// Copyright (c) 2018-2023 The Decred developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.

/*
Package mempool provides a policy-enforced pool of unmined Decred transactions.

A key responsibility of the Decred network is mining transactions – regular
transactions and stake transactions – into blocks.  In order to facilitate
this, the mining process relies on having a readily-available source of
transactions to include in a block that is being solved.

At a high level, this package satisfies that requirement by providing an
in-memory pool of fully validated transactions that can also optionally be
further filtered based upon a configurable policy.

The Policy configuration options has flags that control whether or not
"standard" transactions and old votes are accepted into the mempool.
In essence, a "standard" transaction is one that satisfies a fairly
strict set of requirements that are largely intended to help provide
fair use of the system to all users.  It is important to note that
what is considered to be a "standard" transaction changes over time
as policy and consensus rules evolve. For some insight, at the time
of this writing, an example of _some_ of the criteria that are required
for a transaction to be considered standard are that it is of the
most-recently supported version, finalized, does not exceed a specific size,
and only consists of specific script forms.

Since this package does not deal with other Decred specifics such as network
communication and transaction relay, it returns a list of transactions that were
accepted which gives the caller a high level of flexibility in how they want to
proceed.  Typically, this will involve things such as relaying the transactions
to other peers on the network and notifying the mining process that new
transactions are available.

# Feature Overview

The following is a quick overview of the major features.  It is not intended to
be an exhaustive list.

  - Maintains a pool of fully validated transactions
  - Rejects non-fully-spent duplicate transactions
  - Rejects coinbase transactions
  - Rejects double spends (both from the chain and other transactions in pool)
  - Rejects invalid transactions according to the network consensus rules
  - Full script execution and validation with signature cache support
  - Individual transaction query support
  - Stake transaction support (ticket purchases, votes and revocations)
  - Option to accept or reject old votes
  - Orphan transaction support (transactions that spend from unknown outputs)
  - Configurable limits (see transaction acceptance policy)
  - Automatic addition of orphan transactions that are no longer orphans as new
    transactions are added to the pool
  - Individual orphan transaction query support
  - Configurable transaction acceptance policy
  - Additional metadata tracking for each transaction
  - Manual control of transaction removal
  - Recursive removal of all dependent transactions

# Configurable Transaction Acceptance Policy

  - Option to accept or reject standard transactions
  - Option to accept or reject transactions based on priority calculations
  - Minimum fee threshold
  - Max signature operations per transaction
  - Max orphan transaction size
  - Max number of orphan transactions allowed

# Additional Per-Transaction Metadata Tracking

  - Timestamp when the transaction was added to the pool
  - Most recent block height when the transaction was added to the pool
  - The fee the transaction pays
  - The starting priority for the transaction

# Errors

The errors returned by this package are either the raw errors provided by
underlying calls or of type mempool.RuleError.  Since there are two classes of
rules (mempool acceptance rules and blockchain (consensus) acceptance rules),
the mempool.RuleError type contains a single Err field which will, in turn,
either be a mempool.TxRuleError or a blockchain.RuleError.  The first indicates
a violation of mempool acceptance rules while the latter indicates a violation
of consensus acceptance rules.  This allows the caller to easily differentiate
between unexpected errors, such as database errors, versus errors due to rule
violations through type assertions.  In addition, callers can programmatically
determine the specific rule violation by type asserting the Err field to one of
the aforementioned types and examining their underlying ErrorKind field.
*/
package mempool
