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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 draw provides image composition functions.
//
// See "The Go image/draw package" for an introduction to this package:
// http://blog.golang.org/2011/09/go-imagedraw-package.html
package draw

import (
        "image"
        "image/color"
)

// m is the maximum color value returned by image.Color.RGBA.
const m = 1<<16 - 1

// Op is a Porter-Duff compositing operator.
type Op int

const (
        // Over specifies ``(src in mask) over dst''.
        Over Op = iota
        // Src specifies ``src in mask''.
        Src
)

// A draw.Image is an image.Image with a Set method to change a single pixel.
type Image interface {
        image.Image
        Set(x, y int, c color.Color)
}

// Draw calls DrawMask with a nil mask.
func Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point, op Op) {
        DrawMask(dst, r, src, sp, nil, image.ZP, op)
}

// clip clips r against each image's bounds (after translating into the
// destination image's co-ordinate space) and shifts the points sp and mp by
// the same amount as the change in r.Min.
func clip(dst Image, r *image.Rectangle, src image.Image, sp *image.Point, mask image.Image, mp *image.Point) {
        orig := r.Min
        *r = r.Intersect(dst.Bounds())
        *r = r.Intersect(src.Bounds().Add(orig.Sub(*sp)))
        if mask != nil {
                *r = r.Intersect(mask.Bounds().Add(orig.Sub(*mp)))
        }
        dx := r.Min.X - orig.X
        dy := r.Min.Y - orig.Y
        if dx == 0 && dy == 0 {
                return
        }
        (*sp).X += dx
        (*sp).Y += dy
        (*mp).X += dx
        (*mp).Y += dy
}

// DrawMask aligns r.Min in dst with sp in src and mp in mask and then replaces the rectangle r
// in dst with the result of a Porter-Duff composition. A nil mask is treated as opaque.
func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point, mask image.Image, mp image.Point, op Op) {
        clip(dst, &r, src, &sp, mask, &mp)
        if r.Empty() {
                return
        }

        // Fast paths for special cases. If none of them apply, then we fall back to a general but slow implementation.
        if dst0, ok := dst.(*image.RGBA); ok {
                if op == Over {
                        if mask == nil {
                                switch src0 := src.(type) {
                                case *image.Uniform:
                                        drawFillOver(dst0, r, src0)
                                        return
                                case *image.RGBA:
                                        drawCopyOver(dst0, r, src0, sp)
                                        return
                                case *image.NRGBA:
                                        drawNRGBAOver(dst0, r, src0, sp)
                                        return
                                case *image.YCbCr:
                                        drawYCbCr(dst0, r, src0, sp)
                                        return
                                }
                        } else if mask0, ok := mask.(*image.Alpha); ok {
                                switch src0 := src.(type) {
                                case *image.Uniform:
                                        drawGlyphOver(dst0, r, src0, mask0, mp)
                                        return
                                }
                        }
                } else {
                        if mask == nil {
                                switch src0 := src.(type) {
                                case *image.Uniform:
                                        drawFillSrc(dst0, r, src0)
                                        return
                                case *image.RGBA:
                                        drawCopySrc(dst0, r, src0, sp)
                                        return
                                case *image.NRGBA:
                                        drawNRGBASrc(dst0, r, src0, sp)
                                        return
                                case *image.YCbCr:
                                        drawYCbCr(dst0, r, src0, sp)
                                        return
                                }
                        }
                }
                drawRGBA(dst0, r, src, sp, mask, mp, op)
                return
        }

        x0, x1, dx := r.Min.X, r.Max.X, 1
        y0, y1, dy := r.Min.Y, r.Max.Y, 1
        if image.Image(dst) == src && r.Overlaps(r.Add(sp.Sub(r.Min))) {
                // Rectangles overlap: process backward?
                if sp.Y < r.Min.Y || sp.Y == r.Min.Y && sp.X < r.Min.X {
                        x0, x1, dx = x1-1, x0-1, -1
                        y0, y1, dy = y1-1, y0-1, -1
                }
        }

        var out *color.RGBA64
        sy := sp.Y + y0 - r.Min.Y
        my := mp.Y + y0 - r.Min.Y
        for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
                sx := sp.X + x0 - r.Min.X
                mx := mp.X + x0 - r.Min.X
                for x := x0; x != x1; x, sx, mx = x+dx, sx+dx, mx+dx {
                        ma := uint32(m)
                        if mask != nil {
                                _, _, _, ma = mask.At(mx, my).RGBA()
                        }
                        switch {
                        case ma == 0:
                                if op == Over {
                                        // No-op.
                                } else {
                                        dst.Set(x, y, color.Transparent)
                                }
                        case ma == m && op == Src:
                                dst.Set(x, y, src.At(sx, sy))
                        default:
                                sr, sg, sb, sa := src.At(sx, sy).RGBA()
                                if out == nil {
                                        out = new(color.RGBA64)
                                }
                                if op == Over {
                                        dr, dg, db, da := dst.At(x, y).RGBA()
                                        a := m - (sa * ma / m)
                                        out.R = uint16((dr*a + sr*ma) / m)
                                        out.G = uint16((dg*a + sg*ma) / m)
                                        out.B = uint16((db*a + sb*ma) / m)
                                        out.A = uint16((da*a + sa*ma) / m)
                                } else {
                                        out.R = uint16(sr * ma / m)
                                        out.G = uint16(sg * ma / m)
                                        out.B = uint16(sb * ma / m)
                                        out.A = uint16(sa * ma / m)
                                }
                                dst.Set(x, y, out)
                        }
                }
        }
}

func drawFillOver(dst *image.RGBA, r image.Rectangle, src *image.Uniform) {
        sr, sg, sb, sa := src.RGBA()
        // The 0x101 is here for the same reason as in drawRGBA.
        a := (m - sa) * 0x101
        i0 := dst.PixOffset(r.Min.X, r.Min.Y)
        i1 := i0 + r.Dx()*4
        for y := r.Min.Y; y != r.Max.Y; y++ {
                for i := i0; i < i1; i += 4 {
                        dr := uint32(dst.Pix[i+0])
                        dg := uint32(dst.Pix[i+1])
                        db := uint32(dst.Pix[i+2])
                        da := uint32(dst.Pix[i+3])

                        dst.Pix[i+0] = uint8((dr*a/m + sr) >> 8)
                        dst.Pix[i+1] = uint8((dg*a/m + sg) >> 8)
                        dst.Pix[i+2] = uint8((db*a/m + sb) >> 8)
                        dst.Pix[i+3] = uint8((da*a/m + sa) >> 8)
                }
                i0 += dst.Stride
                i1 += dst.Stride
        }
}

func drawFillSrc(dst *image.RGBA, r image.Rectangle, src *image.Uniform) {
        sr, sg, sb, sa := src.RGBA()
        // The built-in copy function is faster than a straightforward for loop to fill the destination with
        // the color, but copy requires a slice source. We therefore use a for loop to fill the first row, and
        // then use the first row as the slice source for the remaining rows.
        i0 := dst.PixOffset(r.Min.X, r.Min.Y)
        i1 := i0 + r.Dx()*4
        for i := i0; i < i1; i += 4 {
                dst.Pix[i+0] = uint8(sr >> 8)
                dst.Pix[i+1] = uint8(sg >> 8)
                dst.Pix[i+2] = uint8(sb >> 8)
                dst.Pix[i+3] = uint8(sa >> 8)
        }
        firstRow := dst.Pix[i0:i1]
        for y := r.Min.Y + 1; y < r.Max.Y; y++ {
                i0 += dst.Stride
                i1 += dst.Stride
                copy(dst.Pix[i0:i1], firstRow)
        }
}

func drawCopyOver(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point) {
        dx, dy := r.Dx(), r.Dy()
        d0 := dst.PixOffset(r.Min.X, r.Min.Y)
        s0 := src.PixOffset(sp.X, sp.Y)
        var (
                ddelta, sdelta int
                i0, i1, idelta int
        )
        if r.Min.Y < sp.Y || r.Min.Y == sp.Y && r.Min.X <= sp.X {
                ddelta = dst.Stride
                sdelta = src.Stride
                i0, i1, idelta = 0, dx*4, +4
        } else {
                // If the source start point is higher than the destination start point, or equal height but to the left,
                // then we compose the rows in right-to-left, bottom-up order instead of left-to-right, top-down.
                d0 += (dy - 1) * dst.Stride
                s0 += (dy - 1) * src.Stride
                ddelta = -dst.Stride
                sdelta = -src.Stride
                i0, i1, idelta = (dx-1)*4, -4, -4
        }
        for ; dy > 0; dy-- {
                dpix := dst.Pix[d0:]
                spix := src.Pix[s0:]
                for i := i0; i != i1; i += idelta {
                        sr := uint32(spix[i+0]) * 0x101
                        sg := uint32(spix[i+1]) * 0x101
                        sb := uint32(spix[i+2]) * 0x101
                        sa := uint32(spix[i+3]) * 0x101

                        dr := uint32(dpix[i+0])
                        dg := uint32(dpix[i+1])
                        db := uint32(dpix[i+2])
                        da := uint32(dpix[i+3])

                        // The 0x101 is here for the same reason as in drawRGBA.
                        a := (m - sa) * 0x101

                        dpix[i+0] = uint8((dr*a/m + sr) >> 8)
                        dpix[i+1] = uint8((dg*a/m + sg) >> 8)
                        dpix[i+2] = uint8((db*a/m + sb) >> 8)
                        dpix[i+3] = uint8((da*a/m + sa) >> 8)
                }
                d0 += ddelta
                s0 += sdelta
        }
}

func drawCopySrc(dst *image.RGBA, r image.Rectangle, src *image.RGBA, sp image.Point) {
        n, dy := 4*r.Dx(), r.Dy()
        d0 := dst.PixOffset(r.Min.X, r.Min.Y)
        s0 := src.PixOffset(sp.X, sp.Y)
        var ddelta, sdelta int
        if r.Min.Y <= sp.Y {
                ddelta = dst.Stride
                sdelta = src.Stride
        } else {
                // If the source start point is higher than the destination start point, then we compose the rows
                // in bottom-up order instead of top-down. Unlike the drawCopyOver function, we don't have to
                // check the x co-ordinates because the built-in copy function can handle overlapping slices.
                d0 += (dy - 1) * dst.Stride
                s0 += (dy - 1) * src.Stride
                ddelta = -dst.Stride
                sdelta = -src.Stride
        }
        for ; dy > 0; dy-- {
                copy(dst.Pix[d0:d0+n], src.Pix[s0:s0+n])
                d0 += ddelta
                s0 += sdelta
        }
}

func drawNRGBAOver(dst *image.RGBA, r image.Rectangle, src *image.NRGBA, sp image.Point) {
        i0 := (r.Min.X - dst.Rect.Min.X) * 4
        i1 := (r.Max.X - dst.Rect.Min.X) * 4
        si0 := (sp.X - src.Rect.Min.X) * 4
        yMax := r.Max.Y - dst.Rect.Min.Y

        y := r.Min.Y - dst.Rect.Min.Y
        sy := sp.Y - src.Rect.Min.Y
        for ; y != yMax; y, sy = y+1, sy+1 {
                dpix := dst.Pix[y*dst.Stride:]
                spix := src.Pix[sy*src.Stride:]

                for i, si := i0, si0; i < i1; i, si = i+4, si+4 {
                        // Convert from non-premultiplied color to pre-multiplied color.
                        sa := uint32(spix[si+3]) * 0x101
                        sr := uint32(spix[si+0]) * sa / 0xff
                        sg := uint32(spix[si+1]) * sa / 0xff
                        sb := uint32(spix[si+2]) * sa / 0xff

                        dr := uint32(dpix[i+0])
                        dg := uint32(dpix[i+1])
                        db := uint32(dpix[i+2])
                        da := uint32(dpix[i+3])

                        // The 0x101 is here for the same reason as in drawRGBA.
                        a := (m - sa) * 0x101

                        dpix[i+0] = uint8((dr*a/m + sr) >> 8)
                        dpix[i+1] = uint8((dg*a/m + sg) >> 8)
                        dpix[i+2] = uint8((db*a/m + sb) >> 8)
                        dpix[i+3] = uint8((da*a/m + sa) >> 8)
                }
        }
}

func drawNRGBASrc(dst *image.RGBA, r image.Rectangle, src *image.NRGBA, sp image.Point) {
        i0 := (r.Min.X - dst.Rect.Min.X) * 4
        i1 := (r.Max.X - dst.Rect.Min.X) * 4
        si0 := (sp.X - src.Rect.Min.X) * 4
        yMax := r.Max.Y - dst.Rect.Min.Y

        y := r.Min.Y - dst.Rect.Min.Y
        sy := sp.Y - src.Rect.Min.Y
        for ; y != yMax; y, sy = y+1, sy+1 {
                dpix := dst.Pix[y*dst.Stride:]
                spix := src.Pix[sy*src.Stride:]

                for i, si := i0, si0; i < i1; i, si = i+4, si+4 {
                        // Convert from non-premultiplied color to pre-multiplied color.
                        sa := uint32(spix[si+3]) * 0x101
                        sr := uint32(spix[si+0]) * sa / 0xff
                        sg := uint32(spix[si+1]) * sa / 0xff
                        sb := uint32(spix[si+2]) * sa / 0xff

                        dpix[i+0] = uint8(sr >> 8)
                        dpix[i+1] = uint8(sg >> 8)
                        dpix[i+2] = uint8(sb >> 8)
                        dpix[i+3] = uint8(sa >> 8)
                }
        }
}

func drawYCbCr(dst *image.RGBA, r image.Rectangle, src *image.YCbCr, sp image.Point) {
        // An image.YCbCr is always fully opaque, and so if the mask is implicitly nil
        // (i.e. fully opaque) then the op is effectively always Src.
        x0 := (r.Min.X - dst.Rect.Min.X) * 4
        x1 := (r.Max.X - dst.Rect.Min.X) * 4
        y0 := r.Min.Y - dst.Rect.Min.Y
        y1 := r.Max.Y - dst.Rect.Min.Y
        switch src.SubsampleRatio {
        case image.YCbCrSubsampleRatio422:
                for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
                        dpix := dst.Pix[y*dst.Stride:]
                        yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
                        ciBase := (sy-src.Rect.Min.Y)*src.CStride - src.Rect.Min.X/2
                        for x, sx := x0, sp.X; x != x1; x, sx, yi = x+4, sx+1, yi+1 {
                                ci := ciBase + sx/2
                                rr, gg, bb := color.YCbCrToRGB(src.Y[yi], src.Cb[ci], src.Cr[ci])
                                dpix[x+0] = rr
                                dpix[x+1] = gg
                                dpix[x+2] = bb
                                dpix[x+3] = 255
                        }
                }
        case image.YCbCrSubsampleRatio420:
                for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
                        dpix := dst.Pix[y*dst.Stride:]
                        yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
                        ciBase := (sy/2-src.Rect.Min.Y/2)*src.CStride - src.Rect.Min.X/2
                        for x, sx := x0, sp.X; x != x1; x, sx, yi = x+4, sx+1, yi+1 {
                                ci := ciBase + sx/2
                                rr, gg, bb := color.YCbCrToRGB(src.Y[yi], src.Cb[ci], src.Cr[ci])
                                dpix[x+0] = rr
                                dpix[x+1] = gg
                                dpix[x+2] = bb
                                dpix[x+3] = 255
                        }
                }
        default:
                // Default to 4:4:4 subsampling.
                for y, sy := y0, sp.Y; y != y1; y, sy = y+1, sy+1 {
                        dpix := dst.Pix[y*dst.Stride:]
                        yi := (sy-src.Rect.Min.Y)*src.YStride + (sp.X - src.Rect.Min.X)
                        ci := (sy-src.Rect.Min.Y)*src.CStride + (sp.X - src.Rect.Min.X)
                        for x := x0; x != x1; x, yi, ci = x+4, yi+1, ci+1 {
                                rr, gg, bb := color.YCbCrToRGB(src.Y[yi], src.Cb[ci], src.Cr[ci])
                                dpix[x+0] = rr
                                dpix[x+1] = gg
                                dpix[x+2] = bb
                                dpix[x+3] = 255
                        }
                }
        }
}

func drawGlyphOver(dst *image.RGBA, r image.Rectangle, src *image.Uniform, mask *image.Alpha, mp image.Point) {
        i0 := dst.PixOffset(r.Min.X, r.Min.Y)
        i1 := i0 + r.Dx()*4
        mi0 := mask.PixOffset(mp.X, mp.Y)
        sr, sg, sb, sa := src.RGBA()
        for y, my := r.Min.Y, mp.Y; y != r.Max.Y; y, my = y+1, my+1 {
                for i, mi := i0, mi0; i < i1; i, mi = i+4, mi+1 {
                        ma := uint32(mask.Pix[mi])
                        if ma == 0 {
                                continue
                        }
                        ma |= ma << 8

                        dr := uint32(dst.Pix[i+0])
                        dg := uint32(dst.Pix[i+1])
                        db := uint32(dst.Pix[i+2])
                        da := uint32(dst.Pix[i+3])

                        // The 0x101 is here for the same reason as in drawRGBA.
                        a := (m - (sa * ma / m)) * 0x101

                        dst.Pix[i+0] = uint8((dr*a + sr*ma) / m >> 8)
                        dst.Pix[i+1] = uint8((dg*a + sg*ma) / m >> 8)
                        dst.Pix[i+2] = uint8((db*a + sb*ma) / m >> 8)
                        dst.Pix[i+3] = uint8((da*a + sa*ma) / m >> 8)
                }
                i0 += dst.Stride
                i1 += dst.Stride
                mi0 += mask.Stride
        }
}

func drawRGBA(dst *image.RGBA, r image.Rectangle, src image.Image, sp image.Point, mask image.Image, mp image.Point, op Op) {
        x0, x1, dx := r.Min.X, r.Max.X, 1
        y0, y1, dy := r.Min.Y, r.Max.Y, 1
        if image.Image(dst) == src && r.Overlaps(r.Add(sp.Sub(r.Min))) {
                if sp.Y < r.Min.Y || sp.Y == r.Min.Y && sp.X < r.Min.X {
                        x0, x1, dx = x1-1, x0-1, -1
                        y0, y1, dy = y1-1, y0-1, -1
                }
        }

        sy := sp.Y + y0 - r.Min.Y
        my := mp.Y + y0 - r.Min.Y
        sx0 := sp.X + x0 - r.Min.X
        mx0 := mp.X + x0 - r.Min.X
        sx1 := sx0 + (x1 - x0)
        i0 := dst.PixOffset(x0, y0)
        di := dx * 4
        for y := y0; y != y1; y, sy, my = y+dy, sy+dy, my+dy {
                for i, sx, mx := i0, sx0, mx0; sx != sx1; i, sx, mx = i+di, sx+dx, mx+dx {
                        ma := uint32(m)
                        if mask != nil {
                                _, _, _, ma = mask.At(mx, my).RGBA()
                        }
                        sr, sg, sb, sa := src.At(sx, sy).RGBA()
                        if op == Over {
                                dr := uint32(dst.Pix[i+0])
                                dg := uint32(dst.Pix[i+1])
                                db := uint32(dst.Pix[i+2])
                                da := uint32(dst.Pix[i+3])

                                // dr, dg, db and da are all 8-bit color at the moment, ranging in [0,255].
                                // We work in 16-bit color, and so would normally do:
                                // dr |= dr << 8
                                // and similarly for dg, db and da, but instead we multiply a
                                // (which is a 16-bit color, ranging in [0,65535]) by 0x101.
                                // This yields the same result, but is fewer arithmetic operations.
                                a := (m - (sa * ma / m)) * 0x101

                                dst.Pix[i+0] = uint8((dr*a + sr*ma) / m >> 8)
                                dst.Pix[i+1] = uint8((dg*a + sg*ma) / m >> 8)
                                dst.Pix[i+2] = uint8((db*a + sb*ma) / m >> 8)
                                dst.Pix[i+3] = uint8((da*a + sa*ma) / m >> 8)

                        } else {
                                dst.Pix[i+0] = uint8(sr * ma / m >> 8)
                                dst.Pix[i+1] = uint8(sg * ma / m >> 8)
                                dst.Pix[i+2] = uint8(sb * ma / m >> 8)
                                dst.Pix[i+3] = uint8(sa * ma / m >> 8)
                        }
                }
                i0 += dy * dst.Stride
        }
}