Merge aa2f1cd into 560b7a8

README.md

@@ -215,6 +215,7 @@ problems from
 * [Day 202](https://github.com/vaskoz/dailycodingproblem-go/issues/417)
 * [Day 203](https://github.com/vaskoz/dailycodingproblem-go/issues/419)
 * [Day 204](https://github.com/vaskoz/dailycodingproblem-go/issues/421)
+* [Day 205](https://github.com/vaskoz/dailycodingproblem-go/issues/423)
 * [Day 206](https://github.com/vaskoz/dailycodingproblem-go/issues/424)
 * [Day 207](https://github.com/vaskoz/dailycodingproblem-go/issues/426)
 * [Day 208](https://github.com/vaskoz/dailycodingproblem-go/issues/427)

day205/problem.go

@@ -0,0 +1,63 @@
+package day205
+
+import (
+	"errors"
+	"sort"
+)
+
+var errNoGreaterPermutation = errors.New("greater permutation impossible")
+
+// ErrNoGreaterPermutation is the error when the greater
+// permutation doesn't exist.
+func ErrNoGreaterPermutation() error {
+	return errNoGreaterPermutation
+}
+
+// NextPermutationUint find the next greater permutation
+// of it's digits in absolute order.
+// Return an error if a greater permutation isn't possible.
+func NextPermutationUint(num uint) (uint, error) {
+	digits := separateDigits(num)
+	var i int
+	for i = len(digits) - 1; i > 0; i-- {
+		if digits[i] > digits[i-1] {
+			break
+		}
+	}
+	if i == 0 {
+		sort.Ints(digits)
+		return 0, ErrNoGreaterPermutation()
+	}
+	x := digits[i-1]
+	min := i
+	for j := i + 1; j < len(digits); j++ {
+		if digits[j] > x && digits[j] < digits[min] {
+			min = j
+		}
+	}
+	digits[i-1], digits[min] = digits[min], digits[i-1]
+	sort.Ints(digits[i:])
+	return convertToUint(digits), nil
+}
+
+func convertToUint(digits []int) uint {
+	var result uint
+	multiplier := 1
+	for i := range digits {
+		result += uint(digits[len(digits)-1-i] * multiplier)
+		multiplier *= 10
+	}
+	return result
+}
+
+func separateDigits(num uint) []int {
+	var result []int
+	for num != 0 {
+		result = append(result, int(num)%10)
+		num /= 10
+	}
+	for i := 0; i < len(result)/2; i++ {
+		result[i], result[len(result)-1-i] = result[len(result)-1-i], result[i]
+	}
+	return result
+}

day205/problem_test.go

@@ -0,0 +1,32 @@
+package day205
+
+import (
+	"testing"
+)
+
+var testcases = []struct {
+	in  uint
+	out uint
+	err error
+}{
+	{123, 132, nil},
+	{132, 213, nil},
+	{321, 0, ErrNoGreaterPermutation()},
+}
+
+func TestNextPermutationUint(t *testing.T) {
+	t.Parallel()
+	for _, tc := range testcases {
+		if result, err := NextPermutationUint(tc.in); result != tc.out || err != tc.err {
+			t.Errorf("Expected (%v,%v) got (%v,%v)", tc.out, tc.err, result, err)
+		}
+	}
+}
+
+func BenchmarkNextPermutationUint(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			NextPermutationUint(tc.in)
+		}
+	}
+}