项目的完整代码在 C2j-Compiler
前言
在上一篇完成了符号表的构建，下一步就是输出抽象语法树(Abstract Syntax Tree，AST)
抽象语法树（abstract syntax tree 或者缩写为 AST），是源代码的抽象语法结构的树状表现形式，这里特指编程语言的源代码。树上的每个节点都表示源代码中的一种结构。
AST对于编译器是至关重要的，现在的编译型语言一般通过AST来生成IR，解释型语言也可以不用虚拟机而直接遍历AST来解释执行，之后要写解释器和编译器都依赖这个AST
这一篇主要文件有：
AstBuilder.javaAstNode.javaAstNodeImpl.javaNodeKey.javaNodeFactory.java
主要数据结构
AST节点的表示
public interface AstNode {AstNode addChild(AstNode node);AstNode getParent();ArrayList<AstNode> getChildren();void setAttribute(NodeKey key, Object value);Object getAttribute(NodeKey key);boolean isChildrenReverse();void reverseChildren();AstNode copy();}
这是对AstNode接口的实现，并且继承HashMap，这里的NodeKey是
TokenType, VALUE, SYMBOL, PRODUCTION, TEXT
对应的value，
TokenType就是非终结符的类型Text用来存储解析对象的文本信息Symbol对应的就是变量的符号对象Value是对应对象解析的值，比如int a = 1，那么value的值就为1
public class AstNodeImpl extends HashMap<NodeKey, Object> implements AstNode {private Token type;private AstNodeImpl parent;private ArrayList<AstNode> children;String name;private boolean isChildrenReverse = false;public AstNodeImpl(Token type) {this.type = type;this.parent = null;this.children = new ArrayList<>();setAttribute(NodeKey.TokenType, type);}@Overridepublic AstNode addChild(AstNode node) {if (node != null) {children.add(node);((AstNodeImpl) node).parent = this;}return node;}@Overridepublic AstNode getParent() {return parent;}@Overridepublic void reverseChildren() {if (isChildrenReverse) {return;}Collections.reverse(children);isChildrenReverse = true;}@Overridepublic boolean isChildrenReverse() {return isChildrenReverse;}@Overridepublic ArrayList<AstNode> getChildren() {reverseChildren();return children;}@Overridepublic void setAttribute(NodeKey key, Object value) {if (key == NodeKey.TEXT) {name = (String) value;}put(key, value);}@Overridepublic Object getAttribute(NodeKey key) {return get(key);}@Overridepublic String toString() {String info = "";if (get(NodeKey.VALUE) != null) {info += "Node Value is " + get(NodeKey.VALUE).toString();}if (get(NodeKey.TEXT) != null) {info += "\nNode Text is " + get(NodeKey.TEXT).toString();}if (get(NodeKey.SYMBOL) != null) {info += "\nNode Symbol is " + get(NodeKey.SYMBOL).toString();}return info + "\n Node Type is " + type.toString();}@Overridepublic AstNode copy() {AstNodeImpl copy = (AstNodeImpl) NodeFactory.createICodeNode(type);Set<Entry<NodeKey, Object>> attributes = entrySet();Iterator<Map.Entry<NodeKey, Object>> it = attributes.iterator();while (it.hasNext()) {Map.Entry<NodeKey, Object> attribute = it.next();copy.put(attribute.getKey(), attribute.getValue());}return copy;}}
NodeFactory
NodeFactory就是简单的返回一个节点的实现
public class NodeFactory {public static AstNode createICodeNode(Token type) {return new AstNodeImpl(type);}}
构造AST
AST的创建也是需要在语法分析过程中根据reduce操作进行操作的。也就是在takeActionForReduce方法中调用AstBuilder的buildSyntaxTree方法
在AstBuilder里面还是需要两个堆栈来辅助操作
private Stack<AstNode> nodeStack = new Stack<>();private LRStateTableParser parser = null;private TypeSystem typeSystem = null;private Stack<Object> valueStack = null;private String functionName;private HashMap<String, AstNode> funcMap = new HashMap<>();private static AstBuilder instance;
构造AST的主要逻辑在buildSyntaxTree方法里，需要注意的是有一些节点在解释执行和代码生成的时候是不一样的，有时代码生成需要的节点解释执行的话并不需要
在这里提一下UNARY这个非终结符，这个非终结符和NAME很像，但是它一般是代表进行运算和一些操作的时候，比如数组，++，--或者函数调用的时候
其实构建AST的过程和符号表的构建过程有点儿类似，都是根据reduce操作来创建信息和组合信息，符号表是组合修饰符说明符等，而AST则是组合节点间的关系变成一棵树
我们只看几个操作
Specifiers_DeclList_Semi_TO_Def
这个节点需要注意的是，从堆栈的什么地方拿到Symbol，这个需要从reduce次数和推导式中得出* DEF -> SPECIFIERS DECL_LIST SEMI* DECL -> VAR_DECL* VAR_DECL -> NEW_NAME* | VAR_DECL LP RP* | VAR_DECL LP VAR_LIST RP* | LP VAR_DECL RP* | START VAR_DECL
从推导式可以看出，DEF节点的符号应该在valueStack.size() - 3，但是DECL和VAR_DECL没有做reduce操作，所以符号应该在valueStack.size() - 2。这其实和前面的符号表构建算出之前符号的位置是一样的。
TO_UNARY
这里则是变量、数字或者字符串的节点，如果是个变量的号，这个节点就需要一个Symbol的value了
case SyntaxProductionInit.Number_TO_Unary:case SyntaxProductionInit.Name_TO_Unary:case SyntaxProductionInit.String_TO_Unary:node = NodeFactory.createICodeNode(Token.UNARY);if (production == SyntaxProductionInit.Name_TO_Unary) {assignSymbolToNode(node, text);}node.setAttribute(NodeKey.TEXT, text);break;
其余的节点无非是把一些语句拆分它的逻辑然后组成节点，真正的求值部分像Name_TO_Unary比较少，更多是比如把一个if else块分成if节点、判断节点、else节点，之后再按照这棵树进行解释执行或者代码生成
public AstNode buildSyntaxTree(int production, String text) {AstNode node = null;Symbol symbol = null;AstNode child = null;if (Start.STARTTYPE == Start.INTERPRETER) {int p1 = SyntaxProductionInit.Specifiers_DeclList_Semi_TO_Def;int p2 = SyntaxProductionInit.Def_To_DefList;int p3 = SyntaxProductionInit.DefList_Def_TO_DefList;boolean isReturn = production == p1 || production == p2 || production == p3;if (isReturn) {return null;}}switch (production) {case SyntaxProductionInit.Specifiers_DeclList_Semi_TO_Def:node = NodeFactory.createICodeNode(Token.DEF);symbol = (Symbol) valueStack.get(valueStack.size() - 2);node.setAttribute(NodeKey.SYMBOL, symbol);break;case SyntaxProductionInit.Def_To_DefList:node = NodeFactory.createICodeNode(Token.DEF_LIST);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.DefList_Def_TO_DefList:node = NodeFactory.createICodeNode(Token.DEF_LIST);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Number_TO_Unary:case SyntaxProductionInit.Name_TO_Unary:case SyntaxProductionInit.String_TO_Unary:node = NodeFactory.createICodeNode(Token.UNARY);if (production == SyntaxProductionInit.Name_TO_Unary) {assignSymbolToNode(node, text);}node.setAttribute(NodeKey.TEXT, text);break;case SyntaxProductionInit.Unary_LP_RP_TO_Unary:node = NodeFactory.createICodeNode(Token.UNARY);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Unary_LP_ARGS_RP_TO_Unary:node = NodeFactory.createICodeNode(Token.UNARY);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Unary_Incop_TO_Unary:case SyntaxProductionInit.Unary_DecOp_TO_Unary:case SyntaxProductionInit.LP_Expr_RP_TO_Unary:case SyntaxProductionInit.Start_Unary_TO_Unary:node = NodeFactory.createICodeNode(Token.UNARY);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Unary_LB_Expr_RB_TO_Unary:node = NodeFactory.createICodeNode(Token.UNARY);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Uanry_TO_Binary:node = NodeFactory.createICodeNode(Token.BINARY);child = nodeStack.pop();node.setAttribute(NodeKey.TEXT, child.getAttribute(NodeKey.TEXT));node.addChild(child);break;case SyntaxProductionInit.Binary_TO_NoCommaExpr:case SyntaxProductionInit.NoCommaExpr_Equal_NoCommaExpr_TO_NoCommaExpr:node = NodeFactory.createICodeNode(Token.NO_COMMA_EXPR);child = nodeStack.pop();String t = (String) child.getAttribute(NodeKey.TEXT);node.addChild(child);if (production == SyntaxProductionInit.NoCommaExpr_Equal_NoCommaExpr_TO_NoCommaExpr) {child = nodeStack.pop();t = (String) child.getAttribute(NodeKey.TEXT);node.addChild(child);}break;case SyntaxProductionInit.Binary_Plus_Binary_TO_Binary:case SyntaxProductionInit.Binary_DivOp_Binary_TO_Binary:case SyntaxProductionInit.Binary_Minus_Binary_TO_Binary:case SyntaxProductionInit.Binary_Start_Binary_TO_Binary:node = NodeFactory.createICodeNode(Token.BINARY);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Binary_RelOP_Binary_TO_Binray:node = NodeFactory.createICodeNode(Token.BINARY);node.addChild(nodeStack.pop());AstNode operator = NodeFactory.createICodeNode(Token.RELOP);operator.setAttribute(NodeKey.TEXT, parser.getRelOperatorText());node.addChild(operator);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.NoCommaExpr_TO_Expr:node = NodeFactory.createICodeNode(Token.EXPR);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Expr_Semi_TO_Statement:case pountStmt_TO_Statement:node = NodeFactory.createICodeNode(Token.STATEMENT);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.LocalDefs_TO_Statement:node = NodeFactory.createICodeNode(Token.STATEMENT);if (Start.STARTTYPE == Start.CODEGEN) {node.addChild(nodeStack.pop());}break;case SyntaxProductionInit.Statement_TO_StmtList:node = NodeFactory.createICodeNode(Token.STMT_LIST);if (nodeStack.size() > 0) {node.addChild(nodeStack.pop());}break;case SyntaxProductionInit.FOR_OptExpr_Test_EndOptExpr_Statement_TO_Statement:node = NodeFactory.createICodeNode(Token.STATEMENT);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());break;case SyntaxProductionInit.StmtList_Statement_TO_StmtList:node = NodeFactory.createICodeNode(Token.STMT_LIST);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Expr_TO_Test:node = NodeFactory.createICodeNode(Token.TEST);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.If_Test_Statement_TO_IFStatement:node = NodeFactory.createICodeNode(Token.IF_STATEMENT);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());break;case SyntaxProductionInit.IfElseStatemnt_Else_Statemenet_TO_IfElseStatement:node = NodeFactory.createICodeNode(Token.IF_ELSE_STATEMENT);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());break;case SyntaxProductionInit.While_LP_Test_Rp_TO_Statement:case SyntaxProductionInit.Do_Statement_While_Test_To_Statement:node = NodeFactory.createICodeNode(Token.STATEMENT);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Expr_Semi_TO_OptExpr:case SyntaxProductionInit.Semi_TO_OptExpr:node = NodeFactory.createICodeNode(Token.OPT_EXPR);if (production == SyntaxProductionInit.Expr_Semi_TO_OptExpr) {node.addChild(nodeStack.pop());}break;case SyntaxProductionInit.Expr_TO_EndOpt:node = NodeFactory.createICodeNode(Token.END_OPT_EXPR);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.LocalDefs_StmtList_TO_CompoundStmt:node = NodeFactory.createICodeNode(POUND_STMT);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.NewName_LP_RP_TO_FunctDecl:case SyntaxProductionInit.NewName_LP_VarList_RP_TO_FunctDecl:node = NodeFactory.createICodeNode(Token.FUNCT_DECL);node.addChild(nodeStack.pop());child = node.getChildren().get(0);functionName = (String) child.getAttribute(NodeKey.TEXT);symbol = assignSymbolToNode(node, functionName);break;case SyntaxProductionInit.NewName_TO_VarDecl:nodeStack.pop();break;case SyntaxProductionInit.NAME_TO_NewName:node = NodeFactory.createICodeNode(Token.NEW_NAME);node.setAttribute(NodeKey.TEXT, text);break;case SyntaxProductionInit.OptSpecifiers_FunctDecl_CompoundStmt_TO_ExtDef:node = NodeFactory.createICodeNode(Token.EXT_DEF);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());funcMap.put(functionName, node);break;case SyntaxProductionInit.NoCommaExpr_TO_Args:node = NodeFactory.createICodeNode(Token.ARGS);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.NoCommaExpr_Comma_Args_TO_Args:node = NodeFactory.createICodeNode(Token.ARGS);node.addChild(nodeStack.pop());node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Return_Semi_TO_Statement:node = NodeFactory.createICodeNode(Token.STATEMENT);break;case SyntaxProductionInit.Return_Expr_Semi_TO_Statement:node = NodeFactory.createICodeNode(Token.STATEMENT);node.addChild(nodeStack.pop());break;case SyntaxProductionInit.Unary_StructOP_Name_TO_Unary:node = NodeFactory.createICodeNode(Token.UNARY);node.addChild(nodeStack.pop());node.setAttribute(NodeKey.TEXT, text);break;default:break;}if (node != null) {node.setAttribute(NodeKey.PRODUCTION, production);nodeStack.push(node);}return node;}
小结
其实构造AST和创建符号表上非常相似，都是依据reduce操作的信息来完成。在AST的构建中的主要任务就是对源代码语句里的逻辑进行分块，比如对于一个ifelse语句：
上面的图是我依据这个意思话的，和上面构造出来的AST不完全一致
另外我的github博客：/
