Conversion between little and big endian

contracts/Endian.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.8;
+
+contract Endian {
+  /// reverses byte order to allow conversion between little endian and big
+  /// endian integers
+  function _byteSwap(bytes32 input) internal pure returns (bytes32 output) {
+    output = output | bytes1(input);
+    for (uint i = 1; i < 32; i++) {
+      output = output >> 8;
+      output = output | bytes1(input << (i * 8));
+    }
+  }
+}

contracts/TestEndian.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "./Endian.sol";
+
+contract TestEndian is Endian {
+  function byteSwap(bytes32 input) public pure returns (bytes32) {
+    return _byteSwap(input);
+  }
+}

test/Endian.test.js

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+const { expect } = require("chai")
+const { ethers } = require("hardhat")
+
+describe("Endian", function () {
+  const big = "0x000102030405060708090a0b0c0d0e0f101112131415161718191a1b1c1d1e1f"
+  const little = "0x1f1e1d1c1b1a191817161514131211100f0e0d0c0b0a09080706050403020100"
+
+  let endian
+
+  beforeEach(async function () {
+    let Endian = await ethers.getContractFactory("TestEndian")
+    endian = await Endian.deploy()
+  })
+
+  it("converts from little endian to big endian", async function () {
+    expect(await endian.byteSwap(little)).to.equal(big)
+  })
+
+  it("converts from big endian to little endian", async function () {
+    expect(await endian.byteSwap(big)).to.equal(little)
+  })
+})