Add two competing implementations

EIPS/eip-165.md

@@ -51,13 +51,11 @@ contract Selector {
 }
 ```
 
-Note: interfaces do not permit optional functions, therefore, the interface identity will not include them.
-
-Note: an ERC standard may define multiple interfaces to separate core functionality from optional features. For example, [one draft standard defines](https://github.com/ethereum/EIPs/pull/841) ERC721, ERC721Metadata and ERC721Enumerable interfaces.
+Note: interfaces do not permit optional functions, therefore, the interface identity will not them.
 
 ### How a Contract will Publish the Interfaces it Implements
 
-A contract that is compliant with ERC-165 shall implement the following function:
+A contract that is compliant with ERC-165 shall implement the following interface (referred as `ERC165.sol`):
 
 ```solidity
 pragma solidity ^0.4.19;
@@ -116,6 +114,61 @@ XXXXXXXX HELP NEEDED XXXXXXXXX
 
 ## Implementation
 
+This approach uses a `view` function implementation of `supportsInterface`. The execution cost is 478 gas for any input. But contract initialization requires storing each interface (`SSTORE` is 20,000 gas).
+
+```solidity
+pragma solidity ^0.4.19;
+
+import "./ERC165.sol";
+
+interface Simpson {
+    function is2D() external returns (bool);
+    function skinColor() external returns (string);
+}
+
+contract Lisa is ERC165, Simpson {
+    mapping(bytes4 => bool) supportedInterfaces;
+
+    function Lisa() public {
+        supportedInterfaces[this.supportsInterface.selector] = true;
+        supportedInterfaces[this.is2D.selector ^ this.skinColor.selector] = true;
+    }
+
+    function supportsInterface(bytes4 interfaceID) external view returns (bool) {
+        return supportedInterfaces[interfaceID];
+    }
+
+    // ... is usually 2D
+    // skin color is yellow
+}
+```
+
+Following is a `pure` function implementation of `supportsInterface`. The worst-case execution cost is 236 gas, but increases linearly with a higher number of supported interfaces.
+
+```solidity
+pragma solidity ^0.4.19;
+
+import "./ERC165.sol";
+
+interface Simpson {
+    function is2D() external returns (bool);
+    function skinColor() external returns (string);
+}
+
+contract Homer is ERC165, Simpson {
+    function supportsInterface(bytes4 interfaceID) external view returns (bool) {
+        return 
+          interfaceID == this.supportsInterface.selector || // ERC165
+          interfaceID == this.is2D.selector 
+                         ^ this.skinColor.selector; // Simpson
+    }
+}
+```
+
+With three or more supported interfaces (including ERC165 itself as a required supported interface), the mapping table approach (for any case) costs less gas than the worst case for pure approach.
+
+
+
 XXXXXX IN PROGRES XXXXXX [https://github.com/jbaylina/EIP165Cache](https://github.com/jbaylina/EIP165Cache)
 
 ## Copyright