Monotone chain change data structure to doubly linked list - computational-geometry

Question

for (size_t i = 0; i < n; ++i) {
while (k >= 2 && cross(H[k-2], H[k-1], P[i]) <= 0) k--;
H[k++] = P[i];
for (size_t i = n-1, t = k+1; i > 0; --i) {
while (k >= t && cross(H[k-2], H[k-1], P[i-1]) <= 0) k--;
H[k++] = P[i-1];
I found the code for it on wikipedia but for some reasons I prefer to use doubly linked list as data structure The problem is in this first condition for while loop while(k>=2&&...) and while(k>=t && ...)
How can I rewrite these while loops on linked list

Answer 1

unit DoublyLinkedList;
interface
const NULL = NIL;
type TPoint = record
x,y:longint;
end;
PLink = ^TLink;
TLink = record
point:TPoint;
next:PLink;
prev:PLink;
end;
TList = record
first:PLink;
last:PLink;
end;
TFunc = function(A,B:TPoint):integer;
procedure ListInit(var L:TList);
function ListFind(L:TList;key:TPoint):PLink;
function ListIsEmpty(L:TList):boolean;
procedure ListInsertFirst(var L:TList;dd:TPoint);
procedure ListInsertLast(var L:TList;dd:TPoint);
procedure ListDeleteFirst(var L:TList);
procedure ListDeleteLast(var L:TList);
procedure ListInsert(var L:TList;dd:TPoint);
function ListInsertAfter(var L:TList;key,dd:TPoint):boolean;
procedure ListDeleteKey(var L:TList;key:TPoint);
procedure ListDisplayForward(L:TList);
procedure ListDisplayBackward(L:TList);
procedure BSTsort(var L:TList);
implementation
function equals(p1,p2:TPoint):boolean;
begin
equals:=(p1.x = p2.x) and (p1.y = p2.y);
end;
function compare(A,B:TPoint):integer;
var t:integer;
begin
t := 1;
if(A.x < B.x)or((A.x = B.x)and(A.y < B.y))then
t := -1;
if(A.x = B.x)and(A.y = B.y)then
t := 0;
compare := t;
end;
procedure BSTinsert(var root:PLink;x:PLink);
begin
if root = NULL then
begin
root := x;
x^.prev := NULL;
x^.next := NULL;
end
else if compare(root^.point,x^.point) = 0 then
BSTinsert(root^.prev,x)
else if compare(root^.point,x^.point) < 0 then
BSTinsert(root^.next,x)
else
BSTinsert(root^.prev,x);
end;
procedure BSTtoDLL(root:PLink;var L:TList);
begin
if root <> NULL then
begin
BSTtoDLL(root^.prev,L);
if ListIsEmpty(L) then
L.first := root
else
L.last^.next := root;
root^.prev := L.last;
L.last := root;
BSTtoDLL(root^.next,L);
end;
end;
procedure BSTsort(var L:TList);
var root,temp:PLink;
begin
root := NULL; (*This instruction was missing in the code *)
while not ListIsEmpty(L)do
begin
temp := L.first;
if L.first^.next = NULL then
L.last := NULL
else
L.first^.next^.prev := NULL;
L.first := L.first^.next;
BSTinsert(root,temp);
end;
BSTtoDLL(root,L);
end;
procedure ListInit(var L:TList);
begin
L.first := NULL;
L.last := NULL;
end;
function ListFind(L:TList;key:TPoint):PLink;
var p:PLink;
begin
p := L.first;
while(p <> NULL)and(not equals(key,p^.point))do
p := p^.next;
ListFind := p;
end;
function ListIsEmpty(L:TList):boolean;
begin
ListIsEmpty := L.first = NULL;
end;
procedure ListInsertFirst(var L:TList;dd:TPoint);
var newLink:PLink;
begin
new(newLink);
newLink^.point.x := dd.x;
newLink^.point.y := dd.y;
newLink^.next := NULL;
newLink^.prev := NULL;
if ListIsEmpty(L) then
L.last := newLink
else
L.first^.prev := newLink;
newLink^.next := L.first;
L.first := newLink;
end;
procedure ListInsertLast(var L:TList;dd:TPoint);
var newLink:PLink;
begin
new(newLink);
newLink^.point.x := dd.x;
newLink^.point.y := dd.y;
newLink^.next := NULL;
newLink^.prev := NULL;
if ListIsEmpty(L) then
L.first := newLink
else
begin
L.last^.next := newLink;
newLink^.prev := L.last;
end;
L.last := newLink;
end;
procedure ListDeleteFirst(var L:TList);
var temp:PLink;
begin
if not ListIsEmpty(L) then
begin
temp := L.first;
if L.first^.next = NULL then
L.last := NULL
else
L.first^.next^.prev := NULL;
L.first := L.first^.next;
dispose(temp);
end;
end;
procedure ListDeleteLast(var L:TList);
var temp:PLink;
begin
if not ListIsEmpty(L) then
begin
temp := L.last;
if L.first^.next = NULL then
L.first := NULL
else
L.last^.prev^.next := NULL;
L.last := L.last^.prev;
dispose(temp);
end;
end;
procedure ListInsert(var L:TList;dd:TPoint);
var newLink,current:PLink;
begin
new(newLink);
newLink^.point.x := dd.x;
newLink^.point.y := dd.y;
newLink^.next := NULL;
newLink^.prev := NULL;
current := L.first;
while(current <> NULL)and(compare(newLink^.point,current^.point) > 0)do
current := current^.next;
if current = NULL then
begin
if ListIsEmpty(L)then
L.first := newLink
else
begin
L.last^.next := newLink;
newLink^.prev := L.last;
end;
L.last := newLink;
end
else if current^.prev = NULL then
begin
L.first := newLink;
newLink^.next := current;
current^.prev := newLink;
newLink^.prev := NULL;
current := newLink;
end
else
begin
current^.prev^.next := newLink;
newLink^.next := current;
newLink^.prev := current^.prev;
current^.prev := newLink;
end;
end;
function ListInsertAfter(var L:TList;key,dd:TPoint):boolean;
var newLink,current:PLink;
found:boolean;
begin
current := ListFind(L,key);
found := current <> NULL;
if found then
begin
new(newLink);
newLink^.point.x := dd.x;
newLink^.point.y := dd.y;
newLink^.next := NULL;
newLink^.prev := NULL;
if current^.next = NULL then
begin
newLink^.next := NULL;
L.last := newLink;
end
else
begin
newLink^.next := current^.next;
current^.next^.prev := newLink;
end;
newLink^.prev := current;
current^.next := newLink;
end;
ListInsertAfter:= found;
end;
procedure ListDeleteKey(var L:TList;key:TPoint);
var current:PLink;
begin
current := ListFind(L,key);
if current <> NULL then
begin
if current^.prev = NULL then
L.first := current^.next
else
current^.prev^.next := current^.next;
if current^.next = NULL then
L.last := current^.prev
else
current^.next^.prev := current^.prev;
dispose(current);
end;
end;
procedure ListDisplayForward(L:TList);
var current :PLink;
begin
write('List (first-->last): ');
current := L.first;
while current <> NIL do
begin
write('(',current^.point.x,',',current^.point.y,') -> ');
current := current^.next;
end;
writeln('NULL');
end;
procedure ListDisplayBackward(L:TList);
var current :PLink;
begin
write('List (last-->first): ');
current := L.last;
while current <> NIL do
begin
write('(',current^.point.x,',',current^.point.y,') -> ');
current := current^.prev;
end;
writeln('NULL');
end;
begin
end.
program MonotoneChain;
uses crt,doublylinkedlist;
function equals(a,b:TPoint):boolean;
begin
equals := (a.x = b.x) and (a.y = b.y)
end;
function vect(a1,a2,b1,b2:TPoint):longint;
begin
vect := (a2.x - a1.x) * (b2.y - b1.y) - (b2.x - b1.x) * (a2.y - a1.y)
end;
function dist2(a1,a2:TPoint):longint;
begin
dist2 := sqr(a2.x - a1.x) + sqr(a2.y-a1.y)
end;
procedure Solve(var A,B:TList);
var k,t:longint;
pt:PLink;
begin
ListInit(B);
if not ListIsEmpty(A)then
begin
k := 0;
pt := A.first;
while pt <> NULL do
begin
while(k >= 2)and(vect(B.last^.prev^.point,B.last^.point,B.last^.prev^.point,pt^.point) <= 0)do
begin
ListDeleteLast(B);
k := k - 1;
end;
ListInsertLast(B,pt^.point);
k := k + 1;
pt := pt^.next;
end;
t := k + 1;
pt := A.last;
while pt <> NULL do
begin
while(k >= t)and(vect(B.last^.prev^.point,B.last^.point,B.last^.prev^.point,pt^.point) <= 0)do
begin
ListDeleteLast(B);
k := k - 1;
end;
ListInsertLast(B,pt^.point);
k := k + 1;
pt := pt^.prev;
end;
ListDeleteLast(B);
end;
end;
procedure main;
var A,B:TList;
input:text;
p:TPoint;
path:string;
begin
ListInit(A);
writeln('Podaj sciezke do pliku z danymi do wczytania');
readln(path);
path := 'F:\fpc\3.0.4\bin\i386-win32\monotonechain\' + path;
assign(input,path);
{$I-}
reset(input);
{$I+}
if IOResult <> 0 then
writeln('Pliku nie udalo sie wczytac')
else
begin
while not eof(input) do
begin
while not eoln(input) do
begin
read(input,p.x,p.y);
ListInsertLast(A,p);
end;
readln(input);
writeln('List A');
ListDisplayForward(A);
ListDisplayBackward(A);
BSTsort(A);
Solve(A,B);
writeln('List A');
ListDisplayForward(A);
ListDisplayBackward(A);
writeln('List B');
ListDisplayForward(B);
ListDisplayBackward(B);
while not ListIsEmpty(A) do
ListDeleteFirst(A);
while not ListIsEmpty(B) do
ListDeleteFirst(B);
end;
close(input);
end;
readkey;
end;
BEGIN
main;
END.
Here is the code
Segmentation fault appears while I try to sort
the list using BST
Probably when I try to insert the node removed from head of the list to the BST
Segmentation fault appears only when I call it from procedure which finds the convex hull
When I call it from main block of code everything seem to be ok
One of the casese of segmentetion fault is dereferencing null pointer
but i dont know is this segmentation fault caused by dereferencing null pointer
It is strange for me that BSTsort called in main block of code works ok
but BSTsort called in Solve procedure causes segmentation fault
I think i know how to correct this sorting procedure
but I would like to know why segmentation fault occured and why only
in Solve procedure