add decoder precondition comment

include/hpack/decoder.hpp

@@ -121,6 +121,7 @@ struct header_view {
   }
 };
 
+// precondition: in != e
 void decode_string(In& in, In e, decoded_string& out);
 
 struct decoder {
@@ -139,10 +140,12 @@ struct decoder {
    https://www.rfc-editor.org/rfc/rfc7540#section-8.1.2.5
    and protocol error if decoded header name is not lowercase
   */
+  // precondition: in != e
   void decode_header(In& in, In e, header_view& out);
 
   // returns status code
   // its always first header of response, so 'in' must point to first byte of headers block
+  // precondition: in != e
   int decode_response_status(In& in, In e);
 };
 

src/decoder.cpp

@@ -153,6 +153,7 @@ static size_type decode_dynamic_table_size_update(In& in, In e) noexcept {
 }
 
 void decode_string(In& in, In e, decoded_string& out) {
+  assert(in != e);
   bool is_huffman = *in & 0b1000'0000;
   size_type str_len = decode_integer(in, e, 7);
   if (str_len > std::distance(in, e))
@@ -165,6 +166,7 @@ void decode_string(In& in, In e, decoded_string& out) {
 }
 
 void decoder::decode_header(In& in, In e, header_view& out) {
+  assert(in != e);
   if (*in & 0b1000'0000)
     return decode_header_fully_indexed(in, e, dyntab, out);
   if (*in & 0b0100'0000)
@@ -183,6 +185,7 @@ void decoder::decode_header(In& in, In e, header_view& out) {
 }
 
 int decoder::decode_response_status(In& in, In e) {
+  assert(in != e);
   if (*in & 0b1000'0000) {
     // fast path, fully indexed
     auto in_before = in;