package probe

import (
	"encoding/binary"
	"fmt"
	"os"
)

// probeBmp 读取 BMP 文件头中的宽高；自顶向下 BMP 的高度为负值，取绝对值。
// 兼容常见 info 头：BITMAPCOREHEADER(12B, OS/2)、BITMAPINFOHEADER(40B) 及更长的 V4/V5 变体。
func probeBmp(filePath string) (*ProbeResult, error) {
	f, err := os.Open(filePath)
	if err != nil {
		return nil, err
	}
	defer f.Close()

	var fileHeader struct {
		FileType   [2]byte
		FileSize   uint32
		Reserved1  uint16
		Reserved2  uint16
		DataOffset uint32
	}
	if err := binary.Read(f, binary.LittleEndian, &fileHeader); err != nil {
		return nil, err
	}
	if string(fileHeader.FileType[:]) != "BM" {
		return nil, fmt.Errorf("not a valid BMP file")
	}

	var infoSize uint32
	if err := binary.Read(f, binary.LittleEndian, &infoSize); err != nil {
		return nil, err
	}

	switch {
	case infoSize >= 40:
		// BITMAPINFOHEADER(40B)/BITMAPV4HEADER(108B)/BITMAPV5HEADER(124B)/OS2(64B)：宽高为 int32
		var width, height int32
		if err := binary.Read(f, binary.LittleEndian, &width); err != nil {
			return nil, err
		}
		if err := binary.Read(f, binary.LittleEndian, &height); err != nil {
			return nil, err
		}
		return &ProbeResult{Width: abs32(width), Height: abs32(height)}, nil
	case infoSize >= 12:
		// BITMAPCOREHEADER(12B, OS/2 1.x)：宽高为 uint16
		var width, height uint16
		if err := binary.Read(f, binary.LittleEndian, &width); err != nil {
			return nil, err
		}
		if err := binary.Read(f, binary.LittleEndian, &height); err != nil {
			return nil, err
		}
		return &ProbeResult{Width: int(width), Height: int(height)}, nil
	default:
		return nil, fmt.Errorf("unsupported BMP info header size: %d", infoSize)
	}
}

func abs32(v int32) int {
	if v < 0 {
		return int(-v)
	}
	return int(v)
}