[SIDDHANTCOOKIE]

Summary

Phase 3, PR 1 (stacked on #168). This adds a DEX as a Python contract, using a batch auction instead of an AMM or a continuous order book. Orders collect in a batch, and clear settles every crossing order at one uniform price. Everyone in the batch trades at that same price, so the order in which orders arrived, or were placed in a block, doesn't change what anyone gets.

There are no consensus changes. It uses the existing multi-asset msg['value'] and transfer_out.

Contract (examples/batch_auction.py)

• init:<base_asset>:<quote_asset> sets up one market, once. The native coin is "".
• buy:<amount>:<price> requires sending exactly amount × price of the quote asset. sell:<amount>:<price> requires sending exactly amount of the base asset. Prices are integers: quote units per one base unit.
• cancel:<order_id> is owner only, and refunds the escrow.
• clear can be called by anyone:
  • sort buys by price descending and sells ascending, oldest first on ties
  • try each order price as a candidate, keeping demand and supply as running sums, so this is O(k log k) for k orders
  • pick the price that trades the most base; ties go to the smaller demand/supply gap, then to the lower price
  • fill by price priority and then age, with the marginal order partly filled
  • pay buyers the filled base plus any unused quote, and pay sellers filled × price quote plus any unsold base
  • unmatched orders are fully refunded, and the book is emptied
  • base and quote are conserved by construction, and the contract holds nothing after a clear
• MAX_ORDERS = 100 per batch keeps every call's gas bounded.

Sandbox (minichain/contract.py)

• Adds int and sorted to the allowed builtins. Both are pure and add nothing new: range/sum/list already run in C without gas metering.
• The sandbox still rejects *, ** and @, so the contract multiplies integers with a small doubling helper (O(log n) additions).

Measurements

A full batch of 100 random orders:

• each order call used 2,257–92,629 gas (it grows with batch size, because each call is charged for the contract's whole storage)
• the 101st order was refused
• clear used 46,137 gas and left a zero balance in the contract

Known limits

• Whoever calls clear chooses when the batch ends, and can add their own orders before calling it. Uniform pricing limits what that can gain, but a fixed batch schedule needs block height in msg, which would be a separate change.
• One market per deployed contract.

Test Plan

• Added tests/test_batch_auction.py (12 tests, all run end to end through State.apply_transaction):
  • a crossing book clears at one price with the correct fills and refunds
  • a book that doesn't cross refunds everyone
  • the marginal order is partly filled
  • cancel works for the owner only
  • 5 kinds of bad orders are rejected and the funds stay put
  • a second init is rejected
  • the order cap is enforced
  • random books conserve both assets
• Full suite: 434 passed.

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