feat: Add Markov count sample. (#13)

Author: goloroden

README.md

@@ -11,6 +11,7 @@ So far, the following exercises have been covered:
 - [Continous maximum](./continuousmax/) – calculates the maximum of a sliding window
 - [Floyd](./floyd/) – implements Floyd's cycle-finding "Tortoise and Hare" algorithm
 - [Heap](./heap/) – implements a heap from scratch, without using the built-in `container/heap` package
+- [Markov count](./markovcount/) – figures out how often states are reached by a Markov chain
 - [Radix sort](./radixsort/) – implements radix sort
 - [Remove k-th last element](./removekthlastelement/) – removes the k-th last element from a single-linked list
 - [Run-length encoding](./runlengthencoding/) – encodes and decodes strings using run-length encoding

markovcount/documentation.go

@@ -0,0 +1,2 @@
+// Package markovcount figures out how often states are reached by a Markov chain.
+package markovcount

markovcount/markovcount.go

@@ -0,0 +1,52 @@
+package markovcount
+
+import "math/rand"
+
+type Transition struct {
+	From        string
+	To          string
+	Probability float64
+}
+
+func Count(
+	initialState string,
+	transitions []Transition,
+	iterations int,
+) map[string]int {
+	transitionResults := map[string]int{}
+
+	currentState := initialState
+	for i := 0; i < iterations; i++ {
+		nextState := getNextState(currentState, transitions)
+
+		if _, ok := transitionResults[nextState]; !ok {
+			transitionResults[nextState] = 0
+		}
+		transitionResults[nextState]++
+
+		currentState = nextState
+	}
+
+	return transitionResults
+}
+
+func getNextState(
+	currentState string,
+	transitions []Transition,
+) string {
+	randomValue := rand.Float64()
+
+	for _, transition := range transitions {
+		if transition.From != currentState {
+			continue
+		}
+
+		if randomValue <= transition.Probability {
+			return transition.To
+		}
+
+		randomValue -= transition.Probability
+	}
+
+	panic("no transition found")
+}

markovcount/markovcount_test.go

@@ -0,0 +1,35 @@
+package markovcount_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/thenativeweb/codingcircle/markovcount"
+)
+
+func TestCount(t *testing.T) {
+	t.Run("calculates the counts", func(t *testing.T) {
+		transitions := []markovcount.Transition{
+			{From: "a", To: "a", Probability: 0.9},
+			{From: "a", To: "b", Probability: 0.075},
+			{From: "a", To: "c", Probability: 0.025},
+			{From: "b", To: "a", Probability: 0.15},
+			{From: "b", To: "b", Probability: 0.8},
+			{From: "b", To: "c", Probability: 0.05},
+			{From: "c", To: "a", Probability: 0.25},
+			{From: "c", To: "b", Probability: 0.25},
+			{From: "c", To: "c", Probability: 0.5},
+		}
+
+		result := markovcount.Count("a", transitions, 10_000)
+
+		assert.GreaterOrEqual(t, result["a"], 5_700)
+		assert.LessOrEqual(t, result["a"], 6_800)
+
+		assert.GreaterOrEqual(t, result["b"], 2_700)
+		assert.LessOrEqual(t, result["b"], 3_600)
+
+		assert.GreaterOrEqual(t, result["c"], 400)
+		assert.LessOrEqual(t, result["c"], 900)
+	})
+}