from microbit import *
class HCSR04:

    def __init__(self, tpin=pin15, epin=pin15, spin=pin13):
        self.trigger_pin = tpin
        self.echo_pin = epin
        self.sclk_pin = spin

    def distance_mm(self):
        spi.init(baudrate=125000, sclk=self.sclk_pin,
                 mosi=self.trigger_pin, miso=self.echo_pin)
        pre = 0
        post = 0
        k = -1
        length = 500
        resp = bytearray(length)
        resp[0] = 0xFF
        spi.write_readinto(resp, resp)
        # find first non zero value
        try:
            i, value = next((ind, v) for ind, v in enumerate(resp) if v)
        except StopIteration:
            i = -1
        if i > 0:
            pre = bin(value).count("1")
            # find first non full high value afterwards
            try:
                k, value = next((ind, v)
                                for ind, v in enumerate(resp[i:length - 2]) if resp[i + ind + 1] == 0)
                post = bin(value).count("1") if k else 0
                k = k + i
            except StopIteration:
                i = -1
        dist= -1 if i < 0 else round((pre + (k - i) * 8. + post) * 8 * 0.172)
        return dist

sonar = HCSR04()
while True:
    test = sonar.distance_mm()
    display.scroll(str(test))
    sleep(1000)