feat: add blob fee calc functions (#4440)

crates/primitives/src/constants/eip4844.rs

@@ -1,6 +1,6 @@
-//! [EIP-4844](https://eips.ethereum.org/EIPS/eip-4844#parameters) protocol constants for shard Blob Transactions.
+//! [EIP-4844](https://eips.ethereum.org/EIPS/eip-4844#parameters) protocol constants and utils for shard Blob Transactions.
 
-use crate::kzg::KzgSettings;
+use crate::{kzg::KzgSettings, U256};
 use once_cell::sync::Lazy;
 use std::{io::Write, sync::Arc};
 
@@ -25,9 +25,12 @@ pub const MAX_BLOBS_PER_BLOCK: u64 = MAX_DATA_GAS_PER_BLOCK / DATA_GAS_PER_BLOB;
 /// Target number of data blobs in a single block.
 pub const TARGET_BLOBS_PER_BLOCK: u64 = TARGET_DATA_GAS_PER_BLOCK / DATA_GAS_PER_BLOB; // 393216 / 131072 = 3
 
-/// Used to determine the price for next data blob
+/// Determines the maximum rate of change for blob fee
 pub const BLOB_GASPRICE_UPDATE_FRACTION: u64 = 3_338_477u64; // 3338477
 
+/// Minimum gas price for a data blob
+pub const BLOB_TX_MIN_BLOB_GASPRICE: u64 = 1u64;
+
 /// Commitment version of a KZG commitment
 pub const VERSIONED_HASH_VERSION_KZG: u8 = 0x01;
 
@@ -63,6 +66,32 @@ pub enum LoadKzgSettingsError {
     KzgError(c_kzg::Error),
 }
 
+/// Calculates the blob fee for the given excess blob gas.
+pub fn blob_fee(excess_blob_gas: u64) -> U256 {
+    fake_exponential(
+        U256::from(BLOB_TX_MIN_BLOB_GASPRICE),
+        U256::from(excess_blob_gas),
+        U256::from(BLOB_GASPRICE_UPDATE_FRACTION),
+    )
+}
+
+/// Approximates factor * e ** (numerator / denominator) using Taylor expansion.
+///
+/// This is used to calculate the blob price.
+///
+/// See also <https://eips.ethereum.org/EIPS/eip-4844#helpers>
+pub fn fake_exponential(factor: U256, numerator: U256, denominator: U256) -> U256 {
+    let mut output = U256::ZERO;
+    let mut numerator_accum = factor.saturating_mul(denominator);
+    let mut i = U256::from(1u64);
+    while numerator_accum > U256::ZERO {
+        output += numerator_accum;
+        numerator_accum = numerator_accum * numerator / (denominator * i);
+        i += U256::from(1u64);
+    }
+    output / denominator
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -71,4 +100,29 @@ mod tests {
     fn ensure_load_kzg_settings() {
         let _settings = Arc::clone(&MAINNET_KZG_TRUSTED_SETUP);
     }
+
+    #[test]
+    fn test_fake_exp() {
+        // <https://github.com/ethereum/go-ethereum/blob/28857080d732857030eda80c69b9ba2c8926f221/consensus/misc/eip4844/eip4844_test.go#L78-L78>
+        for (factor, num, denom, expected) in &[
+            (1u64, 0u64, 1u64, 1u64),
+            (38493, 0, 1000, 38493),
+            (0, 1234, 2345, 0),
+            (1, 2, 1, 6), // approximate 7.389
+            (1, 4, 2, 6),
+            (1, 3, 1, 16), // approximate 20.09
+            (1, 6, 2, 18),
+            (1, 4, 1, 49), // approximate 54.60
+            (1, 8, 2, 50),
+            (10, 8, 2, 542), // approximate 540.598
+            (11, 8, 2, 596), // approximate 600.58
+            (1, 5, 1, 136),  // approximate 148.4
+            (1, 5, 2, 11),   // approximate 12.18
+            (2, 5, 2, 23),   // approximate 24.36
+            (1, 50000000, 2225652, 5709098764),
+        ] {
+            let res = fake_exponential(U256::from(*factor), U256::from(*num), U256::from(*denom));
+            assert_eq!(res, U256::from(*expected));
+        }
+    }
 }

crates/primitives/src/header.rs

@@ -8,6 +8,7 @@ use crate::{
 };
 use bytes::{Buf, BufMut, BytesMut};
 
+use crate::constants::eip4844::blob_fee;
 use reth_codecs::{add_arbitrary_tests, derive_arbitrary, main_codec, Compact};
 use reth_rlp::{length_of_length, Decodable, Encodable, EMPTY_LIST_CODE, EMPTY_STRING_CODE};
 use serde::{Deserialize, Serialize};
@@ -183,6 +184,22 @@ impl Header {
         }
     }
 
+    /// Returns the blob fee for _this_ block according to the EIP-4844 spec.
+    ///
+    /// Returns `None` if `excess_blob_gas` is None
+    pub fn blob_fee(&self) -> Option<U256> {
+        self.excess_blob_gas.map(blob_fee)
+    }
+
+    /// Returns the blob fee for the next block according to the EIP-4844 spec.
+    ///
+    /// Returns `None` if `excess_blob_gas` is None.
+    ///
+    /// See also [Self::next_block_excess_blob_gas]
+    pub fn next_block_blob_fee(&self) -> Option<U256> {
+        self.next_block_excess_blob_gas().map(blob_fee)
+    }
+
     /// Calculate base fee for next block according to the EIP-1559 spec.
     ///
     /// Returns a `None` if no base fee is set, no EIP-1559 support