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// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package jpeg implements a JPEG image decoder and encoder.
//
// JPEG is defined in ITU-T T.81: http://www.w3.org/Graphics/JPEG/itu-t81.pdf.
package jpeg

import (
        "bufio"
        "image"
        "image/color"
        "io"
)

// TODO(nigeltao): fix up the doc comment style so that sentences start with
// the name of the type or function that they annotate.

// A FormatError reports that the input is not a valid JPEG.
type FormatError string

func (e FormatError) Error() string { return "invalid JPEG format: " + string(e) }

// An UnsupportedError reports that the input uses a valid but unimplemented JPEG feature.
type UnsupportedError string

func (e UnsupportedError) Error() string { return "unsupported JPEG feature: " + string(e) }

// Component specification, specified in section B.2.2.
type component struct {
        h  int   // Horizontal sampling factor.
        v  int   // Vertical sampling factor.
        c  uint8 // Component identifier.
        tq uint8 // Quantization table destination selector.
}

type block [blockSize]int

const (
        blockSize = 64 // A DCT block is 8x8.

        dcTable = 0
        acTable = 1
        maxTc   = 1
        maxTh   = 3
        maxTq   = 3

        // A grayscale JPEG image has only a Y component.
        nGrayComponent = 1
        // A color JPEG image has Y, Cb and Cr components.
        nColorComponent = 3

        // We only support 4:4:4, 4:2:2 and 4:2:0 downsampling, and therefore the
        // number of luma samples per chroma sample is at most 2 in the horizontal
        // and 2 in the vertical direction.
        maxH = 2
        maxV = 2
)

const (
        soiMarker   = 0xd8 // Start Of Image.
        eoiMarker   = 0xd9 // End Of Image.
        sof0Marker  = 0xc0 // Start Of Frame (Baseline).
        sof2Marker  = 0xc2 // Start Of Frame (Progressive).
        dhtMarker   = 0xc4 // Define Huffman Table.
        dqtMarker   = 0xdb // Define Quantization Table.
        sosMarker   = 0xda // Start Of Scan.
        driMarker   = 0xdd // Define Restart Interval.
        rst0Marker  = 0xd0 // ReSTart (0).
        rst7Marker  = 0xd7 // ReSTart (7).
        app0Marker  = 0xe0 // APPlication specific (0).
        app15Marker = 0xef // APPlication specific (15).
        comMarker   = 0xfe // COMment.
)

// Maps from the zig-zag ordering to the natural ordering.
var unzig = [blockSize]int{
        0, 1, 8, 16, 9, 2, 3, 10,
        17, 24, 32, 25, 18, 11, 4, 5,
        12, 19, 26, 33, 40, 48, 41, 34,
        27, 20, 13, 6, 7, 14, 21, 28,
        35, 42, 49, 56, 57, 50, 43, 36,
        29, 22, 15, 23, 30, 37, 44, 51,
        58, 59, 52, 45, 38, 31, 39, 46,
        53, 60, 61, 54, 47, 55, 62, 63,
}

// If the passed in io.Reader does not also have ReadByte, then Decode will introduce its own buffering.
type Reader interface {
        io.Reader
        ReadByte() (c byte, err error)
}

type decoder struct {
        r             Reader
        width, height int
        img1          *image.Gray
        img3          *image.YCbCr
        ri            int // Restart Interval.
        nComp         int
        comp          [nColorComponent]component
        huff          [maxTc + 1][maxTh + 1]huffman
        quant         [maxTq + 1]block
        b             bits
        tmp           [1024]byte
}

// Reads and ignores the next n bytes.
func (d *decoder) ignore(n int) error {
        for n > 0 {
                m := len(d.tmp)
                if m > n {
                        m = n
                }
                _, err := io.ReadFull(d.r, d.tmp[0:m])
                if err != nil {
                        return err
                }
                n -= m
        }
        return nil
}

// Specified in section B.2.2.
func (d *decoder) processSOF(n int) error {
        switch n {
        case 6 + 3*nGrayComponent:
                d.nComp = nGrayComponent
        case 6 + 3*nColorComponent:
                d.nComp = nColorComponent
        default:
                return UnsupportedError("SOF has wrong length")
        }
        _, err := io.ReadFull(d.r, d.tmp[:n])
        if err != nil {
                return err
        }
        // We only support 8-bit precision.
        if d.tmp[0] != 8 {
                return UnsupportedError("precision")
        }
        d.height = int(d.tmp[1])<<8 + int(d.tmp[2])
        d.width = int(d.tmp[3])<<8 + int(d.tmp[4])
        if int(d.tmp[5]) != d.nComp {
                return UnsupportedError("SOF has wrong number of image components")
        }
        for i := 0; i < d.nComp; i++ {
                hv := d.tmp[7+3*i]
                d.comp[i].h = int(hv >> 4)
                d.comp[i].v = int(hv & 0x0f)
                d.comp[i].c = d.tmp[6+3*i]
                d.comp[i].tq = d.tmp[8+3*i]
                if d.nComp == nGrayComponent {
                        continue
                }
                // For color images, we only support 4:4:4, 4:2:2 or 4:2:0 chroma
                // downsampling ratios. This implies that the (h, v) values for the Y
                // component are either (1, 1), (2, 1) or (2, 2), and the (h, v)
                // values for the Cr and Cb components must be (1, 1).
                if i == 0 {
                        if hv != 0x11 && hv != 0x21 && hv != 0x22 {
                                return UnsupportedError("luma downsample ratio")
                        }
                } else if hv != 0x11 {
                        return UnsupportedError("chroma downsample ratio")
                }
        }
        return nil
}

// Specified in section B.2.4.1.
func (d *decoder) processDQT(n int) error {
        const qtLength = 1 + blockSize
        for ; n >= qtLength; n -= qtLength {
                _, err := io.ReadFull(d.r, d.tmp[0:qtLength])
                if err != nil {
                        return err
                }
                pq := d.tmp[0] >> 4
                if pq != 0 {
                        return UnsupportedError("bad Pq value")
                }
                tq := d.tmp[0] & 0x0f
                if tq > maxTq {
                        return FormatError("bad Tq value")
                }
                for i := range d.quant[tq] {
                        d.quant[tq][i] = int(d.tmp[i+1])
                }
        }
        if n != 0 {
                return FormatError("DQT has wrong length")
        }
        return nil
}

// makeImg allocates and initializes the destination image.
func (d *decoder) makeImg(h0, v0, mxx, myy int) {
        if d.nComp == nGrayComponent {
                m := image.NewGray(image.Rect(0, 0, 8*mxx, 8*myy))
                d.img1 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.Gray)
                return
        }
        var subsampleRatio image.YCbCrSubsampleRatio
        switch h0 * v0 {
        case 1:
                subsampleRatio = image.YCbCrSubsampleRatio444
        case 2:
                subsampleRatio = image.YCbCrSubsampleRatio422
        case 4:
                subsampleRatio = image.YCbCrSubsampleRatio420
        default:
                panic("unreachable")
        }
        m := image.NewYCbCr(image.Rect(0, 0, 8*h0*mxx, 8*v0*myy), subsampleRatio)
        d.img3 = m.SubImage(image.Rect(0, 0, d.width, d.height)).(*image.YCbCr)
}

// Specified in section B.2.3.
func (d *decoder) processSOS(n int) error {
        if d.nComp == 0 {
                return FormatError("missing SOF marker")
        }
        if n != 4+2*d.nComp {
                return UnsupportedError("SOS has wrong length")
        }
        _, err := io.ReadFull(d.r, d.tmp[0:4+2*d.nComp])
        if err != nil {
                return err
        }
        if int(d.tmp[0]) != d.nComp {
                return UnsupportedError("SOS has wrong number of image components")
        }
        var scan [nColorComponent]struct {
                td uint8 // DC table selector.
                ta uint8 // AC table selector.
        }
        for i := 0; i < d.nComp; i++ {
                cs := d.tmp[1+2*i] // Component selector.
                if cs != d.comp[i].c {
                        return UnsupportedError("scan components out of order")
                }
                scan[i].td = d.tmp[2+2*i] >> 4
                scan[i].ta = d.tmp[2+2*i] & 0x0f
        }
        // mxx and myy are the number of MCUs (Minimum Coded Units) in the image.
        h0, v0 := d.comp[0].h, d.comp[0].v // The h and v values from the Y components.
        mxx := (d.width + 8*h0 - 1) / (8 * h0)
        myy := (d.height + 8*v0 - 1) / (8 * v0)
        if d.img1 == nil && d.img3 == nil {
                d.makeImg(h0, v0, mxx, myy)
        }

        mcu, expectedRST := 0, uint8(rst0Marker)
        var (
                b  block
                dc [nColorComponent]int
        )
        for my := 0; my < myy; my++ {
                for mx := 0; mx < mxx; mx++ {
                        for i := 0; i < d.nComp; i++ {
                                qt := &d.quant[d.comp[i].tq]
                                for j := 0; j < d.comp[i].h*d.comp[i].v; j++ {
                                        // TODO(nigeltao): make this a "var b block" once the compiler's escape
                                        // analysis is good enough to allocate it on the stack, not the heap.
                                        b = block{}

                                        // Decode the DC coefficient, as specified in section F.2.2.1.
                                        value, err := d.decodeHuffman(&d.huff[dcTable][scan[i].td])
                                        if err != nil {
                                                return err
                                        }
                                        if value > 16 {
                                                return UnsupportedError("excessive DC component")
                                        }
                                        dcDelta, err := d.receiveExtend(value)
                                        if err != nil {
                                                return err
                                        }
                                        dc[i] += dcDelta
                                        b[0] = dc[i] * qt[0]

                                        // Decode the AC coefficients, as specified in section F.2.2.2.
                                        for k := 1; k < blockSize; k++ {
                                                value, err := d.decodeHuffman(&d.huff[acTable][scan[i].ta])
                                                if err != nil {
                                                        return err
                                                }
                                                val0 := value >> 4
                                                val1 := value & 0x0f
                                                if val1 != 0 {
                                                        k += int(val0)
                                                        if k > blockSize {
                                                                return FormatError("bad DCT index")
                                                        }
                                                        ac, err := d.receiveExtend(val1)
                                                        if err != nil {
                                                                return err
                                                        }
                                                        b[unzig[k]] = ac * qt[k]
                                                } else {
                                                        if val0 != 0x0f {
                                                                break
                                                        }
                                                        k += 0x0f
                                                }
                                        }

                                        // Perform the inverse DCT and store the MCU component to the image.
                                        if d.nComp == nGrayComponent {
                                                idct(d.img1.Pix[8*(my*d.img1.Stride+mx):], d.img1.Stride, &b)
                                        } else {
                                                switch i {
                                                case 0:
                                                        mx0 := h0*mx + (j % 2)
                                                        my0 := v0*my + (j / 2)
                                                        idct(d.img3.Y[8*(my0*d.img3.YStride+mx0):], d.img3.YStride, &b)
                                                case 1:
                                                        idct(d.img3.Cb[8*(my*d.img3.CStride+mx):], d.img3.CStride, &b)
                                                case 2:
                                                        idct(d.img3.Cr[8*(my*d.img3.CStride+mx):], d.img3.CStride, &b)
                                                }
                                        }
                                } // for j
                        } // for i
                        mcu++
                        if d.ri > 0 && mcu%d.ri == 0 && mcu < mxx*myy {
                                // A more sophisticated decoder could use RST[0-7] markers to resynchronize from corrupt input,
                                // but this one assumes well-formed input, and hence the restart marker follows immediately.
                                _, err := io.ReadFull(d.r, d.tmp[0:2])
                                if err != nil {
                                        return err
                                }
                                if d.tmp[0] != 0xff || d.tmp[1] != expectedRST {
                                        return FormatError("bad RST marker")
                                }
                                expectedRST++
                                if expectedRST == rst7Marker+1 {
                                        expectedRST = rst0Marker
                                }
                                // Reset the Huffman decoder.
                                d.b = bits{}
                                // Reset the DC components, as per section F.2.1.3.1.
                                dc = [nColorComponent]int{}
                        }
                } // for mx
        } // for my

        return nil
}

// Specified in section B.2.4.4.
func (d *decoder) processDRI(n int) error {
        if n != 2 {
                return FormatError("DRI has wrong length")
        }
        _, err := io.ReadFull(d.r, d.tmp[0:2])
        if err != nil {
                return err
        }
        d.ri = int(d.tmp[0])<<8 + int(d.tmp[1])
        return nil
}

// decode reads a JPEG image from r and returns it as an image.Image.
func (d *decoder) decode(r io.Reader, configOnly bool) (image.Image, error) {
        if rr, ok := r.(Reader); ok {
                d.r = rr
        } else {
                d.r = bufio.NewReader(r)
        }

        // Check for the Start Of Image marker.
        _, err := io.ReadFull(d.r, d.tmp[0:2])
        if err != nil {
                return nil, err
        }
        if d.tmp[0] != 0xff || d.tmp[1] != soiMarker {
                return nil, FormatError("missing SOI marker")
        }

        // Process the remaining segments until the End Of Image marker.
        for {
                _, err := io.ReadFull(d.r, d.tmp[0:2])
                if err != nil {
                        return nil, err
                }
                if d.tmp[0] != 0xff {
                        return nil, FormatError("missing 0xff marker start")
                }
                marker := d.tmp[1]
                if marker == eoiMarker { // End Of Image.
                        break
                }

                // Read the 16-bit length of the segment. The value includes the 2 bytes for the
                // length itself, so we subtract 2 to get the number of remaining bytes.
                _, err = io.ReadFull(d.r, d.tmp[0:2])
                if err != nil {
                        return nil, err
                }
                n := int(d.tmp[0])<<8 + int(d.tmp[1]) - 2
                if n < 0 {
                        return nil, FormatError("short segment length")
                }

                switch {
                case marker == sof0Marker: // Start Of Frame (Baseline).
                        err = d.processSOF(n)
                        if configOnly {
                                return nil, err
                        }
                case marker == sof2Marker: // Start Of Frame (Progressive).
                        err = UnsupportedError("progressive mode")
                case marker == dhtMarker: // Define Huffman Table.
                        err = d.processDHT(n)
                case marker == dqtMarker: // Define Quantization Table.
                        err = d.processDQT(n)
                case marker == sosMarker: // Start Of Scan.
                        err = d.processSOS(n)
                case marker == driMarker: // Define Restart Interval.
                        err = d.processDRI(n)
                case marker >= app0Marker && marker <= app15Marker || marker == comMarker: // APPlication specific, or COMment.
                        err = d.ignore(n)
                default:
                        err = UnsupportedError("unknown marker")
                }
                if err != nil {
                        return nil, err
                }
        }
        if d.img1 != nil {
                return d.img1, nil
        }
        if d.img3 != nil {
                return d.img3, nil
        }
        return nil, FormatError("missing SOS marker")
}

// Decode reads a JPEG image from r and returns it as an image.Image.
func Decode(r io.Reader) (image.Image, error) {
        var d decoder
        return d.decode(r, false)
}

// DecodeConfig returns the color model and dimensions of a JPEG image without
// decoding the entire image.
func DecodeConfig(r io.Reader) (image.Config, error) {
        var d decoder
        if _, err := d.decode(r, true); err != nil {
                return image.Config{}, err
        }
        switch d.nComp {
        case nGrayComponent:
                return image.Config{
                        ColorModel: color.GrayModel,
                        Width:      d.width,
                        Height:     d.height,
                }, nil
        case nColorComponent:
                return image.Config{
                        ColorModel: color.YCbCrModel,
                        Width:      d.width,
                        Height:     d.height,
                }, nil
        }
        return image.Config{}, FormatError("missing SOF marker")
}

func init() {
        image.RegisterFormat("jpeg", "\xff\xd8", Decode, DecodeConfig)
}