evm t8n calculate difficulty

[winsvega]

The current env format is

{
   "currentCoinbase" : "0x2adc25665018aa1fe0e6bc666dac8fc2697ff9ba",
   "currentDifficulty" : "0x020000",
   "currentGasLimit" : "0x05f5e100",
   "currentNumber" : "0x01",
   "currentTimestamp" : "0x03e8",
   "previousHash" : "0xe729de3fec21e30bea3d56adb01ed14bc107273c2775f9355afb10f594a10d9e",
   "blockHashes" : {
       "0" : "0xe729de3fec21e30bea3d56adb01ed14bc107273c2775f9355afb10f594a10d9e"
   }
}

want to extend the following format to be acceptable (as well as the old one)

{
   "currentCoinbase" : "0x2adc25665018aa1fe0e6bc666dac8fc2697ff9ba",
   "parentDifficulty" : "0x020000",
   "currentGasLimit" : "0x05f5e100",
   "currentNumber" : "0x01",
   "parentTimestamp" : "0x03e7",
   "currentTimestamp" : "0x03e8",
   "previousHash" : "0xe729de3fec21e30bea3d56adb01ed14bc107273c2775f9355afb10f594a10d9e",
   "blockHashes" : {
       "0" : "0xe729de3fec21e30bea3d56adb01ed14bc107273c2775f9355afb10f594a10d9e"
   }
}

new fields: parentDifficulty instead of currentDifficulty
and parentTimestamp

the result file then

{
  "currentDifficulty" : "0x020001"
  "stateRoot": "0x1c99b01120e7a2fa1301b3505f20100e72362e5ac3f96854420e56ba8984d716",
  "txRoot": "0xb5eee60b45801179cbde3781b9a5dee9b3111554618c9cda3d6f7e351fd41e0b",
  "receiptRoot": "0x86ceb80cb6bef8fe4ac0f1c99409f67cb2554c4432f374e399b94884eb3e6562",
  "logsHash": "0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347",
  "logsBloom": "0x00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000",
  "receipts": [
  ]
}

would report the new difficulty calculated with geth formula currentDifficulty

Rationale
To avoid point of failure when generating the tests. Currently retesteth calculates the difficulty, but retesteth difficulty formula might be wrong. Since we test the clients, it would make sense that retesteth asks t8ntool what geth evm thinks would be the new difficulty given parentTimestamp and parentDifficulty and currentTimestamp.

[holiman]

SGTM. So just to clarify even further, I'd propose:

1. The currentDifficulty can be given as input. If so, goto 4, If not, then
2. Verify parentDifficulty, parentTimestamp and currentTimestamp are present (otherwise fail), and
3. Calculate the currentDifficulty.
4. Continue the execution

And lastly, the output env always provides the currentDifficulty, regardless of whether it was given or calculated.

[holiman]

Actually, there's one snag; the parent may or may not have uncles. And whether the parent has uncles, affects the difficulty.