1 files changed, 77 insertions, 0 deletions
diff --git a/pngmeta/partsreader.go b/pngmeta/partsreader.go
new file mode 100644
index 0000000..b69e4c3
--- /dev/null
+++ b/pngmeta/partsreader.go
@@ -0,0 +1,77 @@
+package pngmeta
+
+import (
+	"encoding/binary"
+	"errors"
+	"hash"
+	"hash/crc32"
+	"io"
+)
+
+var (
+	ErrCRC32Mismatch = errors.New("crc32 mismatch")
+	ErrNotPNG        = errors.New("not png")
+	ErrBadLength     = errors.New("bad length")
+)
+
+const header = "\x89PNG\r\n\x1a\n"
+
+type PngChunk struct {
+	typ         string
+	length      int32
+	r           io.Reader
+	realR       io.Reader
+	checksummer hash.Hash32
+}
+
+func (c *PngChunk) Read(p []byte) (int, error) {
+	return io.TeeReader(c.r, c.checksummer).Read(p)
+}
+
+func (c *PngChunk) Close() error {
+	var crc32 uint32
+	if err := binary.Read(c.realR, binary.BigEndian, &crc32); err != nil {
+		return err
+	}
+	if crc32 != c.checksummer.Sum32() {
+		return ErrCRC32Mismatch
+	}
+	return nil
+}
+
+func (c *PngChunk) Type() string {
+	return c.typ
+}
+
+type Reader struct {
+	r io.Reader
+}
+
+func NewPNGStepReader(r io.Reader) (*Reader, error) {
+	expectedHeader := make([]byte, len(header))
+	if _, err := io.ReadFull(r, expectedHeader); err != nil {
+		return nil, err
+	}
+	if string(expectedHeader) != header {
+		return nil, ErrNotPNG
+	}
+	return &Reader{r}, nil
+}
+
+func (r *Reader) Next() (*PngChunk, error) {
+	var length int32
+	if err := binary.Read(r.r, binary.BigEndian, &length); err != nil {
+		return nil, err
+	}
+	if length < 0 {
+		return nil, ErrBadLength
+	}
+	var rawTyp [4]byte
+	if _, err := io.ReadFull(r.r, rawTyp[:]); err != nil {
+		return nil, err
+	}
+	typ := string(rawTyp[:])
+	checksummer := crc32.NewIEEE()
+	checksummer.Write([]byte(typ))
+	return &PngChunk{typ, length, io.LimitReader(r.r, int64(length)), r.r, checksummer}, nil
+}