Want to understand the logic of a code

[lkp]

library IEEE;
use IEEE.std_logic_1164.all;
use IEEE.std_logic_arith.all;
use IEEE.std_logic_unsigned.all;
--use work.my_package.all;
entity square_163_7_6_3 is
port (
a: in std_logic_vector(162 downto 0);
z: out std_logic_vector(162 downto 0)
);
end square_163_7_6_3;

architecture circuit of square_163_7_6_3 is

signal s, t, u, s_plus_t: std_logic_vector(162 downto 0);
signal xor1, xor2: std_logic;

begin

vector_s: for i in 0 to 80 generate s(2*i) <= a(i); s(2*i + 1) <= a(i+82); end generate;
s(162) <= a(81);

vector_t1: for j in 0 to 6 generate t(j) <= '0'; end generate;
t(7) <= a(82);
vector_t2: for i in 4 to 80 generate t(2*i) <= a(i+78); t(2*i + 1) <= a(i+79); end generate;
t(162) <= a(159);

xor1 <= a(160) xor a(161); xor2 <= a(161) xor a(162);
u(0) <= a(160); u(1) <= a(160) xor a(162); u(2) <= a(161); u(3) <= xor1;
u(4) <= a(82) xor a(160); u(5) <= xor2; u(6) <= a(83) xor xor1;
u(7) <= '0'; u(8) <= a(84) xor xor1; u(9) <= '0'; u(10) <= a(85) xor xor2;
u(11) <= '0'; u(12) <= a(86) xor a(162);
u(13) <= '0';
vector_u: for i in 7 to 80 generate u(2*i) <= a(i+80); u(2*i + 1) <= '0'; end generate;
u(162) <= a(161);

xor_gates1: for j in 0 to 162 generate s_plus_t(j) <= s(j) xor t(j); end generate;
xor_gates2: for j in 0 to 162 generate z(j) <= s_plus_t(j) xor u(j); end generate;

end circuit;