38f423cc9 — BusyBox

Subject

shell/math: explain the logic, small tweak to make code smaller

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Architecture & Compiler Flags

Architecture: x86_64-linux-gnu Compiler: gcc (Ubuntu 11.4.0-1ubuntu1~22.04.3) 11.4.0

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Commit Diff GitHub →

shell/math.c +25 -10

shell/math.c+25 -10

@@ -133,7 +133,10 @@ typedef unsigned char operator;
	#define tok_decl(prec,id) (((id)<<5) \| (prec))
	#define PREC(op) ((op) & 0x1F)

+	#define PREC_LPAREN 0
	#define TOK_LPAREN tok_decl(0,0)
+	/* Precedence value of RPAREN is used only to distinguish it from LPAREN */
+	#define TOK_RPAREN tok_decl(1,1)

	#define TOK_COMMA tok_decl(1,0)

@@ -223,7 +226,6 @@ typedef unsigned char operator;
	#define SPEC_PREC (UNARYPREC+4)

	#define TOK_NUM tok_decl(SPEC_PREC, 0)
-	#define TOK_RPAREN tok_decl(SPEC_PREC, 1)

	static int
	is_assign_op(operator op)
@@ -789,19 +791,32 @@ evaluate_string(arith_state_t math_state, const char expr)
	* stack until we find an operator with a lesser priority than the
	* one we have just extracted. If op is right-associative,
	* then stop "applying" on the equal priority too.
-	* Left paren is given the lowest priority so it will never be
-	* "applied" in this way.
+	* Left paren will never be "applied" in this way.
	*/
	prec = PREC(op);
-	if ((prec > 0 && prec < UNARYPREC) \|\| prec == SPEC_PREC) {
-	/* not left paren or unary */
+	if (prec != PREC_LPAREN && prec < UNARYPREC) {
+	/* binary, ternary or RPAREN */
	if (lasttok != TOK_NUM) {
-	/* binary op must be preceded by a num */
+	/* must be preceded by a num */
	goto err;
	}
-	/* The algorithm employed here is simple: while we don't
-	* hit an open paren nor the bottom of the stack, pop
-	* tokens and apply them */
+	/* if op is RPAREN:
+	* while opstack is not empty:
+	* pop prev_op
+	* if prev_op is LPAREN (finished evaluating (EXPR)):
+	* goto N
+	* evaluate prev_op on top of numstack
+	* BUG (unpaired RPAREN)
+	* else (op is not RPAREN):
+	* while opstack is not empty:
+	* pop prev_op
+	* if can't evaluate prev_op (it is lower precedence than op):
+	* push prev_op back
+	* goto P
+	* evaluate prev_op on top of numstack
+	* P: push op
+	* N: loop to parse the rest of string
+	*/
	while (opstackptr != opstack) {
	operator prev_op = *--opstackptr;
	if (op == TOK_RPAREN) {
@@ -821,7 +836,7 @@ evaluate_string(arith_state_t math_state, const char expr)
	if (prev_prec < prec
	\|\| (prev_prec == prec && is_right_associative(prec))
	) {
-	/* ...x~y@: push @ on opstack */
+	/* ...x~y@. push @ on opstack */
	opstackptr++; /* undo removal of ~ op */
	goto push_op;
	}