MaxSubarray , kadane's algorithm

algo/maxSubaray.go

@@ -0,0 +1,20 @@
+package algo
+
+// kadane's algorithm
+// https://www.youtube.com/watch?v=qj3CjNEKFeM
+
+// MaxSubarray  -> returns the max sum of possible sub arrays
+func MaxSubarray(array []int) int {
+	minimum := array[0]
+	maximum := 0
+
+	for i := 1; i < len(array); i++ {
+		if array[i] < minimum {
+			minimum = array[i]
+		} else if array[i]-minimum > maximum {
+			maximum = array[i] - minimum
+		}
+	}
+
+	return maximum
+}

tests/maxSubarray_test.go

@@ -0,0 +1,103 @@
+package tests
+
+import (
+	"testing"
+
+	"datastructures/algo"
+)
+
+// --- basic profit cases ---
+
+func TestMaxSubarray_BasicProfit(t *testing.T) {
+	// buy at 1, sell at 6 → 5
+	got := algo.MaxSubarray([]int{7, 1, 5, 3, 6, 4})
+	if got != 5 {
+		t.Errorf("expected 5, got %d", got)
+	}
+}
+
+func TestMaxSubarray_ProfitAtEnd(t *testing.T) {
+	// buy at 1, sell at 9 → 8
+	got := algo.MaxSubarray([]int{3, 1, 4, 1, 5, 9})
+	if got != 8 {
+		t.Errorf("expected 8, got %d", got)
+	}
+}
+
+func TestMaxSubarray_AlreadySorted(t *testing.T) {
+	// buy at 1, sell at 5 → 4
+	got := algo.MaxSubarray([]int{1, 2, 3, 4, 5})
+	if got != 4 {
+		t.Errorf("expected 4, got %d", got)
+	}
+}
+
+// --- no profit cases ---
+
+func TestMaxSubarray_StrictlyDecreasing(t *testing.T) {
+	// prices only fall; no profitable trade
+	got := algo.MaxSubarray([]int{5, 4, 3, 2, 1})
+	if got != 0 {
+		t.Errorf("expected 0, got %d", got)
+	}
+}
+
+func TestMaxSubarray_AllSame(t *testing.T) {
+	got := algo.MaxSubarray([]int{3, 3, 3, 3})
+	if got != 0 {
+		t.Errorf("expected 0, got %d", got)
+	}
+}
+
+// --- single element ---
+
+func TestMaxSubarray_SingleElement(t *testing.T) {
+	got := algo.MaxSubarray([]int{42})
+	if got != 0 {
+		t.Errorf("expected 0, got %d", got)
+	}
+}
+
+// --- two elements ---
+
+func TestMaxSubarray_TwoElementsProfit(t *testing.T) {
+	got := algo.MaxSubarray([]int{1, 10})
+	if got != 9 {
+		t.Errorf("expected 9, got %d", got)
+	}
+}
+
+func TestMaxSubarray_TwoElementsLoss(t *testing.T) {
+	got := algo.MaxSubarray([]int{10, 1})
+	if got != 0 {
+		t.Errorf("expected 0, got %d", got)
+	}
+}
+
+// --- minimum appears late ---
+
+func TestMaxSubarray_MinInMiddle(t *testing.T) {
+	// dip to 2 at index 3, then rises to 8
+	got := algo.MaxSubarray([]int{5, 4, 3, 2, 8})
+	if got != 6 {
+		t.Errorf("expected 6, got %d", got)
+	}
+}
+
+// --- negative values ---
+
+func TestMaxSubarray_AllNegative(t *testing.T) {
+	// minimum tracks most negative; no positive spread possible
+	got := algo.MaxSubarray([]int{-5, -3, -1})
+	if got != 4 {
+		t.Errorf("expected 4, got %d", got)
+	}
+}
+
+func TestMaxSubarray_MixedNegativePositive(t *testing.T) {
+	// buy at -3, sell at 5 → 8
+	got := algo.MaxSubarray([]int{2, -3, 1, 5})
+	if got != 8 {
+		t.Errorf("expected 8, got %d", got)
+	}
+}