Merge 3340768 into 2fc7372

README.md

@@ -369,4 +369,5 @@ problems from
 * [Day 371](https://github.com/vaskoz/dailycodingproblem-go/issues/750)
 * [Day 372](https://github.com/vaskoz/dailycodingproblem-go/issues/748)
 * [Day 373](https://github.com/vaskoz/dailycodingproblem-go/issues/752)
+* [Day 374](https://github.com/vaskoz/dailycodingproblem-go/issues/754)
 

day374/problem.go

@@ -0,0 +1,40 @@
+package day374
+
+import (
+	"errors"
+	"sort"
+)
+
+var (
+	errNoSuchIndex    = errors.New("no index is a fixed point")
+	errInputNotSorted = errors.New("input is not sorted and must be")
+)
+
+// LowestFixedPoint returns the lowest fixed point index.
+// If no such index exists, it returns an error.
+// Also returns an error if input is not sorted.
+// Runs in O(log N) time thanks to binary search.
+// Worse case is O(N/2) because I iterate down from a match
+// to find the lowest fixed point.
+func LowestFixedPoint(sorted []int) (int, error) {
+	if !sort.IntsAreSorted(sorted) {
+		return 0, errInputNotSorted
+	}
+
+	for lo, hi := 0, len(sorted)-1; lo < hi; {
+		switch mid := (lo + hi) / 2; {
+		case sorted[mid] == mid:
+			for sorted[mid] == mid {
+				mid--
+			}
+
+			return mid + 1, nil
+		case sorted[mid] > mid:
+			hi = mid - 1
+		default:
+			lo = mid + 1
+		}
+	}
+
+	return 0, errNoSuchIndex
+}

day374/problem_test.go

@@ -0,0 +1,32 @@
+package day374
+
+import "testing"
+
+// nolint
+var testcases = []struct {
+	input       []int
+	expected    int
+	expectedErr error
+}{
+	{[]int{-5, -3, 2, 3}, 2, nil},
+	{[]int{2, 3, -3}, 0, errInputNotSorted},
+	{[]int{-5, -3, 5, 13}, 0, errNoSuchIndex},
+}
+
+func TestLowestFixedPoint(t *testing.T) {
+	t.Parallel()
+
+	for _, tc := range testcases {
+		if res, err := LowestFixedPoint(tc.input); res != tc.expected || err != tc.expectedErr {
+			t.Errorf("Expected (%v,%v), got (%v,%v)", tc.expected, tc.expectedErr, res, err)
+		}
+	}
+}
+
+func BenchmarkLowestFixedPoint(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, tc := range testcases {
+			LowestFixedPoint(tc.input) // nolint
+		}
+	}
+}