problem14

Author: TipsyPixie

README.md

@@ -30,3 +30,4 @@ Number | Difficulty | Asked By
 [#11](problem011) | MEDIUM | Twitter
 [#12](problem012) | HARD | Amazon
 [#13](problem013) | HARD | Amazon
+[#14](problem014) | MEDIUM | Google

problem011/problem011.go

@@ -48,21 +48,21 @@ func (node *trie) subStrings() []string {
 	if len(*node) == 0 {
 		return []string{""}
 	}
-	childrenChannel := make(chan []string, len(*node))
+	substringChannel := make(chan []string, len(*node))
 	for keyString, subNode := range *node {
 		go func(prefix string, node *trie) {
 			var subStrings []string
 			for _, subSubString := range node.subStrings() {
 				subStrings = append(subStrings, fmt.Sprintf("%s%s", prefix, subSubString))
 			}
-			childrenChannel <- subStrings
+			substringChannel <- subStrings
 		}(keyString, subNode)
 	}
 	finished := 0
 	var result []string
 	for finished < len(*node) {
 		select {
-		case subStrings := <-childrenChannel:
+		case subStrings := <-substringChannel:
 			result = append(result, subStrings...)
 			finished++
 		}

problem013/problem013.go

@@ -5,14 +5,14 @@ func Run(input string, distinctCharactersLimit uint) uint {
 	characterCount := make(map[rune]int)
 	substringStartsAt, substringEndsAt := 0, 0
 	for substringEndsAt < len(input) {
-		substringEndsWith := []rune(input)[substringEndsAt]
-		characterCount[substringEndsWith] += 1
+		startingCharacter := []rune(input)[substringEndsAt]
+		characterCount[startingCharacter] += 1
 		for uint(len(characterCount)) > distinctCharactersLimit {
-			substringStartsWith := []rune(input)[substringStartsAt]
-			if characterCount[substringStartsWith] > 1 {
-				characterCount[substringStartsWith] -= 1
+			endingCharacter := []rune(input)[substringStartsAt]
+			if characterCount[endingCharacter] > 1 {
+				characterCount[endingCharacter]--
 			} else {
-				delete(characterCount, substringStartsWith)
+				delete(characterCount, endingCharacter)
 			}
 			substringStartsAt++
 		}

problem014/README.md

@@ -0,0 +1,9 @@
+## Daily Coding Problem: Problem #14 [Medium]
+
+Good morning! Here's your coding interview problem for today.
+
+This problem was asked by Google.
+
+The area of a circle is defined as πr^2. Estimate π to 3 decimal places using a Monte Carlo method.
+
+Hint: The basic equation of a circle is `x^ + y^2 = r^2`.

problem014/problem014.go

@@ -0,0 +1,49 @@
+package problem014
+
+import (
+	"math"
+	"math/rand"
+	"time"
+)
+
+type tester func(float64, float64) bool
+type generator func() float64
+
+func Run(totalRepeat int) float64 {
+	const concurrency = 8
+	rand.Seed(time.Now().UnixNano())
+	circleTester := func(x float64, y float64) bool { return math.Pow(x, 2)+math.Pow(y, 2) < 1 }
+	xGenerator := func() float64 { return rand.Float64() }
+	subResults := make(chan struct{ successCount, trialCount int }, concurrency)
+	for i := 0; i < concurrency; i++ {
+		margin := i
+		yGenerator := func() float64 { return (float64(margin) + rand.Float64()) / float64(concurrency) }
+		go func(testerFunction tester, xGenerator generator, yGenerator generator, repeat int) {
+			successCount, trialCount := simulate(circleTester, xGenerator, yGenerator, repeat)
+			subResults <- struct{ successCount, trialCount int }{successCount: successCount, trialCount: trialCount}
+		}(circleTester, xGenerator, yGenerator, totalRepeat/concurrency)
+	}
+	finished := 0
+	totalSuccessCount, totalTrialCount := 0, 0
+	for finished < concurrency {
+		select {
+		case result, _ := <-subResults:
+			totalSuccessCount += result.successCount
+			totalTrialCount += result.trialCount
+			finished++
+		}
+	}
+	approxPi := float64(totalSuccessCount*4) / float64(totalTrialCount)
+	return math.Round(1000*approxPi) / 1000
+}
+
+func simulate(testerFunction tester, xGenerator generator, yGenerator generator, repeat int) (int, int) {
+	successCount, trialCount := 0, 0
+	for i := 0; i < repeat; i++ {
+		if testerFunction(xGenerator(), yGenerator()) {
+			successCount++
+		}
+		trialCount++
+	}
+	return successCount, trialCount
+}

problem014/problem014_test.go

@@ -0,0 +1,20 @@
+package problem014
+
+import (
+	"math"
+	"testing"
+)
+
+func TestRun(t *testing.T) {
+	result := Run(10000)
+	if result < math.Pi * 0.95 || result > math.Pi * 1.05 {
+		t.FailNow()
+	}
+}
+
+func TestRun2(t *testing.T) {
+	result := Run(10000000)
+	if result < math.Pi * 0.999 || result > math.Pi * 1.001 {
+		t.FailNow()
+	}
+}