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-// 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"
- "math/rand"
- "os"
- "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.Now().UnixNano()) // 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.
-}