return { sign, integerPart, fractionalPart };
    }

    function integerStringToWords(integerString, sign) {
        if (integerString === '0') {
            return 'null';
        }

        const groups = [];
        for (let index = integerString.length; index > 0; index -= 3) {
            groups.push(integerString.slice(Math.max(0, index - 3), index));
        }

        const parts = [];
        for (let i = groups.length - 1; i >= 0; i -= 1) {
            const groupValue = Number(groups[i]);
            if (groupValue === 0) {
                continue;
            }

            const scale = SCALES[i] || { singular: '', plural: '' };
            if (i >= 2) {
                const prefix = groupValue === 1 ? 'eine' : String(groupValue);
                const label = groupValue === 1 ? scale.singular : scale.plural;
                parts.push(prefix + ' ' + label);
            } else if (i === 1) {
                parts.push(groupValue === 1 ? 'ein ' + scale.singular : String(groupValue) + ' ' + scale.singular);
            } else {
                parts.push(String(groupValue));
            }
        }

        const result = parts.join('\n');
        return sign === '-' ? 'minus ' + result : result;
    }

    function numberToGermanText(value) {
        const parsed = splitNormalizedNumber(value);
        if (!parsed) {
            return '';
        }

        const integerText = integerStringToWords(parsed.integerPart, parsed.sign);
        if (!parsed.fractionalPart || /^0+$/.test(parsed.fractionalPart)) {
            return integerText;
        }

        const decimalDigits = parsed.fractionalPart
            .split('')
            .map(function (digit) {
                return DIGIT_WORDS[Number(digit)];
            })
            .join(' ');

        return integerText + '\nKomma ' + decimalDigits;
    }

    global.numberToGermanText = numberToGermanText;
})(window);