npy 0.4.0

NumPy file format (de-)serialization
Documentation 
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use std::io::{self,Write,BufWriter,Seek,SeekFrom};
use std::fs::File;
use std::path::Path;
use std::marker::PhantomData;

use byteorder::{WriteBytesExt, LittleEndian};

use serializable::Serializable;
use header::DType;

const FILLER: &'static [u8] = &[42; 19];

/// Serialize into a file one row at a time. To serialize an iterator, use the
/// [`to_file`](fn.to_file.html) function.
pub struct OutFile<Row: Serializable> {
    shape_pos: usize,
    len: usize,
    fw: BufWriter<File>,
    _t: PhantomData<Row>
}

impl<Row: Serializable> OutFile<Row> {
    /// Open a file
    pub fn open<P: AsRef<Path>>(path: P) -> io::Result<Self> {
        let dtype = Row::dtype();
        if let &DType::Plain { ref shape, .. } = &dtype {
            assert!(shape.len() == 0, "plain non-scalar dtypes not supported");
        }
        let mut fw = BufWriter::new(File::create(path)?);
        fw.write_all(&[0x93u8])?;
        fw.write_all(b"NUMPY")?;
        fw.write_all(&[0x01u8, 0x00])?;

        let (header, shape_pos) = create_header(&dtype);

        let mut padding: Vec<u8> = vec![];
        padding.extend(&::std::iter::repeat(b' ').take(15 - ((header.len() + 10) % 16)).collect::<Vec<_>>());
        padding.extend(&[b'\n']);

        let len = header.len() + padding.len();
        assert! (len <= ::std::u16::MAX as usize);
        assert_eq!((len + 10) % 16, 0);

        fw.write_u16::<LittleEndian>(len as u16)?;
        fw.write_all(&header)?;
        // Padding to 8 bytes
        fw.write_all(&padding)?;

        Ok(OutFile {
            shape_pos: shape_pos,
            len: 0,
            fw: fw,
            _t: PhantomData,
        })
    }

    /// Append a single row to the file
    pub fn push(&mut self, row: &Row) -> io::Result<()> {
        self.len += 1;
        row.write(&mut self.fw)
    }

    fn close_(&mut self) -> io::Result<()> {
        // Write the size to the header
        self.fw.seek(SeekFrom::Start(self.shape_pos as u64))?;
        let length = format!("{}", self.len);
        self.fw.write_all(length.as_bytes())?;
        self.fw.write_all(&b",), }"[..])?;
        self.fw.write_all(&::std::iter::repeat(b' ').take(FILLER.len() - length.len()).collect::<Vec<_>>())?;
        Ok(())
    }

    /// Finish writing the file by finalizing the header and closing the file.
    ///
    /// If omitted, the file will be closed on drop automatically, but it will panic on error.
    pub fn close(mut self) -> io::Result<()> {
        self.close_()
    }
}

fn create_header(dtype: &DType) -> (Vec<u8>, usize) {
    let mut header: Vec<u8> = vec![];
    header.extend(&b"{'descr': "[..]);
    header.extend(dtype.descr().as_bytes());
    header.extend(&b", 'fortran_order': False, 'shape': ("[..]);
    let shape_pos = header.len() + 10;
    header.extend(FILLER);
    header.extend(&b",), }"[..]);
    (header, shape_pos)
}

impl<Row: Serializable> Drop for OutFile<Row> {
    fn drop(&mut self) {
        let _ = self.close_(); // Ignore the errors
    }
}


// TODO: improve the interface to avoid unnecessary clones
/// Serialize an iterator over a struct to a NPY file
///
/// A single-statement alternative to saving row by row using the [`OutFile`](struct.OutFile.html).
pub fn to_file<'a, S, T, P>(filename: P, data: T) -> ::std::io::Result<()> where
        P: AsRef<Path>,
        S: Serializable + 'a,
        T: IntoIterator<Item=S> {

    let mut of = OutFile::open(filename)?;
    for row in data {
        of.push(&row)?;
    }
    of.close()
}