test: pin mask constants by re-derivation, lone-high-bit case, relative src import

test/src/lib/LibCtPop.ctpop.t.sol

@@ -3,7 +3,18 @@
 pragma solidity =0.8.25;
 
 import {Test} from "forge-std-1.16.1/src/Test.sol";
-import {LibCtPop} from "src/lib/LibCtPop.sol";
+import {
+    LibCtPop,
+    CTPOP_M1,
+    CTPOP_M2,
+    CTPOP_M4,
+    CTPOP_M8,
+    CTPOP_M16,
+    CTPOP_M32,
+    CTPOP_M64,
+    CTPOP_M128,
+    CTPOP_H01
+} from "../../../src/lib/LibCtPop.sol";
 
 /// @title LibCtPopTest
 /// CTPOP (count population) is a function that counts the number of bits set in
@@ -72,4 +83,42 @@ contract LibCtPopTest is Test {
         uint256 ctSlow = LibCtPop.ctpopSlow(x);
         assertEq(ct, ctSlow);
     }
+
+    /// A lone bit in the highest position is counted, deterministically
+    /// pinning the top of the range rather than relying on fuzz coverage.
+    function testCtPopLoneHighBit() external pure {
+        uint256 x = 1 << 255;
+        assertEq(1, LibCtPop.ctpop(x));
+        assertEq(1, LibCtPop.ctpopSlow(x));
+    }
+
+    /// Build the alternating mask with `width` bits set then `width` bits
+    /// clear, starting from the low bit, across the whole word.
+    function altMask(uint256 width) internal pure returns (uint256 mask) {
+        unchecked {
+            uint256 chunk = (1 << width) - 1;
+            for (uint256 i = 0; i < 256; i += 2 * width) {
+                mask |= chunk << i;
+            }
+        }
+    }
+
+    /// Every mask constant is pinned against an independent re-derivation so
+    /// a corrupted constant cannot hide behind ctpop and ctpopSlow sharing it.
+    function testMaskConstantsRederived() external pure {
+        assertEq(altMask(1), CTPOP_M1);
+        assertEq(altMask(2), CTPOP_M2);
+        assertEq(altMask(4), CTPOP_M4);
+        assertEq(altMask(8), CTPOP_M8);
+        assertEq(altMask(16), CTPOP_M16);
+        assertEq(altMask(32), CTPOP_M32);
+        assertEq(altMask(64), CTPOP_M64);
+        assertEq(altMask(128), CTPOP_M128);
+
+        uint256 h01 = 0;
+        for (uint256 i = 0; i < 256; i += 8) {
+            h01 |= uint256(1) << i;
+        }
+        assertEq(h01, CTPOP_H01);
+    }
 }