Merge b271260 into 2c672e9

day271/problem.go

@@ -1,18 +1,60 @@
 package day271
 
-import "sort"
+import (
+	"fmt"
+	"sort"
+	"strconv"
+	"strings"
+)
+
+// divideBy2 performs the following calculation:
+// https://en.wikipedia.org/wiki/Division_by_two#Decimal
+// if n is odd, it drops by 1 to the next lower even digit.
+func divideBy2(n int) int {
+	if n&1 == 1 { // odd check without modulo (no division)
+		n--
+	}
+	str := fmt.Sprintf("0%d", n)
+	var sb strings.Builder
+	for i := 1; i < len(str); i++ {
+		d := str[i] - '0'
+		if (str[i-1]-'0')&1 == 0 { // even
+			switch d {
+			case 0, 1:
+				sb.WriteRune('0')
+			case 2, 3:
+				sb.WriteRune('1')
+			case 4, 5:
+				sb.WriteRune('2')
+			case 6, 7:
+				sb.WriteRune('3')
+			case 8, 9:
+				sb.WriteRune('4')
+			}
+		} else { // odd
+			switch d {
+			case 0, 1:
+				sb.WriteRune('5')
+			case 2, 3:
+				sb.WriteRune('6')
+			case 4, 5:
+				sb.WriteRune('7')
+			case 6, 7:
+				sb.WriteRune('8')
+			case 8, 9:
+				sb.WriteRune('9')
+			}
+		}
+	}
+	res, _ := strconv.Atoi(sb.String())
+	return res
+}
 
 // BinarySearch is a handwritten binary search.
 func BinarySearch(sorted []int, x int) int {
 	left, right := 0, len(sorted)-1
 	for left < right {
-		var mid int
-		dividend := left + right
-		divisor := 2
-		for dividend >= divisor {
-			dividend -= divisor
-			mid++
-		}
+		mid := divideBy2(left + right)
 		val := sorted[mid]
 		switch {
 		case x == val:

day271/problem_test.go

@@ -1,6 +1,11 @@
 package day271
 
-import "testing"
+import (
+	"math/rand"
+	"sort"
+	"testing"
+	"time"
+)
 
 // nolint
 var testcases = []struct {
@@ -45,3 +50,16 @@ func BenchmarkBinarySearchSortPkg(b *testing.B) {
 		}
 	}
 }
+
+func BenchmarkBinarySearchLargeInput(b *testing.B) {
+	input := make([]int, 1000000)
+	rand.Seed(time.Now().UnixNano())
+	for i := range input {
+		input[i] = rand.Int()
+	}
+	sort.Ints(input)
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		BinarySearch(input, 1000)
+	}
+}