Clarify simple and complex prefix codes

* At the beginning of the simple prefix code section, telling us that "a value
of 1 indicates the number of leading zeros" is not very helpful. Instead, it
should indicate that it means a complex prefix code and point the reader to
the relevant section (which repeats this information in more detail)

* Clearly indicate that reusing a value is an error! This seems to be the
behavior of the of the reference implementation.

* Clarify what the termination conditions are while reading the prefix codes.
Also, indicate that it is an error if the prefix tree is over-subscribed or
under-subscribed.

* Clearly state what is the maximum number of individual symbols that may be
read. This ensures that it is forbidden to an stream that continually says that
the symbols have zero length.

docs/draft-alakuijala-brotli-07.nroff

@@ -617,8 +617,9 @@ for the various prefix codes and the sections where they are defined.
 
 The first two bits of the compressed representation of each
 prefix code distinguishes between simple and complex prefix
-codes. If this value is 1, then a simple prefix code follows.
-Otherwise the value indicates the number of leading zeros.
+codes. If this value is 1, then a simple prefix code follows
+as described in this section. Otherwise, a complex prefix code
+follows as described in section 3.5.
 
 A simple prefix code can have only up to four symbols with
 non-zero code length. The format of the simple prefix code is as
@@ -626,7 +627,7 @@ follows:
 
 .nf
    2 bits: value of 1 indicates a simple prefix code
-   2 bits: NSYM - 1, where NSYM = # of symbols coded
+   2 bits: NSYM - 1, # of symbols coded
 
    NSYM symbols, each encoded using ALPHABET_BITS bits
 
@@ -637,9 +638,10 @@ The value of ALPHABET_BITS depends on the alphabet of the prefix
 code: it is the smallest number of bits that can represent all
 symbols in the alphabet. E.g. for the alphabet of literal bytes,
 ALPHABET_BITS is 8. The value of each of the NSYM symbols above is
-the value of the ALPHABETS_BITS width integer value. (If the integer
-value is greater than or equal to the alphabet size, then the stream
-should be rejected as invalid.)
+the value of the ALPHABETS_BITS width integer value. If the integer
+value is greater than or equal to the alphabet size, or the value
+is identical to a previous value, then the stream should be rejected
+as invalid.
 
 Note that the NSYM symbols may not be presented in sorted order. Prefix codes
 of the same bit length must be assigned to the symbols in sorted order.
@@ -756,9 +758,15 @@ We can now define the format of the complex prefix code as follows:
 
       If there are at least two non-zero code lengths, any
       trailing zero code lengths are omitted, i.e. the last
-      code length in the sequence must be non-zero. In this
-      case the sum of (32 >> code length) over all the non-zero
-      code lengths must equal to 32.
+      code length in the sequence must be non-zero. Thus,
+      it is possible to have far less than 18 code lengths.
+
+      From the perspective of the decoder, we can stop reading
+      code lengths once the sum of (32 >> code length) over all
+      the preceding non-zero code lengths equals 32. If the sum
+      is not equal to 32 (and assuming there are at least two
+      non-zero code lengths), then the stream should be
+      rejected as invalid.
 
       If the lengths have been read for the entire code length
       alphabet and there was only one non-zero code length,
@@ -778,13 +786,18 @@ We can now define the format of the complex prefix code as follows:
    Sequence of code lengths symbols, encoded using the code
       length prefix code. Any trailing 0 or 17 must be
       omitted, i.e. the last encoded code length symbol must be
-      between 1 and 16. The sum of (32768 >> code length) over
-      all the non-zero code lengths in the alphabet, including
-      those encoded using repeat code(s) of 16, must equal to
-      32768. If the number of times to repeat the previous length
-      or repeat a zero length would result in more lengths in
-      total than the number of symbols in the alphabet, then the
-      stream should be rejected as invalid.
+      between 1 and 16. The maximum number of symbols to read
+      is the size of the alphabet.
+
+      From the perspective of the decoder, we can stop reading
+      code lengths once the sum of (32768 >> code length) over
+      all the preceding non-zero code lengths, including those
+      encoded using repeat codes(s) of 16, in the alphabet
+      equals 32768. If the sum does not equal 32768, or if the
+      number of times to repeat the previous length or repeat a
+      zero length would result in more lengths in total than
+      the number of symbols in the alphabet, then the stream
+      should be rejected as invalid.
 .fi
 
 .ti 0