Erlang Programming Exercise: 3-10
Exercise 3-10 has you take unstructured text and make filled text, then later, justified text.
Now, it's trivial to justify the text, you could even do it in a single pass, but I wanted to solve both the filled text and the text justification at the same time. To make development faster, I wrote some test stubs (at the bottom of the file) that would load the text from disk, and call the interface functions.
textProcess and justifyText do basically the same thing, with justifyText trying to figure out how many extra spaces to put in each gap. They start out by creating a list of tuples of {Word, WordLength} and then passing that list into makeWordLines/4 which builds lines. At this point, passing the lines into wordLinesToString/2 will solve the filled text part of the problem. To solve the justified text part, the lines are passed into justifyWordLines/2 which will calculate how many extra spaces per gap are needed and call wordLineToString/5 to turn the line into a string.
My implementation suffers from floating point arithmetic problems. Some lines will not get the last space in the last gap because the gap value is 0.999999999999 instead of 1. 
This could be solved in a couple of ways, but I'm ready to move into chapter 4.
Looking at the code now, it's very verbose and can probably be done more succinctly.
-module(threeten).
-export([textProcess/2, justifyText/2, testProcess/1, testJustify/1]).
textProcess(Text, Width) ->
TokenText = tokenizeText(Text),
WordLines = makeWordLines(TokenText, Width, Width, []),
wordLinesToString(WordLines).
justifyText(Text, Width) ->
TokenText = tokenizeText(Text),
WordLines = makeWordLines(TokenText, Width, Width, []),
justifyWordLines(WordLines, Width).
justifyWordLines(WordLines, Width) ->
justifyWordLines(WordLines, Width, []).
justifyWordLines([], _Width, JustifiedLines) ->
lists:flatten(lists:reverse(JustifiedLines));
justifyWordLines([[]|WordLines], Width, JustifiedLines) ->
justifyWordLines(WordLines, Width, JustifiedLines);
justifyWordLines([Line|WordLines], Width, JustifiedLines) ->
SpacesPerGap = case length(WordLines) of
0 -> 0; % the last line isn't justified
_Other -> spacesPerGap(Line, Width)
end,
JustifiedString = lists:reverse(wordLineToString(Line, 0, "", SpacesPerGap, 0)),
justifyWordLines(WordLines, Width, [JustifiedString|JustifiedLines]).
spacesPerGap([], _Width) ->
0;
spacesPerGap(Line, Width) ->
Words = length(Line),
WordLength = lists:foldr(fun({_Word, Length}, Accum) -> Length + Accum end, 0, Line),
ExtraSpaces = Width - WordLength - Words + 1,
Gaps = Words-1,
if
Gaps =< 0 -> 0;
ExtraSpaces =< 0 -> 0;
ExtraSpaces > 0 -> ExtraSpaces / Gaps
end.
wordLinesToString(WordLines) ->
lists:flatten(wordLinesToString(WordLines, [])).
wordLinesToString([], Strings) ->
lists:reverse(Strings);
wordLinesToString([[]|WordLines], String) ->
wordLinesToString(WordLines, String);
wordLinesToString([Line|WordLines], String1) ->
String2 = wordLineToString(Line, 0, String1, 0, 0),
wordLinesToString(WordLines, String2).
wordLineToString([], _WordNumber, String, _SpacesPerGap, _GapLeftover) ->
NewLine = "\n",
[NewLine|String];
wordLineToString([{Word, _Length}|Line], WordNumber, String, SpacesPerGap, _GapLeftover) when WordNumber == 0 ->
wordLineToString(Line, WordNumber+1, [Word|String], SpacesPerGap, SpacesPerGap);
wordLineToString([{Word1, _Length}|Line], WordNumber1, String, SpacesPerGap, GapLeftover1) ->
WordNumber2 = WordNumber1+1, % figure out the current word number
Spaces = erlang:trunc(GapLeftover1) , % drop the decimal places
GapLeftover2 = GapLeftover1 - Spaces + SpacesPerGap, % adjust the remaining leftover
Word2 = [string:copies(" ", Spaces+1)|Word1],
wordLineToString(Line, WordNumber2, [Word2|String], SpacesPerGap, GapLeftover2).
% end case, out of words
makeWordLines([], _Width, _WidthLeft, [LastLine|Lines]) ->
ReversedLastLine = lists:reverse(LastLine),
AllLines = [ReversedLastLine|Lines],
lists:reverse(AllLines);
% if the line is currently empty, and the word length is =< than the length of the line
makeWordLines([{_Word, Length}=Token|Tokens], Width, WidthLeft, [[]|Lines]) when Length =< Width ->
makeWordLines(Tokens, Width, WidthLeft-Length, [[Token]|Lines]);
% line with words, and the word length is =< the length left
makeWordLines([{_Word, Length}=Token|Tokens], Width, WidthLeft, [LastLine|Lines]) when Length < WidthLeft ->
makeWordLines(Tokens, Width, WidthLeft-Length-1, [[Token|LastLine]|Lines]);
% if the line is empty, and the word length is > the width of the line
makeWordLines([{_Word, Length}=Token|Tokens], Width, _WidthLeft, [[]|Lines]) when Length > Width ->
makeWordLines(Tokens, Width, 0, [[Token]|Lines]);
% if there is no more space on the line, reverse the tokens on the last line and push a new one on the stack
makeWordLines(Tokens, Width, _WidthLeft, [LastLine|Lines]) ->
ReversedLastLine = lists:reverse(LastLine),
PreviousLines = [ReversedLastLine|Lines],
makeWordLines(Tokens, Width, Width, [[]|PreviousLines]);
% initial case where we have no existing lines on the stack
makeWordLines(Tokens, Width, WidthLeft, []) ->
makeWordLines(Tokens, Width, WidthLeft, [[]]).
tokenizeText(Text) ->
Tokens = string:tokens(Text, " \t\r\n"),
measureTokens([], Tokens).
measureTokens(Measured, []) ->
lists:reverse(Measured);
measureTokens(Measured, [Token|Tokens]) ->
Length = string:len(Token),
measureTokens([{Token, Length}|Measured], Tokens).
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%% Loading Test Stubs %%%%%%%%%%%%%%%%%%%%%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
testProcess(Width) ->
{ok, Text} = readFileText('threeten.text'),
io:format("~s~n", [textProcess(Text, Width)]).
testJustify(Width) ->
{ok, Text} = readFileText('threeten.text'),
io:format("~s~n", [justifyText(Text, Width)]).
readFileText(Filename) ->
case file:open(Filename, read) of
{ok, IoDevice} ->
Words = loadRawTextLines([], IoDevice),
file:close(IoDevice),
{ok, Words};
{error, Reason} -> {error, Reason}
end.
loadRawTextLines(Lines, IoDevice) ->
case file:read_line(IoDevice) of
{ok, Line} -> loadRawTextLines([Line|Lines], IoDevice);
eof -> lists:flatten(lists:reverse(Lines))
end.
Cheers,
Halzy
