markov: fixed severe bug in Shift()

modules/markov.go

@@ -129,9 +129,13 @@ func (p MarkovPrefix) String() string {
 
 // Shift removes the first word from the Prefix and appends the given word.
 func (p MarkovPrefix) Shift(word string) {
+	if len(p) < *markovPrefixLen {
+		p = append(p, word)
+	} else {
 		copy(p, p[1:])
 		p[len(p)-1] = word
 	}
+}
 
 // MarkovChain contains a map ("chain") of prefixes to a list of suffixes.
 // A prefix is a string of prefixLen words joined with spaces.