ConvertNumberToText

public with sharing class ConvertNumberToText {
    public ConvertNumberToText() {
    }

    private static final List<String> tensNames = new List<String>{
        '',
        ' ten',
        ' twenty',
        ' thirty',
        ' forty',
        ' fifty',
        ' sixty',
        ' seventy',
        ' eighty',
        ' ninety'
    };

    private static final List<String> numNames = new List<String>{
        '',
        ' one',
        ' two',
        ' three',
        ' four',
        ' five',
        ' six',
        ' seven',
        ' eight',
        ' nine',
        ' ten',
        ' eleven',
        ' twelve',
        ' thirteen',
        ' fourteen',
        ' fifteen',
        ' sixteen',
        ' seventeen',
        ' eighteen',
        ' nineteen'
    };

    private static String convertLessThanOneThousand(Integer numberValue) {
        String soFar;

        if (Math.mod(numberValue, 100) < 20) {
            soFar = numNames[Math.mod(numberValue, 100)];
            numberValue = numberValue / 100;
        } else {
            soFar = numNames[Math.mod(numberValue, 10)];
            numberValue = numberValue / 10;

            soFar = tensNames[Math.mod(numberValue, 10)] + soFar;
            numberValue = numberValue / 10;
        }
        if (numberValue == 0)
            return soFar;
        return numNames[numberValue] + ' hundred' + soFar;
    }

    public static String convert(Long numberValue) {
        // 0 to 999 999 999 999
        if (numberValue == 0) {
            return 'zero';
        }
        String snumber = String.valueOf(numberValue);

        // Pad with "0"
        String mask = '000000000000';
        while (snumber.length() < mask.length()) {
            snumber = '0' + snumber;
        }

        // XXXnnnnnnnnn
        Integer billions = Integer.valueOf(snumber.substring(0, 3));
        // nnnXXXnnnnnn
        Integer millions = Integer.valueOf(snumber.substring(3, 6));
        // nnnnnnXXXnnn
        Integer hundredThousands = Integer.valueOf(snumber.substring(6, 9));
        // nnnnnnnnnXXX
        Integer thousands = Integer.valueOf(snumber.substring(9, 12));

        String tradBillions;
        switch on (billions) {
            when 0 {
                tradBillions = '';
            }
            when 1 {
                tradBillions =
                    convertLessThanOneThousand(billions) + ' billion ';
            }
            when else {
                tradBillions =
                    convertLessThanOneThousand(billions) + ' billion ';
            }
        }
        String result = tradBillions;

        String tradMillions;
        switch on (millions) {
            when 0 {
                tradMillions = '';
            }
            when 1 {
                tradMillions =
                    convertLessThanOneThousand(millions) + ' million ';
            }
            when else {
                tradMillions =
                    convertLessThanOneThousand(millions) + ' million ';
            }
        }
        result = result + tradMillions;

        String tradHundredThousands;
        switch on (hundredThousands) {
            when 0 {
                tradHundredThousands = '';
            }
            when 1 {
                tradHundredThousands = 'one thousand ';
            }
            when else {
                tradHundredThousands =
                    convertLessThanOneThousand(hundredThousands) + ' thousand ';
            }
        }
        result = result + tradHundredThousands;

        String tradThousand;
        tradThousand = convertLessThanOneThousand(thousands);
        result = result + tradThousand;

        // remove extra spaces!
        return result.replaceAll('^\\s+', '').replaceAll('\\b\\s{2,}\\b', ' ');
    }

    /*
    String phrase = '12345.67';
    Decimal num = Decimal.valueOf(phrase);
    Integer dollars = num.intValue();
    Integer cent = Integer.valueOf((num - dollars) * 100);

    String s = '$ ' + ConvertNumberToText.convert(dollars) + ' and '
            + ConvertNumberToText.convert(cent) + ' cents';
    System.debug(s);
    */
}