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>

diff --git a/shell/math.c b/shell/math.c
index af1ab55c0500fc5516cc0982cb59de7d1a929ce3..aac5017d01e46ff7438400a43685e5da109d58f6 100644 (file)
--- 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;
 }