Median by Position (#533)

* use median price instead of the highest sustained

* add test for lexicographically reversing a byte slice

* fix pr comments

* fix CI fail

* fix dex tests

* Use a fuzz-tested list of prices

* Working median algorithm based on position + lints

---------

Co-authored-by: akildemir <aeg_asd@hotmail.com>

substrate/dex/pallet/src/mock.rs

@@ -25,7 +25,7 @@ use crate as dex;
 
 use frame_support::{
   construct_runtime,
-  traits::{ConstU32, ConstU64},
+  traits::{ConstU16, ConstU32, ConstU64},
 };
 
 use sp_core::{H256, sr25519::Public};
@@ -40,6 +40,8 @@ pub use coins_pallet as coins;
 
 type Block = frame_system::mocking::MockBlock<Test>;
 
+pub const MEDIAN_PRICE_WINDOW_LENGTH: u16 = 10;
+
 construct_runtime!(
   pub enum Test
   {
@@ -92,6 +94,9 @@ impl Config for Test {
   type WeightInfo = ();
   type LPFee = ConstU32<3>; // means 0.3%
   type MaxSwapPathLength = ConstU32<4>;
+
+  type MedianPriceWindowLength = ConstU16<{ MEDIAN_PRICE_WINDOW_LENGTH }>;
+
   // 100 is good enough when the main currency has 12 decimals.
   type MintMinLiquidity = ConstU64<100>;
 }