shell/math: fix comments about jammed-together num+num corner cases

function                                             old     new   delta
evaluate_string                                     1478    1470      -8

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 16 insertions, 8 deletions

diff --git a/shell/math.c b/shell/math.c
index e5447e767..d9986335a 100644
--- a/shell/math.c
+++ b/shell/math.c
@@ -613,7 +613,7 @@ evaluate_string(arith_state_t *math_state, const char *expr)
         const char *start_expr = expr = skip_whitespace(expr);
 
         {
-                unsigned expr_len = strlen(expr) + 2;
+                unsigned expr_len = strlen(expr);
                 /* If LOTS of whitespace, do not blow up the estimation */
                 const char *p = expr;
                 while (*p) {
@@ -625,14 +625,21 @@ evaluate_string(arith_state_t *math_state, const char *expr)
                                 p++;
                         }
                 }
-                /* There can be no more than expr_len/2
+                dbg("expr:'%s' expr_len:%u", expr, expr_len);
+                /* expr_len deep opstack is needed. Think "------------7".
+                 * Only "?" operator temporarily needs two opstack slots
+                 * (IOW: more than one slot), but its second slot (LPAREN)
+                 * is popped off when ":" is reached.
+                 */
+                opstackptr = opstack = alloca(expr_len * sizeof(opstack[0]));
+                /* There can be no more than (expr_len/2 + 1)
                  * integers/names in any given correct or incorrect expression.
-                 * (modulo "09v09v09v09v09v" case,
+                 * (modulo "09", "0v" cases where 2 chars are 2 ints/names,
                  * but we have code to detect that early)
                  */
-                dbg("expr:'%s' expr_len:%u", expr, expr_len);
-                numstackptr = numstack = alloca((expr_len / 2) * sizeof(numstack[0]));
-                opstackptr = opstack = alloca(expr_len * sizeof(opstack[0]));
+                expr_len = (expr_len / 2)
+                        + 1 /* "1+2" has two nums, 2 = len/2+1, NOT len/2 */;
+                numstackptr = numstack = alloca(expr_len * sizeof(numstack[0]));
         }
 
         /* Start with a left paren */
@@ -714,8 +721,9 @@ evaluate_string(arith_state_t *math_state, const char *expr)
                         dbg("[%d] val:%lld", (int)(numstackptr - numstack), numstackptr->val);
                         /* A number can't be followed by another number, or a variable name.
                          * We'd catch this later anyway, but this would require numstack[]
-                         * to be twice as deep to handle strings where _every_ char is
-                         * a new number or name. Example: 09v09v09v09v09v09v09v09v09v
+                         * to be ~twice as deep to handle strings where _every_ char is
+                         * a new number or name.
+                         * Examples: "09" is two numbers, "0v" is number and name.
                          */
                         if (isalnum(*expr) || *expr == '_')
                                 goto syntax_err;