doc/codewalk: new Markov chain codewalk
R=gri, r, rsc
CC=golang-dev
https://golang.org/cl/4891041
diff --git a/doc/codewalk/markov.go b/doc/codewalk/markov.go
new file mode 100644
index 0000000..959c2b1
--- /dev/null
+++ b/doc/codewalk/markov.go
@@ -0,0 +1,130 @@
+// 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.
+
+/*
+Generating random text: a Markov chain algorithm
+
+Based on the program presented in the "Design and Implementation" chapter
+of The Practice of Programming (Kernighan and Pike, Addison-Wesley 1999).
+See also Computer Recreations, Scientific American 260, 122 - 125 (1989).
+
+A Markov chain algorithm generates text by creating a statistical model of
+potential textual suffixes for a given prefix. Consider this text:
+
+ I am not a number! I am a free man!
+
+Our Markov chain algorithm would arrange this text into this set of prefixes
+and suffixes, or "chain": (This table assumes a prefix length of two words.)
+
+ Prefix Suffix
+
+ "" "" I
+ "" I am
+ I am a
+ I am not
+ a free man!
+ am a free
+ am not a
+ a number! I
+ number! I am
+ not a number!
+
+To generate text using this table we select an initial prefix ("I am", for
+example), choose one of the suffixes associated with that prefix at random
+with probability determined by the input statistics ("a"),
+and then create a new prefix by removing the first word from the prefix
+and appending the suffix (making the new prefix is "am a"). Repeat this process
+until we can't find any suffixes for the current prefix or we exceed the word
+limit. (The word limit is necessary as the chain table may contain cycles.)
+
+Our version of this program reads text from standard input, parsing it into a
+Markov chain, and writes generated text to standard output.
+The prefix and output lengths can be specified using the -prefix and -words
+flags on the command-line.
+*/
+package main
+
+import (
+ "bufio"
+ "flag"
+ "fmt"
+ "io"
+ "os"
+ "rand"
+ "strings"
+ "time"
+)
+
+// Prefix is a Markov chain prefix of one or more words.
+type Prefix []string
+
+// String returns the Prefix as a string (for use as a map key).
+func (p Prefix) String() string {
+ return strings.Join(p, " ")
+}
+
+// Shift removes the first word from the Prefix and appends the given word.
+func (p Prefix) Shift(word string) {
+ copy(p, p[1:])
+ p[len(p)-1] = word
+}
+
+// Chain contains a map ("chain") of prefixes to a list of suffixes.
+// A prefix is a string of prefixLen words joined with spaces.
+// A suffix is a single word. A prefix can have multiple suffixes.
+type Chain struct {
+ chain map[string][]string
+ prefixLen int
+}
+
+// NewChain returns a new Chain with prefixes of prefixLen words.
+func NewChain(prefixLen int) *Chain {
+ return &Chain{make(map[string][]string), prefixLen}
+}
+
+// Build reads text from the provided Reader and
+// parses it into prefixes and suffixes that are stored in Chain.
+func (c *Chain) Build(r io.Reader) {
+ br := bufio.NewReader(r)
+ p := make(Prefix, c.prefixLen)
+ for {
+ var s string
+ if _, err := fmt.Fscan(br, &s); err != nil {
+ break
+ }
+ key := p.String()
+ c.chain[key] = append(c.chain[key], s)
+ p.Shift(s)
+ }
+}
+
+// Generate returns a string of at most n words generated from Chain.
+func (c *Chain) Generate(n int) string {
+ p := make(Prefix, c.prefixLen)
+ var words []string
+ for i := 0; i < n; i++ {
+ choices := c.chain[p.String()]
+ if len(choices) == 0 {
+ break
+ }
+ next := choices[rand.Intn(len(choices))]
+ words = append(words, next)
+ p.Shift(next)
+ }
+ return strings.Join(words, " ")
+}
+
+func main() {
+ // Register command-line flags.
+ numWords := flag.Int("words", 100, "maximum number of words to print")
+ prefixLen := flag.Int("prefix", 2, "prefix length in words")
+
+ flag.Parse() // Parse command-line flags.
+ rand.Seed(time.Nanoseconds()) // Seed the random number generator.
+
+ c := NewChain(*prefixLen) // Initialize a new Chain.
+ c.Build(os.Stdin) // Build chains from standard input.
+ text := c.Generate(*numWords) // Generate text.
+ fmt.Println(text) // Write text to standard output.
+}
