Document how tested prefix lengths are chosen

Author: gilles-peskine-arm

tests/suites/test_suite_asn1parse.function

@@ -188,6 +188,10 @@ void parse_prefixes( const data_t *input,
     size_t buffer_size;
     int ret;
 
+    /* Test every prefix of the input, except the empty string.
+     * The first byte of the string is the tag. Without a tag byte,
+     * we wouldn't know what to parse the input as.
+     */
     for( buffer_size = 1; buffer_size <= input->len; buffer_size++ )
     {
         test_set_step( buffer_size );
@@ -224,6 +228,12 @@ void get_len( const data_t *input, int actual_length_arg )
     size_t actual_length = actual_length_arg;
     size_t buffer_size;
 
+    /* Test prefixes of a buffer containing the given length string
+     * followed by `actual_length` bytes of payload. To save a bit of
+     * time, we skip some "boring" prefixes: we don't test prefixes where
+     * the payload is truncated more than one byte away from either end,
+     * and we only test the empty string on a 1-byte input.
+     */
     for( buffer_size = 1; buffer_size <= input->len + 1; buffer_size++ )
     {
         if( ! get_len_step( input, buffer_size, actual_length ) )