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136 lines
3.8 KiB
C++
136 lines
3.8 KiB
C++
#ifndef _BF_HPP
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#define _BF_HPP
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#include <cassert>
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#include <iostream>
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#include <utility>
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#include <vector>
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#include "graph.hpp"
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static auto _check_vertex(const graph& g,
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std::vector<std::vector<int>>& distances, int v,
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bool check_only = false) -> bool {
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bool improvement_found = false;
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// unpack the vertex coordinates
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int y = v / g.width();
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int x = v % g.width();
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// skip the cells we cannot reach
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if (distances[y][x] == graph::unreachable()) {
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return false;
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}
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// go through the neighbours
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auto u = std::make_pair(x, y);
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for (const auto& [dx, dy] : DIRECTIONS) {
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auto v = std::make_pair(x + dx, y + dy);
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auto cost = g.cost(u, v);
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// if we can move to the cell and it's better, relax¹ it
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if (cost != graph::unreachable() &&
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distances[y][x] + cost < distances[y + dy][x + dx]) {
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if (check_only) {
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return true;
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}
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distances[y + dy][x + dx] = distances[y][x] + cost;
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improvement_found = true;
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}
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}
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return improvement_found;
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}
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auto bf(const graph& g, const vertex_t& source, const vertex_t& destination)
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-> int {
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// ‹source› must be within the bounds
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assert(g.has(source));
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// ‹destination› must be within the bounds
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assert(g.has(destination));
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// we need to initialize the distances
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std::vector<std::vector<int>> distances(
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g.height(), std::vector(g.width(), graph::unreachable()));
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// ‹source› destination denotes the beginning where the cost is 0
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auto [sx, sy] = source;
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distances[sy][sx] = 0;
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// now we need to improve the paths as long as possible
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bool improvement_found;
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do {
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// reset the flag at the beginning
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improvement_found = false;
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// go through all of the vertices
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for (int v = g.height() * g.width() - 1; v >= 0; --v) {
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improvement_found = _check_vertex(g, distances, v) || improvement_found;
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}
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} while (improvement_found);
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return distances[destination.second][destination.first];
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}
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auto bf_finite(const graph& g, const vertex_t& source,
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const vertex_t& destination) -> int {
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// ‹source› must be within the bounds
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assert(g.has(source));
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// ‹destination› must be within the bounds
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assert(g.has(destination));
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// we need to initialize the distances
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std::vector<std::vector<int>> distances(
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g.height(), std::vector(g.width(), graph::unreachable()));
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// ‹source› destination denotes the beginning where the cost is 0
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auto [sx, sy] = source;
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distances[sy][sx] = 0;
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// now we only iterate as many times as cells that we have
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for (int i = g.height() * g.width(); i > 0; --i) {
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// go through all of the vertices
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for (int v = g.height() * g.width() - 1; v >= 0; --v) {
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_check_vertex(g, distances, v);
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}
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}
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return distances[destination.second][destination.first];
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}
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auto bellman_ford(const graph& g, const vertex_t& source)
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-> std::vector<std::vector<int>> {
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// ‹source› must be within the bounds
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assert(g.has(source));
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// we need to initialize the distances
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std::vector<std::vector<int>> distances(
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g.height(), std::vector(g.width(), graph::unreachable()));
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// ‹source› destination denotes the beginning where the cost is 0
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auto [sx, sy] = source;
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distances[sy][sx] = 0;
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// now we only iterate as many times as cells that we have
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for (int i = g.height() * g.width(); i > 0; --i) {
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// go through all of the vertices
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for (int v = g.height() * g.width() - 1; v >= 0; --v) {
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_check_vertex(g, distances, v);
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}
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}
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// now we check for the negative loops
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for (int v = g.height() * g.width() - 1; v >= 0; --v) {
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if (_check_vertex(g, distances, v, true)) {
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std::cerr << "[Bellman-Ford] Found a negative loop\n";
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break;
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}
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}
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return distances;
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}
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#endif /* _BF_HPP */
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