Объясните, как работает программа

const n=15;
var a:array[1..n] of integer;
i,j,x:integer;
begin
randomize;
writeln('Исходный массив');
for i:=1 to n do
begin
a[i]:=-3+random(7);
write(a[i]:4);
end;
writeln;
for i:=1 to n-1 do
for j:=i+1 to n do
if(a[i]<>0)and(a[j]<>0)and(a[i]<a[j]) then
begin
x:=a[i];
a[i]:=a[j];
a[j]:=x;
end;
writeln('Сортировка ненулевых по убыванию');
for i:=1 to n do
write(a[i]:4);
readln
end.

procedure swap(var a,b,k:integer);
var t : integer;
begin
  inc(k);
  t := a; a:=b; b:=t
end;
 
const n = 5;
var 
      d,a : array[1..n] of integer;
  i,j,k,m : integer;
begin
  d[1] := 5;
  d[2] := 4;
  d[3] := 3;
  d[4] := 2;
  d[5] := 1;
 
  a := d;
  Write('original: '); for i:=1 to n do Write(a[i]:3); WriteLn;
  
  {variant 2}
  k := 0;
  for i := 1 to n - 1 do
    for j := i + 1 to n do
      if a[j] < a[i] then swap(a[i], a[j], k);
 
  Write('sorted: '); for i:=1 to n do Write(a[i]:3); WriteLn;
  WriteLn('count of swap: ',k);
 
 
{variant 1}
  a := d;
  k := 0;
  for i := 1 to n - 1 do
    begin
      m := i;
      for j := i + 1 to n do
        if a[j] < a[m] then m := j;
      if m > i then swap(a[m], a[i],k)
    end;
 
 
  Write('sorted: '); for i:=1 to n do Write(a[i]:3); WriteLn;
  WriteLn('count of swap: ',k);
 
end.