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diff --git a/go/golang/go/doc/codewalk/markov.xml b/go/golang/go/doc/codewalk/markov.xml new file mode 100644 index 00000000..7e44840d --- /dev/null +++ b/go/golang/go/doc/codewalk/markov.xml @@ -0,0 +1,307 @@ +<!-- +Copyright 2011 The Go Authors. All rights reserved. +Use of this source code is governed by a BSD-style +license that can be found in the LICENSE file. +--> + +<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> |