import struct

class SaveReader:
    def __init__(self, filename):
        self.successful = False
        try:
            file = open(filename, 'rb').read()
            pointer = 13
            pointer += file[pointer] +1
            if file[pointer] != 0x65:
                print("Error interpreting file - missed first marker.")
                return
            pointer += 1
            while (file[pointer] != 1 or file[pointer+1] != 1 or file[pointer+2] != 5 or file[pointer+3] != 11):
                #In observed files this should advance two or three bytes.
                pointer += 1
                #Actually have a decent idea how to tell if it's going to be two or three bytes (it seems to be a number in the continuation bit + 7-bit int format).
                #But this is a good check that the format is reading as expected.
            pointer += 4
            pointer += file[pointer]+1 #generally skipping two or three bytes
            if file[pointer] == 0x20:
                self.switches = ()
                pointer += 2
            elif file[pointer] == 0x1F:
                pointer += 1
                fieldlen = file[pointer]
                if (fieldlen == 1):
                    switchlen = file[pointer+1]%128
                elif (fieldlen == 2):
                    switchlen = (file[pointer+1]%128)*128+(file[pointer+2]%128)
                pointer += fieldlen*2+2
                self.switches = [file[i] for i in range(pointer, pointer+switchlen)]
                pointer += switchlen
            else:
                print ("Error interpreting file - missed switch-start marker.")
                return
            
            if file[pointer] == 0x22:
                self.variables = ()
                self.successful = True
                return
            elif file[pointer] == 0x21:
                pointer += 1
                pointer += file[pointer]+1
                if file[pointer] != 0x22:
                    print("Error interpreting file - variable-start marker not understood.")
                    return
                pointer += 1
                varbytes = file[pointer]%128
                while (file[pointer] // 128 == 1):
                    varbytes *= 128
                    pointer += 1
                    varbytes += file[pointer]%128
                    #The switch array header and variable array header have similar formats:
                    #(marker byte) (number of bytes next number takes up) (number of switches or variables, in continuation-bit + 7-bit format) (marker byte) (number of bytes in following byte array).
                    #It's just that switches are 1 byte and vars are 4 (little-endian).
                    #And I didn't have the difference fully down until I'd already written the switch interpreter.
                pointer += 1
                self.variables = [struct.unpack('<i', file[pointer+4*i: pointer+4*(i+1)])[0] for i in range(varbytes//4)]
                self.successful = True
                return
            else:
                print ("Error interpreting file - missed variable-start marker.")
                return
            
        except IndexError:
            print("Error interpreting file - reached end of file.")
            return
        except IOError:
            print("IO error - file cannot be read.")
            return
    
    #Returns 0 for OFF, 1 for ON.
    #Switches that haven't been used or allocated default to OFF.
    def switch(self, num):
        if not self.successful:
            print("File read did not complete, don't ask for switches.")
            raise AssertionError
        if num <= 0:
            print("Switches start at 1.")
            raise AssertionError
        if num > len(self.switches):
            return 0
            #2k(3) doesn't necessarily save all the switches or variables if they haven't been used yet.
        else:
            return self.switches[num-1]
            #In-editor labeling of switches starts at 1, not 0.
    
    #Variables that haven't been used or allocated default to 0.
    def variable(self, num):
        if not self.successful:
            print("File read did not complete, don't ask for vars.")
            raise AssertionError
        if num <= 0:
            print("Vars start at 1.")
            raise AssertionError
        if num > len(self.variables):
            return 0
            #2k(3) doesn't necessarily save all the switches or variables if they haven't been used yet.
        else:
            return self.variables[num-1]
            #In-editor labeling starts at 1, not 0.