shell: better comments in BASE#nn code

function                                             old     new   delta
evaluate_string                                      932     930      -2

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

1 files changed, 14 insertions, 10 deletions

diff --git a/shell/math.c b/shell/math.c
index af1ab55c0..aac5017d0 100644
--- a/shell/math.c
+++ b/shell/math.c
@@ -537,25 +537,23 @@ static arith_t strto_arith_t(const char *nptr, char **endptr)
         base = (unsigned)n;
         n = 0;
         nptr = *endptr + 1;
-        /* bash allows "N#" (empty "nnnn" part) */
         for (;;) {
                 unsigned digit = (unsigned)*nptr - '0';
-                if (digit >= 10) {
-                        /* *nptr is not 0..9 */
-                        if (*nptr > 'z')
-                                break; /* this rejects e.g. $((64#~)) */
+                if (digit >= 10 /* not 0..9 */
+                 && digit <= 'z' - '0' /* needed to reject e.g. $((64#~)) */
+                ) {
                         /* in bases up to 36, case does not matter for a-z */
                         digit = (unsigned)(*nptr | 0x20) - ('a' - 10);
                         if (base > 36 && *nptr <= '_') {
-                                /* otherwise, A-Z,@,_ are 36..61,62,63 */
-                                if (*nptr == '@')
-                                        digit = 62;
-                                else if (*nptr == '_')
+                                /* otherwise, A-Z,@,_ are 36-61,62,63 */
+                                if (*nptr == '_')
                                         digit = 63;
+                                else if (*nptr == '@')
+                                        digit = 62;
                                 else if (digit < 36) /* A-Z */
                                         digit += 36 - 10;
                                 else
-                                        break; /* error, such as [ or \ */
+                                        break; /* error: one of [\]^ */
                         }
                         //bb_error_msg("ch:'%c'%d digit:%u", *nptr, *nptr, digit);
                         //if (digit < 10) - example where we need this?
@@ -567,6 +565,12 @@ static arith_t strto_arith_t(const char *nptr, char **endptr)
                 n = n * base + digit;
                 nptr++;
         }
+        /* Note: we do not set errno on bad chars, we just set a pointer
+         * to the first invalid char. For example, this allows
+         * "N#" (empty "nnnn" part): 64#+1 is a valid expression,
+         * it means 64# + 1, whereas 64#~... is not, since ~ is not a valid
+         * operator.
+         */
         *endptr = (char*)nptr;
         return n;
 }