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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() .into_iter()
.rev() .rev()
.zip(forward_costs) .zip(forward_costs)
.map(|((pos_b, cost_b), (pos_f, cost_f))| { .map(|((_, cost_b), (_, cost_f))| cost_f + cost_b)
debug_assert_eq!(pos_f, pos_b);
cost_f + cost_b
})
.min() .min()
.unwrap() .unwrap()
.to_string() .to_string()
@@ -76,11 +72,8 @@ fn cost_at(pos: usize, groups: &[(usize, usize)]) -> usize {
.sum() .sum()
} }
fn ternary_search(groups: &[(usize, usize)], min: usize, max: usize) -> usize { fn ternary_search(groups: &[(usize, usize)], mut min: usize, mut max: usize) -> usize {
if max - min <= 6 { while 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 {
let mid1 = min + (max - min) / 3; let mid1 = min + (max - min) / 3;
let mid2 = max - (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); let cost2 = cost_at(mid2, groups);
if cost1 < cost2 { if cost1 < cost2 {
ternary_search(groups, min, mid2 - 1) max = mid2 - 1
} else { } 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 { 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 min = groups.first().unwrap().1;
let max = groups.last().unwrap().1; let max = groups.last().unwrap().1;
// Brute force approach, better version later
ternary_search(&groups, min, max).to_string() ternary_search(&groups, min, max).to_string()
} }