Further reduce heap allocs (#56)

* work on tracking more heap allocations down

* write own heapless IP AppendFormatAddr functions

* remove potential Error method allocations

* more logging

ipv4/definitions.go

@@ -1,5 +1,7 @@
 package ipv4
 
+import "strconv"
+
 const (
 	sizeHeader = 20
 )
@@ -68,3 +70,15 @@ func b2u8(b bool) uint8 {
 	}
 	return 0
 }
+
+// AppendFormatAddr appends the dotted-decimal text representation of an IPv4 address
+// to dst. Zero heap allocations.
+func AppendFormatAddr(dst []byte, addr [4]byte) []byte {
+	for i, b := range addr {
+		if i != 0 {
+			dst = append(dst, '.')
+		}
+		dst = strconv.AppendUint(dst, uint64(b), 10)
+	}
+	return dst
+}

ipv4/definitions_test.go

@@ -0,0 +1,43 @@
+package ipv4
+
+import (
+	"net/netip"
+	"testing"
+)
+
+func TestAppendFormatAddr(t *testing.T) {
+	tests := []struct {
+		addr [4]byte
+		want string
+	}{
+		{addr: [4]byte{0, 0, 0, 0}, want: "0.0.0.0"},
+		{addr: [4]byte{127, 0, 0, 1}, want: "127.0.0.1"},
+		{addr: [4]byte{192, 168, 1, 1}, want: "192.168.1.1"},
+		{addr: [4]byte{255, 255, 255, 255}, want: "255.255.255.255"},
+		{addr: [4]byte{10, 0, 0, 1}, want: "10.0.0.1"},
+		{addr: [4]byte{1, 2, 3, 4}, want: "1.2.3.4"},
+		{addr: [4]byte{100, 99, 9, 0}, want: "100.99.9.0"},
+	}
+	for _, tc := range tests {
+		got := string(AppendFormatAddr(nil, tc.addr))
+		if got != tc.want {
+			t.Errorf("AppendFormatAddr(%v): got %q, want %q", tc.addr, got, tc.want)
+		}
+		// Cross-check with netip.
+		want := netip.AddrFrom4(tc.addr).String()
+		if got != want {
+			t.Errorf("AppendFormatAddr(%v) disagrees with netip: got %q, want %q", tc.addr, got, want)
+		}
+	}
+}
+
+func TestAppendFormatAddr_noAllocs(t *testing.T) {
+	var buf [24]byte
+	addr := [4]byte{192, 168, 1, 1}
+	allocs := testing.AllocsPerRun(100, func() {
+		_ = AppendFormatAddr(buf[:0], addr)
+	})
+	if allocs != 0 {
+		t.Errorf("expected 0 allocs, got %v", allocs)
+	}
+}