Crate smallnum

Integer optimization: macros return the smallest integer type capable of fitting a static bounds. Both signed (e.g. macro input is maximum) and unsigned (e.g. macro input is maximum or minimum) numbers supported. Saves memory on embedded devices. !#[no_std], #![forbid(unsafe_code)], zero-cost.

Example: Statically-sized Collection Index

When the size of a collection is known at compile-time, the variable used to index it can be size-optimized.

use smallnum::{small_unsigned, SmallUnsigned};
use core::mem::size_of_val;

const MAX_CAPACITY: usize = 500;
let my_array: [u8; MAX_CAPACITY] = [0; MAX_CAPACITY];

let idx: usize = 5;
let small_idx: small_unsigned!(MAX_CAPACITY) = 5;

assert_eq!(idx, small_idx.usize());                     // Equivalent values
assert_eq!(my_array[idx], my_array[small_idx.usize()]); // Equivalent collection indexing
assert!(size_of_val(&idx) > size_of_val(&small_idx));   // Memory savings (6 bytes on 64-bit)

#[cfg(target_pointer_width = "64")]
assert_eq!(size_of_val(&idx), 8);

#[cfg(target_pointer_width = "64")]
assert_eq!(size_of_val(&small_idx), 2);

Example: Tree Node Metadata

When the maximum capacity of a tree is known at compile-time, metadata stored in every node can be size-optimized.

use smallnum::small_unsigned;
use core::mem::size_of;

const MAX_CAPACITY: usize = 500;

// Regular node in a binary tree
struct BinTree<T> {
    value: T,
    left_child: Option<Box<BinTree<T>>>,
    right_child: Option<Box<BinTree<T>>>,
    subtree_size: Option<usize>,
}

// Node with size-optimized metadata
struct SmallBinTree<T> {
    value: T,
    left_child: Option<Box<SmallBinTree<T>>>,
    right_child: Option<Box<SmallBinTree<T>>>,
    subtree_size: Option<small_unsigned!(MAX_CAPACITY)>,
}

// Per-node memory savings (16 bytes on 64-bit)
assert!(size_of::<BinTree<u8>>() > size_of::<SmallBinTree<u8>>());

#[cfg(target_pointer_width = "64")]
assert_eq!(size_of::<BinTree<u8>>(), 40);

#[cfg(target_pointer_width = "64")]
assert_eq!(size_of::<SmallBinTree<u8>>(), 24);

Macros

small_signed	Return smallest signed type capable to representing input value (positive, i.e. maximum, or negative, i.e. minimum)
small_unsigned	Return smallest unsigned type capable to representing input value (positive, i.e. maximum)

Traits

ShrinkSigned	Helper trait for signed type mapping. Internal use only.
ShrinkUnsigned	Helper trait for unsigned type mapping. Internal use only.
SmallSigned	Helper trait for signed normalization (e.g. isize)
SmallUnsigned	Helper trait for unsigned normalization (e.g. usize)