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expr2.c
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expr2.c
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#include "defs.h"
#include "data.h"
#include "decl.h"
// Parsing of expressions with full recursive descent
// Copyright (c) 2019 Warren Toomey, GPL3
// Parse a primary factor and return an
// AST node representing it.
static struct ASTnode *primary(void) {
struct ASTnode *n;
// For an INTLIT token, make a leaf AST node for it
// and scan in the next token. Otherwise, a syntax error
// for any other token type.
switch (Token.token) {
case T_INTLIT:
n = mkastleaf(A_INTLIT, Token.intvalue);
scan(&Token);
return (n);
default:
fprintf(stderr, "syntax error on line %d, token %d\n", Line, Token.token);
exit(1);
}
}
// Convert a binary operator token into an AST operation.
static int arithop(int tok) {
switch (tok) {
case T_PLUS:
return (A_ADD);
case T_MINUS:
return (A_SUBTRACT);
case T_STAR:
return (A_MULTIPLY);
case T_SLASH:
return (A_DIVIDE);
default:
fprintf(stderr, "syntax error on line %d, token %d\n", Line, tok);
exit(1);
}
}
struct ASTnode *additive_expr(void);
// Return an AST tree whose root is a '*' or '/' binary operator
struct ASTnode *multiplicative_expr(void) {
struct ASTnode *left, *right;
int tokentype;
// Get the integer literal on the left.
// Fetch the next token at the same time.
left = primary();
// If no tokens left, return just the left node
tokentype = Token.token;
if (tokentype == T_EOF)
return (left);
// While the token is a '*' or '/'
while ((tokentype == T_STAR) || (tokentype == T_SLASH)) {
// Fetch in the next integer literal
scan(&Token);
right = primary();
// Join that with the left integer literal
left = mkastnode(arithop(tokentype), left, right, 0);
// Update the details of the current token.
// If no tokens left, return just the left node
tokentype = Token.token;
if (tokentype == T_EOF)
break;
}
// Return whatever tree we have created
return (left);
}
// Return an AST tree whose root is a '+' or '-' binary operator
struct ASTnode *additive_expr(void) {
struct ASTnode *left, *right;
int tokentype;
// Get the left sub-tree at a higher precedence than us
left = multiplicative_expr();
// If no tokens left, return just the left node
tokentype = Token.token;
if (tokentype == T_EOF)
return (left);
// Cache the '+' or '-' token type
// Loop working on token at our level of precedence
while (1) {
// Fetch in the next integer literal
scan(&Token);
// Get the right sub-tree at a higher precedence than us
right = multiplicative_expr();
// Join the two sub-trees with our low-precedence operator
left = mkastnode(arithop(tokentype), left, right, 0);
// And get the next token at our precedence
tokentype = Token.token;
if (tokentype == T_EOF)
break;
}
// Return whatever tree we have created
return (left);
}
struct ASTnode *binexpr(int n) {
return (additive_expr());
}