// image_dib.go 将 Windows 剪贴板的 CF_DIB（BITMAPINFO + 像素）转换为 PNG。
//
// 剪贴板截图类内容常见两种来源：现代应用直接提供 "PNG" 注册格式；老应用
// （如 Server 上的画图）只提供 CF_DIB。本转换器覆盖最常见子集：
// BI_RGB / BI_BITFIELDS 压缩、24/32bpp、底行优先（默认）与 top-down。
package cliprdr

import (
	"bytes"
	"encoding/binary"
	"fmt"
	"image"
	"image/png"
)

// DIBToPNG 把 CF_DIB 数据解码并编码为 PNG；不支持的子集返回错误。
func DIBToPNG(dib []byte) ([]byte, error) {
	if len(dib) < 40 {
		return nil, fmt.Errorf("dib too short: %d", len(dib))
	}
	biSize := binary.LittleEndian.Uint32(dib[0:4])
	w := int(int32(binary.LittleEndian.Uint32(dib[4:8])))
	hRaw := int32(binary.LittleEndian.Uint32(dib[8:12]))
	topDown := hRaw < 0
	h := int(hRaw)
	if h < 0 {
		h = -h
	}
	bpp := binary.LittleEndian.Uint16(dib[14:16])
	compression := binary.LittleEndian.Uint32(dib[16:20])
	if w <= 0 || h <= 0 || w > 16384 || h > 16384 {
		return nil, fmt.Errorf("bad dimensions %dx%d", w, h)
	}

	var rMask, gMask, bMask uint32
	pixOff := int(biSize)
	switch compression {
	case 0: // BI_RGB
		rMask, gMask, bMask = 0x00FF0000, 0x0000FF00, 0x000000FF
	case 3: // BI_BITFIELDS
		if biSize >= 108 { // BITMAPV4HEADER/V5HEADER 内含掩码
			rMask = binary.LittleEndian.Uint32(dib[40:44])
			gMask = binary.LittleEndian.Uint32(dib[44:48])
			bMask = binary.LittleEndian.Uint32(dib[48:52])
		} else { // 三个 DWORD 掩码紧跟标准头
			if pixOff+12 > len(dib) {
				return nil, fmt.Errorf("missing bitfields masks")
			}
			rMask = binary.LittleEndian.Uint32(dib[pixOff:])
			gMask = binary.LittleEndian.Uint32(dib[pixOff+4:])
			bMask = binary.LittleEndian.Uint32(dib[pixOff+8:])
			pixOff += 12
		}
	default:
		return nil, fmt.Errorf("unsupported compression %d", compression)
	}
	if bpp != 24 && bpp != 32 {
		return nil, fmt.Errorf("unsupported bpp %d", bpp)
	}

	stride := (w*int(bpp) + 31) / 32 * 4
	if pixOff+stride*h > len(dib) {
		return nil, fmt.Errorf("dib truncated: need %d, have %d", pixOff+stride*h, len(dib))
	}

	img := image.NewRGBA(image.Rect(0, 0, w, h))
	for y := 0; y < h; y++ {
		srcY := y
		if !topDown {
			srcY = h - 1 - y
		}
		row := dib[pixOff+srcY*stride:]
		for x := 0; x < w; x++ {
			var r8, g8, b8 uint8
			if bpp == 32 {
				v := binary.LittleEndian.Uint32(row[x*4:])
				r8, g8, b8 = maskTo8(v, rMask), maskTo8(v, gMask), maskTo8(v, bMask)
			} else {
				o := x * 3
				b8, g8, r8 = row[o], row[o+1], row[o+2]
			}
			i := img.PixOffset(x, y)
			img.Pix[i], img.Pix[i+1], img.Pix[i+2], img.Pix[i+3] = r8, g8, b8, 255
		}
	}

	var buf bytes.Buffer
	if err := png.Encode(&buf, img); err != nil {
		return nil, err
	}
	return buf.Bytes(), nil
}

// maskTo8 把 mask 提取出的位域缩放为 8 位
func maskTo8(v, mask uint32) uint8 {
	if mask == 0 {
		return 0
	}
	shift := 0
	for mask&1 == 0 {
		mask >>= 1
		shift++
	}
	bits := 0
	for mask != 0 {
		mask >>= 1
		bits++
	}
	val := (v >> uint(shift)) & ((1 << uint(bits)) - 1)
	if bits < 8 {
		return uint8(val<<(8-bits) | val>>(2*uint(bits)-8))
	}
	return uint8(val >> uint(bits-8))
}