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
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 infoHeader struct {
		Size          uint32
		Width         int32
		Height        int32
		Planes        uint16
		BitsPerPixel  uint16
		Compression   uint32
		SizeImage     uint32
		XPelsPerMeter int32
		YPelsPerMeter int32
	}
	if err := binary.Read(f, binary.LittleEndian, &infoHeader); err != nil {
		return nil, err
	}
	return &ProbeResult{Width: abs(infoHeader.Width), Height: abs(infoHeader.Height)}, nil
}

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