src/storage/fxfs/src/round.rs

// Copyright 2021 The Fuchsia Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

/// Round `offset` up to next multiple of `block_size`.
/// This function will fail if rounding up leads to an integer overflow.
///
/// (Note that unstable rust is currently adding the same function
/// `{integer}::checked_next_multiple_of()` behind the "int_roundings" feature.)
pub fn round_up<T: Into<U>, U: Copy + num_traits::PrimInt>(offset: U, block_size: T) -> Option<U> {
    let block_size = block_size.into();
    #[allow(clippy::eq_op)]
    let one = block_size / block_size;
    Some(round_down(offset.checked_add(&(block_size - one))?, block_size))
}

/// Round `offset` down to the previous multiple of `block_size`.
pub fn round_down<
    T: Into<U>,
    U: Copy + std::ops::Rem<U, Output = U> + std::ops::Sub<U, Output = U>,
>(
    offset: U,
    block_size: T,
) -> U {
    let block_size = block_size.into();
    offset - offset % block_size
}

/// Computes the rounded division of `numerator` / `denominator`.
///
/// Returns None for an arithmetic error (overflow, or divide-by-zero).
pub fn round_div(numerator: u64, denominator: u64) -> Option<u64> {
    numerator.checked_add(denominator / 2)?.checked_div(denominator)
}

/// Returns how many items of a given size are needed to contain a value.
///
/// TODO(https://github.com/rust-lang/rust/issues/88581): Replace with `{integer}::div_ceil()` when
/// `int_roundings` is available.
pub fn how_many<T: Into<u64>>(value: u64, item_size: T) -> u64 {
    let item_size = item_size.into();
    let items = value / item_size;
    let remainder = value % item_size;
    if remainder != 0 {
        items + 1
    } else {
        items
    }
}

#[cfg(test)]
mod tests {
    use crate::round::{how_many, round_div};

    #[test]
    fn test_how_many() {
        assert_eq!(how_many(0, 3u64), 0);
        assert_eq!(how_many(1, 3u64), 1);
        assert_eq!(how_many(2, 3u64), 1);
        assert_eq!(how_many(3, 3u64), 1);
        assert_eq!(how_many(4, 3u64), 2);
        assert_eq!(how_many(5, 3u64), 2);
        assert_eq!(how_many(6, 3u64), 2);

        assert_eq!(how_many(17u64, 3u8), 6u64);

        assert_eq!(how_many(u64::MAX, 4096u32), u64::MAX / 4096 + 1);
    }

    #[test]
    fn test_round_div() {
        assert_eq!(round_div(9, 4), Some(2));
        assert_eq!(round_div(10, 4), Some(3));
        assert_eq!(round_div(11, 4), Some(3));
        assert_eq!(round_div(12, 4), Some(3));
        assert_eq!(round_div(13, 4), Some(3));
        assert_eq!(round_div(14, 4), Some(4));
        assert_eq!(round_div(0, 1), Some(0));
        assert_eq!(round_div(100, 0), None);
        assert_eq!(round_div(u64::MAX, 2), None);
    }
}