Take a number and give the equivalent number in British English words e.g.

• 1 = one
• 21 = twenty one
• 105 = one hundred and five
• 56945781 = fifty six million, nine hundred and forty five thousand, seven hundred and eighty one

etc. up to 999,999,999.

As a pointer on what we would be looking for:

• Test driven development
• Application of good design principles to solve the problem

One thousand million is equivalent to one billion.

• 1: one
• 10: ten
• 100: hundred
• 1,000: thousand
• 1,000,000: million
• 1,000,000,000: billion

To provide a single algorithm for all the integer values there are six such rule groups:

1. Zero Rule: If the value is zero then the number in words is ‘zero’ and no other rules apply.

2. Three Digit Rule: The integer value is split into groups of three digits starting from the right-hand side. Each set of three digits is then processed individually as a number of hundreds, tens and units. Once converted to text, the three-digit groups are recombined with the addition of the relevant scale number (thousand, million, billion.)

3. Hundreds Rule: If the hundreds portion of a three-digit group is not zero, the number of hundreds is added to the word. If the three-digit group is exactly divisible by one hundred, the text ‘hundred’ is appended. If not, the text ‘hundred and’ is appended e.g. ‘two hundred’ or ‘one hundred and twelve’

4. Tens Rules: If the tens section of a three-digit group is two or higher the appropriate ‘-ty’ word (twenty, thirty, etc) is added to the text and followed by the name of the third digit (unless the third digit is zero, which is ignored). If the tens and the units are both zero, no text is added. For any other value, the name of the one or two-digit number is added as a special case.

5. Recombination Rules: When recombining the translated three-digit groups, each group except the last is followed by a large number name and a comma, unless the group is blank and therefore not included at all. One exception is when the final group does not include any hundreds and there is more than one non-blank group. In this case the final comma is replaced with ‘and’ e.g. ‘one billion, one million and twelve’.

6. Negative Rule: Negative number are always preceded by the text ‘negative’.