feat: add spec of mint manager

specs/protocol/mint-manager.md

@@ -0,0 +1,129 @@
+# MintManager
+
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
+**Table of Contents**
+
+- [MintManager](#mintmanager)
+  - [Overview](#overview)
+  - [Mint Token Transaction Type](#mint-token-transaction-type)
+    - [Mint Token Caller Account](#mint-token-caller-account)
+  - [How to Determine a Mint Amount](#how-to-determine-a-mint-amount)
+  - [Distribution](#distribution)
+  - [Initial Minting and Distribution](#initial-minting-and-distribution)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+
+<!-- All glossary references in this file. -->
+
+[g-transaction-type]: ../glossary.md#transaction-type
+[g-deposited]: ../glossary.md#deposited-transaction
+[g-l1-attr-deposit]: ../glossary.md#l1-attributes-deposited-transaction
+[g-eoa]: ../glossary.md#eoa
+
+## Overview
+
+The `GovernanceToken` of Kroma is minted every block by the `MintManager` contract on L2 and distributed to recipients.
+`MintManager` is a predeployed contract on L2 at address `0x4200000000000000000000000000000000000070`.
+
+## Mint Token Transaction Type
+
+[mint-token-tx-type]: #mint-token-transaction-type
+
+We define a new [EIP-2718] compatible transaction type with the prefix `0x70` to represent a mint token transaction.
+
+[EIP-2718]: https://eips.ethereum.org/EIPS/eip-2718
+
+In contrast to [EIP-155] transactions, this transaction type:
+
+- Does not include signature information, and makes the `from` address explicit.
+  API responses contain zeroed signature `v`, `r`, `s` values for backwards compatibility.
+- Does not include the ETH value. API responses contain zero ETH value for backwards compatibility.
+
+The mint token transaction follows directly after the [L1 attributes deposited transaction][g-l1-attr-deposit].
+his transaction calls the `mint()` function of the `MintManager` contract on L2.
+The `from` of the transaction is always a system account, and the `to` is the `MintManager` contract address.
+Like the [Deposited Transaction][g-deposited], this transaction does not have a signature, and users cannot execute it.
+
+This transaction MUST have the following values:
+
+1. `from` is `0xdeaddeaddeaddeaddeaddeaddeaddeaddead0001` (the address of the
+   [Mint token caller account][mint-token-caller-account])
+2. `to` is `0x420000000000000000000000000000000000070` (the address of the `MintManager` contract).
+3. `gasLimit` is set to `1,000,000`.
+4. `nonce` is set to the current L2 block number.
+5. `data` is an [ABI] encoded call to the `MintManager`'s `mint()` function.
+
+This transaction type is not charged any ETH for its allocated `gasLimit`,
+as it is effectively part of the state-transition processing.
+
+### Mint Token Caller Account
+
+[mint-token-caller-account]: #mint-token-caller-account
+
+The mint token caller account is an [EOA][g-eoa] with no known private key. It has the address
+`0xdeaddeaddeaddeaddeaddeaddeaddeaddead0070`. Its value is returned by the `CALLER` and `ORIGIN`
+opcodes during execution of the mint token transaction.
+
+## How to Determine a Mint Amount
+
+The amount of `GovernanceToken` minted per block decreases periodically.
+The amount to be minted is calculated by repeatedly applying decay based on the current block number for a
+certain number of epochs.
+An epoch can be calculated by dividing the current block number by `SLIDING_WINDOW_BLOCKS` value.
+
+Only up to 8 decimal places are considered for the minting amount.
+
+```python
+def get_mint_amount_per_block(block_number):
+    epoch = block_number // SLIDING_WINDOW_BLOCKS + 1
+
+    if block_number % SLIDING_WINDOW_BLOCKS == 0:
+        epoch = epoch - 1
+
+    mint_amount = INIT_MINT_PER_BLOCK
+    for i in range(1, epoch):
+        mint_amount = (mint_amount * DECAYING_FACTOR) // DECAYING_DENOMINATOR
+        mint_amount = mint_amount // FLOOR_UNIT * FLOOR_UNIT
+
+    return mint_amount
+```
+
+- `SLIDING_WINDOW_BLOCKS`: The period (in blocks) which the mint amount decreases.
+- `INIT_MINT_PER_BLOCK`: The amount minted per block during the first epoch.
+- `DECAYING_FACTOR`: The ratio by which the mint amount decreases from the previous period.
+- `DECAYING_DENOMINATOR`: The denominator of the decaying factor. This value is `10^5`.
+- `FLOOR_UNIT`: The unit for rounding down deciaml places. Since the decimal places for `GovernanceToken` are 18,
+  and the mint amount is considered up to 8 decimal places, this value is `10^10`
+
+## Distribution
+
+After deploying the MintManager contract, the recipient addresses and their respective share percentages
+are set during initialization.
+The minted amount for the current block is then distributed to the configured addresses.
+
+## Initial Minting and Distribution
+
+Mints the amount of tokens that should have been minted from the genesis block to the current block.
+This function operates only if the total supply of the `GovernanceToken` is 0.
+
+```python
+def initial_mint(block_number):
+	if supply > 0:
+        return
+
+	amount = 0
+	mint_per_block = INIT_MINT_PER_BLOCK
+	epoch = block_number // SLIDING_WINDOW + 1
+    offset = block_number % SLIDING_WINDOW
+
+	for i in range(1, epoch):
+		amount = amount + mint_per_block * SLIDING_WINDOW
+		mint_per_block = (mint_per_block * DECAYING_FACTOR) // DECAYING_DENOMINATOR
+		mint_per_block = (mint_per_block // FLOOR_UNIT) * FLOOR_UNIT
+
+    if offset > 0:
+        amount = amount + mint_per_block * offset
+
+```