[wip]

fit_markers.js

// the marker positions, as an array
// of arrays of numbers.
var markers = [[0, 0], [180, 0]];
var D2R = Math.PI / 180,
    R2D = 180 / Math.PI,
    A = 6378137,
    MAXEXTENT = 20037508.34,
    tox = A * D2R;

function getStaticUrl(markers, width, height, maxZoom) {
    // if you don't provide a maxZoom, default to 15. it's a safe number,
    // but fit to taste.
    if (maxZoom === undefined) maxZoom = 15;

    // there's only one marker, the extent is just that, there's nothing to
    // fit, and we just return that marker at the maximum zoom level.
    if (markers.length === 1) {
        return [markers[0][0], markers[0][1], maxZoom];
    }

    var extent = [
        [markers[0][0], markers[0][1]],
        [markers[0][0], markers[0][1]]
    ];

    for (var i = 1; i < markers.length; i++) {
        var x = markers[i][0],
            y = markers[i][1];
        // if this point is outside of the extent, extend it.
        if (x < extent[0][0]) extent[0][0] = x;
        if (x > extent[1][0]) extent[1][0] = x;
        if (y < extent[0][1]) extent[0][1] = y;
        if (y > extent[1][1]) extent[1][1] = y;
    }

    // project this extent into mercator
    var extent_projected = [[
        extent[0][0] * tox,
        A * Math.log(Math.tan((Math.PI * 0.25) + (0.5 * extent[0][1] * D2R)))
    ], [
        extent[1][0] * tox,
        A * Math.log(Math.tan((Math.PI * 0.25) + (0.5 * extent[1][1] * D2R)))
    ]];

    var mwidth = extent_projected[1][0] - extent_projected[0][0],
        mheight = extent_projected[1][1] - extent_projected[0][1];

    var fitted_dimension = Math.max(mwidth, mheight);
    var fit_zoom = maxZoom;

    while (MAXEXTENT / Math.pow(2, fit_zoom) < fitted_dimension) {
        fit_zoom--;
    }
    console.log(fit_zoom);
}

getStaticUrl(markers);