add readme to vrlp, assertion about encode->decode roundtrip

libs/vrlp/README.md

@@ -0,0 +1,61 @@
+# Vanillae RLP library
+
+This implements [Ethereum's RLP codec](https://zxq9.com/archives/2749)
+
+The decoded data ("decoded" from the perspective of the program using the data)
+is arbitrary-depth arbitrary-length lists-of-lists-of-...-binaries (lists and
+binaries are allowed to be empty). The encoded data is of course a single flat
+binary string.
+
+It has been tested against Ethereum's official Python implementation.
+
+You can see the methodology in <tests/vrlp_testgen.erl>:
+
+- 10,000 decoded data structures are generated at random
+- the test generator calls `vrlp:encode/1` to generate an encode/decode pair
+- assert that encode -> decode round trips properly in the `vrlp` module
+- print out an encode-decode pair as a python term to stdout
+- python program tests the encode-decode pairs against Ethereum's library
+
+The heart of the methodology is here:
+
+```erl
+% input: decoded_data
+% format a case as
+% {'decoded_data': <python term for rlist>,
+%  'encoded_bytes': bytes([B1, B2, B3, ...])}
+format_case_py(DecodedData_rlist) ->
+    % EncodedData_bytes = rlp:encode(
+    % DD_js = format_data_js(DecodedData_rlist),
+    EncodedData_bytes          = vrlp:encode(DecodedData_rlist),
+    % assert encode -> decode round trips
+    {DecodedData_rlist, <<>>}  = vrlp:decode(EncodedData_bytes),
+    EncodedBytes_py   = format_bytes_py(EncodedData_bytes),
+    DecodedData_py    = format_data_py(DecodedData_rlist),
+    ["    {'decoded': ", DecodedData_py, ",\n",
+     "     'encoded': ", EncodedBytes_py, "}"].
+```
+
+The Python side:
+
+```python
+def main():
+    all_encode_work = True
+    all_decode_work = True
+    for case in cases:
+        decoded_data       = case['decoded']
+        encoded_bytes      = case['encoded']
+        real_encoded_bytes = rlp.encode(decoded_data)
+        real_decoded_data  = rlp.decode(encoded_bytes)
+        # check if encode/decode works
+        encode_works = (real_encoded_bytes == encoded_bytes)
+        decode_works = (real_decoded_data == decoded_data)
+        print('encode works: %s' % (encode_works))
+        print('decode works: %s' % (decode_works))
+        # update globals
+        all_encode_work = all_encode_work and encode_works
+        all_decode_work = all_decode_work and decode_works
+    # print globals
+    print('all encode work: %s' % (all_encode_work))
+    print('all decode work: %s' % (all_decode_work))
+```

libs/vrlp/tests/vrlp_testgen.erl

@@ -8,7 +8,8 @@ main([]) ->
     % Decoded cases
     DecodedCases = rand_decode_datas(10_000),
     %io:format("~p~n", [DecodedCases]),
-    io:format("~s~n", [format_cases_py(DecodedCases)]),
+    %io:format("~s~n", [format_cases_py(DecodedCases)]),
+    _ = format_cases_py(DecodedCases),
     ok.
 
 
@@ -115,7 +116,9 @@ slcommas([Item | Rest], Acc) ->
 format_case_py(DecodedData_rlist) ->
     % EncodedData_bytes = rlp:encode(
     % DD_js = format_data_js(DecodedData_rlist),
-    EncodedData_bytes = vrlp:encode(DecodedData_rlist),
+    EncodedData_bytes          = vrlp:encode(DecodedData_rlist),
+    % assert encode -> decode round trips
+    {DecodedData_rlist, <<>>}  = vrlp:decode(EncodedData_bytes),
     EncodedBytes_py   = format_bytes_py(EncodedData_bytes),
     DecodedData_py    = format_data_py(DecodedData_rlist),
     ["    {'decoded': ", DecodedData_py, ",\n",