Advent of Code

          2018

--- Day 16: Chronal Classification ---

As you see the Elves defend their hot chocolate successfully, you go back to falling through time. This is going to become a problem.

If you're ever going to return to your own time, you need to understand how this device on your wrist works. You have a little while before you reach your next destination, and with a bit of trial and error, you manage to pull up a programming manual on the device's tiny screen.

According to the manual, the device has four registers (numbered 0 through 3) that can be manipulated by instructions containing one of 16 opcodes. The registers start with the value 0.

Every instruction consists of four values: an opcode, two inputs (named A and B), and an output (named C), in that order. The opcode specifies the behavior of the instruction and how the inputs are interpreted. The output, C, is always treated as a register.

In the opcode descriptions below, if something says "value A", it means to take the number given as A literally. (This is also called an "immediate" value.) If something says "register A", it means to use the number given as A to read from (or write to) the register with that number. So, if the opcode addi adds register A and value B, storing the result in register C, and the instruction addi 0 7 3 is encountered, it would add 7 to the value contained by register 0 and store the sum in register 3, never modifying registers 0, 1, or 2 in the process.

Many opcodes are similar except for how they interpret their arguments. The opcodes fall into seven general categories:

Addition:

Multiplication:

Bitwise AND:

Bitwise OR:

Assignment:

Greater-than testing:

Equality testing:

Unfortunately, while the manual gives the name of each opcode, it doesn't seem to indicate the number. However, you can monitor the CPU to see the contents of the registers before and after instructions are executed to try to work them out. Each opcode has a number from 0 through 15, but the manual doesn't say which is which. For example, suppose you capture the following sample:

Before: [3, 2, 1, 1]
9 2 1 2
After:  [3, 2, 2, 1]

This sample shows the effect of the instruction 9 2 1 2 on the registers. Before the instruction is executed, register 0 has value 3, register 1 has value 2, and registers 2 and 3 have value 1. After the instruction is executed, register 2's value becomes 2.

The instruction itself, 9 2 1 2, means that opcode 9 was executed with A=2, B=1, and C=2. Opcode 9 could be any of the 16 opcodes listed above, but only three of them behave in a way that would cause the result shown in the sample:

None of the other opcodes produce the result captured in the sample. Because of this, the sample above behaves like three opcodes.

You collect many of these samples (the first section of your puzzle input). The manual also includes a small test program (the second section of your puzzle input) - you can ignore it for now.

Ignoring the opcode numbers, how many samples in your puzzle input behave like three or more opcodes?

To play, please identify yourself via one of these services:

[GitHub] [Google] [Twitter] [Reddit] - [How Does Auth Work?]

(Twitter users: if you have auth problems that claim "There is no request token for this page", please clear your twitter.com cookies and try again.)