Merge pull request #10 from clober-dex/feat/msb

feat: msb

Author: JhChoy

contracts/SignificantBit.sol

@@ -1,5 +1,4 @@
-// SPDX-License-Identifier: -
-// License: https://license.clober.io/LICENSE.pdf
+// SPDX-License-Identifier: GPL-2.0-or-later
 
 pragma solidity ^0.8.0;
 
@@ -22,4 +21,35 @@ library SignificantBit {
         }
         return uint8(DEBRUIJN_INDEX[index]); // can optimize with CODECOPY opcode
     }
+
+    function mostSignificantBit(uint256 x) internal pure returns (uint8) {
+        require(x > 0);
+        uint256 msb;
+        assembly {
+            let f := shl(7, gt(x, 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF))
+            msb := or(msb, f)
+            x := shr(f, x)
+            f := shl(6, gt(x, 0xFFFFFFFFFFFFFFFF))
+            msb := or(msb, f)
+            x := shr(f, x)
+            f := shl(5, gt(x, 0xFFFFFFFF))
+            msb := or(msb, f)
+            x := shr(f, x)
+            f := shl(4, gt(x, 0xFFFF))
+            msb := or(msb, f)
+            x := shr(f, x)
+            f := shl(3, gt(x, 0xFF))
+            msb := or(msb, f)
+            x := shr(f, x)
+            f := shl(2, gt(x, 0xF))
+            msb := or(msb, f)
+            x := shr(f, x)
+            f := shl(1, gt(x, 0x3))
+            msb := or(msb, f)
+            x := shr(f, x)
+            f := gt(x, 0x1)
+            msb := or(msb, f)
+        }
+        return uint8(msb);
+    }
 }

contracts/mocks/SignificantBitWrapper.sol

@@ -11,4 +11,8 @@ contract SignificantBitWrapper {
     function leastSignificantBit(uint256 x) external pure returns (uint8) {
         return x.leastSignificantBit();
     }
+
+    function mostSignificantBit(uint256 x) external pure returns (uint8) {
+        return x.mostSignificantBit();
+    }
 }