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src/main/java/Main.java

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+import algorithm.Dijkstras;
+import algorithm.Graph;
+
+import java.util.Map;
+
+public class Main {
+    public static void main(String[] args) {
+        System.out.println("Hello!");
+        var algorithm = new Dijkstras<String>();
+
+        var graph = new Graph<>(Map.of(
+                "A", Map.of("B", 5),
+                "B", Map.of("A", 7)
+        ));
+
+        var route = algorithm.findPath(graph, "A", "A");
+
+        System.out.println(route);
+        System.out.println(graph.getDistance(route));
+
+        route = algorithm.findPath(graph, "A", "B");
+        System.out.println(route);
+        System.out.println(graph.getDistance(route));
+
+        route = algorithm.findPath(graph, "B", "A");
+        System.out.println(route);
+        System.out.println(graph.getDistance(route));
+
+    }
+
+
+}

src/main/java/algorithm/Algorithm.java

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+package algorithm;
+
+import java.util.List;
+
+public interface Algorithm<T> {
+    List<T> findPath(Graph<T> graph, T source, T target);
+}

src/main/java/algorithm/Dijkstras.java

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+package algorithm;
+
+import java.util.*;
+import java.util.stream.Stream;
+
+public class Dijkstras<T> implements Algorithm<T> {
+
+    @Override
+    public List<T> findPath(Graph<T> graph, T source, T target) {
+        final var queue = new LinkedList<T>();
+        final var visited = new HashSet<T>();
+        final var distances = new HashMap<T, Double>();
+        final var previous = new HashMap<T, T>();
+        queue.add(source);
+
+        while (!queue.isEmpty()) {
+            final var prev = queue.pollFirst();
+            final var edges = graph.nodes().get(prev);
+            edges.forEach((node, time) -> {
+                final var distance = distances.getOrDefault(prev, .0) + time.doubleValue();
+                if (!visited.contains(node)) {
+                    queue.add(node);
+                    visited.add(node);
+                }
+                if (distance < distances.getOrDefault(node, Double.MAX_VALUE)) {
+                    previous.put(node, prev);
+                    distances.put(node, distance);
+                }
+            });
+        }
+
+        final var path = new LinkedList<T>();
+        Stream.iterate(target, Objects::nonNull, previous::get).forEach(path::addFirst);
+        return path;
+    }
+
+}

src/main/java/algorithm/Graph.java

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+package algorithm;
+
+import java.util.List;
+import java.util.Map;
+
+public record Graph<T>(Map<T, Map<T, Number>> nodes) {
+
+    public double getDistance(List<T> path) {
+        double distance = 0;
+        for (int i = 1; i < path.size(); ++i) {
+            final var previous = nodes.get(path.get(i - 1));
+            distance += previous.get(path.get(i)).doubleValue();
+        }
+        return distance;
+    }
+}

src/test/groovy/algorithm/DijkstrasSpec.groovy

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+package algorithm
+
+import spock.lang.Specification
+import spock.lang.Subject
+import spock.lang.Unroll
+
+class DijkstrasSpec extends Specification {
+
+    static final SAMPLE_ONE = [
+            A: [B: 7, C: 2],
+            B: [A: 3, C: 5],
+            C: [A: 1, B: 3]
+    ] as Graph<String>
+
+    @Subject
+    def algorithm = new Dijkstras<String>()
+
+    @Unroll("from #source to #target the time is #time and the path is #expected")
+    def 'should find a route for sample one'() {
+        given:
+        def graph = SAMPLE_ONE
+
+        when:
+        def path = algorithm.findPath(graph, source, target)
+
+        then:
+        path == expected
+
+        where:
+        source | target || time | expected
+        'A'    | 'B'    || 7    | ['A', 'B']
+    }
+}

src/test/groovy/algorithm/GraphSpec.groovy

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+package algorithm
+
+import spock.lang.Specification
+import spock.lang.Unroll
+
+class GraphSpec extends Specification {
+
+    @Unroll("distance is #distance for path #path")
+    def "should calculate distance"() {
+        given:
+        def graph = new Graph([
+                A: [B: 7, C: 2],
+                B: [A: 3, C: 5],
+                C: [A: 1, B: 3]
+        ])
+
+        expect:
+        graph.getDistance(path) == distance as double
+
+        where:
+        path                 | distance
+        ['A']                | 0
+        ['B']                | 0
+        ['C']                | 0
+        ['A', 'B']           | 7
+        ['B', 'C']           | 5
+        ['A', 'B', 'C']      | 12
+        ['C', 'A']           | 1
+        ['A', 'B', 'C', 'A'] | 13
+    }
+
+}