STD_LOGIC_VECTOR error

i'm trying to design a game using vhdl. an original source for this game was found in verilog. in that they used this negative indexing inside reg. i tried to chnage the reg indexing as reg[13:0] and game did'nt worked. original verilog code as follows

// Used for drawing, moving and updating of walls

module walls(

input clk,

input update,

input [5:0]data,

output wire hold,

output wire [5:0]data_addr,

input [2:0]quadrant_collision,

input [9:0]radius_collision,

input [2:0]quadrant,

input [9:0]radius,

input reset,

input [1:0]difficulty,

output reg [2:0]quadrant_out,

output reg visible,

output reg island,

output reg visible_collision

);

reg [5:0]walls[0:127];

reg [11:-2]center;

reg [6:0]write_ptr;

wire [11:0]offset = center[11:0] + radius;

wire [11:0]offset_collision = center[11:0] + radius_collision;

integer i;

always @(posedge clk) begin

if(reset) begin

center <= 14'd6144;

end else if(update) begin

case(difficulty)

2'd0: center <= center + 14'd6;

2'd1: center <= center + 14'd9;

2'd2: center <= center + 14'd12;

2'd3: center <= center + 14'd15;

endcase

end

end

always @(posedge clk) begin

quadrant_out <= quadrant;

if(radius < 10'd28) begin

visible <= 1'b0;

visible_collision <= 1'b0;

island <= 1'b1;

end else if(radius < 10'd32) begin

visible <= 1'b1;

visible_collision <= 1'b0;

island <= 1'b0;

end else begin

visible <= walls[offset[11:5]][quadrant+:1];

visible_collision <= walls[offset_collision[11:5]][quadrant_collision+:1];

island <= 1'b0;

end

end

assign data_addr = write_ptr[5:0];

assign hold = write_ptr[5:0]!=6'd0 || (write_ptr==7'b0000000 && center[11]) || (write_ptr==7'b1000000 && !center[11]);

always @(posedge clk) begin

if(reset) begin

write_ptr <= 7'b1000000;

for (i=0; i<128; i=i+1) walls[i] <= 6'b000000;

end else if(hold) begin

walls[write_ptr] <= data;

write_ptr <= write_ptr + 7'd1;

end

end

endmodule