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Fix day 17 edge case

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My algorithm failed with the input in https://www.reddit.com/r/adventofcode/comments/rid0g3/2021_day_17_part_1_an_input_that_might_break_your/ as predicted by the poster. Instead of judging only X values which eventually settle to 0, I am now considering all X values that ever pass through the target area.
This commit is contained in:
Chris Pickett
2021-12-17 11:46:13 -06:00
parent 41f0d167e4
commit 5189bb6734
1 changed files with 11 additions and 10 deletions
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21
src/17.cs
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@ -18,6 +18,10 @@ internal class Day17 : Day
public bool IsFar((int x, int y) pt) => pt.x > maxX; public bool IsFar((int x, int y) pt) => pt.x > maxX;
public bool IsHigh((int x, int y) pt) => pt.y > maxY; public bool IsHigh((int x, int y) pt) => pt.y > maxY;
public bool IsLow((int x, int y) pt) => pt.y < minY; public bool IsLow((int x, int y) pt) => pt.y < minY;
public bool XInRange((int x, int y) pt) => !IsShort(pt) && !IsFar(pt);
public bool XInRange(int x) => XInRange((x, 0));
public bool YInRange((int x, int y) pt) => !IsHigh(pt) && !IsLow(pt);
public bool YInRange(int x) => YInRange((x, 0));
} }
internal override void Go() internal override void Go()
@ -110,24 +114,21 @@ internal class Day17 : Day
private static (int min, int max) GetXVelocityRange(Rectangle bounds) private static (int min, int max) GetXVelocityRange(Rectangle bounds)
{ {
var minSuccessXVel = int.MaxValue; var minSuccessXVel = int.MaxValue;
for (var guess = bounds.maxX; ; guess--) for (var guess = bounds.maxX; guess > 0; guess--)
{ {
var guesspt = (0, 0); var guesspt = (0, 0);
var testVel = (x: guess, 0); var testVel = (x: guess, 0);
while (testVel.x > 0) while (testVel.x > 0)
{ {
guesspt = Step(guesspt, ref testVel); guesspt = Step(guesspt, ref testVel);
}
if (bounds.IsShort(guesspt)) if (bounds.XInRange(guesspt))
{
break;
}
else if (!bounds.IsFar(guesspt))
{
if (guess < minSuccessXVel)
{ {
minSuccessXVel = guess; if (guess < minSuccessXVel)
{
minSuccessXVel = guess;
}
break;
} }
} }
} }