Global W[21,23],F[7,4,4,4],L[7];For S$=Eachin "ABEF,AEFJEFBC,IEFBABFG,BFJNEFGH,AEFGCBFJEFGKIJFB,EFGCBFJKIEFGABFJ,EFBGBFGJEFGJBEFJ".Split(",");L[M]=Len(S)/4-1;For I=0 To L[M];For J=0 To 3;A=S[I*4+J]-65;F[M,I,A&3,A Shr 2]=1;Next;Next;M:+1;Next;Graphics 800,600;Y=19;Repeat;Cls;For I=0 To 20;W[1,I]=1;W[12,I]=1;W[I,20]=1;For J=1 To 12;If W[J,I] Then DrawRect J*8,I*8,8,8

Next;Next;For I=0 To 3;For J=0 To 3;If F[N,R,I,J] Then DrawRect (X+I)*8,(Y+J)*8,8,8
Next;Next;Flip;A=X+KeyHit(39)-KeyHit(37);B=Y+KeyDown(40);Q=(R+KeyHit(87)-KeyHit(81))&L[N];IF C(A,B,N,Q)=0 Then X=A;Y=B;R=Q
M=Millisecs();If M>T Then
T=M+999;If C(X,Y+1,N,R) Then
If Y=0 Then End
For I=0 To 3;For J=0 To 3;If F[N,R,I,J] Then W[X+I,Y+J]=1
Next;Next;D=0;For I=19 To 0 Step -1;Q=0;For J=2 To 11;If W[J,I] Then Q:+1
W[J,I+D]=W[J,I];Next;IF Q>9 Then D:+1
Next;X=5;Y=-1;N=Rand(0,6);EndIf;Y:+1;End If;Until KeyHit(27);Function C(X,Y,N,R);For I=0 To 3;For J=0 To 3;If F(N,R,I,J)*W[I+X,J+Y] Then Return 1

Next;Next;EndFunction

SeedRnd Millisecs()

Global World[ 21, 23 ] ' Game field, stretched right for the shorter way to make glass bottom
Global Figure[ 7, 4, 4, 4 ] ' Stored tetris figures ( 7 pieces, 4x4 matrix, up to 4 phases)
Global Phases[ 7 ] ' Quantity of phases for figure ( 1, 2 or 4 )

For S$ = Eachin "ABEF,AEFJEFBC,IEFBABFG,BFJNEFGH,AEFGCBFJEFGKIJFB,EFGCBFJKIEFGABFJ,EFBGBFGJEFGJBEFJ".Split(",")
        ' There's all phases of figures stored in the sting, using 4 letters per phase, comma separates figure string chunks
        ' Every letter means a quadratic piece of figure in certain position, corresponding to following matrix:
        ' ABCD
        ' EFGH
        ' IJKL
        ' MNOP
       
        Phases[ M ] = Len( S ) / 4 - 1 ' ( Quantity of phases - 1 ) can be calculated by dividing string chunk by 4 and subtracting 1
        For I = 0 To Phases[ M ] ' These cycles fills the Figure matrices
                For J = 0 To 3
                        A = S[ I * 4 + J ] - 65 ' This number converts symbol to number in range from 0 to 15
                        Figure[ M, I, A & 3, A Shr 2 ] = 1 ' 1 means filled square, 0 (by default) means empty
                Next
        Next
        M :+ 1 ' Incrementing figures counter
Next

Graphics 800, 600
Y = 19 ' current y-coordinate of figure, set to 19 to make initialization of new one

Repeat
        Cls
        For I = 0 To 20
                 ' Making glass boundaries
                World[ 1, I ] = 1 ' left wall (it is shifted from the edge to prevent error
                ' when figure matrix is situated partly outside of the game field)

                World[ 12, I ] = 1 ' right wall
                World[ I, 20 ] = 1 ' bottom
               
                ' Drawing game field
                For J = 1 To 12
                        If World[ J, I ] Then DrawRect J * 8, I * 8, 8, 8
                Next
        Next
       
        ' Drawing current figure
        For I = 0 To 3
                For J = 0 To 3
                        If Figure[ N, R, I, J ] Then DrawRect ( X + I ) * 8, ( Y + J ) * 8, 8, 8
                Next
        Next
       
        Flip
       
        ' This is cool controller handing method, that I borrowed from old Speccy magazines
        ' If KEY_RIGHT is pressed then new value of X will be less than old by 1
        ' If KEY_LEFT is pressed then new value of X will be more than old by 1
        A = X + KeyHit( KEY_RIGHT ) - KeyHit( KEY_LEFT )
        B = Y + KeyDown( KEY_DOWN )
       
        ' Phase is "cycling" with (phases quantity - 1) = %0, %1 or %11 - by using logical operator AND
        Q = ( R + KeyHit( KEY_W ) - KeyHit( KEY_Q ) ) & Phases[ N ]
       
        ' If new-positioned figure is not colliding with game field then it will be moved
        IF CheckPosition( A, B, N, Q ) = 0 Then
                X = A
                Y = B
                R = Q
        End If
       
        M = Millisecs()
       
        If M > T Then
                T = M + 999 ' figure will be moved down every second
               
                ' Checking if figure collides with game field when moved down (if true then "finalize" it)
                If CheckPosition( X, Y + 1, N, R ) Then
                        If Y = 0 Then End ' end of game because glass is filled to the top
                       
                        For I = 0 To 3 ' Transferring figure from its matrix to the game field (by filling corresponding squares)
                                For J = 0 To 3
                                        If Figure[ N, R, I, J ] Then World[ X + I, Y + J ] = 1
                                Next
                        Next
                       
                        ' This mechanism will check filled rows and also moving down upper stuff if there's any
                        D = 0 ' Current steps to shift game field contents by
                        For I = 19 To 0 Step -1
                                Q = 0
                                For J = 2 To 11
                                        If World[ J, I ] Then Q :+ 1
                                        World[ J, I + D ] = World[ J, I ]
                                Next
                                IF Q > 9 Then D :+ 1 ' If there's 10 squares in a row then shift steps will be incremented by 1
                        Next
                       
                        ' Initializing new random figure on the top of the glass
                        X = 5
                        Y =  - 1
                        N = Rand( 0, 6 )
                EndIf
               
                Y :+ 1 ' Moving figure down
        End If
Until KeyHit( KEY_ESCAPE )

' This function checks collisions of game field and figure N with phase R in ( X, Y ) coords
Function CheckPosition( X, Y, N, R )
        For I = 0 To 3
                For J = 0 To 3
                        ' Returns 1 if any filled figure square is placed on filled square of game field
                        If Figure[ N, R, I, J ] * World[ I + X, J + Y ] Then Return 1
                Next
        Next
        ' Returns 0 if there's no collision

EndFunction