itmap.sol

library itmap {
    struct entry {
        // Equal to the index of the key of this item in keys, plus 1.
        uint keyIndex;
        uint value;
    }

    struct itmap {
        mapping(uint => entry) data;
        uint[] keys;
    }
    
    function insert(itmap storage self, uint key, uint value) internal returns (bool replaced) {
        entry storage e = self.data[key];
        e.value = value;
        if (e.keyIndex > 0) {
            return true;
        } else {
            e.keyIndex = ++self.keys.length;
            self.keys[e.keyIndex - 1] = key;
            return false;
        }
    }
    
    function remove(itmap storage self, uint key) internal returns (bool success) {
        entry storage e = self.data[key];
        if (e.keyIndex == 0)
            return false;
        
        if (e.keyIndex < self.keys.length) {
            // Move an existing element into the vacated key slot.
            self.data[self.keys[self.keys.length - 1]].keyIndex = e.keyIndex;
            self.keys[e.keyIndex - 1] = self.keys[self.keys.length - 1];
            self.keys.length -= 1;
            delete self.data[key];
        }
    }
    
    function contains(itmap storage self, uint key) internal returns (bool exists) {
        return self.data[key].keyIndex > 0;
    }
    
    function size(itmap storage self) internal returns (uint) {
        return self.keys.length;
    }
    
    function get(itmap storage self, uint key) internal returns (uint value) {
        return self.data[key].value;
    }
    
    function getKey(itmap storage self, uint idx) internal returns (uint value) {
        return self.keys[idx];
    }
}

contract Test {
    // Use itmap for all functions on the struct
    using itmap for itmap.itmap;
    
    // Declare an iterable mapping
    itmap.itmap mymap;
    
    function insert(uint key, uint value) {
        mymap.insert(key, value);
    }
    
    function get(uint key) returns (uint value) {
        return mymap.get(key);
    }
}

This line https://gist.github.com/Arachnid/59159497f124fdbff14bc2ca960b77ba#file-itmap-sol-L36 is missing a return true upon success.

keyIndex should be uint as it by design always ends up 0 or higher? <--- this actually is now already implemented, I had an older version
.

Clearer to make the name of the name of the return value of the function getKey be "key" instead of "value" on line 51 https://gist.github.com/Arachnid/59159497f124fdbff14bc2ca960b77ba#file-itmap-sol-L51

Shouldn't the gets be constant functions as they are not transactions?

Missing a return value at line 63 https://gist.github.com/Arachnid/59159497f124fdbff14bc2ca960b77ba#file-itmap-sol-L63

Hi! I've tweaked this, fixed a bug (it wasn't removing the item when there was only 1 item in map) plus added a few new type mappings.
All of it in a new repo: https://github.com/szerintedmi/solidity-itMapsLib
You don't have any license here, so I've added Apache 2.0. Please let me know if you have any issue with that license.
Suggestions about my changes are welcome too! :)