added few tests

internal/app/app_test.go

@@ -0,0 +1,72 @@
+package app
+
+import (
+	"testing"
+)
+
+func TestHostsFromCIDR(t *testing.T) {
+	tests := []struct {
+		name        string
+		cidr        string
+		length      int
+		first       string
+		last        string
+		shouldError string
+	}{
+		{
+			name:   "subnet/32-singlehost",
+			cidr:   "192.168.10.110/32",
+			length: 1,
+			first:  "192.168.10.110",
+		}, {
+			name:   "subnet/24",
+			cidr:   "192.168.10.0/24",
+			length: 254,
+			first:  "192.168.10.1",
+			last:   "192.168.10.254",
+		},
+		{
+			name:   "subnet/16",
+			cidr:   "192.168.0.0/16",
+			length: 65534,
+			first:  "192.168.0.1",
+			last:   "192.168.255.254",
+		},
+		{
+			name:        "no subnet",
+			cidr:        "192.168.10.0",
+			length:      0,
+			shouldError: "invalid CIDR address: 192.168.10.0",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			hosts, err := hostsFromCIDR(tt.cidr)
+			if err != nil {
+				if tt.shouldError != "" {
+					if err.Error() != tt.shouldError {
+						t.Errorf("hostsFromCIDR() error = %v, want %v", err, tt.shouldError)
+					}
+				} else {
+					t.Errorf("hostsFromCIDR() error = %v", err)
+				}
+			}
+			if len(hosts) != tt.length {
+				t.Errorf("hostsFromCIDR() = %v, want %v", len(hosts), tt.length)
+			}
+			if tt.length > 1 {
+				if hosts[0] != tt.first {
+					t.Errorf("hostsFromCIDR() = %v, want %v", hosts[0], tt.first)
+				}
+				if hosts[len(hosts)-1] != tt.last {
+					t.Errorf("hostsFromCIDR() = %v, want %v", hosts[len(hosts)-1], tt.last)
+				}
+			}
+			if tt.length == 1 {
+				if hosts[0] != tt.first {
+					t.Errorf("hostsFromCIDR() = %v, want %v", hosts[0], tt.first)
+				}
+			}
+		})
+	}
+}