nimbus-eth1/nimbus/vm/interpreter/gas_costs.nim

# Nimbus
# Copyright (c) 2018 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
  math, eth_common/eth_types,
  ./utils/[macros_gen_opcodes, utils_numeric],
  ./opcode_values, ./vm_forks

# Gas Fee Schedule
# Yellow Paper Appendix G - https://ethereum.github.io/yellowpaper/paper.pdf
type
  GasFeeKind = enum
    GasZero,            # Nothing paid for operations of the set Wzero.
    GasBase,            # Amount of gas to pay for operations of the set Wbase.
    GasVeryLow,         # Amount of gas to pay for operations of the set Wverylow.
    GasLow,             # Amount of gas to pay for operations of the set Wlow.
    GasMid,             # Amount of gas to pay for operations of the set Wmid.
    GasHigh,            # Amount of gas to pay for operations of the set Whigh.
    GasExtCode,         # Amount of gas to pay for operations of the set Wextcode.
    GasBalance,         # Amount of gas to pay for a BALANCE operation.
    GasSload,           # Paid for a SLOAD operation.
    GasJumpDest,        # Paid for a JUMPDEST operation.
    GasSset,            # Paid for an SSTORE operation when the storage value is set to non-zero from zero.
    GasSreset,          # Paid for an SSTORE operation when the storage value’s zeroness remains unchanged or is set to zero.
    RefundSclear,       # Refund given (added into refund counter) when the storage value is set to zero from non-zero.
    RefundSelfDestruct, # Refund given (added into refund counter) for self-destructing an account.
    GasSelfDestruct,    # Amount of gas to pay for a SELFDESTRUCT operation.
    GasCreate,          # Paid for a CREATE operation.
    GasCodeDeposit,     # Paid per byte for a CREATE operation to succeed in placing code into state.
    GasCall,            # Paid for a CALL operation.
    GasCallValue,       # Paid for a non-zero value transfer as part of the CALL operation.
    GasCallStipend,     # A stipend for the called contract subtracted from Gcallvalue for a non-zero value transfer.
    GasNewAccount,      # Paid for a CALL or SELFDESTRUCT operation which creates an account.
    GasExp,             # Partial payment for an EXP operation.
    GasExpByte,         # Partial payment when multiplied by ⌈log256(exponent)⌉ for the EXP operation.
    GasMemory,          # Paid for every additional word when expanding memory.
    GasTXCreate,        # Paid by all contract-creating transactions after the Homestead transition.
    GasTXDataZero,      # Paid for every zero byte of data or code for a transaction.
    GasTXDataNonZero,   # Paid for every non-zero byte of data or code for a transaction.
    GasTransaction,     # Paid for every transaction.
    GasLog,             # Partial payment for a LOG operation.
    GasLogData,         # Paid for each byte in a LOG operation’s data.
    GasLogTopic,        # Paid for each topic of a LOG operation.
    GasSha3,            # Paid for each SHA3 operation.
    GasSha3Word,        # Paid for each word (rounded up) for input data to a SHA3 operation.
    GasCopy,            # Partial payment for COPY operations, multiplied by words copied, rounded up.
    GasBlockhash        # Payment for BLOCKHASH operation.
    # GasQuadDivisor      # The quadratic coefficient of the input sizes of the exponentiation-over-modulo precompiled contract.

  GasFeeSchedule = array[GasFeeKind, GasInt]

  GasParams* = object
    # Yellow Paper, Appendix H - https://ethereum.github.io/yellowpaper/paper.pdf
    # GasCost is a function of (σ, μ):
    #   - σ is the full system state
    #   - μ is the machine state
    # In practice, we often require the following from
    #   - σ: an account address
    #   - μ: a value popped from the stack or its size.

    case kind*: Op
    of Sstore:
      s_isStorageEmpty*: bool
    of Call:
      c_isNewAccount*: bool
      c_gasBalance*: GasInt
      c_contractGas*: Gasint
      c_currentMemSize*: Natural
      c_memOffset*: Natural
      c_memLength*: Natural
    else:
      discard

  GasCostKind* = enum
    GckInvalidOp,
    GckFixed,
    GckDynamic,
    GckMemExpansion,
    GckComplex

  GasResult = tuple[gasCost, gasRefund: GasInt]

  GasCost = object
    case kind*: GasCostKind
    of GckInvalidOp:
      discard
    of GckFixed:
      cost*: GasInt
    of GckDynamic:
      d_handler*: proc(value: Uint256): GasInt {.nimcall.}
    of GckMemExpansion:
      m_handler*: proc(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.}
    of GckComplex:
      c_handler*: proc(value: Uint256, gasParams: GasParams): GasResult {.nimcall.}
      # We use gasCost/gasRefund for:
      #   - Properly log and order cost and refund (for Sstore especially)
      #   - Allow to use unsigned integer in the future
      #   - CALL instruction requires passing the child message gas (Ccallgas in yellow paper)

  GasCosts* = array[Op, GasCost]

template gasCosts(FeeSchedule: GasFeeSchedule, prefix, ResultGasCostsName: untyped) =

  ## Generate the gas cost for each forks and store them in a const
  ## named `ResultGasCostsName`

  # ############### Helper functions ##############################

  func `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength: Natural): GasInt {.inline.} =
    # Input: size (in bytes)

    # Yellow Paper:
    #   Memory expansion cost
    #     = Cmem(μ′i) − Cmem(μi)
    #   μi is memory size before opcode execution
    #   μ'i is the memory size after opcode execution

    #   Cmem(a) ≡ Gmemory · a + a² / 512
    #
    #   Except when memLength = 0, where per eq (297),
    #   M(currentMemSize, f, l) = currentMemSize

    let
      prev_words = currentMemSize.wordCount
      new_words = (memOffset + memLength).wordCount

    if memLength == 0 or new_words <= prev_words:
      # Special subcase of memory-expansion cost
      # currentMemSize - currentMemSize = 0
      # "Referencing a zero length range ... does not require memory to be extended
      #  to the beginning of the range."
      #
      # Also, don't credit EVM code for allocating memory
      # then accessing lots of low memory. memoryGasCost,
      # via go-ethereum, checks for this as special case.
      return 0

    let
      prev_cost = prev_words * static(FeeSchedule[GasMemory]) +
        (prev_words ^ 2) shr 9 # div 512
      new_cost = new_words * static(FeeSchedule[GasMemory]) +
        (new_words ^ 2) shr 9 # div 512

    # TODO: add logging
    result = max(new_cost - prev_cost, 0)

  func `prefix all_but_one_64th`(gas: GasInt): GasInt {.inline.} =
    ## Computes all but 1/64th
    ## L(n) ≡ n − ⌊n/64⌋ - (floored(n/64))
    # Introduced in EIP-150 - https://github.com/ethereum/EIPs/blob/master/EIPS/eip-150.md
    # TODO: deactivate it pre-EIP150

    # Note: The all-but-one-64th calculation should occur after the memory expansion fee is taken
    # https://github.com/ethereum/yellowpaper/pull/442

    result = gas - (gas shr 6)

  # ############### Opcode gas functions ##############################

  func `prefix gasExp`(value: Uint256): GasInt {.nimcall.} =
    ## Value is the exponent

    result = static FeeSchedule[GasExp]
    if not value.isZero:
      result += static(FeeSchedule[GasExpByte]) * (1 + log256(value))

  func `prefix gasSha3`(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.} =

    result = `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength)
    result += static(FeeSchedule[GasSha3]) +
      static(FeeSchedule[GasSha3Word]) * (memLength).wordCount

  func `prefix gasCopy`(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.} =
    result = static(FeeSchedule[GasVeryLow]) +
      static(FeeSchedule[GasCopy]) * memLength.wordCount
    result += `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength)

  func `prefix gasExtCodeCopy`(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.} =
    result = static(FeeSchedule[GasExtCode]) +
      static(FeeSchedule[GasCopy]) * memLength.wordCount
    result += `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength)

  func `prefix gasLoadStore`(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.} =
    result = static(FeeSchedule[GasVeryLow])
    result += `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength)

  func `prefix gasSstore`(value: Uint256, gasParams: Gasparams): GasResult {.nimcall.} =
    ## Value is word to save

    # workaround for static evaluation not working for if expression
    const
      gSet = FeeSchedule[GasSset]
      gSreset = FeeSchedule[GasSreset]

    # Gas cost - literal translation of Yellow Paper
    result.gasCost =  if value.isZero.not and gasParams.s_isStorageEmpty:
                        gSet
                      else:
                        gSreset

    # Refund
    if value.isZero and not gasParams.s_isStorageEmpty:
      result.gasRefund = static(FeeSchedule[RefundSclear])

  func `prefix gasLog0`(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.} =
    result = `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength)

    result += static(FeeSchedule[GasLog]) +
      static(FeeSchedule[GasLogData]) * memLength

  func `prefix gasLog1`(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.} =
    result = `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength)

    result += static(FeeSchedule[GasLog]) +
      static(FeeSchedule[GasLogData]) * memLength +
      static(FeeSchedule[GasLogTopic])

  func `prefix gasLog2`(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.} =
    result = `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength)

    result += static(FeeSchedule[GasLog]) +
      static(FeeSchedule[GasLogData]) * memLength +
      static(2 * FeeSchedule[GasLogTopic])

  func `prefix gasLog3`(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.} =
    result = `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength)

    result += static(FeeSchedule[GasLog]) +
      static(FeeSchedule[GasLogData]) * memLength +
      static(3 * FeeSchedule[GasLogTopic])

  func `prefix gasLog4`(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.} =
    result = `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength)

    result += static(FeeSchedule[GasLog]) +
      static(FeeSchedule[GasLogData]) * memLength +
      static(4 * FeeSchedule[GasLogTopic])

  func `prefix gasCall`(value: Uint256, gasParams: Gasparams): GasResult {.nimcall.} =

    # From the Yellow Paper, going through the equation from bottom to top
    # https://ethereum.github.io/yellowpaper/paper.pdf#appendix.H
    #
    # More readable info on the subtleties wiki page: https://github.com/ethereum/wiki/wiki/Subtleties#other-operations
    # CALL has a multi-part gas cost:
    #
    # - 700 base
    # - 9000 additional if the value is nonzero
    # - 25000 additional if the destination account does not yet exist
    #
    # The child message of a nonzero-value CALL operation (NOT the top-level message arising from a transaction!)
    # gains an additional 2300 gas on top of the gas supplied by the calling account;
    # this stipend can be considered to be paid out of the 9000 mandatory additional fee for nonzero-value calls.
    # This ensures that a call recipient will always have enough gas to log that it received funds.
    #
    # EIP150 goes over computation: https://github.com/ethereum/eips/issues/150
    #
    # The discussion for the draft EIP-5, which proposes to change the CALL opcode also goes over
    # the current implementation - https://github.com/ethereum/EIPs/issues/8


    # First we have to take into account the costs of memory expansion:
    # Note there is a "bug" in the Ethereum Yellow Paper
    #   - https://github.com/ethereum/yellowpaper/issues/325
    #     μg already includes memory expansion costs but it is not
    #     plainly explained n the CALL opcode details

    # i.e. Cmem(μ′i) − Cmem(μi)
    #   Yellow Paper: μ′i ≡ M(M(μi,μs[3],μs[4]),μs[5],μs[6])
    #   M is the memory expansion function
    #   μ′i  is passed through gasParams.memRequested
    # TODO:
    #   - Py-EVM has costs for both input and output memory expansion
    #     https://github.com/ethereum/py-evm/blob/eed0bfe4499b394ee58113408e487e7d35ab88d6/evm/vm/logic/call.py#L56-L57
    #   - Parity only for the largest expansion
    #     https://github.com/paritytech/parity/blob/af1088ef61323f171915555023d8e993aaaed755/ethcore/evm/src/interpreter/gasometer.rs#L192-L195
    #   - Go-Ethereum only has one cost
    #     https://github.com/ethereum/go-ethereum/blob/13af27641829f61d1e6b383e37aab6caae22f2c1/core/vm/gas_table.go#L334
    # ⚠⚠ Py-EVM seems wrong if memory is needed for both in and out.
    result.gasCost =  `prefix gasMemoryExpansion`(
                        gasParams.c_currentMemSize,
                        gasParams.c_memOffset,
                        gasParams.c_memLength
                      )

    # Cnew_account - TODO - pre-EIP158 zero-value call consumed 25000 gas
    #                https://github.com/ethereum/eips/issues/158
    if gasParams.c_isNewAccount and not value.isZero:
      result.gasCost += static(FeeSchedule[GasNewAccount])

    # Cxfer
    if not value.isZero:
      result.gasCost += static(FeeSchedule[GasCallValue])

    # Cextra
    result.gasCost += static(FeeSchedule[GasCall])
    let cextra = result.gasCost

    # Cgascap
    result.gasCost =  if gasParams.c_gasBalance >= result.gasCost:
                        min(
                          `prefix all_but_one_64th`(gasParams.c_gasBalance - result.gasCost),
                          gasParams.c_contract_gas
                          )
                      else:
                        gasParams.c_contract_gas

    # Ccallgas - Gas sent to the child message
    result.gasRefund = result.gasCost
    if not value.isZero:
      result.gasRefund += static(FeeSchedule[GasCallStipend])

    # Ccall
    result.gasCost += cextra

  func `prefix gasHalt`(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.} =
    `prefix gasMemoryExpansion`(currentMemSize, memOffset, memLength)

  func `prefix gasSelfDestruct`(value: Uint256, gasParams: Gasparams): GasResult {.nimcall.} =
    # TODO
    discard

  # ###################################################################################################

  # TODO - change this `let` into `const` - pending: https://github.com/nim-lang/Nim/issues/8015
  let `ResultGasCostsName`*{.inject.}: GasCosts = block:
    # We use a block expression to avoid name redefinition conflicts
    # with "fixed" and "dynamic"

    # Syntactic sugar
    func fixed(gasFeeKind: static[GasFeeKind]): GasCost =
      GasCost(kind: GckFixed, cost: static(FeeSchedule[gasFeeKind]))

    func dynamic(handler: proc(value: Uint256): GasInt {.nimcall.}): GasCost =
      GasCost(kind: GckDynamic, d_handler: handler)

    func memExpansion(handler: proc(currentMemSize, memOffset, memLength: Natural): GasInt {.nimcall.}): GasCost =
      GasCost(kind: GckMemExpansion, m_handler: handler)

    func complex(handler: proc(value: Uint256, gasParams: GasParams): GasResult {.nimcall.}): GasCost =
      GasCost(kind: GckComplex, c_handler: handler)

    # Returned value
    fill_enum_table_holes(Op, GasCost(kind: GckInvalidOp)):
      [
          # 0s: Stop and Arithmetic Operations
          Stop:            fixed GasZero,
          Add:             fixed GasVeryLow,
          Mul:             fixed GasLow,
          Sub:             fixed GasVeryLow,
          Div:             fixed GasLow,
          Sdiv:            fixed GasLow,
          Mod:             fixed GasLow,
          Smod:            fixed GasLow,
          Addmod:          fixed GasMid,
          Mulmod:          fixed GasMid,
          Exp:             dynamic `prefix gasExp`,
          SignExtend:      fixed GasLow,

          # 10s: Comparison & Bitwise Logic Operations
          Lt:              fixed GasVeryLow,
          Gt:              fixed GasVeryLow,
          Slt:             fixed GasVeryLow,
          Sgt:             fixed GasVeryLow,
          Eq:              fixed GasVeryLow,
          IsZero:          fixed GasVeryLow,
          And:             fixed GasVeryLow,
          Or:              fixed GasVeryLow,
          Xor:             fixed GasVeryLow,
          Not:             fixed GasVeryLow,
          Byte:            fixed GasVeryLow,

          # 20s: SHA3
          Sha3:            memExpansion `prefix gasSha3`,

          # 30s: Environmental Information
          Address:         fixed GasBase,
          Balance:         fixed GasBalance,
          Origin:          fixed GasBase,
          Caller:          fixed GasBase,
          CallValue:       fixed GasBase,
          CallDataLoad:    fixed GasVeryLow,
          CallDataSize:    fixed GasBase,
          CallDataCopy:    memExpansion `prefix gasCopy`,
          CodeSize:        fixed GasBase,
          CodeCopy:        memExpansion `prefix gasCopy`,
          GasPrice:        fixed GasBase,
          ExtCodeSize:     fixed GasExtcode,
          ExtCodeCopy:     memExpansion `prefix gasExtCodeCopy`,
          ReturnDataSize:  fixed GasBase,
          ReturnDataCopy:  memExpansion `prefix gasCopy`,

          # 40s: Block Information
          Blockhash:       fixed GasBlockhash,
          Coinbase:        fixed GasBase,
          Timestamp:       fixed GasBase,
          Number:          fixed GasBase,
          Difficulty:      fixed GasBase,
          GasLimit:        fixed GasBase,

          # 50s: Stack, Memory, Storage and Flow Operations
          Pop:            fixed GasBase,
          Mload:          memExpansion `prefix gasLoadStore`,
          Mstore:         memExpansion `prefix gasLoadStore`,
          Mstore8:        memExpansion `prefix gasLoadStore`,
          Sload:          fixed GasSload,
          Sstore:         complex `prefix gasSstore`,
          Jump:           fixed GasMid,
          JumpI:          fixed GasHigh,
          Pc:             fixed GasBase,
          Msize:          fixed GasBase,
          Gas:            fixed GasBase,
          JumpDest:       fixed GasJumpDest,

          # 60s & 70s: Push Operations
          Push1:          fixed GasVeryLow,
          Push2:          fixed GasVeryLow,
          Push3:          fixed GasVeryLow,
          Push4:          fixed GasVeryLow,
          Push5:          fixed GasVeryLow,
          Push6:          fixed GasVeryLow,
          Push7:          fixed GasVeryLow,
          Push8:          fixed GasVeryLow,
          Push9:          fixed GasVeryLow,
          Push10:         fixed GasVeryLow,
          Push11:         fixed GasVeryLow,
          Push12:         fixed GasVeryLow,
          Push13:         fixed GasVeryLow,
          Push14:         fixed GasVeryLow,
          Push15:         fixed GasVeryLow,
          Push16:         fixed GasVeryLow,
          Push17:         fixed GasVeryLow,
          Push18:         fixed GasVeryLow,
          Push19:         fixed GasVeryLow,
          Push20:         fixed GasVeryLow,
          Push21:         fixed GasVeryLow,
          Push22:         fixed GasVeryLow,
          Push23:         fixed GasVeryLow,
          Push24:         fixed GasVeryLow,
          Push25:         fixed GasVeryLow,
          Push26:         fixed GasVeryLow,
          Push27:         fixed GasVeryLow,
          Push28:         fixed GasVeryLow,
          Push29:         fixed GasVeryLow,
          Push30:         fixed GasVeryLow,
          Push31:         fixed GasVeryLow,
          Push32:         fixed GasVeryLow,

          # 80s: Duplication Operations
          Dup1:           fixed GasVeryLow,
          Dup2:           fixed GasVeryLow,
          Dup3:           fixed GasVeryLow,
          Dup4:           fixed GasVeryLow,
          Dup5:           fixed GasVeryLow,
          Dup6:           fixed GasVeryLow,
          Dup7:           fixed GasVeryLow,
          Dup8:           fixed GasVeryLow,
          Dup9:           fixed GasVeryLow,
          Dup10:          fixed GasVeryLow,
          Dup11:          fixed GasVeryLow,
          Dup12:          fixed GasVeryLow,
          Dup13:          fixed GasVeryLow,
          Dup14:          fixed GasVeryLow,
          Dup15:          fixed GasVeryLow,
          Dup16:          fixed GasVeryLow,

          # 90s: Exchange Operations
          Swap1:          fixed GasVeryLow,
          Swap2:          fixed GasVeryLow,
          Swap3:          fixed GasVeryLow,
          Swap4:          fixed GasVeryLow,
          Swap5:          fixed GasVeryLow,
          Swap6:          fixed GasVeryLow,
          Swap7:          fixed GasVeryLow,
          Swap8:          fixed GasVeryLow,
          Swap9:          fixed GasVeryLow,
          Swap10:         fixed GasVeryLow,
          Swap11:         fixed GasVeryLow,
          Swap12:         fixed GasVeryLow,
          Swap13:         fixed GasVeryLow,
          Swap14:         fixed GasVeryLow,
          Swap15:         fixed GasVeryLow,
          Swap16:         fixed GasVeryLow,

          # a0s: Logging Operations
          Log0:           memExpansion `prefix gasLog0`,
          Log1:           memExpansion `prefix gasLog1`,
          Log2:           memExpansion `prefix gasLog2`,
          Log3:           memExpansion `prefix gasLog3`,
          Log4:           memExpansion `prefix gasLog4`,

          # f0s: System operations
          Create:         fixed GasCreate, # TODO, dynamic cost
          Call:           complex `prefix gasCall`,
          CallCode:       complex `prefix gasCall`,
          Return:         memExpansion `prefix gasHalt`,
          DelegateCall:   complex `prefix gasCall`,
          StaticCall:     complex `prefix gasCall`,
          Op.Revert:      memExpansion `prefix gasHalt`,
          Invalid:        fixed GasZero,
          SelfDestruct:   complex `prefix gasSelfDestruct`
        ]

# Generate the fork-specific gas costs tables
const
  BaseGasFees: GasFeeSchedule = [
    # Fee Schedule at for the initial Ethereum forks
    GasZero:            0'i64,
    GasBase:            2,
    GasVeryLow:         3,
    GasLow:             5,
    GasMid:             8,
    GasHigh:            10,
    GasExtCode:         20,     # Changed to 700 in Tangerine (EIP150)
    GasBalance:         20,     # Changed to 400 in Tangerine (EIP150)
    GasSload:           50,     # Changed to 200 in Tangerine (EIP150)
    GasJumpDest:        1,
    GasSset:            20_000,
    GasSreset:          5_000,
    RefundSclear:       15_000,
    RefundSelfDestruct: 24_000,
    GasSelfDestruct:    0,      # Changed to 5000 in Tangerine (EIP150)
    GasCreate:          32000,
    GasCodeDeposit:     200,
    GasCall:            40,     # Changed to 700 in Tangerine (EIP150)
    GasCallValue:       9000,
    GasCallStipend:     2300,
    GasNewAccount:      25_000,
    GasExp:             10,
    GasExpByte:         10,     # Changed to 50 in Spurious Dragon (EIP160)
    GasMemory:          3,
    GasTXCreate:        32000,
    GasTXDataZero:      4,
    GasTXDataNonZero:   68,
    GasTransaction:     21000,
    GasLog:             375,
    GasLogData:         8,
    GasLogTopic:        375,
    GasSha3:            30,
    GasSha3Word:        6,
    GasCopy:            3,
    GasBlockhash:       20
    # GasQuadDivisor:     100     # Unused, do not confuse with the quadratic coefficient 512 for memory expansion
  ]

# Create the schedule for each forks
func tangerineGasFees(previous_fees: GasFeeSchedule): GasFeeSchedule =
  # https://github.com/ethereum/EIPs/blob/master/EIPS/eip-150.md
  result = previous_fees
  result[GasSload]        = 200
  result[GasSelfDestruct] = 5000
  result[GasBalance]      = 400
  result[GasCall]         = 40

func spuriousGasFees(previous_fees: GasFeeSchedule): GasFeeSchedule =
  # https://github.com/ethereum/EIPs/blob/master/EIPS/eip-160.md
  result = previous_fees
  result[GasExpByte]      = 50

const
  TangerineGasFees = BaseGasFees.tangerineGasFees
  SpuriousGasFees = TangerineGasFees.spuriousGasFees

gasCosts(BaseGasFees, base, BaseGasCosts)
gasCosts(TangerineGasFees, tangerine, TangerineGasCosts)

proc forkToSchedule*(fork: Fork): GasCosts =
  if fork < FkTangerine:
    BaseGasCosts
  else:
    TangerineGasCosts