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Implement iter::Chain::{advance_by, advance_back_by}

This commit is contained in:
Tim Vermeulen 2020-10-02 05:26:36 +02:00
parent 9cba260df0
commit c5d6a0dd96
1 changed files with 64 additions and 12 deletions
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76
library/core/src/iter/adapters/chain.rs
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@ -115,16 +115,42 @@ where
}
#[inline]
fn nth(&mut self, mut n: usize) -> Option<A::Item> {
fn advance_by(&mut self, n: usize) -> Result<(), usize> {
let mut rem = n;
if let Some(ref mut a) = self.a {
while let Some(x) = a.next() {
if n == 0 {
return Some(x);
}
n -= 1;
match a.advance_by(rem) {
Ok(()) => return Ok(()),
Err(k) => rem -= k,
}
self.a = None;
}
if let Some(ref mut b) = self.b {
match b.advance_by(rem) {
Ok(()) => return Ok(()),
Err(k) => rem -= k,
}
// we don't fuse the second iterator
}
if rem == 0 { Ok(()) } else { Err(n - rem) }
}
#[inline]
fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
if let Some(ref mut a) = self.a {
match a.advance_by(n) {
Ok(()) => match a.next() {
None => n = 0,
x => return x,
},
Err(k) => n -= k,
}
self.a = None;
}
maybe!(self.b.nth(n))
}
@ -191,16 +217,42 @@ where
}
#[inline]
fn nth_back(&mut self, mut n: usize) -> Option<A::Item> {
fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
let mut rem = n;
if let Some(ref mut b) = self.b {
while let Some(x) = b.next_back() {
if n == 0 {
return Some(x);
}
n -= 1;
match b.advance_back_by(rem) {
Ok(()) => return Ok(()),
Err(k) => rem -= k,
}
self.b = None;
}
if let Some(ref mut a) = self.a {
match a.advance_back_by(rem) {
Ok(()) => return Ok(()),
Err(k) => rem -= k,
}
// we don't fuse the second iterator
}
if rem == 0 { Ok(()) } else { Err(n - rem) }
}
#[inline]
fn nth_back(&mut self, mut n: usize) -> Option<Self::Item> {
if let Some(ref mut b) = self.b {
match b.advance_back_by(n) {
Ok(()) => match b.next_back() {
None => n = 0,
x => return x,
},
Err(k) => n -= k,
}
self.b = None;
}
maybe!(self.a.nth_back(n))
}