Giải thích giúp mình thuật toán của bài này cho minh với( khó quá)

var left, right : longint;


function pp(n:longint):boolean;
var i, S : longint;
begin
    s:=1;
    for i:=2 to trunc(sqrt(n)) do
        if n mod i = 0 then
            begin
                s := s + i;
                if i<>n div i then
                    s := s + n div i;
                if s>n then exit(true);
            end;
    exit(s>n);
end;


function countPP2(left, right : longint) : longint;
var i, count : longint;
begin
    count := 0;
    for i := left to right do
        if PP(i) then
            count := count + 1;
    exit(count);
end;


function countPP1(left, right : longint) : longint;
var i, j, count, t : longint;
    sum : array[2..100001] of longint;
begin
    for i := 2 to right do
        sum[i] := 1;
    count := 0;
    t := right div 2 + 1;
    for i := 2 to t do
        begin
            j := i * 2;
            while (j <= right) do
                begin
                    sum[j] := sum[j] + i;
                    j := j + i;
                end;
            if (i >= left) and (sum[i] > i) then
                inc(count);
        end;
    exit(count);
end;


begin
    left := 2;
    right := 100000;


    writeln(countPP2(left, right));
    writeln(countPP1(left, right));
end.

type numArray = array[1..32000] of shortint;

function compare(a: numArray; na: integer; b: numArray; nb: integer) : shortint;
var i : integer;
begin
    if (na < nb) then exit(-1);
    if (na > nb) then exit(1);
    for i := na downto 1 do
        begin
            if (a[i] > b[i]) then exit(1);
            if (a[i] < b[i]) then exit(-1);
        end;
    exit(0);
end;

procedure differences(var a: numArray; var na: integer; b: numArray; nb: integer);
var i, index : integer;
    nho : shortint;
begin
    nho := 0;
    index := 1;
    for i := 1 to na do
        begin
            if (a[i] >= b[i] + nho) then
                begin
                    a[i] := a[i] - b[i] - nho;
                    nho := 0;
                end
            else
                begin
                    a[i] := a[i] + 10 - b[i] - nho;
                    nho := 1;
                end;
            if (a[i] > 0) then
                index := i;
        end;
    na := index;
end;

procedure greatestCommonDivisor(var a: numArray; var na: integer; var b: numArray; var nb: integer);
var t : shortint;
begin
    t := compare(a, na, b, nb);
    while (t <> 0) do
        begin
            if (t > 0) then
                differences(a, na, b, nb);
            if (t < 0) then
                differences(b, nb, a, na);
            t := compare(a, na, b, nb);
        end;
end;

for i := na downto 1 do

a[1] := 8; a[2] := 1; na := 2;    // Số 18
b[1] := 2; b[2] := 1; nb := 2;    // Số 12
greatestCommonDivisor(a, na, b, nb);     // UCLN của 18 và 12
for i := na downto 1 do write(a[i]);         // Ghi kết quả

const   MAX = 32001;
        fi = 'UC.INP';
        fo = 'UC.OUT';
type numArray = array[1..MAX] of shortint;
var a, b : numArray;
    n, m : integer;
    f : text;


procedure doc;
var x : char;
    i, so, code : integer;
begin
    assign(f, fi); reset(f);
    readln(f, n, m);
    for i:=1 to MAX do begin a[i]:=0; b[i]:=0; end;
    for i:=n downto 1 do
        begin
            read(f, x);
            val(x, so, code);
            a[i] := so;
        end;
    readln(f);
    for i:=m downto 1 do
        begin
            read(f, x);
            val(x, so, code);
            b[i] := so;
        end;
    close(f);
end;


function compare(a: numArray; na: integer; b: numArray; nb: integer) : shortint;
var i : integer;
begin
    if (na < nb) then exit(-1);
    if (na > nb) then exit(1);
    for i := na downto 1 do
        begin
            if (a[i] > b[i]) then exit(1);
            if (a[i] < b[i]) then exit(-1);
        end;
    exit(0);
end;


procedure differences(var a: numArray; var na: integer; b: numArray; nb: integer);
var i, index : integer;
    nho : shortint;
begin
    nho := 0;
    index := 1;
    for i := 1 to na do
        begin
            if (a[i] >= b[i] + nho) then
                begin
                    a[i] := a[i] - b[i] - nho;
                    nho := 0;
                end
            else
                begin
                    a[i] := a[i] + 10 - b[i] - nho;
                    nho := 1;
                end;
            if (a[i] > 0) then
                index := i;
        end;
    na := index;
end;


procedure greatestCommonDivisor(var a: numArray; var na: integer; var b: numArray; var nb: integer);
var t : shortint;
begin
    t := compare(a, na, b, nb);
    while (t <> 0) do
        begin
            if (t > 0) then
                differences(a, na, b, nb);
            if (t < 0) then
                differences(b, nb, a, na);
            t := compare(a, na, b, nb);
        end;
end;


procedure xuat;
var i : integer;
begin
    assign(f,fo); rewrite(f);
    for i := n downto 1 do write(a[i]);
    close(f);
end;


begin
    doc;
    greatestCommonDivisor(a, n, b, m);
    xuat;
end.

begin
    a[1] := 8; a[2] := 0; a[3] := 1; n := 3;     // số 108
    b[1] := 2; b[2] := 7; m := 2;     // số 72
    greatestCommonDivisor(a, n, b, m);
    for i := n downto 1 do write(a[i]);     // in ra số 36
    writeln();
end.

while (a <> b) do
   begin
      if (a > b) then a := a - b
      else b := b - a;
   end;