我正在做Coursera NLP类(class)，第一个编程任务是构建一个Viterbi解码器。我想我真的很接近完成它，但是有一些难以捉摸的错误，我似乎无法追踪。这是我的代码:
http://pastie.org/private/ksmbns3gjctedu1zxrehw
http://pastie.org/private/ssv6tc8dwnamn2qegdvww
到目前为止，我已经调试了与“教学”相关的功能，因此可以说算法参数已正确估算。特别感兴趣的是viterbi()和findW()方法。我正在使用的算法的定义可以在这里找到:第18页的http://www.cs.columbia.edu/~mcollins/hmms-spring2013.pdf。
我很难解决的一件事是，当K = {1，2}(在我的情况下为0和1，因为我为零，在这些情况下，我使用的参数分别是q({TAGSET} | *，*)和q({TAGSET} | *，{TAGSET})。
提示而不是用汤匙喂食的答案也将受到高度赞赏!
最佳答案
这是Yusuke Shunyama =)http://cs.nyu.edu/yusuke/course/NLP/viterbi/Viterbi.java的viterbi解码器的简单实现
/*
* Viterbi.java
* Toy Viterbi Decorder
*
* by Yusuke Shinyama <yusuke at cs . nyu . edu>
*
* Permission to use, copy, modify, distribute this software
* for any purpose is hereby granted without fee, provided
* that the above copyright notice appear in all copies and
* that both that copyright notice and this permission notice
* appear in supporting documentation.
*/
import java.awt.*;
import java.util.*;
import java.text.*;
import java.awt.event.*;
import java.applet.*;
class Symbol {
public String name;
public Symbol(String s) {
name = s;
}
}
class SymbolTable {
Hashtable table;
public SymbolTable() {
table = new Hashtable();
}
public Symbol intern(String s) {
s = s.toLowerCase();
Object sym = table.get(s);
if (sym == null) {
sym = new Symbol(s);
table.put(s, sym);
}
return (Symbol)sym;
}
}
class SymbolList {
Vector list;
public SymbolList() {
list = new Vector();
}
public int size() {
return list.size();
}
public void set(int index, Symbol sym) {
list.setElementAt(sym, index);
}
public void add(Symbol sym) {
list.addElement(sym);
}
public Symbol get(int index) {
return (Symbol) list.elementAt(index);
}
}
class IntegerList {
Vector list;
public IntegerList() {
list = new Vector();
}
public int size() {
return list.size();
}
public void set(int index, int i) {
list.setElementAt(new Integer(i), index);
}
public void add(int i) {
list.addElement(new Integer(i));
}
public int get(int index) {
return ((Integer)list.elementAt(index)).intValue();
}
}
class ProbTable {
Hashtable table;
public ProbTable() {
table = new Hashtable();
}
public void put(Object obj, double prob) {
table.put(obj, new Double(prob));
}
public double get(Object obj) {
Double prob = (Double)table.get(obj);
if (prob == null) {
return 0.0;
}
return prob.doubleValue();
}
// normalize probability
public void normalize() {
double total = 0.0;
for(Enumeration e = table.elements() ; e.hasMoreElements() ;) {
total += ((Double)e.nextElement()).doubleValue();
}
if (total == 0.0) {
return; // div by zero!
}
for(Enumeration e = table.keys() ; e.hasMoreElements() ;) {
Object k = e.nextElement();
double prob = ((Double)table.get(k)).doubleValue();
table.put(k, new Double(prob / total));
}
}
}
class State {
public String name;
ProbTable emits;
ProbTable linksto;
public State(String s) {
name = s;
emits = new ProbTable();
linksto = new ProbTable();
}
public void normalize() {
emits.normalize();
linksto.normalize();
}
public void addSymbol(Symbol sym, double prob) {
emits.put(sym, prob);
}
public double emitprob(Symbol sym) {
return emits.get(sym);
}
public void addLink(State st, double prob) {
linksto.put(st, prob);
}
public double transprob(State st) {
return linksto.get(st);
}
}
class StateTable {
Hashtable table;
public StateTable() {
table = new Hashtable();
}
public State get(String s) {
s = s.toUpperCase();
State st = (State)table.get(s);
if (st == null) {
st = new State(s);
table.put(s, st);
}
return st;
}
}
class StateIDTable {
Hashtable table;
public StateIDTable() {
table = new Hashtable();
}
public void put(State obj, int i) {
table.put(obj, new Integer(i));
}
public int get(State obj) {
Integer i = (Integer)table.get(obj);
if (i == null) {
return 0;
}
return i.intValue();
}
}
class StateList {
Vector list;
public StateList() {
list = new Vector();
}
public int size() {
return list.size();
}
public void set(int index, State st) {
list.setElementAt(st, index);
}
public void add(State st) {
list.addElement(st);
}
public State get(int index) {
return (State) list.elementAt(index);
}
}
class HMMCanvas extends Canvas {
static final int grid_x = 60;
static final int grid_y = 40;
static final int offset_x = 70;
static final int offset_y = 30;
static final int offset_y2 = 10;
static final int offset_y3 = 65;
static final int col_x = 40;
static final int col_y = 10;
static final int state_r = 10;
static final Color state_fill = Color.white;
static final Color state_fill_maximum = Color.yellow;
static final Color state_fill_best = Color.red;
static final Color state_boundery = Color.black;
static final Color link_normal = Color.green;
static final Color link_processed = Color.blue;
static final Color link_maximum = Color.red;
HMMDecoder hmm;
public HMMCanvas() {
setBackground(Color.white);
setSize(400,300);
}
public void setHMM(HMMDecoder h) {
hmm = h;
}
private void drawState(Graphics g, int x, int y, Color c) {
x = x * grid_x + offset_x;
y = y * grid_y + offset_y;
g.setColor(c);
g.fillOval(x-state_r, y-state_r, state_r*2, state_r*2);
g.setColor(state_boundery);
g.drawOval(x-state_r, y-state_r, state_r*2, state_r*2);
}
private void drawLink(Graphics g, int x, int y0, int y1, Color c) {
int x0 = grid_x * x + offset_x;
int x1 = grid_x * (x+1) + offset_x;
y0 = y0 * grid_y + offset_y;
y1 = y1 * grid_y + offset_y;
g.setColor(c);
g.drawLine(x0, y0, x1, y1);
}
private void drawCenterString(Graphics g, String s, int x, int y) {
x = x - g.getFontMetrics().stringWidth(s)/2;
g.setColor(Color.black);
g.drawString(s, x, y+5);
}
private void drawRightString(Graphics g, String s, int x, int y) {
x = x - g.getFontMetrics().stringWidth(s);
g.setColor(Color.black);
g.drawString(s, x, y+5);
}
public void paint(Graphics g) {
if (hmm == null) {
return;
}
DecimalFormat form = new DecimalFormat("0.0000");
int nsymbols = hmm.symbols.size();
int nstates = hmm.states.size();
// complete graph.
for(int i = 0; i < nsymbols; i++) {
int offset_ymax = offset_y2+nstates*grid_y;
if (i < nsymbols-1) {
for(int y1 = 0; y1 < nstates; y1++) {
for(int y0 = 0; y0 < nstates; y0++) {
Color c = link_normal;
if (hmm.stage == i+1 && hmm.i0 == y0 && hmm.i1 == y1) {
c = link_processed;
}
if (hmm.matrix_prevstate[i+1][y1] == y0) {
c = link_maximum;
}
drawLink(g, i, y0, y1, c);
if (c == link_maximum && 0 < i) {
double transprob = hmm.states.get(y0).transprob(hmm.states.get(y1));
drawCenterString(g, form.format(transprob),
offset_x + i*grid_x + grid_x/2, offset_ymax);
offset_ymax = offset_ymax + 16;
}
}
}
}
// state circles.
for(int y = 0; y < nstates; y++) {
Color c = state_fill;
if (hmm.matrix_prevstate[i][y] != -1) {
c = state_fill_maximum;
}
if (hmm.sequence.size() == nsymbols &&
hmm.sequence.get(nsymbols-1-i) == y) {
c = state_fill_best;
}
drawState(g, i, y, c);
}
}
// max probability.
for(int i = 0; i < nsymbols; i++) {
for(int y1 = 0; y1 < nstates; y1++) {
if (hmm.matrix_prevstate[i][y1] != -1) {
drawCenterString(g, form.format(hmm.matrix_maxprob[i][y1]),
offset_x+i*grid_x, offset_y+y1*grid_y);
}
}
}
// captions (symbols atop)
for(int i = 0; i < nsymbols; i++) {
drawCenterString(g, hmm.symbols.get(i).name, offset_x+i*grid_x, col_y);
}
// captions (states in left)
for(int y = 0; y < nstates; y++) {
drawRightString(g, hmm.states.get(y).name, col_x, offset_y+y*grid_y);
}
// status bar
g.setColor(Color.black);
g.drawString(hmm.status, col_x, offset_y3+nstates*grid_y);
g.drawString(hmm.status2, col_x, offset_y3+nstates*grid_y+16);
}
}
class HMMDecoder {
StateList states;
int state_start;
int state_end;
public IntegerList sequence;
public double[][] matrix_maxprob;
public int[][] matrix_prevstate;
public SymbolList symbols;
public double probmax;
public int stage, i0, i1;
public boolean laststage;
public String status, status2;
public HMMDecoder() {
status = "Not initialized.";
status2 = "";
states = new StateList();
}
public void addStartState(State st) {
state_start = states.size(); // get current index
states.add(st);
}
public void addNormalState(State st) {
states.add(st);
}
public void addEndState(State st) {
state_end = states.size(); // get current index
states.add(st);
}
// for debugging.
public void showmatrix() {
for(int i = 0; i < symbols.size(); i++) {
for(int j = 0; j < states.size(); j++) {
System.out.print(matrix_maxprob[i][j]+" "+matrix_prevstate[i][j]+", ");
}
System.out.println();
}
}
// initialize for decoding
public void initialize(SymbolList syms) {
// symbols[syms.length] should be END
symbols = syms;
matrix_maxprob = new double[symbols.size()][states.size()];
matrix_prevstate = new int[symbols.size()][states.size()];
for(int i = 0; i < symbols.size(); i++) {
for(int j = 0; j < states.size(); j++) {
matrix_prevstate[i][j] = -1;
}
}
State start = states.get(state_start);
for(int i = 0; i < states.size(); i++) {
matrix_maxprob[0][i] = start.transprob(states.get(i));
matrix_prevstate[0][i] = 0;
}
stage = 0;
i0 = -1;
i1 = -1;
sequence = new IntegerList();
status = "Ok, let's get started...";
status2 = "";
}
// forward procedure
public boolean proceed_decoding() {
status2 = "";
// already end?
if (symbols.size() <= stage) {
return false;
}
// not started?
if (stage == 0) {
stage = 1;
i0 = 0;
i1 = 0;
matrix_maxprob[stage][i1] = 0.0;
} else {
i0++;
if (states.size() <= i0) {
// i0 should be reinitialized.
i0 = 0;
i1++;
if (states.size() <= i1) {
// i1 should be reinitialized.
// goto next stage.
stage++;
if (symbols.size() <= stage) {
// done.
status = "Decoding finished.";
return false;
}
laststage = (stage == symbols.size()-1);
i1 = 0;
}
matrix_maxprob[stage][i1] = 0.0;
}
}
// sym1: next symbol
Symbol sym1 = symbols.get(stage);
State s0 = states.get(i0);
State s1 = states.get(i1);
// precond: 1 <= stage.
double prob = matrix_maxprob[stage-1][i0];
DecimalFormat form = new DecimalFormat("0.0000");
status = "Prob:" + form.format(prob);
if (1 < stage) {
// skip first stage.
double transprob = s0.transprob(s1);
prob = prob * transprob;
status = status + " x " + form.format(transprob);
}
double emitprob = s1.emitprob(sym1);
prob = prob * emitprob;
status = status + " x " + form.format(emitprob) + "(" + s1.name+":"+sym1.name + ")";
status = status + " = " + form.format(prob);
// System.out.println("stage: "+stage+", i0:"+i0+", i1:"+i1+", prob:"+prob);
if (matrix_maxprob[stage][i1] < prob) {
matrix_maxprob[stage][i1] = prob;
matrix_prevstate[stage][i1] = i0;
status2 = "(new maximum found)";
}
return true;
}
// backward proc
public void backward() {
int probmaxstate = state_end;
sequence.add(probmaxstate);
for(int i = symbols.size()-1; 0 < i; i--) {
probmaxstate = matrix_prevstate[i][probmaxstate];
if (probmaxstate == -1) {
status2 = "Decoding failed.";
return;
}
sequence.add(probmaxstate);
//System.out.println("stage: "+i+", state:"+probmaxstate);
}
}
}
public class Viterbi extends Applet implements ActionListener, Runnable {
SymbolTable symtab;
StateTable sttab;
HMMDecoder myhmm = null;
HMMCanvas canvas;
Panel p;
TextArea hmmdesc;
TextField sentence;
Button bstart, bskip;
static final String initialHMM =
"start: go(cow,1.0)\n" +
"cow: emit(moo,0.9) emit(hello,0.1) go(cow,0.5) go(duck,0.3) go(end,0.2)\n" +
"duck: emit(quack,0.6) emit(hello,0.4) go(duck,0.5) go(cow,0.3) go(end,0.2)\n";
final int sleepmillisec = 100; // 0.1s
// setup hmm
// success:true.
boolean setupHMM(String s) {
myhmm = new HMMDecoder();
symtab = new SymbolTable();
sttab = new StateTable();
State start = sttab.get("start");
State end = sttab.get("end");
myhmm.addStartState(start);
boolean success = true;
StringTokenizer lines = new StringTokenizer(s, "\n");
while (lines.hasMoreTokens()) {
// foreach line.
String line = lines.nextToken();
int i = line.indexOf(':');
if (i == -1) break;
State st0 = sttab.get(line.substring(0,i).trim());
if (st0 != start && st0 != end) {
myhmm.addNormalState(st0);
}
//System.out.println(st0.name+":"+line.substring(i+1));
StringTokenizer tokenz = new StringTokenizer(line.substring(i+1), ", ");
while (tokenz.hasMoreTokens()) {
// foreach token.
String t = tokenz.nextToken().toLowerCase();
if (t.startsWith("go(")) {
State st1 = sttab.get(t.substring(3).trim());
// fetch another token.
if (!tokenz.hasMoreTokens()) {
success = false; // err. nomoretoken
break;
}
String n = tokenz.nextToken().replace(')', ' ');
double prob;
try {
prob = Double.valueOf(n).doubleValue();
} catch (NumberFormatException e) {
success = false; // err.
prob = 0.0;
}
st0.addLink(st1, prob);
//System.out.println("go:"+st1.name+","+prob);
} else if (t.startsWith("emit(")) {
Symbol sym = symtab.intern(t.substring(5).trim());
// fetch another token.
if (!tokenz.hasMoreTokens()) {
success = false; // err. nomoretoken
break;
}
String n = tokenz.nextToken().replace(')', ' ');
double prob;
try {
prob = Double.valueOf(n).doubleValue();
} catch (NumberFormatException e) {
success = false; // err.
prob = 0.0;
}
st0.addSymbol(sym, prob);
//System.out.println("emit:"+sym.name+","+prob);
} else {
// illegal syntax, just ignore
break;
}
}
st0.normalize(); // normalize probability
}
end.addSymbol(symtab.intern("end"), 1.0);
myhmm.addEndState(end);
return success;
}
// success:true.
boolean setup() {
if (! setupHMM(hmmdesc.getText()))
return false;
// initialize words
SymbolList words = new SymbolList();
StringTokenizer tokenz = new StringTokenizer(sentence.getText());
words.add(symtab.intern("start"));
while (tokenz.hasMoreTokens()) {
words.add(symtab.intern(tokenz.nextToken()));
}
words.add(symtab.intern("end"));
myhmm.initialize(words);
canvas.setHMM(myhmm);
return true;
}
public void init() {
canvas = new HMMCanvas();
setLayout(new BorderLayout());
p = new Panel();
sentence = new TextField("moo hello quack", 20);
bstart = new Button(" Start ");
bskip = new Button("Auto");
bstart.addActionListener(this);
bskip.addActionListener(this);
p.add(sentence);
p.add(bstart);
p.add(bskip);
hmmdesc = new TextArea(initialHMM, 4, 20);
add("North", canvas);
add("Center", p);
add("South", hmmdesc);
}
void setup_fallback() {
// adjustable
State cow = sttab.get("cow");
State duck = sttab.get("duck");
State end = sttab.get("end");
cow.addLink (cow, 0.5);
cow.addLink (duck, 0.3);
cow.addLink (end, 0.2);
duck.addLink (cow, 0.3);
duck.addLink (duck, 0.5);
duck.addLink (end, 0.2);
cow.addSymbol(symtab.intern("moo"), 0.9);
cow.addSymbol(symtab.intern("hello"), 0.1);
duck.addSymbol(symtab.intern("quack"), 0.6);
duck.addSymbol(symtab.intern("hello"), 0.4);
}
public void destroy() {
remove(p);
remove(canvas);
}
public void processEvent(AWTEvent e) {
if (e.getID() == Event.WINDOW_DESTROY) {
System.exit(0);
}
}
public void run() {
if (myhmm != null) {
while (myhmm.proceed_decoding()) {
canvas.repaint();
try {
Thread.sleep(sleepmillisec);
} catch (InterruptedException e) {
;
}
}
myhmm.backward();
canvas.repaint();
bstart.setLabel(" Start ");
bstart.setEnabled(true);
bskip.setEnabled(true);
myhmm = null;
}
}
public void actionPerformed(ActionEvent ev) {
String label = ev.getActionCommand();
if (label.equalsIgnoreCase(" start ")) {
if (!setup()) {
// error
return;
}
bstart.setLabel("Proceed");
canvas.repaint();
} else if (label.equalsIgnoreCase("proceed")) {
// next
if (! myhmm.proceed_decoding()) {
myhmm.backward();
bstart.setLabel(" Start ");
myhmm = null;
}
canvas.repaint();
} else if (label.equalsIgnoreCase("auto")) {
// skip
if (myhmm == null) {
if (!setup()) {
// error
return;
}
}
bstart.setEnabled(false);
bskip.setEnabled(false);
Thread me = new Thread(this);
me.setPriority(Thread.MIN_PRIORITY);
// start animation.
me.start();
}
}
public static void main(String args[]) {
Frame f = new Frame("Viterbi");
Viterbi v = new Viterbi();
f.add("Center", v);
f.setSize(400, 400);
f.show();
v.init();
v.start();
}
public String getAppletInfo() {
return "A Sample Viterbi Decoder Applet";
}
}
关于java - Java中的维特比算法，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/15348351/