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+# Nimbus - Types and helpers used at the boundary of transactions/RPC and EVMC/EVM
+#
+# Copyright (c) 2019-2021 Status Research & Development GmbH
+# Licensed under either of
+#  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
+#  * MIT license ([LICENSE-MIT](LICENSE-MIT) or http://opensource.org/licenses/MIT)
+# at your option. This file may not be copied, modified, or distributed except according to those terms.
+
+import
+  sets, stint, evmc/evmc, eth/common/eth_types, ../vm_types
+
+# Object `TransactionHost` represents "EVMC host" to the EVM.  "Host services"
+# manage account state outside EVM such as balance transfers, storage, logs and
+# gas refunds.  This object holds transaction state hidden from the EVM, and
+# the EVM passes this around as opaque `evmc_host_context`.  To the application
+# outside the EVM, this object represents a computation for a transaction.
+#
+# `Host..` types are like EVMC types, but used in `TransactionHost` code.  They
+# occupy the same positions in EVMC functions and objects as the type they map
+# to/from.  But `Host..` types match internal APIs in the rest of Nimbus, to
+# minimise the amount of explicit conversions when using them.  We could just
+# use the Nimbus types, but these names document their use and also role.
+#
+# `Evmc..` types named here are actual EVMC types.  They play a similar role to
+# `Host..` types, except the `Evmc` prefix indicates they are the actual EVMC
+# types and more care applies.  Byte order (big/little-endian) may need to be
+# swapped in object fields of these types, and enums respected.
+#
+# When crossing the EVMC API boundary, between internal APIs and EVMC binary
+# interface there have to be type-conversions and some big/little-endian byte
+# swapping.  Those conversions are mostly kept out of `TransactionHost` logic
+# and delegated to glue code.
+
+type
+  HostAddress*       = EthAddress        # Mapped to/from evmc_address.
+  HostKey*           = UInt256           # Mapped to/from evmc_bytes32.
+  HostValue*         = UInt256           # Mapped to/from evmc_bytes32.
+  HostBalance*       = UInt256           # Mapped to/from evmc_uint256be.
+  HostSize*          = uint              # Mapped to/from csize_t - unsigned!
+  HostHash*          = Hash256           # Mapped to/from evmc_bytes32.
+  HostTopic*         = Topic             # Mapped to/from evmc_bytes32.
+  HostBlockNumber*   = BlockNumber       # Mapped to/from int64.
+  HostGasInt*        = GasInt            # Mapped to/from int64.
+  HostGasPrice*      = GasInt            # Mapped to/from evmc_uint256be.
+
+  EvmcStatusCode*    = evmc_status_code
+  EvmcCallKind*      = evmc_call_kind
+  EvmcStorageStatus* = evmc_storage_status
+  EvmcTxContext*     = evmc_tx_context
+  EvmcMessage*       = evmc_message
+  EvmcResult*        = evmc_result
+
+  TransactionHost* = ref object
+    vmState*:         BaseVMState
+    computation*:     Computation
+    msg*:             EvmcMessage
+    input*:           seq[byte]
+    cachedTxContext*: bool
+    txContext*:       EvmcTxContext
+    logEntries*:      seq[Log]
+    touchedAccounts*: HashSet[EthAddress]
+    selfDestructs*:   HashSet[EthAddress]
+
+# These versions of `toEvmc` and `fromEvmc` don't flip big/little-endian like
+# the older functions in `evmc_helpers`.  New code only flips with _explicit_
+# calls to `flip256` where it is wanted.
+
+template toEvmc*(n: Uint256): evmc_uint256be =
+  cast[evmc_uint256be](n)
+
+template toEvmc*(n: Hash256): evmc_bytes32 =
+  cast[evmc_bytes32](n)
+
+template toEvmc*(address: EthAddress): evmc_address =
+  cast[evmc_address](address)
+
+template fromEvmc*(n: evmc_uint256be): UInt256 =
+  cast[UInt256](n)
+
+template fromEvmc*(address: evmc_address): EthAddress =
+  cast[EthAddress](address)
+
+template flip256*(word256: evmc_uint256be): evmc_uint256be =
+  cast[evmc_uint256be](Uint256.fromBytesBe(word256.bytes).toBytes)
+
+template isCreate*(kind: EvmcCallKind): bool =
+  kind in {EVMC_CREATE, EVMC_CREATE2}
+
+template isStatic*(msg: EvmcMessage): bool =
+  EVMC_STATIC in msg.flags
+
+# Nim quirks: Exporting `evmc_status_code` (etc) are needed to access the enum
+# values, even though alias `EnumStatusCode` is already exported.  Exporting
+# `evmc_flags` won't export the flags, `evmc_flag_bit_shifts` must be used.
+export
+  evmc_status_code, evmc_call_kind,
+  evmc_flag_bit_shifts, evmc_storage_status