import smbus
import time

ADDR_L=0x38 # 7bit address of the VEML6070 (write, read)
ADDR_H=0x39 # 7bit address of the VEML6070 (read)

RSET_240K=240000
RSET_270K=270000
RSET_300K=300000
RSET_600K=600000

SHUTDOWN_DISABLE=0x00
SHUTDOWN_ENABLE=0x01

INTEGRATIONTIME_1_2T=0x00
INTEGRATIONTIME_1T=0x01
INTEGRATIONTIME_2T=0x02
INTEGRATIONTIME_4T=0x03

class Veml6070:

    def __init__(self, i2c_bus=1, sensor_address=ADDR_L, rset=RSET_270K, integration_time=INTEGRATIONTIME_1T):
        self.bus = smbus.SMBus(i2c_bus)
        self.sendor_address = sensor_address
        self.rset = rset
        self.shutdown = SHUTDOWN_DISABLE # before set_integration_time()
        self.set_integration_time(integration_time)
        self.disable()

    def set_integration_time(self, integration_time):
        self.integration_time = integration_time
        self.bus.write_byte(self.sendor_address, self.get_command_byte())
        # constant offset determined experimentally to allow sensor to readjust
        time.sleep(0.2)

    def get_integration_time(self):
        return self.integration_time

    def enable(self):
        self.shutdown = SHUTDOWN_DISABLE
        self.bus.write_byte(self.sendor_address, self.get_command_byte())

    def disable(self):
        self.shutdown = SHUTDOWN_ENABLE
        self.bus.write_byte(self.sendor_address, self.get_command_byte())

    def get_uva_light_intensity_raw(self):
        self.enable()
        # wait two times the refresh time to allow completion of a previous cycle with old settings (worst case)
        time.sleep(self.get_refresh_time()*2)
        msb = self.bus.read_byte(self.sendor_address+(ADDR_H-ADDR_L))
        lsb = self.bus.read_byte(self.sendor_address)
        self.disable()
        return (msb << 8) | lsb

    def get_uva_light_intensity(self):
        uv = self.get_uva_light_intensity_raw()
        return uv * self.get_uva_light_sensitivity()

    def get_command_byte(self):
        """
        assembles the command byte for the current state
        """
        cmd = (self.shutdown & 0x01) << 0 # SD
        cmd = (self.integration_time & 0x03) << 2 # IT
        cmd = ((cmd | 0x02) & 0x3F) # reserved bits
        return cmd

    def get_refresh_time(self):
        """
        returns time needed to perform a complete measurement using current settings (in s)
        """
        case_refresh_rset = {
            RSET_240K: 0.1,
            RSET_270K: 0.1125,
            RSET_300K: 0.125,
            RSET_600K: 0.25
        }
        case_refresh_it = {
            INTEGRATIONTIME_1_2T: 0.5,
            INTEGRATIONTIME_1T: 1,
            INTEGRATIONTIME_2T: 2,
            INTEGRATIONTIME_4T: 4
        }
        return case_refresh_rset[self.rset] * case_refresh_it[self.integration_time]

    def get_uva_light_sensitivity(self):
        """
        returns UVA light sensitivity in W/(m*m)/step
        """
        case_sens_rset = {
            RSET_240K: 0.05,
            RSET_270K: 0.05625,
            RSET_300K: 0.0625,
            RSET_600K: 0.125
        }
        case_sens_it = {
            INTEGRATIONTIME_1_2T: 0.5,
            INTEGRATIONTIME_1T: 1,
            INTEGRATIONTIME_2T: 2,
            INTEGRATIONTIME_4T: 4
        }
        return case_sens_rset[self.rset] / case_sens_it[self.integration_time]