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Convert recursion to iteration

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Bert Peters
2021-12-07 11:00:26 +01:00
parent d099614217
commit c02f1e11c5
1 changed files with 8 additions and 13 deletions
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21
2021/src/day07.rs
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@@ -51,11 +51,7 @@ pub fn part1(input: &mut dyn Read) -> String {
.into_iter()
.rev()
.zip(forward_costs)
.map(|((pos_b, cost_b), (pos_f, cost_f))| {
debug_assert_eq!(pos_f, pos_b);
cost_f + cost_b
})
.map(|((_, cost_b), (_, cost_f))| cost_f + cost_b)
.min()
.unwrap()
.to_string()
@@ -76,11 +72,8 @@ fn cost_at(pos: usize, groups: &[(usize, usize)]) -> usize {
.sum()
}
fn ternary_search(groups: &[(usize, usize)], min: usize, max: usize) -> usize {
if max - min <= 6 {
// ternary search isn't effective in small ranges, just iterate instead.
(min..=max).map(|pos| cost_at(pos, groups)).min().unwrap()
} else {
fn ternary_search(groups: &[(usize, usize)], mut min: usize, mut max: usize) -> usize {
while max - min > 6 {
let mid1 = min + (max - min) / 3;
let mid2 = max - (max - min) / 3;
@@ -88,11 +81,14 @@ fn ternary_search(groups: &[(usize, usize)], min: usize, max: usize) -> usize {
let cost2 = cost_at(mid2, groups);
if cost1 < cost2 {
ternary_search(groups, min, mid2 - 1)
max = mid2 - 1
} else {
ternary_search(groups, mid1 + 1, max)
min = mid1 + 1
}
}
// Ternary search isn't effective at such small intervals so we iterate the remaining part
(min..=max).map(|pos| cost_at(pos, groups)).min().unwrap()
}
pub fn part2(input: &mut dyn Read) -> String {
@@ -102,7 +98,6 @@ pub fn part2(input: &mut dyn Read) -> String {
let min = groups.first().unwrap().1;
let max = groups.last().unwrap().1;
// Brute force approach, better version later
ternary_search(&groups, min, max).to_string()
}