parameter (pair %decodeBigMapDiff (bytes %key) (option (bytes %value)))  # value can be null, so we made it optional

storage (map (address %holder) 
             (pair (nat %balance) 
                   (pair (nat %decimals) 
                         (string %symbol))))

BEGIN %decodeBigMapDiff (Pair 0x05070701000000066c65646765720a00000016000016e64994c2ddbd293695b63e4cade029d3c8b5e3 
                              (Some 0x05070700ac9a010200000000)) 
                        {}  # empty map

DUP ; DIP { CDR @storage } ;  # save storage on the bottom of the stack
CAR @parameter

DUP ; CAR @key  # first let's check the key

UNPACK (pair string address) ; # trying to unpack big map key (they can be of different types underneath)
ASSERT_SOME

DUP ; CAR @label

PUSH string "ledger" ; ASSERT_CMPEQ ;  # make sure it's balance data

DUMP

CDR @holder ;

SWAP ; CDR @value ; 
ASSERT_SOME  # actually we need to handle cases when a key is removed from big_map, will do later



UNPACK (pair nat (map address nat)) ; 
ASSERT_SOME  # here we can fail, since we know exactly how data is packed

CAR @balance

PUSH @symbol string "TZBTC" ;  # adding metadata
PUSH @decimals nat 8 ;

DUMP

PAIR ; SWAP ; PAIR ; SOME ;

SWAP ; UPDATE  # writing to the storage

NIL operation ; PAIR ; COMMIT

parameter (pair %decodeBigMapDiff (bytes %key) (option (bytes %value))) ;
storage (map (address %holder) 
             (pair (nat %balance) 
                   (pair (nat %decimals) 
                         (string %symbol)))) ;
code {
    DUP ; CAR ; DIP { CDR } ;
    DUP ; CAR ;
    UNPACK (pair string address) ;
    IF_SOME { 
        DUP ; CAR ;
        PUSH string "ledger" ;
        IFCMPEQ { 
            CDR @holder ; 
            SWAP ; CDR ;
            IF_SOME { 
                UNPACK (pair nat (map address nat)) ;
                ASSERT_SOME ;
                CAR @balance ;
            } { 
                PUSH @balance nat 0;
            } ;
            PUSH @symbol string "TZBTC" ;
            PUSH @decimals nat 8 ;
            PAIR ; SWAP ; PAIR ; SOME ;
            SWAP ; UPDATE                
        } {
            DROP 2
        } ;
    } { 
        DROP
    } ;
    NIL operation ; PAIR ;
}

DEBUG False

RUN %decodeBigMapDiff (Pair 0x05070701000000066c65646765720a00000016000016e64994c2ddbd293695b63e4cade029d3c8b5e3 
                            (Some 0x05070700ac9a010200000000)) 
                      {}

RUN %decodeBigMapDiff (Pair 0x05070701000000066c65646765720a00000016000016e64994c2ddbd293695b63e4cade029d3c8b5e3 
                            None) 
                      {}

RUN %decodeBigMapDiff (Pair 0xdeadbeef None) {}