Move description of escaping to code that does the escaping rather than the…

Move description of escaping to code that does the escaping rather than the CalculateBase64EscapedLenInternal helper method.
Note that output padding is conditional on do_padding.

PiperOrigin-RevId: 499901986
Change-Id: I8c1d28fe372b3e0e2216654db83f949caa297892

absl/strings/internal/escaping.cc

@@ -28,19 +28,11 @@ size_t CalculateBase64EscapedLenInternal(size_t input_len, bool do_padding) {
   // Base64 encodes three bytes of input at a time. If the input is not
   // Base64 encodes three bytes of input at a time. If the input is not
   // divisible by three, we pad as appropriate.
   // divisible by three, we pad as appropriate.
   //
   //
-  // (from https://tools.ietf.org/html/rfc3548)
-  // Special processing is performed if fewer than 24 bits are available
-  // at the end of the data being encoded.  A full encoding quantum is
-  // always completed at the end of a quantity.  When fewer than 24 input
-  // bits are available in an input group, zero bits are added (on the
-  // right) to form an integral number of 6-bit groups.  Padding at the
-  // end of the data is performed using the '=' character.  Since all base
-  // 64 input is an integral number of octets, only the following cases
-  // can arise:
-
   // Base64 encodes each three bytes of input into four bytes of output.
   // Base64 encodes each three bytes of input into four bytes of output.
   size_t len = (input_len / 3) * 4;
   size_t len = (input_len / 3) * 4;
 
 
+  // Since all base 64 input is an integral number of octets, only the following
+  // cases can arise:
   if (input_len % 3 == 0) {
   if (input_len % 3 == 0) {
     // (from https://tools.ietf.org/html/rfc3548)
     // (from https://tools.ietf.org/html/rfc3548)
     // (1) the final quantum of encoding input is an integral multiple of 24
     // (1) the final quantum of encoding input is an integral multiple of 24
@@ -83,6 +75,16 @@ size_t Base64EscapeInternal(const unsigned char* src, size_t szsrc, char* dest,
   char* const limit_dest = dest + szdest;
   char* const limit_dest = dest + szdest;
   const unsigned char* const limit_src = src + szsrc;
   const unsigned char* const limit_src = src + szsrc;
 
 
+  // (from https://tools.ietf.org/html/rfc3548)
+  // Special processing is performed if fewer than 24 bits are available
+  // at the end of the data being encoded.  A full encoding quantum is
+  // always completed at the end of a quantity.  When fewer than 24 input
+  // bits are available in an input group, zero bits are added (on the
+  // right) to form an integral number of 6-bit groups.
+  //
+  // If do_padding is true, padding at the end of the data is performed. This
+  // output padding uses the '=' character.
+
   // Three bytes of data encodes to four characters of cyphertext.
   // Three bytes of data encodes to four characters of cyphertext.
   // So we can pump through three-byte chunks atomically.
   // So we can pump through three-byte chunks atomically.
   if (szsrc >= 3) {                    // "limit_src - 3" is UB if szsrc < 3.
   if (szsrc >= 3) {                    // "limit_src - 3" is UB if szsrc < 3.