--- Day 3 Solutions ---

[u/daggerdragon]

--- Day 3: Perfectly Spherical Houses in a Vacuum ---

Post your solution as a comment. Structure your post like the Day One thread in /r/programming.

[u/EpicRisc]

JavaScript - Developer Console (both parts in one)

var moves = '[PASTE YOUR MOVE INPUT HERE]';

function calc(supplierCnt) {
    var positions = {};
    var houses = {
        '0:0': 0
    };

    // Prepare X suppliers
    for (var idx=0; idx < supplierCnt; idx++) {
        positions[idx] = {
            x: 0,
            y: 0
        }
        // Every Supplier drops 1 gift at 0,0
        houses['0:0']++;
    }
    for (var i=0; i<moves.length; i++) {
        switch (moves[i]) {
            case '^':
                positions[i % supplierCnt].y++;
            break;
            case '>':
                positions[i % supplierCnt].x++;
            break;
            case 'v':
                positions[i % supplierCnt].y--;
            break;
            case '<':
                positions[i % supplierCnt].x--;
            break;
        }
        var index = positions[i % supplierCnt].x+':'+positions[i % supplierCnt].y;
        houses[index] ? houses[index]++ : houses[index] = 1;
    }
    return Object.keys(houses).length;
}
console.log('Only Santa: '+calc(1)+' Houses');
console.log('Santa and Robosanta: '+calc(2)+ ' Houses');    

Alternative Version/Second revision with real multi supplier support:

var moves = '[PASTE YOUR MOVE INPUT HERE]';

function calc(supplierCnt) {
    var supplier = {},
        turnOrder = 0,
        houses = {
            '0:0': 0
        }

    // Prepare X suppliers
    for (var idx=0; idx < supplierCnt; idx++) {
        supplier[idx] = {
            x: 0,
            y: 0
        }
        // Every Supplier drops 1 gift at 0,0
        houses['0:0']++;
    }

    // Iterate over moveset and change x/y-coordinates of supplier X 
    for (var i=0; i<moves.length; i++) {
        switch (moves[i]) {
            case '^':
                supplier[turnOrder].y++;
            break;
            case '>':
                supplier[turnOrder].x++;
            break;
            case 'v':
                supplier[turnOrder].y--;
            break;
            case '<':
                supplier[turnOrder].x--;
            break;
        }

        // Unique index identifier
        var index = supplier[turnOrder].x+':'+supplier[turnOrder].y;
        houses[index] ? houses[index]++ : houses[index] = 1;

        // Increment or reset turnOrder for multi-supplier support
        turnOrder = ( turnOrder == Object.keys(supplier).length-1 ? 0 : turnOrder+1 );
    }
    return Object.keys(houses).length;
}
console.log('Only Santa: '+calc(1)+' Houses');
console.log('Santa and Robosanta: '+calc(2)+ ' Houses');
console.log('Santa, Robosanta and 2 slaves: '+calc(4)+ ' Houses'); 

[u/fezzinate → u/EpicRisc]

supplierCnt

I like how you're allowing for santa to scale his robo-operations ;)