small tweaks, added bench.sh for benchmark

posseg/bench.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+go test -run=XXX -v -bench=BenchmarkViterbi -benchtime 10s  -benchmem -memprofile viterbi-mem.out -cpuprofile viterbi-cpu.out
+go tool pprof -png -output ~/tmp/viterbi-cpu.png posseg.test viterbi-cpu.out
+go tool pprof -png -output ~/tmp/viterbi-mem.png posseg.test viterbi-mem.out

posseg/prob_trans.go

@@ -4,13 +4,15 @@ import (
 	"math"
 )
 
+var inf = math.Inf(-1)
+
 type probTransMap map[string]float64
 
 func (m probTransMap) Get(key string) float64 {
 	if value, ok := m[key]; ok {
 		return value
 	} else {
-		return math.Inf(-1)
+		return inf
 	}
 }