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// This program implements a Markov chain algorithm. It generates pseudo-random
// prose based on the input text. It works by breaking each phrase into two parts:
// a multi-word prefix, and a single word suffix. It generates output by randomly
// choosing a suffix that follows the prefix.
//
// The program is based on Chapter 3 of "The Practice of Programming" by Brian Kernighan
// and Rob Pike.
package main
import (
"bufio"
"flag"
"fmt"
"io"
"math/rand"
"os"
)
const (
nPref = 2 // number of prefix words
maxGenDefault = 10000 // maximum number of words to generate
)
type Prefix struct {
words [nPref]string
}
// push advances the prefix to contain `word'.
func (p *Prefix) push(word string) {
if nPref < 1 {
return
} else if nPref == 1 {
p.words[0] = word
} else {
p.words = [nPref]string(append(p.words[1:], word))
}
}
// StateMap associates each multi-word Prefix with a list of suffix words. The starting
// state is an empty Prefix{} with the suffix list containing the first word of the
// input text. The ending state has the last few words of the input as prefix, and
// the suffix list contains an empty string.
type StateMap map[Prefix][]string
func main() {
maxGen := flag.Int("w", maxGenDefault, "maximum number of words to generate")
flag.Parse()
states := build(os.Stdin)
generate(states, *maxGen)
}
// build populates a StateMap with words read from `in'.
func build(in io.Reader) StateMap {
states := make(StateMap)
prefix := Prefix{}
scanner := bufio.NewScanner(in)
scanner.Split(bufio.ScanWords)
for scanner.Scan() {
suffix := scanner.Text()
states[prefix] = append(states[prefix], suffix)
prefix.push(suffix)
}
states[prefix] = append(states[prefix], "") // ending state
return states
}
// generate produces at most `maxWords' of output, one word per line.
func generate(states StateMap, maxWords int) {
prefix := Prefix{}
for i := 0; i < maxWords; i++ {
suffixes := states[prefix]
suffix := suffixes[rand.Intn(len(suffixes))]
if suffix == "" { // ending state
return
}
fmt.Println(suffix)
prefix.push(suffix)
}
}
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