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wrapping add #9

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merged 11 commits into from
Sep 25, 2023
Merged

wrapping add #9

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e8bb70b
wrapping add
Easyoakland Sep 1, 2023
c6ff5fd
Fix potential overflow by forwarding to std impl
Easyoakland Sep 5, 2023
dd5ed24
const fn, use MAX, MIN directly
Easyoakland Sep 6, 2023
d23d13f
Formatting
Easyoakland Sep 6, 2023
fba6c18
Wrapping add method that doesn't require panic on any input.
Easyoakland Sep 6, 2023
ef0d93c
Nearly done with explanation for algorithm
Easyoakland Sep 7, 2023
086928b
Fixed no_panic wrapping_add impl explanation.
Easyoakland Sep 7, 2023
db4b833
minor comment fix
Easyoakland Sep 7, 2023
66dad44
fix wrapping_add(signed::MIN) and minor formatting
Easyoakland Sep 25, 2023
210a6a8
fix fmt errors
Easyoakland Sep 25, 2023
ef0b60c
clarify safety comment
Easyoakland Sep 25, 2023
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69 changes: 69 additions & 0 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -155,6 +155,7 @@ macro_rules! impl_ranged {
mod_name: $mod_name:ident
internal: $internal:ident
signed: $is_signed:ident
unsigned: $unsigned_type:ident
optional: $optional_type:ident
}
)*) => {$(
Expand Down Expand Up @@ -792,6 +793,62 @@ macro_rules! impl_ranged {
<Self as $crate::traits::RangeIsValid>::ASSERT;
Self::new_saturating(self.get().saturating_pow(exp))
}

/// Wrapping integer addition. Computes `self + rhs`, wrapping around the numeric
/// bounds.
#[must_use = "this returns the result of the operation, without modifying the original"]
#[inline]
#[allow(trivial_casts, trivial_numeric_casts)] // needed since some casts have to send unsigned -> unsigned to handle signed -> unsigned
pub const fn wrapping_add(self, rhs: $internal) -> Self {
// Forward to type impl if same as type.
if MIN == $internal::MIN && MAX == $internal::MAX {
// Safety: std impl is safe
return unsafe { Self::new_unchecked(self.get().wrapping_add(rhs)) }
}

<Self as $crate::traits::RangeIsValid>::ASSERT;
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let inner = self.get();
let range_len = MAX.abs_diff(MIN) + 1;
// Calculate the offset with proper handling for negative rhs
#[allow(unused_comparisons)]
// equivalent to `rem_euclid_unsigned()` if that method existed
let offset = if rhs >= 0 {
(rhs as $unsigned_type) % range_len
} else {
// Let ux refer to an n bit unsigned and ix refer to an n bit signed integer.
// Can't write -ux or ux::abs() method. This gets around compilation error.
let rhs_abs = (rhs - rhs - rhs) as $unsigned_type;
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// Largest multiple of range_len <= type::MAX is lowest if range_len * 2 > ux::MAX -> range_len >= ux::MAX / 2 + 1
// Also = 0 in mod range_len arithmetic.
// Sub from this large number rhs_abs (-rhs) to get rhs % range_len
// ix::MIN = -2^(n-1) so 0 <= rhs_abs <= 2^(n-1)
// ux::MAX / 2 + 1 = 2^(n-1) so this subtraction will always be a >= 0 after subtraction
// Thus converting rhs signed negative to equivalent positive value in mod range_len arithmetic
((($unsigned_type::MAX / range_len) * range_len) - (rhs_abs)) % range_len
};

let greater_vals = MAX.abs_diff(inner);
// No wrap
if offset <= greater_vals {
// Safety:
// if inner >= 0 -> No overflow beyond range (offset <= greater_vals)
// if inner < 0: Same as >=0 with caveat:
// `(signed as unsigned).wrapping_add(unsigned) as signed` is the same as
// `signed::checked_add_unsigned(unsigned).unwrap()` or `wrapping_add_unsigned`
// (the difference doesn't matter since it won't overflow the signed type),
// but unsigned integers don't have either method so it won't compile that way.
unsafe { Self::new_unchecked(((inner as $unsigned_type).wrapping_add(offset)) as $internal) }
}
// Wrap
else {
// Safety:
// - offset < range_len by rem_euclid (MIN + ... safe)
// - offset > greater_values from if statement (offset - (greater_values + 1) safe)
//
// again using `(signed as unsigned).wrapping_add(unsigned) as signed` = `checked_add_unsigned` trick
unsafe { Self::new_unchecked(((MIN as $unsigned_type).wrapping_add(offset - ((greater_vals + 1)))) as $internal) }
}
}
}

impl<const MIN: $internal, const MAX: $internal> $optional_type<MIN, MAX> {
Expand Down Expand Up @@ -1210,72 +1267,84 @@ impl_ranged! {
mod_name: ranged_u8
internal: u8
signed: false
unsigned: u8
optional: OptionRangedU8
}
RangedU16 {
mod_name: ranged_u16
internal: u16
signed: false
unsigned: u16
optional: OptionRangedU16
}
RangedU32 {
mod_name: ranged_u32
internal: u32
signed: false
unsigned: u32
optional: OptionRangedU32
}
RangedU64 {
mod_name: ranged_u64
internal: u64
signed: false
unsigned: u64
optional: OptionRangedU64
}
RangedU128 {
mod_name: ranged_u128
internal: u128
signed: false
unsigned: u128
optional: OptionRangedU128
}
RangedUsize {
mod_name: ranged_usize
internal: usize
signed: false
unsigned: usize
optional: OptionRangedUsize
}
RangedI8 {
mod_name: ranged_i8
internal: i8
signed: true
unsigned: u8
optional: OptionRangedI8
}
RangedI16 {
mod_name: ranged_i16
internal: i16
signed: true
unsigned: u16
optional: OptionRangedI16
}
RangedI32 {
mod_name: ranged_i32
internal: i32
signed: true
unsigned: u32
optional: OptionRangedI32
}
RangedI64 {
mod_name: ranged_i64
internal: i64
signed: true
unsigned: u64
optional: OptionRangedI64
}
RangedI128 {
mod_name: ranged_i128
internal: i128
signed: true
unsigned: u128
optional: OptionRangedI128
}
RangedIsize {
mod_name: ranged_isize
internal: isize
signed: true
unsigned: usize
optional: OptionRangedIsize
}
}
32 changes: 32 additions & 0 deletions src/tests.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -385,6 +385,38 @@ macro_rules! tests {
assert_eq!($t::<5, 10>::MAX.saturating_add(1), $t::<5, 10>::MAX);
)*}

#[test]
fn wrapping_add() {$(
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assert_eq!($t::<5, 10>::MAX.wrapping_add(0), $t::<5, 10>::MAX);
assert_eq!($t::<5, 10>::MAX.wrapping_add(1), $t::<5, 10>::MIN);
assert_eq!($t::<{ $inner::MIN }, { $inner::MAX }>::MAX.wrapping_add(1),
$t::<{ $inner::MIN }, { $inner::MAX }>::MIN);
for i in 1..127 {
assert_eq!($t::<{ $inner::MIN}, { $inner::MAX - 1 }>::MAX.wrapping_add(i),
$t::<{ $inner::MIN}, { $inner::MAX - 1 }>::new($inner::MIN + i - 1).unwrap_or_else(|| panic!("{i} not {}", $inner::MAX + i )));
}
)*
$(if_signed! { $signed
for i in 1..=127 {
assert_eq!($t::<-5, 126>::MIN.wrapping_add(-i), $t::<-5,126>::new(126-i+1).unwrap_or_else(|| panic!("{i} not {}", 126-i+1)), "{i}");
assert_eq!($t::<-5, 126>::MIN.wrapping_add(i), $t::<-5,126>::new(-5+i).unwrap_or_else(|| panic!("{i} not {}", 126-i+1)), "{i}");
}
assert_eq!($t::<-5, 10>::MAX.wrapping_add(0), $t::<-5, 10>::MAX);
assert_eq!($t::<-5, -3>::MIN.wrapping_add(-1-3), $t::<-5, -3>::MAX);
assert_eq!($t::<-5, -3>::MIN.wrapping_add(-1-30), $t::<-5, -3>::MAX);
assert_eq!($t::<-5, -3>::MIN.wrapping_add(30), $t::<-5, -3>::MIN);
assert_eq!($t::<-5, -3>::MIN.wrapping_add(-30), $t::<-5, -3>::MIN);
assert_eq!($t::<-5, 10>::MAX.wrapping_add(25), $t::<-5, 10>::MIN.wrapping_add(24));
assert_eq!($t::<-5, 10>::MIN.wrapping_add(24), $t::<-5, 10>::MIN.wrapping_add(8));
assert_eq!($t::<-5, 10>::MAX.wrapping_add(1), $t::<-5, 10>::MIN);
assert_eq!($t::<-5, 10>::MIN.wrapping_add(-1), $t::<-5, 10>::MAX);
assert_eq!($t::<-5, 127>::MIN.wrapping_add(-1), $t::<-5, 127>::MAX);
assert_eq!($t::<-127, 126>::MIN.wrapping_add(-1), $t::<-127, 126>::MAX);
assert_eq!($t::<{ $inner::MIN }, { $inner::MAX }>::MIN.wrapping_add(-1),
$t::<{ $inner::MIN }, { $inner::MAX }>::MAX);
})*
}

#[test]
fn saturating_sub() {$(
assert_eq!($t::<5, 10>::MIN.saturating_sub(0), $t::<5, 10>::MIN);
Expand Down