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

ipv6/definitions.go

@@ -7,3 +7,58 @@ const (
 )
 
 type ToS = ipv4.ToS
+
+// AppendFormatAddr appends the canonical text representation of an IPv6 address
+// to dst following RFC 5952 conventions (lowercase hex, :: compression for the
+// longest run of consecutive zero groups of length ≥ 2). Zero heap allocations.
+func AppendFormatAddr(dst []byte, addr [16]byte) []byte {
+	const hexDigits = "0123456789abcdef"
+
+	// Find the longest run of consecutive all-zero 16-bit groups for :: compression.
+	bestStart, bestLen := 0, 0
+	curStart := -1
+	for i := 0; i < 8; i++ {
+		if addr[i*2] == 0 && addr[i*2+1] == 0 {
+			if curStart < 0 {
+				curStart = i
+			}
+			if i-curStart+1 > bestLen {
+				bestStart = curStart
+				bestLen = i - curStart + 1
+			}
+		} else {
+			curStart = -1
+		}
+	}
+	if bestLen < 2 {
+		bestLen = 0 // RFC 5952 §4.2.2: do not compress a single 16-bit group.
+	}
+
+	needColon := false
+	for i := 0; i < 8; i++ {
+		if bestLen > 0 && i == bestStart {
+			dst = append(dst, ':', ':')
+			i += bestLen - 1 // skip compressed groups; loop increments i.
+			needColon = false
+			continue
+		}
+		if needColon {
+			dst = append(dst, ':')
+		}
+		needColon = true
+		hi := addr[i*2]
+		lo := addr[i*2+1]
+		v := uint16(hi)<<8 | uint16(lo)
+		if v >= 0x1000 {
+			dst = append(dst, hexDigits[hi>>4])
+		}
+		if v >= 0x100 {
+			dst = append(dst, hexDigits[hi&0xf])
+		}
+		if v >= 0x10 {
+			dst = append(dst, hexDigits[lo>>4])
+		}
+		dst = append(dst, hexDigits[lo&0xf])
+	}
+	return dst
+}