Convert Monetary Amount to Swahili Converter

NumberToSwahiliConverter.java

import java.util.*;
import java.lang.*;
import java.io.*;

import java.text.DecimalFormat;

class NumberToSwahiliConverter {
    private static final String[] scalesInSwahili = {"", "laki", "milioni", "bilioni"};

    private static final String[] units = {
            "", "moja", "mbili", "tatu", "nne", "tano", "sita", "saba", "nane", "tisa"
    };

    private static final String[] teens = {
            "kumi", "kumi na moja", "kumi na mbili", "kumi na tatu", "kumi na nne", "kumi na tano",
            "kumi na sita", "kumi na saba", "kumi na nane", "kumi na tisa"
    };

    private static final String[] tens = {
            "", "kumi", "ishirini", "thelathini", "arobaini", "hamsini", "sitini", "sabini", "themanini", "tisini"
    };

    public static String convert(long number) {
        if (number == 0) {
            return "sifuri".toUpperCase();
        }

        // pad with "0"
        String mask = "000000000000";
        DecimalFormat df = new DecimalFormat(mask);
        String maskedNumber = df.format(number);

        final var result = new StringBuilder();
        LinkedList<int[]> partitionIndices = new LinkedList<>();

        partitionIndices.add(new int[]{0, 3});
        partitionIndices.add(new int[]{3, 6});
        partitionIndices.add(new int[]{6, 9});
        partitionIndices.add(new int[]{9, 12});

        for (int i = 0; i < partitionIndices.size(); i++) {
            int start = partitionIndices.get(i)[0];
            int end = partitionIndices.get(i)[1];
            int partition = Integer.parseInt(maskedNumber.substring(start, end));
            if (partition != 0) {
                String partInWords = i == partitionIndices.size() - 2
                        ? convertHundredThousands(partition)
                        : convertHundreds(partition);
                String scale = i == partitionIndices.size() - 2 && (partition / 100) == 0
                ? ""
                : scalesInSwahili[partitionIndices.size() - 1 - i];

                if (!scale.isEmpty()) {
                    if (partition == 1 && scale.equals("milioni")) {
                        result.append("milioni moja ");
                    } else {
                        result.append(scale).append(" ").append(partInWords).append(" ");
                    }
                } else {
                    result.append(partInWords).append(" ");
                }

                // if (partition > 0 && i == partitionIndices.size() - 2) {
                //     result.append(" na ");
                // }
            }
        }

        // remove extra spaces
        return result
                .toString()
                .replaceAll("^\\s+", "")
                .replaceAll("\\b\\s{2,}\\b", " ")
                .trim()
                .toUpperCase()
                .strip();
    }


    private static String convertHundreds(int num) {
        StringBuilder result = new StringBuilder();

        if (num >= 100) {
            int hundreds = num / 100;
            if (hundreds == 1) {
                result.append("mia ").append(" ");
            } else {
                result.append(" mia ").append(units[hundreds]).append(" ");
            }
            num %= 100;
        }

        if (num >= 20) {
            result.append(tens[num / 10]).append(" ");
            num %= 10;
            if (num > 0) {
                result.append("na ").append(units[num]).append(" ");
            }
        } else if (num >= 10) {
            result.append(teens[num - 10]).append(" ");
        } else if (num > 0) {
            result.append(units[num]).append(" ");
        }

        return result.toString().trim();
    }

    private static String convertHundredThousands(int num) {
        StringBuilder result = new StringBuilder();

        if (num >= 100) {
            int hundreds = num / 100;
            if (hundreds == 1) {
                result.append("moja ");
            } else {
                result.append(units[hundreds]).append(" ");
            }
            num %= 100;
        }

        if (num > 0) {
            result.append("elfu ");
        }

        if (num >= 20) {
            result.append(tens[num / 10]).append(" ");
            num %= 10;
            if (num > 0) {
                result.append("na ").append(units[num]).append(" ");
            }
        } else if (num >= 10) {
            result.append(teens[num - 10]).append(" ");
        } else if (num > 0) {
            result.append(units[num]).append(" ");
        }

        return result.toString().trim();
    }
   
    public static void main(String[] args) {
        System.out.println("*** " + NumberToWordsConverter.convert(123456789));
        System.out.println("*** " + NumberToWordsConverter.convert(123456989));
        System.out.println("*** " + NumberToWordsConverter.convert(0));
        System.out.println("*** " + NumberToWordsConverter.convert(1000));
        System.out.println("*** " + NumberToWordsConverter.convert(10000));
        System.out.println("*** " + NumberToWordsConverter.convert(10001));
        System.out.println("*** " + NumberToWordsConverter.convert(11000));
        System.out.println("*** " + NumberToWordsConverter.convert(100000));
        System.out.println("*** " + NumberToWordsConverter.convert(1000000));
        System.out.println("*** " + NumberToWordsConverter.convert(1000000000));
    }
}