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80 lines
2 KiB
C++
80 lines
2 KiB
C++
#ifndef _DIJKSTRA_HPP
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#define _DIJKSTRA_HPP
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#include <algorithm>
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#include <cassert>
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#include <functional>
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#include <optional>
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#include <utility>
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#include <vector>
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#include "graph.hpp"
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namespace {
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using pqueue_item_t = std::pair<int, vertex_t>;
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using pqueue_t = std::vector<pqueue_item_t>;
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auto pushq(pqueue_t& q, pqueue_item_t v) -> void {
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q.push_back(v);
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std::push_heap(q.begin(), q.end(), std::greater<>{});
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}
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auto popq(pqueue_t& q) -> std::optional<pqueue_item_t> {
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if (q.empty()) {
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return {};
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}
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std::pop_heap(q.begin(), q.end(), std::greater<>{});
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pqueue_item_t top = q.back();
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q.pop_back();
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return std::make_optional(top);
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}
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} // namespace
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auto dijkstra(const graph& g, const vertex_t& source)
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-> std::vector<std::vector<int>> {
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// make sure that ‹source› exists
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assert(g.has(source));
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// 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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// initialize the visited
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std::vector<std::vector<bool>> visited(g.height(),
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std::vector(g.width(), false));
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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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pqueue_t priority_queue{std::make_pair(0, source)};
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std::optional<pqueue_item_t> item{};
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while ((item = popq(priority_queue))) {
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auto [cost, u] = *item;
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auto [x, y] = u;
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// we have already found the shortest path
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if (visited[y][x]) {
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continue;
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}
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visited[y][x] = true;
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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 and update queue
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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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distances[y + dy][x + dx] = distances[y][x] + cost;
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pushq(priority_queue, std::make_pair(distances[y + dy][x + dx], v));
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}
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}
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}
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return distances;
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}
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#endif /* _DIJKSTRA_HPP */
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