Generating a MongoDB ObjectId in SalesForce Apex

MongoObjectId.cls

public with sharing class MongoObjectId {
    private static final Integer COUNTER_MASK = 16777215; // 0xFFFFFF
    private static final Integer PID_MASK = 65535; // 0xFFFF
    private static final String MACHINE_PART = '050135';
    private static final String[] hexMap = '0123456789abcdef'.split('');
    private static Integer counter = -1;

    public static void seed() {
    	// Seed once or force reseed when testing
      	if(-1 == counter || Test.isRunningTest()) {
      		counter = Integer.valueOf(Math.mod(
	    		Integer.valueOf(TestStubs.random() * COUNTER_MASK),
	    		COUNTER_MASK
	    	));
      	}
    	System.debug('counter');
    	System.debug(counter);
    }

    public static String decToHex(Long x, Integer length) {
        String result = '';

        while (x != 0) {
            Integer modulus = Integer.valueOf(Math.mod(x, 16));
            result = hexMap[modulus] + result;
            x /= 16;
        }

		while(result.length() < length) {
		    result = '0' + result;
		}

        return result;
    }

    public static String userIdToPid(String userId) {
        Integer pid = 0;

        for(Integer x = userId.length() - 1; x >= 0; x--) {
            pid = Math.mod(pid + userId.codePointAt(x), PID_MASK);
        }

        return decToHex(pid, 4);
    }

    public static String generate() {
    	// Ensure the counter was seeded
    	seed();
    	// Get current timestamp
    	Integer timestamp = Integer.valueOf(TestStubs.now().getTime() / 1000);
    	// Compute pid from userId
    	String pid = userIdToPid(TestStubs.getUserId());
    	// Increment the internal counter
    	counter = counter + 1;
    	counter = counter & COUNTER_MASK;
    	List<String> parts = new List<String> {
    		decToHex(timestamp, 8),
    		MACHINE_PART,
    		pid,
    		decToHex(counter, 6)
    	};
    	System.debug('counter');
    	System.debug(counter);
    	System.debug('Timestamp');
    	System.debug(timestamp);
    	return String.join(parts, '');
    }

}

TestMongoObjectId.cls

@isTest(seeAllData=false)
private class TestMongoObjectId {

	static testMethod void testToHex() {
	   	Test.startTest();

	   	System.assertEquals('000ff', MongoObjectId.decToHex(
	   		255,
	   		5
	   	));

	   	System.assertEquals('000fe', MongoObjectId.decToHex(
	   		254,
	   		5
	   	));

	   	System.assertEquals('0012730c57800', MongoObjectId.decToHex(
	   		1267833600000L,
	   		13
	   	));

	   	Test.stopTest();
	}

	static testMethod void testUserIdToPid() {
	   	Test.startTest();

	   	System.assertEquals('01d7', MongoObjectId.userIdToPid(
	   	    '1664BABA'
	   	));

	   	Test.stopTest();
	}

	static testMethod void testGenerate() {
	   	Test.startTest();

	   	TestStubs.testRandom = 0.5;
	   	TestStubs.testNow = 1267833600000L;
	   	TestStubs.testUserId = '1664BABA';

	   	String id = MongoObjectId.generate();

	   	System.assertEquals(24, id.length());
	   	System.assertEquals('4b919b0005013501d7800000', id);

	   	Test.stopTest();
	}
}

TestStubs.cls

public with sharing class TestStubs {
	@TestVisible
	private static Long testNow {get; set;}
	@TestVisible
	private static Decimal testRandom {get; set;}
	@TestVisible
	private static String testUserId {get; set;}

	public static DateTime now() {
		if(Test.isRunningTest() && (null != testNow)) {
        	return DateTime.newInstance(testNow);
		} else {
			return System.now();
		}
	}

	public static Decimal random() {
		if(Test.isRunningTest() && (null != testRandom)) {
        	return testRandom;
		} else {
			return Math.random();
		}
	}

	public static String getUserId() {
		if(Test.isRunningTest() && (null != testUserId)) {
        	return testUserId;
		} else {
			return UserInfo.getUserId();
		}
	}

}

TestTestStubs.cls

@isTest(seeAllData=false)
private class TestTestStubs {

	static testMethod void testRandomStub() {
	   	Test.startTest();

	   	TestStubs.testRandom = 0.5;

	   	System.assertEquals(TestStubs.testRandom, TestStubs.random());

	   	Test.stopTest();
	}

	static testMethod void testNowStub() {
	   	Test.startTest();

	   	TestStubs.testNow = 1267833600000L;

	   	System.assertEquals(TestStubs.testNow, TestStubs.now().getTime());

	   	Test.stopTest();
	}

	static testMethod void testUserIdStub() {
	   	Test.startTest();

	   	TestStubs.testUserId = '1664BABA';

	   	System.assertEquals((TestStubs.testUserId), TestStubs.getUserId());

	   	Test.stopTest();
	}

}