[MINOR] add encode/decode function for 30-bit integers from/to base64

These functions only require 5 chars to encode 30 bits, and don't expect
any padding. They will be used to encode dates in cookies.
(cherry picked from commit a7e2b5fc4612994c7b13bcb103a4a2c3ecd6438a)

include/common/base64.h

@@ -2,7 +2,7 @@
  * include/common/base64.h
  * Ascii to Base64 conversion as described in RFC1421.
  *
- * Copyright 2006 Willy Tarreau <w@1wt.eu>
+ * Copyright 2006-2010 Willy Tarreau <w@1wt.eu>
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License
@@ -18,6 +18,8 @@
 
 int a2base64(char *in, int ilen, char *out, int olen);
 int base64dec(const char *in, size_t ilen, char *out, size_t olen);
+const char *s30tob64(int in, char *out);
+int b64tos30(const char *in);
 
 extern const char base64tab[];
 

src/base64.c

@@ -1,7 +1,7 @@
 /*
  * ASCII <-> Base64 conversion as described in RFC1421.
  *
- * Copyright 2006-2008 Willy Tarreau <w@1wt.eu>
+ * Copyright 2006-2010 Willy Tarreau <w@1wt.eu>
  * Copyright 2009-2010 Krzysztof Piotr Oledzki <ole@ans.pl>
  *
  * This program is free software; you can redistribute it and/or
@@ -135,3 +135,50 @@ int base64dec(const char *in, size_t ilen, char *out, size_t olen) {
 
 	return convlen;
 }
+
+
+/* Converts the lower 30 bits of an integer to a 5-char base64 string. The
+ * caller is responsible for ensuring that the output buffer can accept 6 bytes
+ * (5 + the trailing zero). The pointer to the string is returned. The
+ * conversion is performed with MSB first and in a format that can be
+ * decoded with b64tos30(). This format is not padded and thus is not
+ * compatible with usual base64 routines.
+ */
+const char *s30tob64(int in, char *out)
+{
+	int i;
+	for (i = 0; i < 5; i++) {
+		out[i] = base64tab[(in >> 24) & 0x3F];
+		in <<= 6;
+	}
+	out[5] = '\0';
+	return out;
+}
+
+/* Converts a 5-char base64 string encoded by s30tob64() into a 30-bit integer.
+ * The caller is responsible for ensuring that the input contains at least 5
+ * chars. If any unexpected character is encountered, a negative value is
+ * returned.  Otherwise the decoded value is returned.
+ */
+int b64tos30(const char *in)
+{
+	int i, out;
+	signed char b;
+
+	out = 0;
+	for (i = 0; i < 5; i++) {
+		b = (signed char)in[i] - B64CMIN;
+		if ((unsigned char)b > (B64CMAX - B64CMIN))
+			return -1;  /* input character out of range */
+
+		b = base64rev[b] - B64BASE - 1;
+		if (b < 0)          /* invalid character */
+			return -1;
+
+		if (b == B64PADV)   /* padding not allowed */
+			return -1;
+
+		out = (out << 6) + b;
+	}
+	return out;
+}