libbb: smaller and faster decode_base64()

function                                             old     new   delta
decode_base64                                        195     180     -15

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

2 files changed, 36 insertions, 101 deletions

diff --git a/libbb/uuencode.c b/libbb/uuencode.c
index b4ee20c20..2e9edb219 100644
--- a/libbb/uuencode.c
+++ b/libbb/uuencode.c
@@ -82,7 +82,7 @@ void FAST_FUNC bb_uuencode(char *p, const void *src, int length, const char *tbl
 }
 
 /*
- * Decode base64 encoded string. Stops on '\0'.
+ * Decode base64 encoded string. Stops on NUL after terminating "=" or "==".
  *
  * Returns: pointer to the undecoded part of source.
  * If points to '\0', then the source was fully decoded.
@@ -91,76 +91,48 @@ void FAST_FUNC bb_uuencode(char *p, const void *src, int length, const char *tbl
 const char* FAST_FUNC decode_base64(char **pp_dst, const char *src)
 {
         char *dst = *pp_dst;
-        const char *src_tail;
-
-        while (1) {
-                unsigned char six_bit[4];
-                int count = 0;
+        unsigned ch = 0;
+        int i = 0;
 
-                /* Fetch up to four 6-bit values */
-                src_tail = src;
-                while (count < 4) {
-                        char *table_ptr;
-                        int ch;
+        while (*src) {
+                int t = (unsigned char)*src++;
 
-                        /* Get next _valid_ character.
-                         * bb_uuenc_tbl_base64[] contains this string:
-                         *  0         1         2         3         4         5         6
-                         *  01234567890123456789012345678901234567890123456789012345678901234
-                         * "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="
-                         */
-                        do {
-                                ch = *src;
-                                if (ch == '\0') {
-                                        if (count == 0) {
-                                                /* Example:
-                                                 * If we decode "QUJD <NUL>", we want
-                                                 * to return ptr to NUL, not to ' ',
-                                                 * because we did fully decode
-                                                 * the string (to "ABC").
-                                                 */
-                                                src_tail = src;
-                                        }
-                                        goto ret;
-                                }
-                                src++;
-                                table_ptr = strchr(bb_uuenc_tbl_base64, ch);
+                /* "if" forest is faster than strchr(bb_uuenc_tbl_base64, t) */
+                if (t >= '0' && t <= '9')
+                        t = t - '0' + 52;
+                else if (t >= 'A' && t <= 'Z')
+                        t = t - 'A';
+                else if (t >= 'a' && t <= 'z')
+                        t = t - 'a' + 26;
+                else if (t == '+')
+                        t = 62;
+                else if (t == '/')
+                        t = 63;
+                else if (t == '=' && (i == 3 || (i == 2 && *src == '=')))
+                        /* the above disallows "==AA", "A===", "AA=A" etc */
+                        t = 0x1000000;
+                else
 //TODO: add BASE64_FLAG_foo to die on bad char?
-                        } while (!table_ptr);
-
-                        /* Convert encoded character to decimal */
-                        ch = table_ptr - bb_uuenc_tbl_base64;
+                        continue;
 
-                        /* ch is 64 if char was '=', otherwise 0..63 */
-                        if (ch == 64)
+                ch = (ch << 6) | t;
+                if (++i == 4) {
+                        *dst++ = (char) (ch >> 16);
+                        *dst++ = (char) (ch >> 8);
+                        *dst++ = (char) ch;
+                        i = 0;
+                        if (ch & 0x1000000) { /* was last input char '='? */
+                                dst--;
+                                if (ch & (0x1000000 << 6)) /* was it "=="? */
+                                        dst--;
                                 break;
-                        six_bit[count] = ch;
-                        count++;
+                        }
+                        ch = 0;
                 }
-
-                /* Transform 6-bit values to 8-bit ones.
-                 * count can be < 4 when we decode the tail:
-                 * "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++ = six_bit[0] << 2 | six_bit[1] >> 4;
-                if (count > 2)
-                        *dst++ = six_bit[1] << 4 | six_bit[2] >> 2;
-                if (count > 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;
-        return src_tail;
+        /* i should be zero here if full 4-char block was decoded */
+        return src - i; /* -i rejects truncations: e.g. "MQ" and "MQ=" (correct encoding is "MQ==" -> "1") */
 }
 
 #if ENABLE_BASE32
diff --git a/networking/httpd.c b/networking/httpd.c
index 961f8cab4..4ffd89c48 100644
--- a/networking/httpd.c
+++ b/networking/httpd.c
@@ -1017,46 +1017,9 @@ static char *encodeString(const char *string)
  */
 static void decodeBase64(char *Data)
 {
-# if ENABLE_BASE64 || ENABLE_UUDECODE
-        /* Call decode_base64() from uuencode.c */
         char *eptr = Data;
         decode_base64(&eptr, Data);
         *eptr = '\0';
-# else
-        const unsigned char *in = (const unsigned char *)Data;
-        /* The decoded size will be at most 3/4 the size of the encoded */
-        unsigned ch = 0;
-        int i = 0;
-
-        while (*in) {
-                int t = *in++;
-
-                if (t >= '0' && t <= '9')
-                        t = t - '0' + 52;
-                else if (t >= 'A' && t <= 'Z')
-                        t = t - 'A';
-                else if (t >= 'a' && t <= 'z')
-                        t = t - 'a' + 26;
-                else if (t == '+')
-                        t = 62;
-                else if (t == '/')
-                        t = 63;
-                else if (t == '=')
-                        t = 0;
-                else
-                        continue;
-
-                ch = (ch << 6) | t;
-                i++;
-                if (i == 4) {
-                        *Data++ = (char) (ch >> 16);
-                        *Data++ = (char) (ch >> 8);
-                        *Data++ = (char) ch;
-                        i = 0;
-                }
-        }
-        *Data = '\0';
-# endif
 }
 #endif