本篇博文是作为陈老师的编译器课程学习笔记
课程地址：
https://study.163.com/course/courseLearn.htm?courseId=1002830012#/learn/video?lessonId=1003433001&courseId=1002830012
下面，我们先给出first set 的算法流程，然后我们再以上面的语法推导为例子，走一步算法。
- 如果A是一个终结符，那么FIRST(A)={A}
- 对于以下形式的语法推导:
s -> A a
s是非终结符，A是终结符，a 是零个或多个终结符或非终结符的组合，那么 A 属于 FIRST(s).
3.对于推导表达式：
s -> b a
s 和 b 是非终结符，而且b 不是nullable的，那么first(s) = first(b)
4.对于推导表达式:
s -> a1 a2 … an b
如果a1, a2 … an 是nullable 的非终结符，b是非终结符但不是nullable的，或者b是终结符，那么first(s) 是 first(a1)… first(an) 以及first(b)的集合。
总结起来FIRST set就是：下一个合法输入的终结符的集合，也就是指在 | 后面的非终结符的头部的终结符。
public class ParseTableBuilder {
private HashMap<Integer, Symbols> symbolMap = new HashMap<Integer, Symbols>();
private ArrayList<Symbols> symbolArray = new ArrayList<Symbols>();
private boolean runFirstSetPass = true;
private boolean runFollowSetPass = true;
public ParseTableBuilder() {
initProductions();
}
private void initProductions() {
ArrayList<int[]> productions = new ArrayList<int[]>();
productions.add(new int[]{SymbolDefine.EXPR, SymbolDefine.SEMI});
Symbols stmt = new Symbols(SymbolDefine.STMT, false, productions);
symbolMap.put(SymbolDefine.STMT, stmt);
symbolArray.add(stmt);
productions = new ArrayList<int[]>();
productions.add(new int[] {SymbolDefine.TERM, SymbolDefine.EXPR_PRIME});
Symbols expr = new Symbols(SymbolDefine.EXPR, true, productions);
symbolMap.put(SymbolDefine.EXPR, expr);
symbolArray.add(expr);
productions = new ArrayList<int[]>();
productions.add(new int[] {SymbolDefine.PLUS, SymbolDefine.TERM, SymbolDefine.EXPR_PRIME});
Symbols expr_prime = new Symbols(SymbolDefine.EXPR_PRIME, true, productions);
symbolMap.put(SymbolDefine.EXPR_PRIME, expr_prime);
symbolArray.add(expr_prime);
productions = new ArrayList<int[]>();
productions.add(new int[]{SymbolDefine.FACTOR, SymbolDefine.TERM_PRIME});
Symbols term = new Symbols(SymbolDefine.TERM, false, productions);
symbolMap.put(SymbolDefine.TERM, term);
symbolArray.add(term);
productions = new ArrayList<int[]>();
productions.add(new int[] {SymbolDefine.TIMES, SymbolDefine.FACTOR, SymbolDefine.TERM_PRIME});
Symbols term_prime = new Symbols(SymbolDefine.TERM_PRIME, true, productions);
symbolMap.put(SymbolDefine.TERM_PRIME, term_prime);
symbolArray.add(term_prime);
productions = new ArrayList<int[]>();
productions.add(new int[] {SymbolDefine.LP, SymbolDefine.EXPR, SymbolDefine.RP});
productions.add(new int[] {SymbolDefine.NUM_OR_ID});
Symbols factor = new Symbols(SymbolDefine.FACTOR, false, productions);
symbolMap.put(SymbolDefine.FACTOR, factor);
symbolArray.add(factor);
Symbols semi = new Symbols(SymbolDefine.SEMI, false, null);
symbolMap.put(SymbolDefine.SEMI, semi);
symbolArray.add(semi);
Symbols plus = new Symbols(SymbolDefine.PLUS, false, null);
symbolMap.put(SymbolDefine.PLUS, plus);
symbolArray.add(plus);
Symbols times = new Symbols(SymbolDefine.TIMES, false, null);
symbolMap.put(SymbolDefine.TIMES, times);
symbolArray.add(times);
Symbols lp = new Symbols(SymbolDefine.LP, false, null);
symbolMap.put(SymbolDefine.LP, lp);
symbolArray.add(lp);
Symbols rp = new Symbols(SymbolDefine.RP, false, null);
symbolMap.put(SymbolDefine.RP, rp);
symbolArray.add(rp);
Symbols num_or_id = new Symbols(SymbolDefine.NUM_OR_ID, false, null);
symbolMap.put(SymbolDefine.NUM_OR_ID, num_or_id);
symbolArray.add(num_or_id);
}
public void runFirstSets() {
while (runFirstSetPass) {
runFirstSetPass = false;
Iterator<Symbols> it = symbolArray.iterator();
while (it.hasNext()) {
Symbols symbol = it.next();
addSymbolFirstSet(symbol);
}
printAllFirstSet();
System.out.println("============");
}
}
private void addSymbolFirstSet(Symbols symbol) {
if (isSymbolTerminals(symbol.value) == true) {
return;
}
for (int i = 0; i < symbol.productions.size(); i++) {
int[] rightSize = symbol.productions.get(i);
if (isSymbolTerminals(rightSize[0]) && symbol.firstSet.contains(rightSize[0]) == false) {
runFirstSetPass = true;
symbol.firstSet.add(rightSize[0]);
}
else if (isSymbolTerminals(rightSize[0]) == false) {
//add first set of nullable
int pos = 0;
Symbols curSymbol = null;
do {
curSymbol = symbolMap.get(rightSize[pos]);
if (symbol.firstSet.containsAll(curSymbol.firstSet) == false) {
runFirstSetPass = true;
for (int j = 0; j < curSymbol.firstSet.size(); j++) {
if (symbol.firstSet.contains(curSymbol.firstSet.get(j)) == false) {
symbol.firstSet.add(curSymbol.firstSet.get(j));
}
}//for (int j = 0; j < curSymbol.firstSet.size(); j++)
}//if (symbol.firstSet.containsAll(curSymbol.firstSet) == false)
pos++;
}while(pos < rightSize.length && curSymbol.isNullable);
} // else
}//for (int i = 0; i < symbol.productions.size(); i++)
}
private boolean isSymbolTerminals(int symbol) {
return symbol < 256;
}
}