## Thursday, November 6, 2014

### SPOJ Problem Set (classical) - Fishmonger

Problem:

Solution:

I started the problem using the a simple complete search of all path. Of course, we are not going to be able to enumerate through all the paths. But fortunately we don't have to, once we have tried a path we can memoize the result as follow.

If I am at a certain node n1 with certain an initial time constraint T1before, and with time t12 I can get to node n2, recursively we know the best path to get to market from n2 with budget T-t12 take time T2after and has minimal cost C2after, then we have one best path candidate from n1 to n2 within time constraint T1before that actually spend time T2after + t12 with cost C2after + c12. Try all candidate and find the solution.

The key to the above idea is memoization - we save the fact that if we are going from node n1, the budget given is between T1before and T2after + t12, then the best path has the be the best path found, no point to find a best path again.

I have coded the solution, unforunately it is Wrong Answer. Removed memoization, clearly Time Limit Exceeded.  Looking in retrospect, could be similar bug that I hit in the approach below.

Now I tried something else, based on Floyd Warshall. Floyd Warshall is that we can find the shortest path between every two nodes by allowing incrementally more intermediate nodes. I thought, if instead of the shortest path, what if we maintain the set of non dominated paths? By non-dominated, I mean paths that has either lower cost higher time, or path with higher time lower cost, but not higher time, higher cost ones. We also constraint the time of those path within time budget. At the end of the algorithm, we will get, between all pairs of nodes, the set of all non-dominated paths, and therefore we can pick the one with least cost within time budget at the end of the algorithm.

For non-dominated filtering, I just used the simple $O(p^2)$ algorithm, where $p$ is the number of candidate paths. There exists $O(p \log p)$ algorithm, and maybe even better, I just didn't do it since the time is good enough for the problem to accept.

All is fine, except one bug I just can't find myself. When I initialized the table, I put the initial distance and initial cost between two nodes into the table. I didn't check if those values are within time budget, as a result, if there is a graph with a low cost path within short time budget that reach the market directly, I would have returned that path - wrong.

If I got more time, I can try modifying Dijkstra's algorithm or Bellman Ford's algorithm, basically the same idea of keeping non-dominated paths. These single source shortest path algorithm should be faster than the all pair shortest path problem, I believe.

As an aside - thanks a lot for those who helped me to find out the tricky test case that I missed. I guess I need to be a better adversary of my own code to find that out. I also got the feedback that the code is hard to read, I tried to make my code as descriptive as it can be, but I guess everybody have a different taste of what does it mean by readable, or well, code are just not fun to read, wrapping it with sugar just doesn't cover the bitterness.

Code:

#include "stdafx.h"

// http://www.spoj.com/problems/FISHER/

// #define LOG

#include <iostream>
#include <vector>
#include <map>
#include <algorithm>

using namespace std;

int SPOJ_FISHER()
{
while (true)
{
int num_cities;
int time_budget;
vector<vector<int> > distances;
vector<vector<int> > tolls;

cin >> num_cities;
cin >> time_budget;
if (num_cities == 0 && time_budget == 0)
{
break;
}
distances.resize(num_cities);
tolls.resize(num_cities);
for (int i = 0; i < num_cities; i++)
{
distances[i].resize(num_cities);
tolls[i].resize(num_cities);
}
for (int i = 0; i < num_cities; i++)
{
for (int j = 0; j < num_cities; j++)
{
int distance;
cin >> distance;
distances[i][j] = distance;
}
}
for (int i = 0; i < num_cities; i++)
{
for (int j = 0; j < num_cities; j++)
{
int toll;
cin >> toll;
tolls[i][j] = toll;
}
}

// Try Floyd Warshall
// Denote the set of shortest paths from i to j going through {0,1,...k - 1} be shortest_paths[i][j][k],
// It is a set of shortest paths because there can be multiple shortest paths with different time used.
// We should record if using longer time can lead to lower cost, or similarly higher cost but less time
// The first element in the pair is the cost, the second element in the pair is time used
vector<vector<vector<vector<pair<int, int> > > > > shortest_paths;
shortest_paths.resize(num_cities);
for (int i = 0; i < num_cities; i++)
{
shortest_paths[i].resize(num_cities);
for (int j = 0; j < num_cities; j++)
{
shortest_paths[i][j].resize(num_cities + 1);
}
}

// Initialization - there is only one path without going through any node
#ifdef LOG
cout << "k = " << 0 << endl;
cout << "<table border='1'>" << endl;
#endif
for (int i = 0; i < num_cities; i++)
{
#ifdef LOG
cout << "<tr>" << endl;
#endif

for (int j = 0; j < num_cities; j++)
{
if (distances[i][j] <= time_budget)
{
#ifdef LOG
cout << "<td>(" << tolls[i][j] << ", " << distances[i][j] << ")</td>";
#endif
shortest_paths[i][j][0].push_back(pair<int, int>(tolls[i][j], distances[i][j]));
}
}
#ifdef LOG
cout << "</tr>" << endl;
#endif
}
#ifdef LOG
cout << "</table>" << endl;
#endif
// Iteration - the shortest path
for (int k = 1; k <= num_cities; k++)
{
#ifdef LOG
cout << "k = " << k << endl;
cout << "<table border='1'>" << endl;
#endif
for (int i = 0; i < num_cities; i++)
{
#ifdef LOG
cout << "<tr>";
#endif
for (int j = 0; j < num_cities; j++)
{
// Step 1: Generate all candidate shortest paths
map<pair<int, int>, bool> candidates;
for (vector<pair<int, int> >::iterator pi = shortest_paths[i][j][k - 1].begin(); pi != shortest_paths[i][j][k - 1].end(); pi++)
{
candidates.insert(pair<pair<int, int>, bool>(*pi, false));
}
for (vector<pair<int, int> >::iterator fi = shortest_paths[i][k - 1][k - 1].begin(); fi != shortest_paths[i][k - 1][k - 1].end(); fi++)
{
for (vector<pair<int, int> >::iterator si = shortest_paths[k - 1][j][k - 1].begin(); si != shortest_paths[k - 1][j][k - 1].end(); si++)
{
int first_path_cost = fi->first;
int first_path_time_used = fi->second;
int second_path_cost = si->first;
int second_path_time_used = si->second;

int new_path_cost = first_path_cost + second_path_cost;
int new_path_time_used = first_path_time_used + second_path_time_used;

if (new_path_time_used <= time_budget)
{
candidates.insert(pair<pair<int, int>, bool>(pair<int, int>(new_path_cost, new_path_time_used), false));
}
}
}

vector<pair<pair<int, int>, bool> > candidates_list;
for (map<pair<int,int>, bool>::iterator ci = candidates.begin(); ci != candidates.end(); ci++)
{
candidates_list.push_back(*ci);
}

for (unsigned int p = 0; p < candidates_list.size(); p++)
{
for (unsigned int q = 0; q < candidates_list.size(); q++)
{
if (p != q)
{
int first_path_cost = candidates_list[p].first.first;
int first_path_time_used = candidates_list[p].first.second;
int second_path_cost = candidates_list[q].first.first;
int second_path_time_used = candidates_list[q].first.second;

// First take less time and less cost than second, second is eliminated
if (first_path_time_used <= second_path_time_used && first_path_cost <= second_path_cost)
{
candidates_list[q].second = true;
}
}
}
}
#ifdef LOG
cout << "<td>";
#endif
for (unsigned int p = 0; p < candidates_list.size(); p++)
{
if (candidates_list[p].second == false)
{
#ifdef LOG
cout << "(" << candidates_list[p].first.first << ", " << candidates_list[p].first.second << ")<br>";
#endif
shortest_paths[i][j][k].push_back(candidates_list[p].first);
}
}
#ifdef LOG
cout << "</td>";
#endif

}
#ifdef LOG
cout << "</tr>" << endl;;
#endif
}
#ifdef LOG
cout << "</table>" << endl;
#endif
}

bool first = true;
int best_cost = -1;
int best_cost_time = -1;
for (vector<pair<int, int> >::iterator pi = shortest_paths[0][num_cities - 1][num_cities].begin(); pi != shortest_paths[0][num_cities - 1][num_cities].end(); pi++)
{
if (first)
{
best_cost = pi->first;
best_cost_time = pi->second;
first = false;
}
else
{
if (pi->first < best_cost)
{
best_cost = pi->first;
best_cost_time = pi->second;
}
if (pi->first == best_cost && pi->second < best_cost_time)
{
best_cost_time = pi->second;
}
}
}
cout << best_cost << " " << best_cost_time << endl;
}

return 0;

}