Need help understanding some parts of "15 Puzzle Game" in C++ [closed] - c++

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I was reading the code of "15 Puzzle Game" implemented in C++ (Link).
Code
#include <time.h>
#include <stdlib.h>
#include <vector>
#include <string>
#include <iostream>
class p15 {
public :
void play() {
bool p = true;
std::string a;
while( p ) {
createBrd();
while( !isDone() ) { drawBrd();getMove(); }
drawBrd();
std::cout << "\n\nCongratulations!\nPlay again (Y/N)?";
std::cin >> a; if( a != "Y" && a != "y" ) break;
}
}
private:
void createBrd() {
int i = 1; std::vector<int> v;
for( ; i < 16; i++ ) { brd[i - 1] = i; }
brd[15] = 0; x = y = 3;
for( i = 0; i < 1000; i++ ) {
getCandidates( v );
move( v[rand() % v.size()] );
v.clear();
}
}
void move( int d ) {
int t = x + y * 4;
switch( d ) {
case 1: y--; break;
case 2: x++; break;
case 4: y++; break;
case 8: x--;
}
brd[t] = brd[x + y * 4];
brd[x + y * 4] = 0;
}
void getCandidates( std::vector<int>& v ) {
if( x < 3 ) v.push_back( 2 ); if( x > 0 ) v.push_back( 8 );
if( y < 3 ) v.push_back( 4 ); if( y > 0 ) v.push_back( 1 );
}
void drawBrd() {
int r; std::cout << "\n\n";
for( int y = 0; y < 4; y++ ) {
std::cout << "+----+----+----+----+\n";
for( int x = 0; x < 4; x++ ) {
r = brd[x + y * 4];
std::cout << "| ";
if( r < 10 ) std::cout << " ";
if( !r ) std::cout << " ";
else std::cout << r << " ";
}
std::cout << "|\n";
}
std::cout << "+----+----+----+----+\n";
}
void getMove() {
std::vector<int> v; getCandidates( v );
std::vector<int> p; getTiles( p, v ); unsigned int i;
while( true ) {
std::cout << "\nPossible moves: ";
for( i = 0; i < p.size(); i++ ) std::cout << p[i] << " ";
int z; std::cin >> z;
for( i = 0; i < p.size(); i++ )
if( z == p[i] ) { move( v[i] ); return; }
}
}
void getTiles( std::vector<int>& p, std::vector<int>& v ) {
for( unsigned int t = 0; t < v.size(); t++ ) {
int xx = x, yy = y;
switch( v[t] ) {
case 1: yy--; break;
case 2: xx++; break;
case 4: yy++; break;
case 8: xx--;
}
p.push_back( brd[xx + yy * 4] );
}
}
bool isDone() {
for( int i = 0; i < 15; i++ ) {
if( brd[i] != i + 1 ) return false;
}
return true;
}
int brd[16], x, y;
};
int main( int argc, char* argv[] ) {
srand( ( unsigned )time( 0 ) );
p15 p; p.play(); return 0;
}
/*
Possible Output:
+----+----+----+----+
| 11 | 5 | 12 | 3 |
+----+----+----+----+
| 10 | 7 | 6 | 4 |
+----+----+----+----+
| 13 | | 2 | 1 |
+----+----+----+----+
| 15 | 14 | 8 | 9 |
+----+----+----+----+
Possible moves: 2 13 14 7
*/
While I understood most of the code, some parts seemed unclear to me:
A. Why does the for loop iterate a 1000 times? Why is the number 1000 chosen?
for( i = 0; i < 1000; i++ )
B. In the above loop, why is v.clear(); called once in every iteration?
C. What is the significance of the numbers 1, 2, 4 and 8 in the code (used in the switch blocks)?
D. How does the getCandidates() function work?

Answer 1

A. The board is being shuffled, so 1000 means 'a lot, but not that much that you really have to start waiting for the board being shuffled'
B. getCandidates() returns the candidates by filling them in the vector, v.clear() resets the vector for new candidates.
C. Here 1,2,4,8 just means up,down,left,right (not in particular order). This is filled in by getCandidate(). There seem to be no reason for these specific values, they could be used as bits in an byte, but that doesn't seem to be the case here.
D. getCandidate() fills up,down,left,right in a vector based on whether the empty spot is near which border.