Add a helper for calculating the EIP1559 base fee

eth/eip1559.nim

@@ -0,0 +1,47 @@
+import
+  common/eth_types
+
+const
+  EIP1559_BASE_FEE_CHANGE_DENOMINATOR* = ##\
+    ## Bounds the amount the base fee can change between blocks.
+    8
+
+  EIP1559_ELASTICITY_MULTIPLIER* = ##\
+    ## Bounds the maximum gas limit an EIP-1559 block may have.
+    2
+
+  EIP1559_INITIAL_BASE_FEE* = ##\
+    ## Initial base fee for Eip1559 blocks.
+    1000000000.u256
+
+proc calcEip1599BaseFee*(parentGasLimit, parentGasUsed: GasInt;
+                         parentBaseFee: UInt256): UInt256 =
+  ## calculates the basefee of the header.
+
+  let parentGasTarget = parentGasLimit div EIP1559_ELASTICITY_MULTIPLIER
+
+  # If the parent gasUsed is the same as the target, the baseFee remains
+  # unchanged.
+  if parentGasUsed == parentGasTarget:
+    return parentBaseFee
+
+  let parentGasDenom = parentGasTarget.u256 *
+                         EIP1559_BASE_FEE_CHANGE_DENOMINATOR.u256
+
+  if parentGasTarget < parentGasUsed:
+    # If the parent block used more gas than its target, the baseFee should
+    # increase.
+    let
+      gasUsedDelta = (parentGasUsed - parentGasTarget).u256
+      baseFeeDelta = (parentBaseFee * gasUsedDelta) div parentGasDenom
+
+    return parentBaseFee + max(baseFeeDelta, 1.u256)
+
+  else:
+    # Otherwise if the parent block used less gas than its target, the
+    # baseFee should decrease.
+    let
+      gasUsedDelta = (parentGasTarget - parentGasUsed).u256
+      baseFeeDelta = (parentBaseFee * gasUsedDelta) div parentGasDenom
+
+    return max(parentBaseFee - baseFeeDelta, 0.u256)