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309 lines
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309 lines
13 KiB
XML
<!--
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Copyright 2011 The Go Authors. All rights reserved.
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Use of this source code is governed by a BSD-style
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license that can be found in the LICENSE file.
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-->
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<codewalk title="Generating arbitrary text: a Markov chain algorithm">
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<step title="Introduction" src="doc/codewalk/markov.go:/Generating/,/line\./">
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This codewalk describes a program that generates random text using
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a Markov chain algorithm. The package comment describes the algorithm
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and the operation of the program. Please read it before continuing.
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</step>
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<step title="Modeling Markov chains" src="doc/codewalk/markov.go:/ chain/">
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A chain consists of a prefix and a suffix. Each prefix is a set
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number of words, while a suffix is a single word.
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A prefix can have an arbitrary number of suffixes.
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To model this data, we use a <code>map[string][]string</code>.
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Each map key is a prefix (a <code>string</code>) and its values are
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lists of suffixes (a slice of strings, <code>[]string</code>).
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<br/><br/>
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Here is the example table from the package comment
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as modeled by this data structure:
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<pre>
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map[string][]string{
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" ": {"I"},
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" I": {"am"},
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"I am": {"a", "not"},
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"a free": {"man!"},
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"am a": {"free"},
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"am not": {"a"},
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"a number!": {"I"},
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"number! I": {"am"},
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"not a": {"number!"},
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}</pre>
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While each prefix consists of multiple words, we
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store prefixes in the map as a single <code>string</code>.
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It would seem more natural to store the prefix as a
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<code>[]string</code>, but we can't do this with a map because the
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key type of a map must implement equality (and slices do not).
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<br/><br/>
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Therefore, in most of our code we will model prefixes as a
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<code>[]string</code> and join the strings together with a space
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to generate the map key:
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<pre>
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Prefix Map key
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[]string{"", ""} " "
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[]string{"", "I"} " I"
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[]string{"I", "am"} "I am"
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</pre>
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</step>
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<step title="The Chain struct" src="doc/codewalk/markov.go:/type Chain/,/}/">
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The complete state of the chain table consists of the table itself and
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the word length of the prefixes. The <code>Chain</code> struct stores
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this data.
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</step>
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<step title="The NewChain constructor function" src="doc/codewalk/markov.go:/func New/,/}/">
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The <code>Chain</code> struct has two unexported fields (those that
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do not begin with an upper case character), and so we write a
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<code>NewChain</code> constructor function that initializes the
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<code>chain</code> map with <code>make</code> and sets the
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<code>prefixLen</code> field.
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<br/><br/>
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This is constructor function is not strictly necessary as this entire
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program is within a single package (<code>main</code>) and therefore
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there is little practical difference between exported and unexported
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fields. We could just as easily write out the contents of this function
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when we want to construct a new Chain.
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But using these unexported fields is good practice; it clearly denotes
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that only methods of Chain and its constructor function should access
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those fields. Also, structuring <code>Chain</code> like this means we
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could easily move it into its own package at some later date.
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</step>
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<step title="The Prefix type" src="doc/codewalk/markov.go:/type Prefix/">
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Since we'll be working with prefixes often, we define a
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<code>Prefix</code> type with the concrete type <code>[]string</code>.
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Defining a named type clearly allows us to be explicit when we are
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working with a prefix instead of just a <code>[]string</code>.
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Also, in Go we can define methods on any named type (not just structs),
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so we can add methods that operate on <code>Prefix</code> if we need to.
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</step>
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<step title="The String method" src="doc/codewalk/markov.go:/func[^\n]+String/,/}/">
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The first method we define on <code>Prefix</code> is
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<code>String</code>. It returns a <code>string</code> representation
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of a <code>Prefix</code> by joining the slice elements together with
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spaces. We will use this method to generate keys when working with
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the chain map.
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</step>
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<step title="Building the chain" src="doc/codewalk/markov.go:/func[^\n]+Build/,/\n}/">
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The <code>Build</code> method reads text from an <code>io.Reader</code>
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and parses it into prefixes and suffixes that are stored in the
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<code>Chain</code>.
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<br/><br/>
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The <code><a href="/pkg/io/#Reader">io.Reader</a></code> is an
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interface type that is widely used by the standard library and
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other Go code. Our code uses the
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<code><a href="/pkg/fmt/#Fscan">fmt.Fscan</a></code> function, which
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reads space-separated values from an <code>io.Reader</code>.
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<br/><br/>
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The <code>Build</code> method returns once the <code>Reader</code>'s
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<code>Read</code> method returns <code>io.EOF</code> (end of file)
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or some other read error occurs.
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</step>
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<step title="Buffering the input" src="doc/codewalk/markov.go:/bufio\.NewReader/">
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This function does many small reads, which can be inefficient for some
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<code>Readers</code>. For efficiency we wrap the provided
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<code>io.Reader</code> with
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<code><a href="/pkg/bufio/">bufio.NewReader</a></code> to create a
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new <code>io.Reader</code> that provides buffering.
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</step>
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<step title="The Prefix variable" src="doc/codewalk/markov.go:/make\(Prefix/">
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At the top of the function we make a <code>Prefix</code> slice
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<code>p</code> using the <code>Chain</code>'s <code>prefixLen</code>
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field as its length.
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We'll use this variable to hold the current prefix and mutate it with
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each new word we encounter.
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</step>
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<step title="Scanning words" src="doc/codewalk/markov.go:/var s string/,/\n }/">
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In our loop we read words from the <code>Reader</code> into a
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<code>string</code> variable <code>s</code> using
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<code>fmt.Fscan</code>. Since <code>Fscan</code> uses space to
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separate each input value, each call will yield just one word
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(including punctuation), which is exactly what we need.
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<br/><br/>
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<code>Fscan</code> returns an error if it encounters a read error
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(<code>io.EOF</code>, for example) or if it can't scan the requested
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value (in our case, a single string). In either case we just want to
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stop scanning, so we <code>break</code> out of the loop.
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</step>
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<step title="Adding a prefix and suffix to the chain" src="doc/codewalk/markov.go:/ key/,/key\], s\)">
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The word stored in <code>s</code> is a new suffix. We add the new
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prefix/suffix combination to the <code>chain</code> map by computing
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the map key with <code>p.String</code> and appending the suffix
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to the slice stored under that key.
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<br/><br/>
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The built-in <code>append</code> function appends elements to a slice
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and allocates new storage when necessary. When the provided slice is
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<code>nil</code>, <code>append</code> allocates a new slice.
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This behavior conveniently ties in with the semantics of our map:
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retrieving an unset key returns the zero value of the value type and
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the zero value of <code>[]string</code> is <code>nil</code>.
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When our program encounters a new prefix (yielding a <code>nil</code>
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value in the map) <code>append</code> will allocate a new slice.
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<br/><br/>
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For more information about the <code>append</code> function and slices
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in general see the
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<a href="http://blog.golang.org/2011/01/go-slices-usage-and-internals.html">Slices: usage and internals</a> article.
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</step>
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<step title="Pushing the suffix onto the prefix" src="doc/codewalk/markov.go:/p\.Shift/">
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Before reading the next word our algorithm requires us to drop the
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first word from the prefix and push the current suffix onto the prefix.
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<br/><br/>
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When in this state
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<pre>
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p == Prefix{"I", "am"}
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s == "not" </pre>
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the new value for <code>p</code> would be
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<pre>
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p == Prefix{"am", "not"}</pre>
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This operation is also required during text generation so we put
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the code to perform this mutation of the slice inside a method on
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<code>Prefix</code> named <code>Shift</code>.
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</step>
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<step title="The Shift method" src="doc/codewalk/markov.go:/func[^\n]+Shift/,/\n}/">
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The <code>Shift</code> method uses the built-in <code>copy</code>
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function to copy the last len(p)-1 elements of <code>p</code> to
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the start of the slice, effectively moving the elements
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one index to the left (if you consider zero as the leftmost index).
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<pre>
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p := Prefix{"I", "am"}
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copy(p, p[:1])
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// p == Prefix{"am", "am"}</pre>
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We then assign the provided <code>word</code> to the last index
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of the slice:
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<pre>
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// suffix == "not"
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p[len(p)-1] = suffix
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// p == Prefix{"am", "not"}</pre>
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</step>
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<step title="Generating text" src="doc/codewalk/markov.go:/func[^\n]+Generate/,/\n}/">
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The <code>Generate</code> method is similar to <code>Build</code>
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except that instead of reading words from a <code>Reader</code>
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and storing them in a map, it reads words from the map and
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appends them to a slice (<code>words</code>).
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<br/><br/>
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<code>Generate</code> uses a conditional for loop to generate
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up to <code>n</code> words.
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</step>
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<step title="Getting potential suffixes" src="doc/codewalk/markov.go:/choices/,/}\n/">
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At each iteration of the loop we retrieve a list of potential suffixes
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for the current prefix. We access the <code>chain</code> map at key
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<code>p.String()</code> and assign its contents to <code>choices</code>.
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<br/><br/>
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If <code>len(choices)</code> is zero we break out of the loop as there
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are no potential suffixes for that prefix.
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This test also works if the key isn't present in the map at all:
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in that case, <code>choices</code> will be <code>nil</code> and the
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length of a <code>nil</code> slice is zero.
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</step>
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<step title="Choosing a suffix at random" src="doc/codewalk/markov.go:/next := choices/,/Shift/">
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To choose a suffix we use the
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<code><a href="/pkg/rand/#Intn">rand.Intn</a></code> function.
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It returns a random integer up to (but not including) the provided
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value. Passing in <code>len(choices)</code> gives us a random index
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into the full length of the list.
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<br/><br/>
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We use that index to pick our new suffix, assign it to
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<code>next</code> and append it to the <code>words</code> slice.
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<br/><br/>
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Next, we <code>Shift</code> the new suffix onto the prefix just as
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we did in the <code>Build</code> method.
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</step>
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<step title="Returning the generated text" src="doc/codewalk/markov.go:/Join\(words/">
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Before returning the generated text as a string, we use the
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<code>strings.Join</code> function to join the elements of
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the <code>words</code> slice together, separated by spaces.
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</step>
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<step title="Command-line flags" src="doc/codewalk/markov.go:/Register command-line flags/,/prefixLen/">
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To make it easy to tweak the prefix and generated text lengths we
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use the <code><a href="/pkg/flag/">flag</a></code> package to parse
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command-line flags.
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<br/><br/>
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These calls to <code>flag.Int</code> register new flags with the
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<code>flag</code> package. The arguments to <code>Int</code> are the
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flag name, its default value, and a description. The <code>Int</code>
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function returns a pointer to an integer that will contain the
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user-supplied value (or the default value if the flag was omitted on
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the command-line).
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</step>
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<step title="Program set up" src="doc/codewalk/markov.go:/flag.Parse/,/rand.Seed/">
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The <code>main</code> function begins by parsing the command-line
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flags with <code>flag.Parse</code> and seeding the <code>rand</code>
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package's random number generator with the current time.
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<br/><br/>
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If the command-line flags provided by the user are invalid the
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<code>flag.Parse</code> function will print an informative usage
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message and terminate the program.
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</step>
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<step title="Creating and building a new Chain" src="doc/codewalk/markov.go:/c := NewChain/,/c\.Build/">
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To create the new <code>Chain</code> we call <code>NewChain</code>
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with the value of the <code>prefix</code> flag.
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<br/><br/>
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To build the chain we call <code>Build</code> with
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<code>os.Stdin</code> (which implements <code>io.Reader</code>) so
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that it will read its input from standard input.
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</step>
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<step title="Generating and printing text" src="doc/codewalk/markov.go:/c\.Generate/,/fmt.Println/">
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Finally, to generate text we call <code>Generate</code> with
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the value of the <code>words</code> flag and assigning the result
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to the variable <code>text</code>.
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<br/><br/>
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Then we call <code>fmt.Println</code> to write the text to standard
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output, followed by a carriage return.
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</step>
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<step title="Using this program" src="doc/codewalk/markov.go">
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To use this program, first compile and link it.
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If you are using <code>6g</code> as your compiler, the command
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would look something like this:
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<pre>
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$ 6g markov.go && 6l -o markov markov.6</pre>
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And then execute it while piping in some input text:
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<pre>
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$ echo "a man a plan a canal panama" | ./markov -prefix=1
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a plan a man a plan a canal panama
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</pre>
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Here's a transcript of generating some text using the Go distribution's
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README file as source material:
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<pre>
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$ ./markov -words=10 < $GOROOT/go/README
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This is the source code repository for the Go source
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$ ./markov -prefix=1 -words=10 < $GOROOT/go/README
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This is the go directory (the one containing this README).
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$ ./markov -prefix=1 -words=10 < $GOROOT/go/README
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This is the variable if you have just untarred a</pre>
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</step>
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<step title="An exercise for the reader" src="doc/codewalk/markov.go">
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The <code>Generate</code> function does a lot of allocations when it
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builds the <code>words</code> slice. As an exercise, modify it to
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take an <code>io.Writer</code> to which it incrementally writes the
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generated text with <code>Fprint</code>.
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Aside from being more efficient this makes <code>Generate</code>
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more symmetrical to <code>Build</code>.
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</step>
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</codewalk>
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