imagesize 0.15.0

Quick probing of image dimensions without loading the entire file.
Documentation 
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use std::io::{BufRead, Seek, SeekFrom};

use crate::{ImageResult, ImageSize};

pub fn size<R: BufRead + Seek>(reader: &mut R) -> ImageResult<ImageSize> {
    // ASTC header is 16 bytes total
    // Magic number: 0x13, 0x12, 0x10, 0x5C (4 bytes)
    // Block dimensions: blockdim_x, blockdim_y, blockdim_z (3 bytes) - skip these
    // Image dimensions: xsize (3 bytes), ysize (3 bytes), zsize (3 bytes)

    reader.seek(SeekFrom::Start(7))?; // Skip magic + block dimensions

    // Read 3-byte little-endian values for image dimensions
    let mut xsize_bytes = [0u8; 4];
    reader.read_exact(&mut xsize_bytes[0..3])?;
    let width = u32::from_le_bytes(xsize_bytes) as usize;

    let mut ysize_bytes = [0u8; 4];
    reader.read_exact(&mut ysize_bytes[0..3])?;
    let height = u32::from_le_bytes(ysize_bytes) as usize;

    Ok(ImageSize { width, height })
}

pub fn matches(header: &[u8]) -> bool {
    // ASTC magic number is 0x13 0xAB 0xA? 0x5C (third byte can be A0 or A1)
    header.len() >= 4
        && header[0] == 0x13
        && header[1] == 0xAB
        && (header[2] == 0xA0 || header[2] == 0xA1)
        && header[3] == 0x5C
}