refactor: use ordered publisher list

This change brings down the average cost of a price update without
aggregation from 5k to 2k because binary search requires much less
lookups. There are mechanims implemented in the code to make sure
that upon the upgrade we sort the array without adding overhead to
the happy path (working binary search). Also add_publisher now does
an insertion sort to keep the list sorted and del_publisher won't
change the order of the list.

Author: ali-bahjati

program/rust/src/processor/add_publisher.rs

@@ -73,13 +73,21 @@ pub fn add_publisher(
         }
     }
 
-    let current_index: usize = try_convert(price_data.num_)?;
+    let mut current_index: usize = try_convert(price_data.num_)?;
     sol_memset(
         bytes_of_mut(&mut price_data.comp_[current_index]),
         0,
         size_of::<PriceComponent>(),
     );
     price_data.comp_[current_index].pub_ = cmd_args.publisher;
+
+    while current_index > 0
+        && price_data.comp_[current_index].pub_ < price_data.comp_[current_index - 1].pub_
+    {
+        price_data.comp_.swap(current_index, current_index - 1);
+        current_index -= 1;
+    }
+
     price_data.num_ += 1;
     price_data.header.size = try_convert::<_, u32>(PriceAccount::INITIAL_SIZE)?;
     Ok(())

program/rust/src/processor/upd_price.rs

@@ -127,27 +127,64 @@ pub fn upd_price(
     // Check clock
     let clock = Clock::from_account_info(clock_account)?;
 
-    let mut publisher_index: usize = 0;
+    let publisher_index: usize;
     let latest_aggregate_price: PriceInfo;
 
     // The price_data borrow happens in a scope because it must be
     // dropped before we borrow again as raw data pointer for the C
     // aggregation logic.
     {
         // Verify that symbol account is initialized
-        let price_data = load_checked::<PriceAccount>(price_account, cmd_args.header.version)?;
+        let mut price_data = load_checked::<PriceAccount>(price_account, cmd_args.header.version)?;
 
-        // Verify that publisher is authorized
-        while publisher_index < try_convert::<u32, usize>(price_data.num_)? {
-            if price_data.comp_[publisher_index].pub_ == *funding_account.key {
-                break;
+        // Verify that publisher is authorized by initially binary searching
+        // for the publisher's component in the price account. The binary
+        // search might not work if the publisher list is not sorted; therefore
+        // we fall back to a linear search and during the search we try to bring
+        // the publishers list closer to a sorted state. (We do not sort them
+        // entirely to avoid exceeding the compute budget.)
+        let mut lo = 0;
+        let mut hi = try_convert::<u32, usize>(price_data.num_)?;
+
+        let mut index = None;
+        while lo < hi {
+            let mid = lo + (hi - lo) / 2;
+            match price_data.comp_[mid].pub_.cmp(funding_account.key) {
+                std::cmp::Ordering::Less => {
+                    lo = mid + 1;
+                }
+                std::cmp::Ordering::Greater => {
+                    hi = mid;
+                }
+                std::cmp::Ordering::Equal => {
+                    index = Some(mid);
+                    break;
+                }
             }
-            publisher_index += 1;
         }
-        pyth_assert(
-            publisher_index < try_convert::<u32, usize>(price_data.num_)?,
-            OracleError::PermissionViolation.into(),
-        )?;
+
+        publisher_index = match index {
+            Some(index) => index,
+            None => {
+                let mut index = 0;
+                while index < try_convert::<u32, usize>(price_data.num_)? {
+                    if price_data.comp_[index].pub_ == *funding_account.key {
+                        break;
+                    }
+                    if index > 0 && price_data.comp_[index].pub_ < price_data.comp_[index - 1].pub_
+                    {
+                        price_data.comp_.swap(index, index - 1);
+                    }
+                    index += 1;
+                }
+                pyth_assert(
+                    index < try_convert::<u32, usize>(price_data.num_)?,
+                    OracleError::PermissionViolation.into(),
+                )?;
+                index
+            }
+        };
+
 
         latest_aggregate_price = price_data.agg_;
         let latest_publisher_price = price_data.comp_[publisher_index].latest_;