move codec-specific utilities into pkg/codecs

pkg/codecs/jpeg/define_quantization_table.go

@@ -0,0 +1,65 @@
+package jpeg
+
+import (
+	"fmt"
+)
+
+// QuantizationTable is a DQT quantization table.
+type QuantizationTable struct {
+	ID        uint8
+	Precision uint8
+	Data      []byte
+}
+
+// DefineQuantizationTable is a DQT marker.
+type DefineQuantizationTable struct {
+	Tables []QuantizationTable
+}
+
+// Unmarshal decodes the marker.
+func (m *DefineQuantizationTable) Unmarshal(buf []byte) error {
+	for len(buf) != 0 {
+		if len(buf) < 1 {
+			return fmt.Errorf("image is too short")
+		}
+
+		id := buf[0] & 0x0F
+		precision := buf[0] >> 4
+		buf = buf[1:]
+		if precision != 0 {
+			return fmt.Errorf("Precision %d is not supported", precision)
+		}
+
+		if len(buf) < 64 {
+			return fmt.Errorf("image is too short")
+		}
+
+		m.Tables = append(m.Tables, QuantizationTable{
+			ID:        id,
+			Precision: precision,
+			Data:      buf[:64],
+		})
+		buf = buf[64:]
+	}
+
+	return nil
+}
+
+// Marshal encodes the marker.
+func (m DefineQuantizationTable) Marshal(buf []byte) []byte {
+	buf = append(buf, []byte{0xFF, MarkerDefineQuantizationTable}...)
+
+	// length
+	s := 2
+	for _, t := range m.Tables {
+		s += 1 + len(t.Data)
+	}
+	buf = append(buf, []byte{byte(s >> 8), byte(s)}...)
+
+	for _, t := range m.Tables {
+		buf = append(buf, []byte{(t.ID)}...)
+		buf = append(buf, t.Data...)
+	}
+
+	return buf
+}