nim-eth-contracts/examples/wrc20.nim

## ewasm “WRC20” token contract coding challenge
## https://gist.github.com/axic/16158c5c88fbc7b1d09dfa8c658bc363

import ../eth_contracts, stint

proc do_balance() =
  if getCallDataSize() != 24:
    revert(nil, 0)

  var address: array[20, byte]
  callDataCopy(address, 4)

  var balance: array[32, byte]
  storageLoad(address, addr balance)
  ret(addr balance, sizeof(balance).int32)

proc do_transfer() =
  if getCallDataSize() != 32:
    revert(nil, 0)

  var sender: array[20, byte]
  getCaller(addr sender)
  var recipient: array[20, byte]
  callDataCopy(recipient, 4)
  var value: uint64
  callDataCopy(value, 24)

  var senderBalance: array[32, byte]
  storageLoad(sender, addr senderBalance)
  var recipientBalance: array[32, byte]
  storageLoad(recipient, addr recipientBalance)

  var sb = readUintBE[256](senderBalance)
  var rb = readUintBE[256](recipientBalance)
  let v = value.u256

  block:
    # This is a quick stub to avoid using stint for now. Works only with
    # single byte values.
    if sb < v:
      revert(nil, 0)

    sb -= v
    rb += v

  senderBalance = sb.toByteArrayBE()
  recipientBalance = rb.toByteArrayBE()
  storageStore(sender, addr senderBalance)
  storageStore(recipient, addr recipientBalance)

proc main() {.exportwasm.} =
  if getCallDataSize() < 4:
    revert(nil, 0)
  var selector: uint32
  callDataCopy(selector, 0)
  case selector
  of 0x9993021a'u32:
    do_balance()
  of 0x5d359fbd'u32:
    do_transfer()
  else:
    revert(nil, 0)