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

function                                             old     new   delta
evaluate_string                                     1258    1257      -1

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
master
Denys Vlasenko 2023-06-15 16:46:31 +02:00
parent ea6dcbe283
commit 38f423cc9c
1 changed files with 25 additions and 10 deletions
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35
shell/math.c
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@ -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;
}