commit 685d8014897d5e8d2cc8d407cc2d3d1b01cad59d
Author: Dustin Oprea <myselfasunder@gmail.com>
Date:   Sat Apr 14 14:38:35 2018 -0400

    Initial commit.
    
    - Parsing works.
    - Not yet resolving values.
    - Not yet resolving the actual IDs.
    - Not yet able to make changes.

diff --git a/assets/NDM_8901.jpg b/assets/NDM_8901.jpg
new file mode 100644
index 0000000..b892262
Binary files /dev/null and b/assets/NDM_8901.jpg differ
diff --git a/exif.go b/exif.go
new file mode 100644
index 0000000..14d4187
--- /dev/null
+++ b/exif.go
@@ -0,0 +1,95 @@
+package exif
+
+import (
+    "fmt"
+    "errors"
+    "bytes"
+
+    "encoding/binary"
+
+    "github.com/dsoprea/go-logging"
+)
+
+var (
+    exifLogger = log.NewLogger("exif.exif")
+    ErrNotExif = errors.New("not exif data")
+)
+
+type Exif struct {
+
+}
+
+func NewExif() *Exif {
+    return new(Exif)
+}
+
+func (e *Exif) IsExif(data []byte) (ok bool) {
+    if bytes.Compare(data[:6], []byte("Exif\000\000")) == 0 {
+        return true
+    }
+
+    return false
+}
+
+func (e *Exif) Parse(data []byte) (err error) {
+    defer func() {
+        if state := recover(); state != nil {
+            err = log.Wrap(state.(error))
+        }
+    }()
+
+    if e.IsExif(data) == false {
+
+// TODO(dustin): !! Debugging.
+        fmt.Printf("AppData doesn't look like EXIF.  BYTES=(%d)\n", len(data))
+
+        return ErrNotExif
+    }
+
+    // Good reference:
+    //
+    //      CIPA DC-008-2016; JEITA CP-3451D
+    //      -> http://www.cipa.jp/std/documents/e/DC-008-Translation-2016-E.pdf
+
+fmt.Printf("AppData DOES look like EXIF.  BYTES=(%d)\n", len(data))
+    byteOrderSignature := data[6:8]
+    byteOrder := IfdByteOrder(BigEndianByteOrder)
+    if string(byteOrderSignature) == "II" {
+        byteOrder = IfdByteOrder(LittleEndianByteOrder)
+    } else if string(byteOrderSignature) != "MM" {
+        log.Panicf("byte-order not recognized: [%v]", byteOrderSignature)
+    }
+
+    fmt.Printf("BYTE-ORDER: [%s]\n", byteOrderSignature)
+
+    fixedBytes := data[8:10]
+    if fixedBytes[0] != 0x2a || fixedBytes[1] != 0x00 {
+        exifLogger.Warningf(nil, "EXIF app-data header fixed-bytes should be 0x002a but are: [%v]", fixedBytes)
+
+// TODO(dustin): Debugging.
+        fmt.Printf("EXIF app-data header fixed-bytes should be 0x002a but are: [%v]\n", fixedBytes)
+
+        return nil
+    }
+
+    firstIfdOffset := uint32(0)
+    if byteOrder.IsLittleEndian() == true {
+        firstIfdOffset = binary.LittleEndian.Uint32(data[10:14])
+    } else {
+        firstIfdOffset = binary.BigEndian.Uint32(data[10:14])
+    }
+
+    ifd := NewIfd(data, byteOrder)
+
+    visitor := func() error {
+// TODO(dustin): !! Debugging.
+
+        fmt.Printf("IFD visitor.\n")
+        return nil
+    }
+
+    err = ifd.Scan(visitor, firstIfdOffset)
+    log.PanicIf(err)
+
+    return nil
+}
diff --git a/exif_test.go b/exif_test.go
new file mode 100644
index 0000000..2ed915b
--- /dev/null
+++ b/exif_test.go
@@ -0,0 +1,77 @@
+package exif
+
+import (
+    "testing"
+    "os"
+    "path"
+
+    "io/ioutil"
+
+    "github.com/dsoprea/go-logging"
+)
+
+var (
+    assetsPath = ""
+)
+
+
+func TestIsExif_True(t *testing.T) {
+    e := NewExif()
+
+    if ok := e.IsExif([]byte("Exif\000\000")); ok != true {
+        t.Fatalf("expected true")
+    }
+}
+
+func TestIsExif_False(t *testing.T) {
+    e := NewExif()
+
+    if ok := e.IsExif([]byte("something unexpected")); ok != false {
+        t.Fatalf("expected false")
+    }
+}
+
+func TestParse(t *testing.T) {
+    // Open the file.
+
+    filepath := path.Join(assetsPath, "NDM_8901.jpg")
+    f, err := os.Open(filepath)
+    log.PanicIf(err)
+
+    defer f.Close()
+
+    data, err := ioutil.ReadAll(f)
+    log.PanicIf(err)
+
+    // Search for the beginning of the EXIF information. The EXIF is near the
+    // very beginning of our/most JPEGs, so this has a very low cost.
+
+    e := NewExif()
+
+    foundAt := -1
+    for i := 0; i < len(data); i++ {
+        if e.IsExif(data[i:i + 6]) == true {
+            foundAt = i
+            break
+        }
+    }
+
+    if foundAt == -1 {
+        log.Panicf("EXIF start not found")
+    }
+
+    // Run the parse.
+
+    err = e.Parse(data[foundAt:])
+    log.PanicIf(err)
+}
+
+func init() {
+    goPath := os.Getenv("GOPATH")
+    if goPath == "" {
+        log.Panicf("GOPATH is empty")
+    }
+
+    assetsPath = path.Join(goPath, "src", "github.com", "dsoprea", "go-exif", "assets")
+}
+
diff --git a/ifd.go b/ifd.go
new file mode 100644
index 0000000..272c037
--- /dev/null
+++ b/ifd.go
@@ -0,0 +1,216 @@
+package exif
+
+import (
+    "fmt"
+    "bytes"
+    "io"
+
+    "encoding/binary"
+
+    "github.com/dsoprea/go-logging"
+)
+
+const (
+    BigEndianByteOrder = iota
+    LittleEndianByteOrder = iota
+)
+
+var (
+    ifdLogger = log.NewLogger("exifjpeg.ifd")
+)
+
+type IfdByteOrder int
+
+func (ibo IfdByteOrder) IsBigEndian() bool {
+    return ibo == BigEndianByteOrder
+}
+
+func (ibo IfdByteOrder) IsLittleEndian() bool {
+    return ibo == LittleEndianByteOrder
+}
+
+type Ifd struct {
+    data []byte
+    buffer *bytes.Buffer
+    byteOrder IfdByteOrder
+    currentOffset uint32
+    ifdTopOffset uint32
+}
+
+func NewIfd(data []byte, byteOrder IfdByteOrder) *Ifd {
+    return &Ifd{
+        data: data,
+        buffer: bytes.NewBuffer(data),
+        byteOrder: byteOrder,
+        ifdTopOffset: 6,
+    }
+}
+
+// read is a wrapper around the built-in reader that applies which endianness
+// we are.
+func (ifd *Ifd) read(r io.Reader, into interface{}) (err error) {
+    defer func() {
+        if state := recover(); state != nil {
+            err = log.Wrap(state.(error))
+        }
+    }()
+
+    if ifd.byteOrder.IsLittleEndian() == true {
+        err := binary.Read(r, binary.LittleEndian, into)
+        log.PanicIf(err)
+    } else {
+        err := binary.Read(r, binary.BigEndian, into)
+        log.PanicIf(err)
+    }
+
+    return nil
+}
+
+// getUint16 reads a uint16 and advances both our current and our current
+// accumulator (which allows us to know how far to seek to the beginning of the
+// next IFD when it's time to jump).
+func (ifd *Ifd) getUint16() (value uint16, err error) {
+    defer func() {
+        if state := recover(); state != nil {
+            err = log.Wrap(state.(error))
+        }
+    }()
+
+    err = ifd.read(ifd.buffer, &value)
+    log.PanicIf(err)
+
+    ifd.currentOffset += 2
+
+    return value, nil
+}
+
+// getUint32 reads a uint32 and advances both our current and our current
+// accumulator (which allows us to know how far to seek to the beginning of the
+// next IFD when it's time to jump).
+func (ifd *Ifd) getUint32() (value uint32, err error) {
+    defer func() {
+        if state := recover(); state != nil {
+            err = log.Wrap(state.(error))
+        }
+    }()
+
+    err = ifd.read(ifd.buffer, &value)
+    log.PanicIf(err)
+
+    ifd.currentOffset += 4
+
+    return value, nil
+}
+
+// parseCurrentIfd decodes the IFD block that we're currently sitting on the
+// first byte of.
+func (ifd *Ifd) parseCurrentIfd() (nextIfdOffset uint32, err error) {
+    defer func() {
+        if state := recover(); state != nil {
+            err = log.Wrap(state.(error))
+        }
+    }()
+
+
+    tagCount, err := ifd.getUint16()
+    log.PanicIf(err)
+
+    fmt.Printf("IFD: TOTAL TAG COUNT=(%02x)\n", tagCount)
+
+    for i := uint16(0); i < tagCount; i++ {
+// TODO(dustin): !! 0x8769 tag-IDs are child IFDs.
+        tagId, err := ifd.getUint16()
+        log.PanicIf(err)
+
+        fmt.Printf("IFD: Tag (%d) ID=(%02x)\n", i, tagId)
+
+
+        tagType, err := ifd.getUint16()
+        log.PanicIf(err)
+
+        fmt.Printf("IFD: Tag (%d) TYPE=(%d)\n", i, tagType)
+
+
+        tagCount, err := ifd.getUint32()
+        log.PanicIf(err)
+
+        fmt.Printf("IFD: Tag (%d) COUNT=(%02x)\n", i, tagCount)
+
+
+        valueOffset, err := ifd.getUint32()
+        log.PanicIf(err)
+
+        fmt.Printf("IFD: Tag (%d) VALUE-OFFSET=(%x)\n", i, valueOffset)
+
+// Notes on the tag-value's value (we'll have to use this as a pointer if the type potentially requires more than four bytes):
+//
+// This tag records the offset from the start of the TIFF header to the position where the value itself is
+// recorded. In cases where the value fits in 4 Bytes, the value itself is recorded. If the value is smaller
+// than 4 Bytes, the value is stored in the 4-Byte area starting from the left, i.e., from the lower end of
+// the byte offset area. For example, in big endian format, if the type is SHORT and the value is 1, it is
+// recorded as 00010000.H
+
+    }
+
+    fmt.Printf("\n")
+
+    nextIfdOffset, err = ifd.getUint32()
+    log.PanicIf(err)
+
+    fmt.Printf("IFD: NEXT-IFD-OFFSET=(%x)\n", nextIfdOffset)
+
+    fmt.Printf("\n")
+
+    return nextIfdOffset, nil
+}
+
+// forwardToIfd jumps to the beginning of an IFD block that starts on or after
+// the current position.
+func (ifd *Ifd) forwardToIfd(ifdOffset uint32) (err error) {
+    defer func() {
+        if state := recover(); state != nil {
+            err = log.Wrap(state.(error))
+        }
+    }()
+
+    fmt.Printf("IFD: Forwarding to IFD. TOP-OFFSET=(%d) IFD-OFFSET=(%d)\n", ifd.ifdTopOffset, ifdOffset)
+
+    nextOffset := ifd.ifdTopOffset + ifdOffset
+
+    // We're assuming the guarantee that the next IFD will follow the
+    // current one. So, figure out how far it is from our current position.
+    delta := nextOffset - ifd.currentOffset
+    ifd.buffer.Next(int(delta))
+
+    ifd.currentOffset = nextOffset
+
+    return nil
+}
+
+type IfdVisitor func() error
+
+// Scan enumerates the different EXIF blocks (called IFDs).
+func (ifd *Ifd) Scan(v IfdVisitor, firstIfdOffset uint32) (err error) {
+    defer func() {
+        if state := recover(); state != nil {
+            err = log.Wrap(state.(error))
+        }
+    }()
+
+    err = ifd.forwardToIfd(firstIfdOffset)
+    log.PanicIf(err)
+
+    for {
+        nextIfdOffset, err := ifd.parseCurrentIfd()
+        log.PanicIf(err)
+
+        if nextIfdOffset == 0 {
+            break
+        }
+
+        err = ifd.forwardToIfd(nextIfdOffset)
+        log.PanicIf(err)
+    }
+
+    return nil
+}