shell/math: avoid $((3**999999999999999999)) to take years

function                                             old     new   delta
arith_apply                                          991    1030     +39

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

1 files changed, 14 insertions, 3 deletions

diff --git a/shell/math.c b/shell/math.c
index b06d1ae3c..e90a38f05 100644
--- a/shell/math.c
+++ b/shell/math.c
@@ -155,7 +155,7 @@ typedef unsigned char operator;
 #define PREC_ASSIGN2            3
 #define TOK_MUL_ASSIGN          tok_decl(3,0)
 /* "/" and "/=" ops have the same id bits */
-#define DIV_ID1                  1
+#define DIV_ID1                 1
 #define TOK_DIV_ASSIGN          tok_decl(3,DIV_ID1)
 #define TOK_REM_ASSIGN          tok_decl(3,2)
 
@@ -422,8 +422,19 @@ arith_apply(arith_state_t *math_state, operator op, var_or_num_t *numstack, var_
                         if (right_side_val < 0)
                                 return "exponent less than 0";
                         c = 1;
-                        while (--right_side_val >= 0)
+                        while (right_side_val != 0) {
+                                if ((right_side_val & 1) == 0) {
+                                        /* this if() block is not necessary for correctness,
+                                         * but otherwise echo $((3**999999999999999999))
+                                         * takes a VERY LONG time
+                                         * (and it's not interruptible by ^C)
+                                         */
+                                        rez *= rez;
+                                        right_side_val >>= 1;
+                                }
                                 c *= rez;
+                                right_side_val--;
+                        }
                         rez = c;
                 }
                 else /*if (op == TOK_DIV || op == TOK_DIV_ASSIGN
@@ -550,7 +561,7 @@ static arith_t parse_with_base(const char *nptr, char **endptr, unsigned base)
                         /* current char is one of :;<=>?@A..Z[\]^_`a..z */
 
                         /* in bases up to 36, case does not matter for a-z,
-                         * map A..Z and a..z to 10..35: */
+                         * map @A..Z and `a..z to 9..35: */
                         digit = (unsigned)(*nptr | 0x20) - ('a' - 10);
                         if (base > 36 && *nptr <= '_') {
                                 /* base > 36: A-Z,@,_ are 36-61,62,63 */