I'm trying to implement a 4x4 sliding-block puzzle solver that utilizes the A* algorithm. Even though was trying to code in as neat a way as possible, a segfault slipped into the code that points to a failed attempt to push a pointer to an std::priority_queue instance. Here's the full code followed by my debugging attempts, I had no idea how to cut it into a working test case:
#include <iostream>
#include <iomanip>
#include <cstring>
#include <cmath>
#include <vector>
#include <set>
#include <queue>
#include <functional>
#include <ctime>
#include <cstdlib>
using namespace std;
const char GOAL_STATE[] = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,0};
class Puzzle
{
protected:
char puzzle[16];
int index(char searched) const
{
for(int i=0; i<16; i++)
{
if (puzzle[i]==searched)
return i;
}
return -1;
}
public:
Puzzle(const char init[16] = NULL)
{
if(init!=NULL)
{
memcpy(puzzle, init, sizeof(char)*16);
}
else
{
memcpy(puzzle, GOAL_STATE, sizeof(char)*16);
}
}
void show() const
{
for(int i=0; i<4; i++)
{
for(int j=0; j<4; j++)
{
cout << setw(3) << (int)puzzle[i*4+j] << " ";
}
cout << endl;
}
}
bool move(int m_pos)
{
int b_pos = index(0);
int b_x = b_pos / 4;
int b_y = b_pos % 4;
int x = m_pos / 4;
int y = m_pos % 4;
if (m_pos < 16 && (
(x == b_x && (y == b_y + 1 || y == b_y - 1)) ||
(y == b_y && (x == b_x + 1 || x == b_x - 1))
)
)
{
char tmp = puzzle[m_pos];
puzzle[m_pos] = puzzle[b_pos];
puzzle[b_pos] = tmp;
return true;
}
else
return false;
}
int count_correct() const
{
int ret = 0;
for(int i=0; i<16; i++)
if ((i<15 && (puzzle[i])==i+1) || (i==15 && puzzle[i]==0))
ret++;
return ret;
}
vector<int> possible_moves() const
{
vector<int> ret;
int b_pos = index(0);
int b_x = b_pos / 4;
int b_y = b_pos % 4;
ret.push_back(b_y * 4 + b_x - 1);
ret.push_back((b_y - 1) * 4 + b_x);
if (b_x<3)
ret.push_back(b_y * 4 + b_x + 1);
if (b_y<3)
ret.push_back((b_y + 1) * 4 + b_x);
return ret;
}
};
class AStarPuzzle : public Puzzle
{
int H;
int f;
public:
int tried;
AStarPuzzle* previous;
AStarPuzzle(const char init[16] = NULL,
int _f = 0, int _tried = 0, AStarPuzzle* _previous = NULL) : Puzzle(init)
{
f = _f;
tried = _tried;
previous = _previous;
}
AStarPuzzle (const AStarPuzzle& old) : Puzzle(old.puzzle)
{
f = old.f;
tried = old.tried;
previous = old.previous;
}
double heur() const
{
double ret = 0.0;
for (int goalIndex = 0 ; goalIndex<16; goalIndex++)
{
int tileIndex = index( GOAL_STATE[goalIndex] );
ret += abs((double)((goalIndex % 4) - (tileIndex % 4)));
ret += abs(floor(goalIndex / 4.0) - floor(tileIndex / 4.0));
}
return 0.1*f + max(ret, (double)count_correct());
}
bool operator<(const AStarPuzzle& other) const
{
return heur()>other.heur();
}
int get_f()
{
return f;
}
void increase_f()
{
f++;
}
AStarPuzzle operator=(const AStarPuzzle& rhs)
{
cout << "STUB: AStarPuzzle::operator=" << endl;
}
~AStarPuzzle()
{
cout << "STUB: AStarPuzzle::~AStarPuzzle" << endl;
}
void operator delete(void *p)
{
cout << "Attempting to delete AStarPuzzle?" << endl;
}
};
struct CompareHeuristics : public std::binary_function<AStarPuzzle*, AStarPuzzle*, bool >
{
bool operator()(AStarPuzzle* lhs, AStarPuzzle* rhs) const
{
return lhs->heur() > rhs->heur();
}
};
vector<int> retracePath(AStarPuzzle* c)
{
vector<int> ret;
while (c->previous!=NULL)
{
c = c->previous;
ret.push_back(c->tried);
}
return ret;
}
vector<int> aStar(AStarPuzzle* current)
{
set<AStarPuzzle*> openList;
set<AStarPuzzle*> closedList;
std::priority_queue<AStarPuzzle*, std::vector<AStarPuzzle*>, CompareHeuristics> openHeap;
int max_corr = 0;
int min_step = 0;
openList.insert(current);
openHeap.push(current);
while (openList.size()!=0)
{
current = openHeap.top();
cout << "An iteration. heur()==" << current->heur() << endl;
current->show();
openHeap.top();
if (current->count_correct() == 16)
{
//return vector<int>();
return retracePath(current);
}
openList.erase(current);
closedList.insert(current);
vector<int> directions = current->possible_moves();
for(unsigned int i=0; i<directions.size(); i++)
{
AStarPuzzle* tile = new AStarPuzzle(*current);
tile->move(directions[i]);
tile->increase_f();
if (closedList.count(tile)==0)
{
int corr = tile->count_correct();
int f = tile->get_f();
if (corr > max_corr or (corr == max_corr && f < min_step))
{
max_corr = corr;
min_step = f;
cout << corr << "," << f << endl;
}
tile->previous = current;
if (openList.count(tile)==0)
{
openList.insert(tile);
openHeap.push(tile);
}
}
}
}
return vector<int>();
}
int main(int argc, char **argv) {
char shuffled[16];
memcpy(shuffled, GOAL_STATE, sizeof(char)*16);
srand(time(0));
for (int i=0; i<16; i++) {
int r = i + (rand() % (16-i));
char temp = shuffled[i];
shuffled[i] = shuffled[r];
shuffled[r] = temp;
}
shuffled = {0, 3, 7, 5, 9, 12 , 13 , 11, 8, 6 , 14, 4 , 15, 2 , 10 , 1 };
AStarPuzzle* p = new AStarPuzzle(shuffled);
p->show();
aStar(p);
return 0;
}
And the gdb output:
> gdb ./si-proj2cpp
GNU gdb (GDB) 7.0.1-debian
Copyright (C) 2009 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i486-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /home/d33tah/workspace/si-proj2cpp/Debug/si-proj2cpp...done.
(gdb) r
Starting program: /home/d33tah/workspace/si-proj2cpp/Debug/si-proj2cpp
0 3 7 5
9 12 13 11
8 6 14 4
15 2 10 1
An iteration. heur()==44
0 3 7 5
9 12 13 11
8 6 14 4
15 2 10 1
An iteration. heur()==44
0 3 7 5
9 12 13 11
8 6 14 4
15 2 10 1
An iteration. heur()==44
0 3 7 5
9 12 13 11
8 6 14 4
15 2 10 1
An iteration. heur()==44
0 3 7 5
9 12 13 11
8 6 14 4
15 2 10 1
Program received signal SIGSEGV, Segmentation fault.
0xb7dab5fc in _int_free (av=<value optimized out>, p=0x80502b0) at malloc.c:4957
4957 malloc.c: No such file or directory.
in malloc.c
Current language: auto
The current source language is "auto; currently c".
(gdb) bt
#0 0xb7dab5fc in _int_free (av=<value optimized out>, p=0x80502b0) at malloc.c:4957
#1 0xb7dae8ad in *__GI___libc_free (mem=0x80502b8) at malloc.c:3739
#2 0xb7f86701 in operator delete(void*) () from /usr/lib/libstdc++.so.6
#3 0x0804b97b in __gnu_cxx::new_allocator<AStarPuzzle*>::deallocate (this=0xbffff564, __p=0x80502b8) at /usr/include/c++/4.4/ext/new_allocator.h:95
#4 0x0804ab4b in std::_Vector_base<AStarPuzzle*, std::allocator<AStarPuzzle*> >::_M_deallocate (this=0xbffff564, __p=0x80502b8, __n=16)
at /usr/include/c++/4.4/bits/stl_vector.h:146
#5 0x0804b2a9 in std::vector<AStarPuzzle*, std::allocator<AStarPuzzle*> >::_M_insert_aux (this=0xbffff564, __position=..., __x=@0xbffff554)
at /usr/include/c++/4.4/bits/vector.tcc:361
#6 0x0804a5cf in std::vector<AStarPuzzle*, std::allocator<AStarPuzzle*> >::push_back (this=0xbffff564, __x=@0xbffff554) at /usr/include/c++/4.4/bits/stl_vector.h:741
#7 0x08049bbb in std::priority_queue<AStarPuzzle*, std::vector<AStarPuzzle*, std::allocator<AStarPuzzle*> >, CompareHeuristics>::push (this=0xbffff564, __x=@0xbffff554)
at /usr/include/c++/4.4/bits/stl_queue.h:511
#8 0x08049054 in aStar (current=0x8050008) at ../main.cpp:248
#9 0x08049273 in main (argc=1, argv=0xbffff704) at ../main.cpp:275
As somebody suggested, I used valgrind to see what happens:
> valgrind ./si-proj2cpp
==26655== Memcheck, a memory error detector
==26655== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al.
==26655== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==26655== Command: ./si-proj2cpp
==26655==
0 3 7 5
9 12 13 11
8 6 14 4
15 2 10 1
An iteration. heur()==44
0 3 7 5
9 12 13 11
8 6 14 4
15 2 10 1
==26655== Invalid read of size 1
==26655== at 0x804950E: Puzzle::move(int) (main.cpp:74)
==26655== by 0x8048F3A: aStar(AStarPuzzle*) (main.cpp:231)
==26655== by 0x8049272: main (main.cpp:275)
==26655== Address 0x42ca1ef is 1 bytes before a block of size 32 alloc'd
==26655== at 0x402471C: operator new(unsigned int) (vg_replace_malloc.c:255)
==26655== by 0x8048EF6: aStar(AStarPuzzle*) (main.cpp:230)
==26655== by 0x8049272: main (main.cpp:275)
==26655==
==26655== Invalid write of size 1
==26655== at 0x8049525: Puzzle::move(int) (main.cpp:75)
==26655== by 0x8048F3A: aStar(AStarPuzzle*) (main.cpp:231)
==26655== by 0x8049272: main (main.cpp:275)
==26655== Address 0x42ca1ef is 1 bytes before a block of size 32 alloc'd
==26655== at 0x402471C: operator new(unsigned int) (vg_replace_malloc.c:255)
==26655== by 0x8048EF6: aStar(AStarPuzzle*) (main.cpp:230)
==26655== by 0x8049272: main (main.cpp:275)
==26655==
An iteration. heur()==44
0 3 7 5
9 12 13 11
8 6 14 4
15 2 10 1
An iteration. heur()==44
0 3 7 5
9 12 13 11
8 6 14 4
15 2 10 1
(...looks like an infinite loop, so I'm stopping the program with ctr+c)
0 3 7 5
9 12 13 11
8 6 14 4
15 2 10 1
^CAn iteration. heur()==44
==26655==
==26655== HEAP SUMMARY:
==26655== in use at exit: 17,076 bytes in 579 blocks
==26655== total heap usage: 805 allocs, 226 frees, 21,156 bytes allocated
==26655==
==26655== LEAK SUMMARY:
==26655== definitely lost: 0 bytes in 0 blocks
==26655== indirectly lost: 0 bytes in 0 blocks
==26655== possibly lost: 0 bytes in 0 blocks
==26655== still reachable: 17,076 bytes in 579 blocks
==26655== suppressed: 0 bytes in 0 blocks
==26655== Rerun with --leak-check=full to see details of leaked memory
==26655==
==26655== For counts of detected and suppressed errors, rerun with: -v
==26655== ERROR SUMMARY: 288 errors from 2 contexts (suppressed: 19 from 8)
The problem was actually not memory-related, it was - as @Synxis pointed out - negative m_pos being passed to Puzzle::move. The reason it happened was that possible_moves() didn't account for the situation when the 0 block was in (0,0) - fixing possible_moves helped. Another bug there was related to x and y being the other way round and openHeap.top() being called twice instead of openHeap.top(), openHeap.pop().