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+---
+title: "I Heard You Like Registers — Python — #adventofcode Day 8"
+description: "In which I implement a very simple assembly-type language."
+slug: day-08
+date: 2017-12-10T19:33:46+00:00
+tags:
+- Technology
+- Learning
+- Advent of Code
+- Python
+series: aoc2017
+---
+
+[Today's challenge](http://adventofcode.com/2017/day/8) describes a simple instruction set for a CPU, incrementing and decrementing values in registers according to simple conditions. We have to interpret a stream of these instructions, and to prove that we've done so, give the highest value of any register, both at the end of the program and throughout the whole program.
+
+[→ Full code on GitHub](https://github.com/jezcope/aoc2017/blob/master/08-i-heard-you-like-registers.py)
+
+!!! commentary
+    This turned out to be a nice straightforward one to implement, as the instruction format was easily parsed by regular expression, and Python provides the `eval` function which made evaluating the conditions a doddle.
+
+Import various standard library bits that we'll use later.
+
+```python
+import re
+import fileinput as fi
+from math import inf
+from collections import defaultdict
+```
+
+We could just parse the instructions by splitting the string, but using a regular expression is a little bit more robust because it won't match at all if given an invalid instruction.
+
+```python
+INSTRUCTION_RE = re.compile(r'(\w+) (inc|dec) (-?\d+) if (.+)\s*')
+
+def parse_instruction(instruction):
+    match = INSTRUCTION_RE.match(instruction)
+    return match.group(1, 2, 3, 4)
+```
+
+Executing an instruction simply checks the condition and if it evaluates to `True` updates the relevant register.
+
+```python
+def exec_instruction(registers, instruction):
+    name, op, value, cond = instruction
+
+    value = int(value)
+    if op == 'dec':
+        value = -value
+
+    if eval(cond, globals(), registers):
+        registers[name] += value
+```
+
+`highest_value` returns the maximum value found in any register.
+
+```python
+def highest_value(registers):
+    return sorted(registers.items(), key=lambda x: x[1], reverse=True)[0][1]
+```
+
+Finally, loop through all the instructions and carry them out, updating `global_max` as we go. We need to be able to deal with registers that haven't been accessed before. Keeping the registers in a dictionary means that we can evaluate the conditions directly using `eval` above, passing it as the `locals` argument. The standard `dict` will raise an exception if we try to access a key that doesn't exist, so instead we use `collections.defaultdict`, which allows us to specify what the default value for a non-existent key will be. New registers start at 0, so we use a simple lambda to define a function that always returns 0.
+
+```python
+global_max = -inf
+registers = defaultdict(lambda: 0)
+for i in map(parse_instruction, fi.input()):
+    exec_instruction(registers, i)
+    global_max = max(global_max, highest_value(registers))
+
+print('Max value:', highest_value(registers))
+print('All-time max:', global_max)
+```
+
+