Quick Navigator

 Search Site Miscellaneous Server Agreement Year 2038 Credits

# Manual Reference Pages  -  MATH::STRING::CHARSET (3)

.ds Aq ’

### NAME

Math::String::Charset - A simple charset for Math::String objects.

### SYNOPSIS

```

use Math::String::Charset;

\$a = new Math::String::Charset;             # default a-z
\$b = new Math::String::Charset [a..z];  # same
\$c = new Math::String::Charset
{ start => [a..z], sep =>   };    # with   between chars

print \$b->length();                         # a-z => 26

# construct a charset from bigram table, and an initial set (containing
# valid start-characters)
# Note: After an a, either an b, c or a can follow, in this order
#       After an d only an a can follow
\$bi = new Math::String::Charset ( {
start => a..d,
bi => {
a => [ b, c, a ],
b => [ c, b ],
c => [ a, c ],
d => [ a, ],
q => [ ],                     # q will be automatically in end
}
end => [ a, b, ],
} );
print \$bi->length();                # a,b => 2 (cross of end and start)
print scalar \$bi->class(2);         # count of combinations with 2 letters
# will be 3+2+2+1 => 8

\$d = new Math::String::Charset ( { start => [a..z],
minlen => 2, maxlen => 4, } );

print \$d->first(0),"\n";            # undef, too short
print \$d->first(1),"\n";            # undef, to short
print \$d->first(2),"\n";            # aa

\$d = new Math::String::Charset ( { start => [a..z] } );

print \$d->first(0),"\n";            #
print \$d->first(1),"\n";            # a
print \$d->last(1),"\n";             # z
print \$d->first(2),"\n";            # aa

```

### REQUIRES

perl5.005, Exporter, Math::BigInt

### EXPORTS

Exports nothing on default, can export analyze.

### DESCRIPTION

This module lets you create an charset object, which is used to contruct Math::String objects. This object knows how to handle simple charsets as well as complex onex consisting of bi-grams (later tri and more).

In case of more complex charsets, a reference to a Math::String::Charset::Nested or Math::String::Charset::grouped will be returned.
Default charset The default charset is the set containing abcdefghijklmnopqrstuvwxyz (thus producing always lower case output).

### ERORRS

Upon error, the field _error stores the error message, then die() is called with this message. If you do not want the program to die (f.i. to catch the errors), then use the following:

```

use Math::String::Charset;

\$Math::String::Charset::die_on_error = 0;

\$a = new Math::String::Charset ();      # error, empty set!
print \$a->error(),"\n";

```

### INTERNAL DETAILS

This object caches certain calculation results (f.i. the number of possible combinations for a certain string length), thus greatly speeding up sequentiell Math::String conversations from string to number, and vice versa.

#### CHARACTER LENGTH

All characters used to construct the charset must have the same length, but need not neccessarily be one byte/char long.

#### COMPLEXITY

The complexity for converting from number to string, and vice versa, is O(N), with N beeing the number of characters in the string.

Actually, it is a bit higher, since the underlying Math::BigInt needs more time for longer numbers than for shorts. But usually the practically string length limit is reached before this effect shows up.

See BENCHMARKS in Math::String for run-time details.

#### STRING ORDERING

With a simple charset, converting between the number and string is relatively simple and straightforward, albeit slow.

With bigrams, this becomes even more complex. But since all the information on how to convert between number and string in inside the charset definition, Math::String::Charset will produce (and sometimes cache) this information. Thus Math::String is simple a hull around Math::String::Charset and Math::BigInt.

#### SIMPLE CHARSETS

Depending on the charset, the order in which Math::String ’sees’ the strings is different. Example with charset ’A’..’D’:

```

A      1
B      2
C      3
D      4
AA      5
AB      6
AC      7
BA      9
BB     10
BC     11
..
AAA     20
AAB     21 etc

```

The order of characters does not matter, ’B’,’D’,’C’,’A’ will produce similiar results, though in a different order inside Math::String:

```

B      1
D      2
C      3
A      4
BB      5
BD      6
BC      7
..
BBB     20
BBD     21 etc

```

Here is an example with characters of length 3:

```

foo      1
bar      2
baz      3
foofoo      4
foobar      5
foobaz      6
barfoo      7
barbar      8
barbaz      9
bazfoo     10
bazbar     11
bazbaz     12
foofoofoo     13 etc

```

All charset items must have the same length, unless you use a separator string:

```

use Math::String;

\$a = Math::String->new(,
{ start => [ qw/ the green car a/ ], sep =>   } );

while (\$b ne the green car)
{
\$a ++;
print "\$a\t";         # print "a green car" etc
}

```

The separator is a string, not a regexp and it must not be present in any of the characters of the charset.

The old way was using a fill character, which is more complicated:

```

use Math::String;

\$a = Math::String->new(, [ qw/ the::: green: car::: a:::::/ ]);

while (\$b ne the green car)
{
\$a ++;
print "\$a\t";         # print "a:::::green:car:::" etc

\$b = "\$a"; \$b =~ s/:+/ /g; \$b =~ s/\s+\$//;
print "\$b\n";         # print "a green car" etc
}

```

This produces:

```

the:::  the
green:  green
car:::  car
a:::::  a
the:::the:::    the the
the:::green:    the green
the:::car:::    the car
the:::a:::::    the a
green:the:::    green the
green:green:    green green
green:car:::    green car
green:a:::::    green a
car:::the:::    car the
car:::green:    car green
car:::car:::    car car
car:::a:::::    car a
a:::::the:::    a the
a:::::green:    a green
a:::::car:::    a car
a:::::a:::::    a a
the:::the:::the:::      the the the
the:::the:::green:      the the green
the:::the:::car:::      the the car
the:::the:::a:::::      the the a
the:::green:the:::      the green the
the:::green:green:      the green green
the:::green:car:::      the green car

```

#### HIGHER ORDERS

Now imagine a charset that is defined as follows:

Starting characters for each string can be ’a’,’c’,’b’ and ’d’ (in that order). Each ’a’ can be followed by either ’b’, ’c’ or ’a’ (again in that order), each ’c can be followed by either ’c’, ’d’ (again in that order), and each ’b’ or ’d’ can be followed by an ’a’ (and nothing else).

The definition is thus:

```

use Math::String::Charset;

\$cs = Math::String::Charset->new( {
start => [ a, c, b, d ],
bi => {
a => [ b,c,a ],
b => [ a, ],
d => [ a, ],
c => [ c,d ],
}
} );

```

This means that each character in a string depends on the previous character. Please note that the probabilities on which characters follows how often which character do not concern us here. We simple enumerate them all. Or put differently: each probability is 1.

With the charset above, the string sequence runs as follows:

```

string  number  count of strings
with length

a       1
c       2
b       3
d       4     1=4
ab       5
ac       6
aa       7
cc       8
cd       9
ba      10
da      11     2=7
aba      12
acc      13
acd      14
aab      15
aac      16
aaa      17
ccc      18
ccd      19
cda      20
bab      21
bac      22
baa      23
dab      24
dac      25
daa      26     3=15
abab      27
abac      28
abaa      29
accc      30
accd      31
acda      32
aaba      33
aacc      34
aacd      35     etc

```

There are 4 strings with length 1, 7 with length 2, 15 with length 3 etc. Here is an example for first() and last():

```

\$charset->first(3);     # gives aba
\$charset->last(3);      # gives daa

```

#### RESTRICTING STRING ENDINGS

Sometimes, you want to specify that a string can end only in certain characters. There are two ways:

```

use Math::String::Charset;

\$cs = Math::String::Charset->new( {
start => [ a, c, b, d ],
bi => {
a => [ b,c,a ],
b => [ a, ],
d => [ a, ],
c => [ c,d ],
}
end => [ a,b ],
} );

```

This defines any string ending not in ’a’ or ’b’ as invalid. The sequence runs thus:

```

string  number  count of strings
with length

a       1
b       2     2
ab       4
aa       5
ba       6
da       7     4
aba       8
aab       9
aaa      10
cda      11
bab      12
baa      13
dab      14
daa      15     8
abab      16
abaa      17     etc

```

There are now only 2 strings with length 1, 4 with length 2, 8 with length 3 etc.

The other way is to specify the (additional) ending restrictions implicit by using chars that are not followed by other characters:

```

use Math::String::Charset;

\$cs = Math::String::Charset->new( {
start => [ a, c, b, d ],
bi => {
a => [ b,c,a ],
b => [ a, ],
d => [ a, ],
c => [  ],
}
} );

```

Since ’c’ is not followed by any characters, there are no strings with a ’c’ in the middle (which means strings can end in ’c’):

```

string  number  count of strings
with length

a       1
c       2
b       3
d       4     4
ab       5
ac       6
aa       7
ba       8
da       9     5
aba      10
aab      11
aac      12
aaa      13
bab      14
bac      15
baa      16
dab      17
dac      18
daa      19     10
abab      20
abac      21 etc

```

There are now 4 strings with length 1, 5 with length 2, 10 with length 3 etc.

Any character that is not followed by another character is automatically added to end. This is because otherwise you would have created a rendundand character which could never appear in any string:

Let’s assume ’q’ is not in the end set, and not followed by any other character:
1. There can no string q, since strings of lenght 1 start <B>andB> end with their only character. Since ’q’ is not in end, the string q is invalid (no matter wether ’q’ appears in start or not).
2. No string longer than 1 could start with ’q’ or have a ’q’ in the middle, since ’q’ is not followed by anything. This leaves only strings with length 1 and these are invalid according to rule 1.

#### CONVERTING (STRING <=> NUMBER)

From now on, a ’class’ refers to all strings with the same length. The order or length of a class is the length of all strings in it.

With a simple charset, each class has exactly M times more strings than the previous class (e.g. the class with a length - 1). M is in this case the length of the charset.

#### SIMPLE CHARSET

To convert between string and number, we must simple know which string has which number and which number is which string. Although this sounds very difficult, it is not so. With ’simple’ charsets, it only involves a bit of math.

First we need to know how many string are in the class. From this information we can determine the lenght of a string given it’s number, and get the range inside which the number to a string lies:

Let’s stick to the example with 4 characters above, ’A’..’D’:

```

Stringlenght    strings with that length        first in range
1               4                               1
2               16 (4*4)                        5
3               64 (4*4*4)                      21
4               4**4                            85
5               4**5 etc                        341

```

You see that this is easy to calculate. Now, given the number 66, we can determine how long the string must be:

66 is greater than 21, but lower than 85, so the string must be 3 characters long. This information is determined in O(N) steps, wheras N is the length of the string by successive comparing the number to the elements in all string of a certain length.

If we then subtract from 66 the 21, we get 45 and thus know it must be the fourty-fifth string of the 3 character long ones.

The math involved to determine which 3 character-string it actually is equally to converting between decimal and hexadecimal numbers. Please see source for the gory, but boring details.

#### HIGHER ORDER CHARSETS

For charsets of higher order, even determining the number of all strings in a class becomes more difficult. Fortunately, there is a way to do it in N steps just like with a simple charset.

#### BASED ON ENDING COUNTS

The first way is based on the observation that the number of strings in class n+1 only depends on the number of ending chars in class n, and nothing else.

This is, however, not used in the current implemenation, since there is a slightly faster/simpler way based on the count of strings that start with a given character in class n, n-1, n-2 etc. See below for a description.

Here is for reference the example with ending char counts:

```

use Math::String::Charset;

\$cs = Math::String::Charset->new( {
start => [ a, c, b, d ],
bi => {
a => [ b,c,a ],
c => [ c,d ],
b => [ a, ],
d => [ a, ],
}
} );

Class 1:
a       1
c       2
b       3
d       4     4

```

As you can see, there is one ’a’, one ’c’, one ’b’ and one ’d’. To determine how many strings are in class 2, we must multiply the occurances of each character by the number of how many characters it is followed:

```

a * 3 + c * 2 + d * 1 + b * 1

```

which equals

```

1 * 3 + 1 * 2 + 1 * 1 + 1 * 1

```

If we summ this all up, we get 3+2+1+1 = 7, which is exactly the number of strings in class 2. But to determine now the number of strings in class 3, we must now how many strings in class 2 end on ’a’, how many on ’b’ etc.

We can do this in the same loop, by not only keeping a sum, but by counting all the different endings. F.i. exactly one string ended in ’a’ in class 1. Since ’a’ can be followed by 3 characters, for each character we know that it will occure at least 1 time. So we add the 1 to the character in question.

```

\$new_count->{b} += \$count->{a};

```

This yields the amounts of strings that end in ’b’ in the next class.

We have to do this for every different starting character, and for each of the characters that follows each starting character. In the worst case this means M*M steps, while M is the length of the charset. We must repeat this for each of the classes, so that the complexity becomes O(N*M*M) in the worst case. For strings of higher order this gets worse, adding a *M for each higher order.

For our example, after processing ’a’, we will have the following counts for ending chars in class 2:

```

b => 1
c => 1
a => 1

```

After processing ’c’, it is:

```

b => 1
c => 2 (+1)
a => 1
d => 1 (+1)

```

because ’c’ is followed by ’d’ or ’c’. When we are done with all characters, the following count’s are in our \$new_count hash:

```

b => 1
c => 2
a => 3
d => 1

```

When we sum them up, we get the count of strings in class 2. For class 3, we start with an empty count hash again, and then again for each character process the ones that follow it. Example for a:

```

b => 0
c => 0
a => 0
d => 0

```

3 times ending in ’a’ followed by ’b’,’c’ or ’d’:

```

b => 3  (+3)
c => 3  (+3)
a => 3  (+3)
d => 0

```

2 times ending ’c’ followed by ’c’ or ’d’:

```

b => 3
c => 5  (+2)
a => 3
d => 2  (+2)

```

After processing ’b’ and ’d’ in a similiar manner we get:

```

b => 3
c => 5
a => 5
d => 2

```

The sum is 15, and we know now that we have 15 different strings in class 3. The process for higher classes is the same again, re-using the counts from the lower class.

#### BASED ON STARTING COUNTS

The second, and implemented method counts for each class how many strings start with a given character. This gives us two information at once:
o A string of length N and a starting char of X, which number it must have at minimum (by summing up the counts of all strings that come before X) and how many strings are there starting with X (although this is not used for X, but only for all strings that come after X).
o How many strings are there with a given length, by summing up all the counts for the different starting chars.
This method also has the advantage that it doesn’t need to re-calculate the count for each level. If we have cached the information for class 7, we can calculate class 8 right-away. The old method would either need to start at class 1, working up to 8 again, or cache additional information of the order N (where N is the number of different characters in the charset).

Here is how the second method works, based on the example above:

```

start => [ a, c, b, d ],
bi => {
a => [ b,c,a ],
c => [ c,d ],
b => [ a, ],
d => [ a, ],
}

```

The sequence runs as follows:

```

String  Strings starting with
this character in this level

a     1
c     1
b     1
d     1
ab
ac
aa     3       (1+1+1)
cc
cd     2       (1+1)
ba     1
da     1
aba
acc
acd
aab
aac
aaa     6       1 (b) + 2 (c) + 3 (a)
ccc
ccd
cda     3       2 (c) + 1 (d)
bab
bac
baa     3
dab
dac
daa     3
abab
abac
abaa
accc     etc

```

As you can see, for length one, there is exactly one string for each starting character.

For the next class, we can find out how many strings start with a given char, by adding together all the counts of strings in the previous class.

F.i. in class 3, there are 6 strings starting with ’a’. We find this out by adding together 1 (there is 1 string starting with ’b’ in class 2), 2 (there are two strings starting with ’c’ in class 2) and 3 (three strings starting with ’a’ in class 2).

As a special case we must throw away all strings in class 2 that have invalid ending characters. By doing this, we automatically have restricted <B>allB> strings to only valid ending characters. Therefore, class 1 and 2 are setup upon creating the charset object, the others are calculated on-demand and then cached.

Since we are calculating the strings in the order of the starting characters, we can sum up all strings up to this character.

```

String  First string in that class

a     0
c     1
b     2
d     3

ab     0
ac
aa
cc     3
cd
ba     5
da     6

aba     0
acc
acd
aab
aac
aaa
ccc     6
ccd
cda
bab     9
bac
baa
dab     12
dac
daa
abab     0
abac
abaa
accc     etc

```

When we add to the number of the last character (f.i. 12 in case of ’d’ in class 3) the amount of strings with that character (here 3), we end up with the number of all strings in that class.

Thus in the same loop we calculate:
what is the first number of a string starting with ’x’ in that class
how many strings are in this class at all
That should be all we need to know to convert a string to it’s number.

#### HIGHER ORDER CHARSETS, FINDING THE RIGHT NUMBER

From the section above we know that we can find out which number a string of a certain class has at minimum and at maximum. But what number has the string in that range, actually?

Well, given the information it is easy. First, find out which minimum number a string has with the given starting character in the class. Add this to it’s base number. Then reduce the class by one, look at the next character and repeat this. In pseudo code:

```

\$class = length (\$string); \$base = base_number->[\$class];
foreach (\$character)
{
\$base += \$sum->[\$class]->{\$character};
\$class --;
}

```

So, after N simple steps (where N is the number of characters in the string), we have found the number of the string.

#### MULTIPLE MULTIWAY TREES

It helps to imagine the strings like a couple of trees (ASCII art is crude):

```

class:  1   2    3   etc

number
1       a
5     +--ab
12   |   +--aba
6     +--ac
13   |   +--acc
14   |   +--acd
7     +--aa
15       +--aab
16       +--aac
17       +--aaa

2       c
8     +--cc
18   |   +--ccc
19   |   +--ccd
9     +--cd
20       +--cda

3       b
10     +--ba
21       +--bab
22       +--bac
23       +--baa

4       d
11     +--da
24       +--dab
25       +--dac
26       +--daa

```

As you can see, there is a (independend) tree for each of the starting characters, which in turn contains independed sub-trees for each string in the next class etc. It is interesting to note that each string deeper in the tree starts with the same common starting string, aka ’d’, ’da’, ’dab’ etc.

With a simple charset, all these trees contain the same number of nodes. With higher order charsets, this is no longer true.

### METHODS

#### BInew()

```

new();

```

Create a new Math::String::Charset object.

The constructor takes either an ARRAY or a HASH reference. In case of the array, all elements in that array will be used as characters in the charset, and the charset will be of order 0, type 0.

If given a HASH reference, the following keys can be used for all charsets:

```

minlen          Minimum string length, -inf if not defined
maxlen          Maximum string length, +inf if not defined

```

The following keys can only be used in certain combinations, which will be explained below:

```

bi              hash,  table with bi-grams
sets            hash, table with charsets for the different places
start           array ref to list of all valid (starting) characters
end             array ref to list of all valid ending characters
sep             separator character, none if undef (only for order 1)

```

If you use neither <B>biB> nor <B>setsB>, the charset will be of order 1, type 0. If you use a hash key named <B>biB>, the charset will be of order 2, type 0. If you use a hash key named <B>setsB>, the charset will be of order 1, type 1.

For a charset of type 0, order 1 (simpel set) the following keys are valid:

```

start           required
end             optional (to restrict number of 1-character strings)
sep             optional

```

For a charset of type 0, order 2 (bi-gram set) the following keys are valid:

```

start           optional
end             optional
bi              required

```

For a charset of type 1, order 1 (grouped set) the following keys are valid:

```

sets            required

```
start start contains an array reference to all valid starting characters, e.g. no valid string can start with a character not listed here.
bi bi contains a hash reference, each key of the hash points to an array, which in turn contains all the valid combinations of two letters.
sets sets contains a hash reference, each key of the hash indicates an index. Each of the hash entries points either to an ARRAY reference or a Math::String::Charset of order 1, type 0.

Positive indices count from the left side, negative from the right. 0 denotes the default.

At each of the position indexed by a key, the appropriate charset will be used.

Example for specifying that strings must start with upper case letters, followed by lower case letters and can end in either a lower case letter or a number:

```

sets => {
0 => [a..z],              # the default
1 => [A..Z],              # first character is always A..Z
-1 => [a..z,0..9],     # last is q..z,0..9
}

```
end start contains an array reference to all valid ending characters, e.g. no valid string can end with a character not listed here. Note that strings of length 1 start <B>andB> end with their only character, so the character must be listed in end and start to produce a string with one character. Also all characters that are not followed by any other character are added silently to the end set.
minlen Optional minimum string length. Any string shorter than this will be invalid. Must be shorter than a (possible defined) maxlen. If not given is set to -inf. Note that the minlen might be adjusted to a greater number, if it is set to 1 or greater, but there are not valid strings with 2,3 etc. In this case the minlen will be set to the first non-empty class of the charset.
maxlen Optional maximum string length. Any string longer than this will be invalid. Must be longer than a (possible defined) minlen. If not given is set to +inf.
scale Optional input/output scale. See scale().

#### BIcopy()

```

\$copy = \$charset->copy();

```

Create a new charset as a copy from an existing one.

#### BIscale()

```

\$scale = \$charset->scale();
\$charset->scale(120);

```

Get/set the (optional) scale for all strings. A scale is an integer factor that will be applied to each as_number() output. Also, all from_number() will use the scale to modularize the input, e.g. dividing by the scale, then taking the integer result, and the multiplying with the scale again.

E.g. for a scale of 3, the string to number mapping would be changed from the left to the right column:

```

string form             normal number   scaled number
0               0
a                     1               3
b                     2               6
c                     3               9

```

And so on. Input like 8 will be divided by 3, which results in 2 due to rounding down to the nearest integer, this multiplied by 3 again gives 6. So:

```

my \$cs = Math::String::Charset->new([a..z]); # a..z
\$string = Math::String->new( a,\$cs );          # a..z
print \$string->as_number();                      # 1
\$cs->scale(3);
print \$string->as_number();                      # 3
\$string = Math::String->from_number(10,\$cs);     # [10/3] => 3 *3 == 9

```

#### BIminlen()

```

\$charset->minlen();

```

Return minimum string length.

#### BImaxlen()

```

\$charset->maxlen();

```

Return maximum string length.

#### BIlength()

```

\$charset->length();

```

Return the number of items in the charset, for higher order charsets the number of valid 1-character long strings. Shortcut for \$charset->class(1).

#### BIcount()

Returns the count of all possible strings described by the charset as a positive BigInt. Returns ’inf’ if no maxlen is defined, because there should be no upper bound on how many strings are possible. (This might change if we can calculate an upper bound - not sure if this is possible with bigrams).

If maxlen is defined, forces a calculation of all possible class() values and may therefore be very slow on the first call, it also caches possible lot’s of values.

#### BIclass()

```

\$charset->class(\$order);

```

Return the number of items in a class.

```

print \$charset->class(5);       # how many strings with length 5?

```

#### BImap()

```

\$charset->map(\$char);

```

Map a character to it’s number, counting from 0 .. N-1 where N is the length of the charset:

```

\$charset = Math::String::Charset->new([A..Z]);

print \$charset->map(A),"\n";          # prints 0
print \$charset->map(Z),"\n";          # prints 25

```

#### BIchar()

```

\$charset->char(\$nr);

```

Returns the character number \$nr from the set, or undef.

```

print \$charset->char(0);        # first char
print \$charset->char(1);        # second char
print \$charset->char(-1);       # last one

```

#### BIlowest()

```

\$charset->lowest(\$length);

```

Return the number of the first string of length \$length. This is equivalent to (but much faster):

```

\$str = \$charset->first(\$length);
\$number = \$charset->str2num(\$str);

```

#### BIhighest()

```

\$charset->highest(\$length);

```

Return the number of the last string of length \$length. This is equivalent to (but much faster):

```

\$str = \$charset->first(\$length+1);
\$number = \$charset->str2num(\$str);
\$number--;

```

#### BIorder()

```

\$order = \$charset->order();

```

Return the order of the charset: 1 for simple charsets, 2 (bi-grams), 3 etc for higher orders. See also type().

#### BItype()

```

\$type = \$charset->type();

```

Return the type of the charset: 0 for simple charsets, 1 for grouped ones. If the type is 0, the order can be 1,23 etc, with type 1 the order is always 1, too. See also order.

#### BIcharlen()

```

\$character_length = \$charset->charlen();

```

Return the length of one character in the set. 1 or greater.

#### BIchars()

```

\$chars = \$charset->chars( \$bigint );

```

Returns the number of characters that the string would have, when you would convert \$bigint (Math::BigInt or Math::String object) back to a string. This is much faster than doing

```

\$chars = length ("\$math_string");

```

since it does not need to actually construct the string.

#### BIfirst()

```

\$charset->first( \$length );

```

Return the first string with a length of \$length, according to the charset. See lowest() for the corrospending number.

#### BIlast()

```

\$charset->last( \$length );

```

Return the last string with a length of \$length, according to the charset. See highest() for the corrospending number.

#### BIis_valid()

```

\$charset->is_valid();

```

Check wether a string conforms to the charset set or not. Returns 1 for okay, 0 for invalid strings.

#### BInorm()

```

\$charset->norm();

```

Normalize a string by removing separator char at front/end. Does nothing if no separator is defined.

#### BIerror()

```

\$charset->error();

```

Returns "" for no error or an error message that occured if construction of the charset failed. Set \$Math::String::Charset::die_on_error to 0 to get the error message, otherwise the program will die.

#### BIstart()

```

\$charset->start();

```

In list context, returns a list of all characters in the start set, for simple charsets (e.g. no bi, tri-grams etc) simple returns the charset. In scalar context returns the lenght of the start set.

Note that the returned end set can be differen from what you specified upon constructing the charset, because characters that are not followed by any other character will be excluded from the start set (they can’t possible start a string longer than one character).

Think of the start set as the set of all characters that can start a string with more than one character. The set for one character strings is called <B>onesB> and you can access if via ones().

#### BIend()

```

\$charset->end();

```

In list context, returns a list of all characters in the end set, aka all characters a string can end with. For simple charsets (e.g. no bi, tri-grams etc) simple returns the charset. In scalar context returns the lenght of the end set.

Note that the returned end set can be differen from what you specified upon constructing the charset, because characters that are not followed by any other character will be included in the end set, too.

#### BIones()

```

\$charset->ones();

```

In list context, returns a list of all strings consisting of one character, for simple charsets (e.g. no bi, tri-grams etc) simple returns the charset. In scalar context returns the lenght of the <B>onesB> set.

This list is the cross of <B>startB> and <B>endB> that is calculated after adding characters with no followers to <B>endB>, but before removing the characters with no followers from <B>startB>.

Think of a string of only one character as if it starts with and ends in this character at the same time. For instance, if you have the following definition:

```

cs = {
start => [ a, b, c, q ],
end => [ b, c, x ],
bi => {
q => [ ],
a => [ b, c ]
b => [ a ]
}
}

```

The ’q’ is not followed by any other character, so it can only end strings. And since it is not in the <B>endB> set, it is first added to this set:

```

cs = {
start => [ a, b, c, q ],
end => [ b, c, x, q ],
bi => {
q => [ ],
a => [ b, c ]
b => [ a ]
}
}

```

Now the cross of start and end is build. Since only ’b’, ’c’ and ’q’ appear in both end and start, ones consists of:

```

_ones => [ b, c, q ]

```

The order of the chars in ones is the same ordering as in start.

After this, any character that is not followed by an other character is removed from start:

```

start => [ a, b, ],

```

Thus a string with only one character can be ’b’, ’c’, or ’q’, and any string with more than one character must start with either ’a’ or ’b’.

#### BIprev()

```

\$string = Math::String->new( );
\$charset->prev(\$string);

```

Give the charset and a string, calculates the previous string in the sequence. This is faster than decrementing the number of the string and converting the new number to a string. This routine is mainly used internally by Math::String and updates the cache of the given Math::String.

#### BInext()

```

\$string = Math::String->new( );
\$charset->next(\$string);

```

Give the charset and a string, calculates the next string in the sequence. This is faster than incrementing the number of the string and converting the new number to a string. This routine is mainly used internally by Math::String and updates the cache of the given Math::String.

#### BIstudy()

```

\$hash = Math::String::Charset::study( {
order => \$order, words => \@words, sep => separator,
charlen => 1, hist => 1 } );

```

Studies the given list of strings/words and builds a hash that you can use to construct a charset of. The order is 1 for simple charsets, 2 for bigrams and so on. The key depth is a synonym for order.

separator (can be undef) is the sting that separates characters. charlen is the length of a character, and defaults to 1. Use this if you have characters longer than one and no separator string.

If you set the parameter hist to a value different from zero, the returned hash will contain a key hist, too. This will be a reference to a hash containing the histogram of letters or n-grams, depending on the depth of the analysis.

Some example:

```

use Math::String::Charset;
use Data::Dumper;

\$hash = Math::String::Charset::study( {
depth => 1, words => [ hocuspocus], hist => 1 } );
print Dumper (\$hash),"\n";

```

This will produce (slightly contracted here):

```

\$VAR1 = {
end   => [ s ],
hist  => { u => 2, o => 2, p => 1, h => 1,
s => 2, c => 2 },
chars => [ u, o, s, c, p, h ],
start => [ h ]
};

```

Using depth = 2>, you would get (slightly ontracted again):

```

\$VAR1 = {
end  => [ s ],
hist => { u => { s => 2 },
o => { c => 2 },
p => { o => 1 },
h => { o => 1 },
s => { p => 1 },
c => { u => 2 }
},
bi   => {
u => [ s ],
o => [ c ],
h => [ o ],
p => [ o ],
c => [ u ],
s => [ p ]
},
start => [ h ]
};

```

Instead passing an ARRAY ref as words, you can as well pass a HASH ref. The keys in the hash will be used as words then. This is so that you can clean out doubles by using a hash and pass it to study without converting it back to an array first.

#### BIanalyze()

Is an exportable alias for study().

```

use Math::String::Charset qw/analyze/;

\$hash = Math::String::Charset::analyze(
words => [Perl,Hacker,Just,Another], depth => 2,
);

```

### EXAMPLES

```

use Math::String::Charset;

# construct a charset from bigram table, and an initial set (containing
# valid start-characters)
# Note: After an a, either an b, c or a can follow, in this order
#       After an d only an a can follow
#       There is no q as start character, but q can follow d!
#       You need to define followers for q!
\$bi = new Math::String::Charset ( {
start => a..d,
bi => {
a => [ b, ],
b => [ c, b ],
c => [ a, c ],
d => [ a, q ],
q => [ a, b ],
}
} );
print \$bi->length(),"\n";                   # 4
print scalar \$bi->combinations(2),"\n";     # count of combos with 2 chars
# will be 1+2+2+2+2 => 9
my @comb = \$bi->combinations(3);
foreach (@comb)
{
print "\$_\n";
}

```

This will print:

```

4
7
abc
abb
bca
bcc
bbc
bbb
cab
cca
ccc
dab
dqa
dqb

```

Another example using characters of different lengths to find all combinations of words in a list:

```

#!/usr/bin/perl -w

# test for Math::String and Math::String::Charset

BEGIN { unshift @INC, ../lib; }

use Math::String;
use Math::String::Charset;
use strict;

my \$count = shift || 4000;

my \$words = {};
open FILE, wordlist.txt or die "Cant read wordlist.txt: \$!\n";
while (<FILE>)
{
chomp; \$words->{lc(\$_)} ++;   # clean out doubles
}
close FILE;
my \$cs = new Math::String::Charset ( { sep =>  ,
words => \$words,
} );

my \$string = Math::String->new(,\$cs);

print "# Generating first \$count strings:\n";
for (my \$i = 0; \$i < \$count; \$i++)
{
print ++\$string,"\n";
}
print "# Done.\n";

```

### TODO

 o Currently only bigrams are supported. This should be generic and arbitrarily deeply nested. o study() does not yet work with separator chars and chars longer than 1. o str2num and num2str do not work fully for bigrams yet.

### BUGS

None doscovered yet.

### AUTHOR

If you use this module in one of your projects, then please email me. I want to hear about how my code helps you ;)

This module is (C) Copyright by Tels http://bloodgate.com 2000-2008.

Search for    or go to Top of page |  Section 3 |  Main Index

 perl v5.20.3 MATH::STRING::CHARSET (3) 2008-04-03

Visit the GSP FreeBSD Man Page Interface.
Output converted with manServer 1.07.