day-eight.js

// Lessons learned - use Clojure
const fs = require('fs')

function parseInstruction(instruction) {
    const regex = /([a-z]+) ([a-z]+) ([-]*[0-9]+) if ([a-z]+) ([><=!]+) ([-]*[0-9]+)/g
    return regex.exec(instruction)
}

function parseInput(filePath) {
    const input = fs.readFileSync(filePath, 'utf8').trim().split("\n");
    const parsedInput = input.map(function(instruction) {
        const parsedInstruction = parseInstruction(instruction);
        return {"register": parsedInstruction[1],
                "action": parsedInstruction[2],
                "amount": parseInt(parsedInstruction[3]),
                "condition-op": parsedInstruction[5],
                "condition-operand-1": parsedInstruction[4],
                "condition-operand-2": parseInt(parsedInstruction[6])}
    })
    return parsedInput;
}

function partOne(input) {
    let registers = {}

    const registerNames = new Set(input.map(function(data) {
        return data["register"]
    }))

    for (let registerName of registerNames) {
        registers[registerName] = 0
    }

    const processedInput = input.reduce(function(regs, instruction) {
        const conditionOperator = instruction["condition-op"]
        let evaluatedCondition;
        if (conditionOperator === ">") {
            evaluatedCondition = regs[instruction["condition-operand-1"]] > instruction["condition-operand-2"]
        } else if (conditionOperator === "<") {
            evaluatedCondition = regs[instruction["condition-operand-1"]] < instruction["condition-operand-2"]
        } else if (conditionOperator === "==") {
            evaluatedCondition = regs[instruction["condition-operand-1"]] == instruction["condition-operand-2"]
        } else if (conditionOperator == "!=") {
            evaluatedCondition = regs[instruction["condition-operand-1"]] != instruction["condition-operand-2"]
        } else if (conditionOperator == "<=") {
            evaluatedCondition = regs[instruction["condition-operand-1"]] <= instruction["condition-operand-2"]
        } else {
            evaluatedCondition = regs[instruction["condition-operand-1"]] >= instruction["condition-operand-2"]
        }

        if (evaluatedCondition) {
            if (instruction["action"] == "inc") {
                regs[instruction["register"]] = regs[instruction["register"]] + instruction["amount"]
            } else {
                regs[instruction["register"]] = regs[instruction["register"]] - instruction["amount"]
            }
        }
        return regs
    }, registers)

    return Object.values(processedInput).reduce(function(a, b) {
        return Math.max(a, b);
    });
}

console.log(partOne(parseInput("./input.txt")))