How to decode the Google Directions API polylines field into lat long points in Objective-C for iPhone?

Question

I want to draw routes on a map corresponding to directions JSON which I am getting through the Google Directions API: https://developers.google.com/maps/documentation/directions/start

I have figured out how to extract the latitude and longitude from the steps field, however this doesn't follow curvy roads very well. I think what I need is to decode the polyline information, I found Googles instructions on how to encode polylines: https://developers.google.com/maps/documentation/utilities/polylinealgorithm

I did find some code here for Android and also Javascript on decoding the polylines, for example:

Map View draw directions using google Directions API - decoding polylines

android get and parse Google Directions

But I can't find same for Objective-C iPhone code, can anybody help me with this? I'm sure I can do it myself if I have to, but it sure would save me some time if it's already available somewhere.

EDIT: the key here is being able to decode the base64 encoding on a character by character basis. To be more specific, I get something like this in JSON from Google which is encoded using base64 encoding among other things:

...   "overview_polyline" : {
        "points" : "ydelDz~vpN_@NO@QEKWIYIIO?YCS@WFGBEBICCAE?G@y@RKBEBEBAD?HTpB@LALALCNEJEFSP_@LyDv@aB\GBMB"
       },
...

Note: I should mention that this question refers to Google Maps API v1, it is much easier to do this in v2 using GMSPolyLine polyLineWithPath as many answers below will tell you (thanks to @cdescours).

Answer 1

I hope it's not against the rules to link to my own blog post if it's relevant to the question, but I've solved this problem in the past. Stand-alone answer from linked post:

@implementation MKPolyline (MKPolyline_EncodedString)

+ (MKPolyline *)polylineWithEncodedString:(NSString *)encodedString {
    const char *bytes = [encodedString UTF8String];
    NSUInteger length = [encodedString lengthOfBytesUsingEncoding:NSUTF8StringEncoding];
    NSUInteger idx = 0;

    NSUInteger count = length / 4;
    CLLocationCoordinate2D *coords = calloc(count, sizeof(CLLocationCoordinate2D));
    NSUInteger coordIdx = 0;

    float latitude = 0;
    float longitude = 0;
    while (idx < length) {
        char byte = 0;
        int res = 0;
        char shift = 0;

        do {
            byte = bytes[idx++] - 63;
            res |= (byte & 0x1F) << shift;
            shift += 5;
        } while (byte >= 0x20);

        float deltaLat = ((res & 1) ? ~(res >> 1) : (res >> 1));
        latitude += deltaLat;

        shift = 0;
        res = 0;

        do {
            byte = bytes[idx++] - 0x3F;
            res |= (byte & 0x1F) << shift;
            shift += 5;
        } while (byte >= 0x20);

        float deltaLon = ((res & 1) ? ~(res >> 1) : (res >> 1));
        longitude += deltaLon;

        float finalLat = latitude * 1E-5;
        float finalLon = longitude * 1E-5;

        CLLocationCoordinate2D coord = CLLocationCoordinate2DMake(finalLat, finalLon);
        coords[coordIdx++] = coord;

        if (coordIdx == count) {
            NSUInteger newCount = count + 10;
            coords = realloc(coords, newCount * sizeof(CLLocationCoordinate2D));
            count = newCount;
        }
    }

    MKPolyline *polyline = [MKPolyline polylineWithCoordinates:coords count:coordIdx];
    free(coords);

    return polyline;
}

@end