07704c1 Add some tests for getchaintxstats (Akio Nakamura)
3336676 Fix getchaintxstats() (Akio Nakamura)
Pull request description:
1. calculate nblocks more adaptive.
-> set default nblocks to min (blocks for 1 month, target block's height - 1)
-> before PR: if not specify nblocks-parameter, illegal parameter error will happen when target block height is below nblocks.
2. correct error message.
-> nblocks accepts [1 .. block's height -1] . so add a word "-1".
3. add check 0-divide.
-> if nTimeDiff = 0 then use UniValue(UniValue::VNULL) and returns {... "txrate": null} .
-> before PR: if nTimeDiff = 0 then returns {... "txrate":} and bitcoin-cli cannot handle the response.
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