How to convert numbers to words in Erlang?

Question

I found this interesting question about converting numbers into "words":

Code Golf: Number to Words

I would really like to see how you would implement this efficiently in Erlang.

Answer 1

-module(int2txt).
-export([convert/1]).

convert(0) -> "zero";
convert(N) -> convert1(N).

convert1(0) -> "";
convert1(N) when N>=1000 ->
    join(convert_thou(thoubit(N), 0),convert1(N rem 1000));
convert1(N) when N>=100 ->
    join(convert1(N div 100),"hundred",convert1(N rem 100));
convert1(N) when N>=20 ->
    join(tens((N div 10)-1) ++"ty",convert1(N rem 10));
convert1(N) when N>=13 -> teens(N-12) ++ "teen";
convert1(N) -> ones(N).

convert_thou({0,0},_) -> "";
convert_thou({0,N2},C) -> convert_thou(thoubit(N2),C+1);
convert_thou({N,N2},C) -> join(convert_thou(thoubit(N2),C+1),
                   convert1(N),thouword(C)).

thoubit(N) -> {(N div 1000) rem 1000,N div 1000}.

ones(N) -> element(N,{"one","two","three","four","five","six","seven",
              "eight","nine","ten","eleven","twelve"}).
tens(N) -> element(N,{"twen","thir","for","fif","six","seven","eigh","nine"}).
teens(2) -> "four";
teens(N) -> tens(N+1).
thouword(0) -> "thousand";
thouword(C) -> illions(C) ++ "illion".
illions(N) -> element(N,{"m","b","tr","quadr","quint","sex","sept",
                         "oct","non","dec"}).

join(X,Y,Z) -> join(join(X,Y),Z).
join("",X) -> X;
join(X,"") -> X;
join(X,Y) -> X++" "++Y.

Tests:

1> int2txt:convert(0).
"zero"
2> int2txt:convert(1024).
"one thousand twenty four"
3> int2txt:convert(1048576).
"one million forty eight thousand five hundred seventy six"
4> int2txt:convert(1073741824).
"one billion seventy three million seven hundred forty one thousand
eight hundred twenty four"
5> int2txt:convert(1000001).   
"one million one"