libbb: shrink base64 decoding a bit

function                                             old     new   delta
bb_uuenc_tbl_base64                                   67      66      -1
decode_base64                                        182     161     -21

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

1 files changed, 23 insertions, 19 deletions

diff --git a/libbb/uuencode.c b/libbb/uuencode.c
index 46ca79654..f7b248492 100644
--- a/libbb/uuencode.c
+++ b/libbb/uuencode.c
@@ -10,7 +10,7 @@
 #include "libbb.h"
 
 /* Conversion table.  for base 64 */
-const char bb_uuenc_tbl_base64[65 + 2] ALIGN1 = {
+const char bb_uuenc_tbl_base64[65 + 1] ALIGN1 = {
         'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H',
         'I', 'J', 'K', 'L', 'M', 'N', 'O', 'P',
         'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X',
@@ -20,7 +20,7 @@ const char bb_uuenc_tbl_base64[65 + 2] ALIGN1 = {
         'w', 'x', 'y', 'z', '0', '1', '2', '3',
         '4', '5', '6', '7', '8', '9', '+', '/',
         '=' /* termination character */,
-        '\n', '\0' /* needed for uudecode.c */
+        '\0' /* needed for uudecode.c only */
 };
 
 const char bb_uuenc_tbl_std[65] ALIGN1 = {
@@ -77,7 +77,7 @@ void FAST_FUNC bb_uuencode(char *p, const void *src, int length, const char *tbl
  *
  * Returns: pointer to the undecoded part of source.
  * If points to '\0', then the source was fully decoded.
- * (*dst): advanced past the last written byte.
+ * (*pp_dst): advanced past the last written byte.
  */
 const char* FAST_FUNC decode_base64(char **pp_dst, const char *src)
 {
@@ -85,10 +85,10 @@ const char* FAST_FUNC decode_base64(char **pp_dst, const char *src)
         const char *src_tail;
 
         while (1) {
-                unsigned char translated[4];
+                unsigned char six_bit[4];
                 int count = 0;
 
-                /* Process one group of 4 chars */
+                /* Fetch up to four 6-bit values */
                 src_tail = src;
                 while (count < 4) {
                         char *table_ptr;
@@ -97,8 +97,8 @@ const char* FAST_FUNC decode_base64(char **pp_dst, const char *src)
                         /* Get next _valid_ character.
                          * bb_uuenc_tbl_base64[] contains this string:
                          *  0         1         2         3         4         5         6
-                         *  012345678901234567890123456789012345678901234567890123456789012345
-                         * "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=\n"
+                         *  01234567890123456789012345678901234567890123456789012345678901234
+                         * "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="
                          */
                         do {
                                 ch = *src;
@@ -117,33 +117,37 @@ const char* FAST_FUNC decode_base64(char **pp_dst, const char *src)
                                 src++;
                                 table_ptr = strchr(bb_uuenc_tbl_base64, ch);
 //TODO: add BASE64_FLAG_foo to die on bad char?
-//Note that then we may need to still allow '\r' (for mail processing)
                         } while (!table_ptr);
 
                         /* Convert encoded character to decimal */
                         ch = table_ptr - bb_uuenc_tbl_base64;
 
-                        if (ch == 65) {  /* '\n' */
-                                continue;
-                        }
                         /* ch is 64 if char was '=', otherwise 0..63 */
-                        translated[count] = ch & 63; /* 64 -> 0 */
-                        if (ch == 64) {  /* '=' */
+                        if (ch == 64)
                                 break;
-                        }
+                        six_bit[count] = ch;
                         count++;
                 }
 
-                /* Merge 6 bit chars to 8 bit.
+                /* Transform 6-bit values to 8-bit ones.
                  * count can be < 4 when we decode the tail:
-                 * "eQ==" -> "y", not "y NUL NUL"
+                 * "eQ==" -> "y", not "y NUL NUL".
+                 * Note that (count > 1) is always true,
+                 * "x===" encoding is not valid:
+                 * even a single zero byte encodes as "AA==".
+                 * However, with current logic we come here with count == 1
+                 * when we decode "==" tail.
                  */
                 if (count > 1)
-                        *dst++ = translated[0] << 2 | translated[1] >> 4;
+                        *dst++ = six_bit[0] << 2 | six_bit[1] >> 4;
                 if (count > 2)
-                        *dst++ = translated[1] << 4 | translated[2] >> 2;
+                        *dst++ = six_bit[1] << 4 | six_bit[2] >> 2;
                 if (count > 3)
-                        *dst++ = translated[2] << 6 | translated[3];
+                        *dst++ = six_bit[2] << 6 | six_bit[3];
+                /* Note that if we decode "AA==" and ate first '=',
+                 * we just decoded one char (count == 2) and now we'll
+                 * do the loop once more to decode second '='.
+                 */
         } /* while (1) */
  ret:
         *pp_dst = dst;