Deposit collateral

contracts/Collateral.sol

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+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+import "@openzeppelin/contracts/token/ERC20/IERC20.sol";
+
+contract Collateral {
+  IERC20 private immutable token;
+  mapping(address => uint256) private balances;
+
+  constructor(IERC20 _token) {
+    token = _token;
+  }
+
+  function balanceOf(address account) public view returns (uint256) {
+    return balances[account];
+  }
+
+  function deposit(uint256 amount) public {
+    token.transferFrom(msg.sender, address(this), amount);
+    balances[msg.sender] += amount;
+  }
+}

test/Collateral.js

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+const { expect } = require("chai")
+
+describe("Collateral", function () {
+  let collateral, token
+  let account0, account1
+
+  beforeEach(async function () {
+    let Collateral = await ethers.getContractFactory("Collateral")
+    let TestToken = await ethers.getContractFactory("TestToken")
+    token = await TestToken.deploy()
+    collateral = await Collateral.deploy(token.address)
+    ;[account0, account1] = await ethers.getSigners()
+    await token.mint(account0.address, 1000)
+    await token.mint(account1.address, 1000)
+  })
+
+  it("assigns zero collateral by default", async function () {
+    expect(await collateral.balanceOf(account0.address)).to.equal(0)
+    expect(await collateral.balanceOf(account1.address)).to.equal(0)
+  })
+
+  describe("depositing", function () {
+    beforeEach(async function () {
+      await token.connect(account0).approve(collateral.address, 100)
+      await token.connect(account1).approve(collateral.address, 100)
+    })
+
+    it("updates the amount of collateral", async function () {
+      await collateral.connect(account0).deposit(40)
+      await collateral.connect(account1).deposit(2)
+      expect(await collateral.balanceOf(account0.address)).to.equal(40)
+      expect(await collateral.balanceOf(account1.address)).to.equal(2)
+    })
+
+    it("transfers tokens to the contract", async function () {
+      let before = await token.balanceOf(collateral.address)
+      await collateral.deposit(42)
+      let after = await token.balanceOf(collateral.address)
+      expect(after - before).to.equal(42)
+    })
+
+    it("fails when token transfer fails", async function () {
+      let allowed = await token.allowance(account0.address, collateral.address)
+      let invalidAmount = allowed.toNumber() + 1
+      await expect(collateral.deposit(invalidAmount)).to.be.revertedWith(
+        "ERC20: transfer amount exceeds allowance"
+      )
+    })
+  })
+})