Advent-of-Code/2020/src/main/kotlin/Day18.kt

package be.vandewalleh.aoc.days

import be.vandewalleh.aoc.utils.input.Day
import be.vandewalleh.aoc.utils.input.Lines
import java.util.*
import org.slf4j.Logger
import org.slf4j.LoggerFactory

private enum class Operator { Add, Multiply }

private operator fun Operator.invoke(a: Long, b: Long) = when (this) {
    Operator.Add -> a + b
    Operator.Multiply -> a * b
}

private inline fun Logger.debug(msg: () -> Any) {
    if (isDebugEnabled) debug(msg().toString())
}

@Day(18)
class Day18(@Lines val input: List<String>) {

    private val logger = LoggerFactory.getLogger("Day18")
    private val lines = input.map { it.replace(" ", "") }

    private fun parseGroups(line: String): Map<Int, List<IntRange>> {
        var depth = 0
        val groups = mutableMapOf<Int, MutableList<IntRange>>()
        val openingPars = Stack<Int>()

        for ((index, char) in line.withIndex()) {
            when (char) {
                '(' -> {
                    openingPars.push(index)
                    depth++
                }
                ')' -> {
                    depth--
                    val group = groups.getOrPut(depth) { mutableListOf() }
                    group.add(openingPars.pop()..index)
                }
            }
        }

        return groups.also { logger.debug { it } }
    }

    private fun solveGroup(group: String, precedence: Boolean) = solveGroup(parseGroup(group), precedence)

    private fun solveGroup(group: Pair<LinkedList<Long>, LinkedList<Operator>>, precedence: Boolean): Long {
        val (operands, operators) = group
        logger.debug { operands }
        logger.debug { operators }

        if (!precedence) {
            for (i in operators.indices) {
                val a = operands[i]
                val b = operands[i + 1]

                val result = operators[i](a, b)

                operands[i + 1] = result
            }

            return operands.peekLast()
        } else {

            var i = 0
            while (operators.any { it == Operator.Add }) {

                val operator = operators[i]

                if (operator == Operator.Add) {
                    val a = operands[i]
                    val b = operands[i + 1]
                    val res = operator(a, b)
                    operands.removeAt(i)
                    operands[i] = res
                    operators.removeAt(i)
                } else {
                    i++
                }
            }

            return operands.reduce { a, b -> Operator.Multiply(a, b) }
        }
    }

    private fun parseGroup(group: String): Pair<LinkedList<Long>, LinkedList<Operator>> {
        val operands = LinkedList<Long>()
        val operators = LinkedList<Operator>()
        val operand = StringBuilder()

        for (char in group) {
            if (char.isDigit()) {
                operand.append(char)
            } else {
                val operator = when (char) {
                    '+' -> Operator.Add
                    '*' -> Operator.Multiply
                    else -> error("Unknown operator $char")
                }
                operators.add(operator)
                operands.add(operand.toString().toLong())
                operand.clear()
            }
        }

        operands.add(operand.toString().toLong())

        return Pair(operands, operators)
    }

    private fun solveLine(line: String, precedence: Boolean = false): Long {
        var l = line

        while (true) {
            val groups = parseGroups(l)
            if (groups.isEmpty()) break

            val highestDepth = groups.keys.maxOrNull()!!
            val solvableRanges = groups[highestDepth]!!

            val solved = ArrayDeque<Pair<IntRange, Long>>()

            for (range in solvableRanges) {
                val rangeWithoutPars = range.first + 1 until range.last
                val answer = solveGroup(l.substring(rangeWithoutPars), precedence)
                solved.addLast(range to answer)
            }

            var solution = solved.removeFirst()
            var inside = false

            val builder = StringBuilder()

            for (i in l.indices) {
                if (!inside && i in solution.first) {
                    inside = true
                    builder.append(solution.second)
                } else if (inside && i !in solution.first) {
                    inside = false

                    solution = if (solved.isEmpty()) -1..-1 to -1  // avoid null checks..
                    else solved.removeFirst()

                    builder.append(l[i])
                } else if (i !in solution.first) {
                    builder.append(l[i])
                }
            }

            l = builder.toString()
        }

        return solveGroup(l, precedence)
    }

    fun part1() = lines
        .parallelStream()
        .map { solveLine(it) }
        .reduce { t, u -> t + u }
        .get()

    fun part2() = lines
        .parallelStream()
        .map { solveLine(it, precedence = true) }
        .reduce { t, u -> t + u }
        .get()
}