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<codewalk title="Generating arbitrary text: a Markov chain algorithm">
<step title="Introduction" src="doc/codewalk/markov.go:/Generating/,/line\./">
This codewalk describes a program that generates random text using
a Markov chain algorithm. The package comment describes the algorithm
and the operation of the program. Please read it before continuing.
</step>
<step title="Modeling Markov chains" src="doc/codewalk/markov.go:/ chain/">
A chain consists of a prefix and a suffix. Each prefix is a set
number of words, while a suffix is a single word.
A prefix can have an arbitrary number of suffixes.
To model this data, we use a <code>map[string][]string</code>.
Each map key is a prefix (a <code>string</code>) and its values are
lists of suffixes (a slice of strings, <code>[]string</code>).
<br/><br/>
Here is the example table from the package comment
as modeled by this data structure:
<pre>
map[string][]string{
" ": {"I"},
" I": {"am"},
"I am": {"a", "not"},
"a free": {"man!"},
"am a": {"free"},
"am not": {"a"},
"a number!": {"I"},
"number! I": {"am"},
"not a": {"number!"},
}</pre>
While each prefix consists of multiple words, we
store prefixes in the map as a single <code>string</code>.
It would seem more natural to store the prefix as a
<code>[]string</code>, but we can't do this with a map because the
key type of a map must implement equality (and slices do not).
<br/><br/>
Therefore, in most of our code we will model prefixes as a
<code>[]string</code> and join the strings together with a space
to generate the map key:
<pre>
Prefix Map key
[]string{"", ""} " "
[]string{"", "I"} " I"
[]string{"I", "am"} "I am"
</pre>
</step>
<step title="The Chain struct" src="doc/codewalk/markov.go:/type Chain/,/}/">
The complete state of the chain table consists of the table itself and
the word length of the prefixes. The <code>Chain</code> struct stores
this data.
</step>
<step title="The NewChain constructor function" src="doc/codewalk/markov.go:/func New/,/\n}/">
The <code>Chain</code> struct has two unexported fields (those that
do not begin with an upper case character), and so we write a
<code>NewChain</code> constructor function that initializes the
<code>chain</code> map with <code>make</code> and sets the
<code>prefixLen</code> field.
<br/><br/>
This is constructor function is not strictly necessary as this entire
program is within a single package (<code>main</code>) and therefore
there is little practical difference between exported and unexported
fields. We could just as easily write out the contents of this function
when we want to construct a new Chain.
But using these unexported fields is good practice; it clearly denotes
that only methods of Chain and its constructor function should access
those fields. Also, structuring <code>Chain</code> like this means we
could easily move it into its own package at some later date.
</step>
<step title="The Prefix type" src="doc/codewalk/markov.go:/type Prefix/">
Since we'll be working with prefixes often, we define a
<code>Prefix</code> type with the concrete type <code>[]string</code>.
Defining a named type clearly allows us to be explicit when we are
working with a prefix instead of just a <code>[]string</code>.
Also, in Go we can define methods on any named type (not just structs),
so we can add methods that operate on <code>Prefix</code> if we need to.
</step>
<step title="The String method" src="doc/codewalk/markov.go:/func[^\n]+String/,/}/">
The first method we define on <code>Prefix</code> is
<code>String</code>. It returns a <code>string</code> representation
of a <code>Prefix</code> by joining the slice elements together with
spaces. We will use this method to generate keys when working with
the chain map.
</step>
<step title="Building the chain" src="doc/codewalk/markov.go:/func[^\n]+Build/,/\n}/">
The <code>Build</code> method reads text from an <code>io.Reader</code>
and parses it into prefixes and suffixes that are stored in the
<code>Chain</code>.
<br/><br/>
The <code><a href="/pkg/io/#Reader">io.Reader</a></code> is an
interface type that is widely used by the standard library and
other Go code. Our code uses the
<code><a href="/pkg/fmt/#Fscan">fmt.Fscan</a></code> function, which
reads space-separated values from an <code>io.Reader</code>.
<br/><br/>
The <code>Build</code> method returns once the <code>Reader</code>'s
<code>Read</code> method returns <code>io.EOF</code> (end of file)
or some other read error occurs.
</step>
<step title="Buffering the input" src="doc/codewalk/markov.go:/bufio\.NewReader/">
This function does many small reads, which can be inefficient for some
<code>Readers</code>. For efficiency we wrap the provided
<code>io.Reader</code> with
<code><a href="/pkg/bufio/">bufio.NewReader</a></code> to create a
new <code>io.Reader</code> that provides buffering.
</step>
<step title="The Prefix variable" src="doc/codewalk/markov.go:/make\(Prefix/">
At the top of the function we make a <code>Prefix</code> slice
<code>p</code> using the <code>Chain</code>'s <code>prefixLen</code>
field as its length.
We'll use this variable to hold the current prefix and mutate it with
each new word we encounter.
</step>
<step title="Scanning words" src="doc/codewalk/markov.go:/var s string/,/\n }/">
In our loop we read words from the <code>Reader</code> into a
<code>string</code> variable <code>s</code> using
<code>fmt.Fscan</code>. Since <code>Fscan</code> uses space to
separate each input value, each call will yield just one word
(including punctuation), which is exactly what we need.
<br/><br/>
<code>Fscan</code> returns an error if it encounters a read error
(<code>io.EOF</code>, for example) or if it can't scan the requested
value (in our case, a single string). In either case we just want to
stop scanning, so we <code>break</code> out of the loop.
</step>
<step title="Adding a prefix and suffix to the chain" src="doc/codewalk/markov.go:/ key/,/key\], s\)">
The word stored in <code>s</code> is a new suffix. We add the new
prefix/suffix combination to the <code>chain</code> map by computing
the map key with <code>p.String</code> and appending the suffix
to the slice stored under that key.
<br/><br/>
The built-in <code>append</code> function appends elements to a slice
and allocates new storage when necessary. When the provided slice is
<code>nil</code>, <code>append</code> allocates a new slice.
This behavior conveniently ties in with the semantics of our map:
retrieving an unset key returns the zero value of the value type and
the zero value of <code>[]string</code> is <code>nil</code>.
When our program encounters a new prefix (yielding a <code>nil</code>
value in the map) <code>append</code> will allocate a new slice.
<br/><br/>
For more information about the <code>append</code> function and slices
in general see the
<a href="/doc/articles/slices_usage_and_internals.html">Slices: usage and internals</a> article.
</step>
<step title="Pushing the suffix onto the prefix" src="doc/codewalk/markov.go:/p\.Shift/">
Before reading the next word our algorithm requires us to drop the
first word from the prefix and push the current suffix onto the prefix.
<br/><br/>
When in this state
<pre>
p == Prefix{"I", "am"}
s == "not" </pre>
the new value for <code>p</code> would be
<pre>
p == Prefix{"am", "not"}</pre>
This operation is also required during text generation so we put
the code to perform this mutation of the slice inside a method on
<code>Prefix</code> named <code>Shift</code>.
</step>
<step title="The Shift method" src="doc/codewalk/markov.go:/func[^\n]+Shift/,/\n}/">
The <code>Shift</code> method uses the built-in <code>copy</code>
function to copy the last len(p)-1 elements of <code>p</code> to
the start of the slice, effectively moving the elements
one index to the left (if you consider zero as the leftmost index).
<pre>
p := Prefix{"I", "am"}
copy(p, p[1:])
// p == Prefix{"am", "am"}</pre>
We then assign the provided <code>word</code> to the last index
of the slice:
<pre>
// suffix == "not"
p[len(p)-1] = suffix
// p == Prefix{"am", "not"}</pre>
</step>
<step title="Generating text" src="doc/codewalk/markov.go:/func[^\n]+Generate/,/\n}/">
The <code>Generate</code> method is similar to <code>Build</code>
except that instead of reading words from a <code>Reader</code>
and storing them in a map, it reads words from the map and
appends them to a slice (<code>words</code>).
<br/><br/>
<code>Generate</code> uses a conditional for loop to generate
up to <code>n</code> words.
</step>
<step title="Getting potential suffixes" src="doc/codewalk/markov.go:/choices/,/}\n/">
At each iteration of the loop we retrieve a list of potential suffixes
for the current prefix. We access the <code>chain</code> map at key
<code>p.String()</code> and assign its contents to <code>choices</code>.
<br/><br/>
If <code>len(choices)</code> is zero we break out of the loop as there
are no potential suffixes for that prefix.
This test also works if the key isn't present in the map at all:
in that case, <code>choices</code> will be <code>nil</code> and the
length of a <code>nil</code> slice is zero.
</step>
<step title="Choosing a suffix at random" src="doc/codewalk/markov.go:/next := choices/,/Shift/">
To choose a suffix we use the
<code><a href="/pkg/math/rand/#Intn">rand.Intn</a></code> function.
It returns a random integer up to (but not including) the provided
value. Passing in <code>len(choices)</code> gives us a random index
into the full length of the list.
<br/><br/>
We use that index to pick our new suffix, assign it to
<code>next</code> and append it to the <code>words</code> slice.
<br/><br/>
Next, we <code>Shift</code> the new suffix onto the prefix just as
we did in the <code>Build</code> method.
</step>
<step title="Returning the generated text" src="doc/codewalk/markov.go:/Join\(words/">
Before returning the generated text as a string, we use the
<code>strings.Join</code> function to join the elements of
the <code>words</code> slice together, separated by spaces.
</step>
<step title="Command-line flags" src="doc/codewalk/markov.go:/Register command-line flags/,/prefixLen/">
To make it easy to tweak the prefix and generated text lengths we
use the <code><a href="/pkg/flag/">flag</a></code> package to parse
command-line flags.
<br/><br/>
These calls to <code>flag.Int</code> register new flags with the
<code>flag</code> package. The arguments to <code>Int</code> are the
flag name, its default value, and a description. The <code>Int</code>
function returns a pointer to an integer that will contain the
user-supplied value (or the default value if the flag was omitted on
the command-line).
</step>
<step title="Program set up" src="doc/codewalk/markov.go:/flag.Parse/,/rand.Seed/">
The <code>main</code> function begins by parsing the command-line
flags with <code>flag.Parse</code> and seeding the <code>rand</code>
package's random number generator with the current time.
<br/><br/>
If the command-line flags provided by the user are invalid the
<code>flag.Parse</code> function will print an informative usage
message and terminate the program.
</step>
<step title="Creating and building a new Chain" src="doc/codewalk/markov.go:/c := NewChain/,/c\.Build/">
To create the new <code>Chain</code> we call <code>NewChain</code>
with the value of the <code>prefix</code> flag.
<br/><br/>
To build the chain we call <code>Build</code> with
<code>os.Stdin</code> (which implements <code>io.Reader</code>) so
that it will read its input from standard input.
</step>
<step title="Generating and printing text" src="doc/codewalk/markov.go:/c\.Generate/,/fmt.Println/">
Finally, to generate text we call <code>Generate</code> with
the value of the <code>words</code> flag and assigning the result
to the variable <code>text</code>.
<br/><br/>
Then we call <code>fmt.Println</code> to write the text to standard
output, followed by a carriage return.
</step>
<step title="Using this program" src="doc/codewalk/markov.go">
To use this program, first build it with the
<a href="/cmd/go/">go</a> command:
<pre>
$ go build markov.go</pre>
And then execute it while piping in some input text:
<pre>
$ echo "a man a plan a canal panama" \
| ./markov -prefix=1
a plan a man a plan a canal panama</pre>
Here's a transcript of generating some text using the Go distribution's
README file as source material:
<pre>
$ ./markov -words=10 < $GOROOT/README
This is the source code repository for the Go source
$ ./markov -prefix=1 -words=10 < $GOROOT/README
This is the go directory (the one containing this README).
$ ./markov -prefix=1 -words=10 < $GOROOT/README
This is the variable if you have just untarred a</pre>
</step>
<step title="An exercise for the reader" src="doc/codewalk/markov.go">
The <code>Generate</code> function does a lot of allocations when it
builds the <code>words</code> slice. As an exercise, modify it to
take an <code>io.Writer</code> to which it incrementally writes the
generated text with <code>Fprint</code>.
Aside from being more efficient this makes <code>Generate</code>
more symmetrical to <code>Build</code>.
</step>
</codewalk>