From 7bab5d4691fcca427609db3f105bfb84365dd922 Mon Sep 17 00:00:00 2001
From: Mohammad Nassar <mohammad@starkware.co>
Date: Tue, 30 Jul 2024 14:32:48 +0300
Subject: [PATCH] feat(mempool): impl calculate-gas-price according to EIP 1559

---
 crates/batcher/src/fee_market.rs | 70 ++++++++++++++++++++++++++++++++
 crates/batcher/src/lib.rs        |  1 +
 2 files changed, 71 insertions(+)
 create mode 100644 crates/batcher/src/fee_market.rs

diff --git a/crates/batcher/src/fee_market.rs b/crates/batcher/src/fee_market.rs
new file mode 100644
index 0000000000..2ae88bf04c
--- /dev/null
+++ b/crates/batcher/src/fee_market.rs
@@ -0,0 +1,70 @@
+use std::cmp::max;
+
+//  This constant is used to calculate the base gas price for the next block according to EIP-1559
+// and serves as a sensitivity parameter that limits the maximum rate of change of the gas price
+// between consecutive blocks.
+const GAS_PRICE_MAX_CHANGE_DENOMINATOR: u128 = 48;
+const MIN_GAS_PRICE: u64 = 100000; // In fri.
+// TODO(Mohammad): Check the exact value for maximum block size in StarkNet.
+const MAX_BLOCK_SIZE: u64 = 4000000000; // In gas units. It's equivalent to 40M gas steps, with 100 gas units per step.
+
+/// Calculate the base gas price for the next block according to EIP-1559.
+///
+/// # Parameters
+/// - `price`: The base fee of the current block.
+/// - `gas_used`: The total gas used in the current block.
+/// - `gas_target`: The target gas usage per block (usually half of the gas limit).
+pub fn calculate_next_base_gas_price(price: u64, gas_used: u64, gas_target: u64) -> u64 {
+    // Setting the target at 50% of the max block size balances the rate of gas price changes,
+    // helping to prevent sudden spikes, particularly during increases, for a better user
+    // experience.
+    assert_eq!(
+        gas_target,
+        MAX_BLOCK_SIZE / 2,
+        "Gas target must be 50% of max block size to balance price changes."
+    );
+    // To prevent precision loss during multiplication and division, we set a minimum gas price.
+    // Additionally, a minimum gas price is established to prevent prolonged periods before the
+    // price reaches a higher value.
+    assert!(
+        price >= MIN_GAS_PRICE,
+        "The gas price must be at least the minimum to prevent precision loss during \
+         multiplication and division."
+    );
+
+    // We use unsigned integers (u64 and u128) to avoid overflow issues, as the input values are
+    // naturally unsigned and i256 is unstable in Rust. This approach allows safe handling of
+    // all inputs using u128 for intermediate calculations.
+
+    // The absolute difference between gas_used and gas_target is always u64.
+    let gas_delta = gas_used.abs_diff(gas_target);
+    // Convert to u128 to prevent overflow, as a product of two u64 fits inside a u128.
+    let price_u128 = u128::from(price);
+    let gas_delta_u128 = u128::from(gas_delta);
+    let gas_target_u128 = u128::from(gas_target);
+
+    // Calculate the gas change as u128 to handle potential overflow during multiplication.
+    let gas_delta_cost =
+        price_u128.checked_mul(gas_delta_u128).expect("Both variables originate from u64");
+    // Calculate the price change, maintaining precision by dividing after scaling up.
+    // This avoids significant precision loss that would occur if dividing before
+    // multiplication.
+    let price_change_u128 = gas_delta_cost / gas_target_u128 / GAS_PRICE_MAX_CHANGE_DENOMINATOR;
+
+    // Convert back to u64, as the price change should fit within the u64 range.
+    // Since the target is half the maximum block size (which fits within a u64), the gas delta
+    // is bounded by half the maximum block size. Therefore, after dividing by the gas target
+    // (which is half the maximum block size), the result is guaranteed to fit within a u64.
+    let price_change = u64::try_from(price_change_u128)
+        .expect("Result fits u64 after division of a bounded gas delta");
+
+    let adjusted_price =
+        if gas_used > gas_target { price + price_change } else { price - price_change };
+
+    assert!(
+        gas_used > gas_target && adjusted_price >= price
+            || gas_used <= gas_target && adjusted_price <= price
+    );
+
+    max(adjusted_price, MIN_GAS_PRICE)
+}
diff --git a/crates/batcher/src/lib.rs b/crates/batcher/src/lib.rs
index 5e5c03af26..5055c49d42 100644
--- a/crates/batcher/src/lib.rs
+++ b/crates/batcher/src/lib.rs
@@ -1,6 +1,7 @@
 pub mod batcher;
 pub mod communication;
 pub mod config;
+pub mod fee_market;
 pub mod proposals_manager;
 #[cfg(test)]
 mod proposals_manager_test;