implement calcEIP1559GasLimit

CalcGasLimit1559 calculates the next block gas limit under 1559 rules.
this function is needed in upcoming sealing engine implementation

nimbus/utils/header.nim

@@ -80,3 +80,26 @@ proc generateHeaderFromParentHeader*(config: ChainConfig, parent: BlockHeader,
     extraData: extraData,
     fee: baseFee
   )
+
+# CalcGasLimit1559 calculates the next block gas limit under 1559 rules.
+func calcGasLimit1559*(parentGasLimit, desiredLimit: GasInt): GasInt =
+  let delta = parentGasLimit div GAS_LIMIT_EMA_DENOMINATOR - 1.GasInt
+  var limit = parentGasLimit
+  var desiredLimit = desiredLimit
+
+  if desiredLimit < GAS_LIMIT_MINIMUM:
+    desiredLimit = GAS_LIMIT_MINIMUM
+
+  # If we're outside our allowed gas range, we try to hone towards them
+  if limit < desiredLimit:
+    limit = parentGasLimit + delta
+    if limit > desiredLimit:
+      limit = desiredLimit
+    return limit
+
+  if limit > desiredLimit:
+    limit = parentGasLimit - delta
+    if limit < desiredLimit:
+      limit = desiredLimit
+
+  return limit

tests/test_misc.nim

@@ -4,7 +4,8 @@ import
   eth/common/eth_types,
   eth/p2p,
   ../nimbus/vm_internals,
-  ../nimbus/config
+  ../nimbus/config,
+  ../nimbus/utils/header
 
 func toAddress(n: int): EthAddress =
   result[19] = n.byte
@@ -60,5 +61,43 @@ proc miscMain*() =
 
       check conf.net.networkId == 123.NetworkId
 
+    test "calcGasLimitEIP1559":
+      type
+        GLT = object
+          limit: GasInt
+          max  : GasInt
+          min  : GasInt
+
+      const testData = [
+        GLT(limit: 20000000, max: 20019530, min: 19980470),
+        GLT(limit: 40000000, max: 40039061, min: 39960939)
+      ]
+
+      for x in testData:
+        # Increase
+        var have = calcGasLimit1559(x.limit, 2*x.limit)
+        var want = x.max
+        check have == want
+
+        # Decrease
+        have = calcGasLimit1559(x.limit, 0)
+        want = x.min
+        check have == want
+
+        # Small decrease
+        have = calcGasLimit1559(x.limit, x.limit-1)
+        want = x.limit-1
+        check have == want
+
+        # Small increase
+        have = calcGasLimit1559(x.limit, x.limit+1)
+        want = x.limit+1
+        check have == want
+
+        # No change
+        have = calcGasLimit1559(x.limit, x.limit)
+        want = x.limit
+        check have == want
+
 when isMainModule:
   miscMain()