blog/static/files/algorithms/paths/bf-to-astar/dijkstra.hpp

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#ifndef _DIJKSTRA_HPP
#define _DIJKSTRA_HPP
#include <algorithm>
#include <cassert>
#include <functional>
#include <optional>
#include <utility>
#include <vector>
#include "graph.hpp"
auto dijkstra(const graph& g, const vertex_t& source)
-> std::vector<std::vector<int>> {
// make sure that source exists
assert(g.has(source));
// initialize the distances
std::vector<std::vector<int>> distances(
g.height(), std::vector(g.width(), graph::unreachable()));
// initialize the visited
std::vector<std::vector<bool>> visited(g.height(),
std::vector(g.width(), false));
// source destination denotes the beginning where the cost is 0
auto [sx, sy] = source;
distances[sy][sx] = 0;
pqueue_t priority_queue{std::make_pair(0, source)};
std::optional<pqueue_item_t> item{};
while ((item = popq(priority_queue))) {
auto [cost, u] = *item;
auto [x, y] = u;
// we have already found the shortest path
if (visited[y][x]) {
continue;
}
visited[y][x] = true;
for (const auto& [dx, dy] : DIRECTIONS) {
auto v = std::make_pair(x + dx, y + dy);
auto cost = g.cost(u, v);
// if we can move to the cell and it's better, relax¹ it and update queue
if (cost != graph::unreachable() &&
distances[y][x] + cost < distances[y + dy][x + dx]) {
distances[y + dy][x + dx] = distances[y][x] + cost;
pushq(priority_queue, std::make_pair(distances[y + dy][x + dx], v));
}
}
}
return distances;
}
#endif /* _DIJKSTRA_HPP */