arrow2 0.6.1

Unofficial implementation of Apache Arrow spec in safe Rust
Documentation 
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use std::io::Read;

use super::{ByteRecord, Reader};

use crate::{
    datatypes::*,
    error::{ArrowError, Result},
};

/// Returns a new [`Schema`] whereby the fields are selected based on `projection`.
pub fn projected_schema(schema: &Schema, projection: Option<&[usize]>) -> Schema {
    match &projection {
        Some(projection) => {
            let fields = schema.fields();
            let projected_fields: Vec<Field> =
                projection.iter().map(|i| fields[*i].clone()).collect();
            Schema::new_from(projected_fields, schema.metadata().clone())
        }
        None => schema.clone(),
    }
}

/// Reads `len` rows from `reader` into `row`, skiping `skip`.
/// This operation has minimal CPU work and is thus the fastest way to read through a CSV
/// without deserializing the contents to Arrow.
pub fn read_rows<R: Read>(
    reader: &mut Reader<R>,
    skip: usize,
    rows: &mut [ByteRecord],
) -> Result<usize> {
    // skip first `start` rows.
    let mut row = ByteRecord::new();
    for _ in 0..skip {
        let res = reader.read_byte_record(&mut row);
        if !res.unwrap_or(false) {
            break;
        }
    }

    let mut row_number = 0;
    for row in rows.iter_mut() {
        let has_more = reader.read_byte_record(row).map_err(|e| {
            ArrowError::External(format!(" at line {}", skip + row_number), Box::new(e))
        })?;
        if !has_more {
            break;
        }
        row_number += 1;
    }
    Ok(row_number)
}