# Elastic collisions between blocks

The 3Blue1Brown Youtube channel released a video a year ago called "Why do colliding blocks compute pi?" which describes a model where a block A of mass $$\a\$$ slides into a block B of mass $$\b\$$, which then pushes block B into a wall, causing it to bounce off the wall and then collide again with block A.

The miracle of this process is that if $$\a/b = 10^{2n-2}\$$ the number of total collisions (both between A and B and between B with the wall) is given by the first $$\n\$$ digits of $$\\pi\$$.

# Example output

+-------+---+--------+
| a     | b | output |
+-------+---+--------+
| 1     | 1 | 3      |
| 2     | 1 | 5      |
| 3     | 1 | 5      |
| 4     | 1 | 6      |
| 5     | 1 | 7      |
| 10    | 3 | 6      |
| 7     | 2 | 6      |
| 9     | 2 | 7      |
| 1     | 2 | 3      |
| 1     | 5 | 2      |
| 100   | 1 | 31     |
| 10000 | 1 | 314    |
+-------+---+--------+


(These values were calculated using this web applet from Reddit user KyleCow1. Please let me know if I've made any mistakes.)

# Challenge

Your challenge is to take two positive integers $$\a, b \in \mathbb N_{>0}\$$, and output the number of collisions in this scenario. Your program should be able to handle all $$\a, b \leq 10\,000\$$. This is a challenge, so the shortest program wins.

• can we input b,a instead of a,b? – ngn Jan 29 '20 at 3:00
• ..or a+ib as a complex number? – ngn Jan 29 '20 at 3:06
• Sure, either of these inputs is fine. – Peter Kagey Jan 29 '20 at 7:15
• @ngn in what way would the complex number help? – RGS Jan 29 '20 at 7:51
• @RGS if your language has a concise way of getting the argument of a complex number (the "theta"), then arctg(b/a) could be theta(a+ib), but i'm not sure it would help much in this case, as b/a is under a sqrt – ngn Jan 29 '20 at 8:00

# Jelly, 10 9 bytes

My first Jelly submission :')

÷½ÆṬØP÷Ċ’


-1 byte thanks to Mr.Xcoder

Uses the formula as in the video. Receives the input flipped; OP gave permission. It probably has room for further golfing, so be sure to give me feedback!

÷½        divide b by a and take square root
ÆṬ      take the ArcTan of that; then
÷   divide
ØP    pi
Ċ  Round up
’ and subtract one


Try it online

• 9 bytes. This gives a different result for the pair 3,1->5 (test case #3), but the current 10-byter wrongly gives 6 due to floating point inaccuracies, so it's actually a "fix" too :) – Mr. Xcoder Jan 29 '20 at 11:00
• @Mr.Xcoder thanks for your feedback! – RGS Jan 29 '20 at 13:21

# APL (Dyalog Classic), 18 16 bytes

¯1+⌈○÷¯3○.5*⍨⎕÷⎕


Try it online!

rendered as the equivalent {¯1+⌈○÷¯3○.5*⍨⍵÷⍺} in the tio link, to facilitate testing; ⎕ means evaluated input; ⍺ and ⍵ are the arguments to an anonymous function

$$\left\lceil\frac\pi{\textrm{arctg}\sqrt\frac ba}\right\rceil-1$$

(that's the formula from the video. i recommend watching it. it's well explained and the animations are great for building intuition.)

• You could go Extended for .5*⍨√ – Adám Jan 29 '20 at 13:41

# JavaScript (ES7),  41  39 bytes

Takes input as (a)(b).

a=>b=>3.14159265/Math.atan((b/a)**.5)|0


Try it online!

### How?

Instead of using a ceil function and subtracting $$\1\$$, we deliberately use a slightly underestimated approximation of $$\\pi\$$ and floor the result with a bitwise OR.

For $$\a,b\le 10000\$$, it was empirically proven to give the same results as this safer 44-byte version:

with(Math)f=a=>b=>ceil(PI/atan((b/a)**.5))-1


Try it online!

• I'm assuming you tried reducing the number of decimal places in your pi approximation? I'm quite sad because 355/113 is a really nice approximation of pi and would allow you to golf even more. – RGS Jan 29 '20 at 14:30
• @RGS That's correct. Going from memory here, but I think I need either 3.14159264 or 3.14159265. – Arnauld Jan 29 '20 at 14:36
• The simplest fraction in that range is 99378/31633, which isn't short enough to golf, unfortunately – isaacg Jan 29 '20 at 20:40

a#b=ceiling(pi/atan(sqrt$b/a))-1  Try it online! # Mathematica, 30 29 bytes, 25 characters ⌈Pi/ArcTan@Sqrt[#2/#]⌉-1&  Try it online -1 byte thanks to ExpiredData Uses the formula explained in the video. • You can just use # instead of #1 – Expired Data Jan 29 '20 at 12:05 • @ExpiredData thanks :D – RGS Jan 29 '20 at 13:20 # Excel, 35 37 bytes -2 bytes thanks to @Chronocidal =CEILING(PI()/(ATAN((B1/A1)^.5)),1)-1  • You seem to have an unnecessary set of brackets around ATAN, for 35 bytes – Chronocidal Jan 31 '20 at 15:11 # C (gcc), 63 $$\\cdots\$$ 47 46 + 3 (compiler flags) = 49 bytes f(a,b){a=ceil(acos(-1)/atan(sqrt(b*1./a)))-1;}  Try it online! Saved 11 12 bytes thanks to ceilingcat!!! Using the formula from the video ngn recommended. # Ruby, 49 43 bytes ->a,b{(-1.arg/((a*b)**0.5+b.i).arg).ceil-1}  Try it online! -5 thans to G B, nicely done -1 replacing 1i*b with b.i • I suppose it would be acceptable to take floats as input so that ->a,b{(Math::PI/Math.atan((b/a)**0.5)).ceil-1} works -- but you may want to ask the OP. – Arnauld Jan 29 '20 at 10:03 • You can use -1.arg for Math::PI – G B Jan 29 '20 at 15:45 • And ((a*b)**0.5+1i*b).arg is the same as Math.atan(b**0.5/a**0.5) – G B Jan 29 '20 at 16:24 # 05AB1E, 30 bytes /t©ažq;‚Δ{ÐO;Å¼®‹èsO;‚}нžqs/î<  Try it online! Almost definitely golfable.. Implements the formula but because 05AB1E doesn't have arctan we have to calculate it, this does it by bisection. # Explanation /t - sqrt(a/b) © - store this value ažq;‚ - the array [0, pi/2] Δ - repeat until the array passed in does not change {Ð - sort then triplicate array O;Å¼ - arctan of the average of the array (bisection) ®‹ - is this greater than the sqrt(a/b)? 0 if so 1 if not èsO;‚ - Take the index from the current two guesses and combine with their average }н - Stop looping and get the first value - (this should be close enough to arctan(sqrt(a/b)) žqs/ - push pi then divide it by the atan(sqrt(a/b)) guess î< - ceil and decrement  # Python 3, 55 bytes lambda a,b:ceil(pi/atan((b/a)**.5)-1) from math import*  Try it online! # Rust, 59 bytes |a:f64,b:f64|(f64::acos(-1.)/(b/a).sqrt().atan()).ceil()-1.  Try it online! # AWK, 56 55 bytes {x=atan2(0,-1)/atan2(($2/$1)^.5,1);$0=int(x);$0-=$0~x}1


Try it online!

I know I can save a few bytes by hardcoding Pi to some arbitrary precision, but I'm not sure how accurate it is outside of the provided test cases.

# PHP, 46 bytes

<?=ceil(pi()/atan(($argv/$argv)**.5))-1;


Try it online!

Not very original I guess, applying the formula from the video..