package main

import (
	"errors"
	"fmt"
	"math"
	"slices"
	"strconv"
	"strings"
	"time"

	h "git.bullercodeworks.com/brian/adventofcode/helpers"
)

func main() {
	inp := h.StdinToStringSlice()
	part1(inp)
	fmt.Println()
	part2(inp)
}

func part1(inp []string) {
	vm := InitVM(inp)
	fmt.Println("# Part 1")
	fmt.Println(run(vm.RegA, vm.RegB, vm.RegC, vm.Prog))
}

func part2(inp []string) {
	fmt.Println("# Part 2")

	vm := InitVM(inp)
	a, b, c, prog := 0, vm.RegB, vm.RegC, vm.Prog
	for pos := len(prog) - 1; pos >= 0; pos-- {
		a <<= 3
		for !slices.Equal(run(a, b, c, prog), prog[pos:]) {
			a++
		}
	}

	fmt.Println("Corrected A:", a)
}

func run(a, b, c int, program []int) []int {
	out := make([]int, 0)
	for ptr := 0; ptr < len(program); ptr += 2 {
		oc, oa := program[ptr], program[ptr+1]
		value := oa
		switch oa {
		case 4:
			value = a
		case 5:
			value = b
		case 6:
			value = c
		}
		switch oc {
		case 0:
			a >>= value
		case 1:
			b ^= oa
		case 2:
			b = value % 8
		case 3:
			if a != 0 {
				ptr = oa - 2
			}
		case 4:
			b ^= c
		case 5:
			out = append(out, value%8)
		case 6:
			b = a >> value
		case 7:
			c = a >> value
		}
	}
	return out
}

type VM struct {
	RegA, RegB, RegC int
	Prog             []int
	ProgStr          string
	ptr              int
	Running          bool

	StdOut    []int
	StdOutStr string
	Debug     bool

	ProcHistory map[string]bool
	Err         error
}

func InitVM(inp []string) *VM {
	vm := &VM{
		ProcHistory: make(map[string]bool),
	}
	fmt.Sscanf(inp[0], "Register A: %d", &vm.RegA)
	fmt.Sscanf(inp[1], "Register B: %d", &vm.RegB)
	fmt.Sscanf(inp[2], "Register C: %d", &vm.RegC)
	vm.ProgStr = inp[4][9:]
	prog := strings.Split(vm.ProgStr, ",")
	for i := range prog {
		p, _ := strconv.Atoi(prog[i])
		vm.Prog = append(vm.Prog, p)
	}
	return vm
}

func (vm *VM) DetectLoop() {
	if _, ok := vm.ProcHistory[vm.String()]; ok {
		vm.Err = errors.New("loop detected")
		vm.Running = false
	}
}

func (vm *VM) Run() {
	do := func(op, operand int) {
		vm.PrintDebug(op, operand)
		switch op {
		case 0: // adv (divide A by 2^combo -> int -> A)
			vm.RegA = int(float64(vm.RegA) / math.Pow(2, float64(vm.GetVal(operand))))
		case 1: // bxl (bitwise XOR of B and literal operand -> B)
			vm.RegB = int(vm.RegB ^ operand)
		case 2: // bst (combo % 8 -> B)
			vm.RegB = int(vm.GetVal(operand) % 8)
		case 3: // jnz (if A is = 0 do nothing, else jump ptr to literal operand)
			if vm.RegA != 0 {
				vm.ptr = operand
			}
		case 4: // bxc (bitwise XOR of B and C -> B) (reads operand but ignore it)
			vm.RegB = vm.RegB ^ vm.RegC
		case 5: // out (calc value of combo operand % 8, output that value)
			v := vm.GetVal(operand) % 8
			vm.StdOut = append(vm.StdOut, v)
			if vm.StdOutStr == "" {
				vm.StdOutStr = fmt.Sprintf("%d", v)
			} else {
				vm.StdOutStr = fmt.Sprintf("%s,%d", vm.StdOut, v)
			}
		case 6: // bdv (exactly like 0, but store into B)
			vm.RegB = int(float64(vm.RegA) / math.Pow(2, float64(vm.GetVal(operand))))
		case 7: // cdv (exactly like 0, but store into C)
			vm.RegC = int(float64(vm.RegA) / math.Pow(2, float64(vm.GetVal(operand))))
		}
	}
	vm.Running = true
	for vm.Running {
		if vm.ptr >= int(len(vm.Prog)) {
			break
		}
		if vm.Debug {
			fmt.Printf("Do: %d %d\n", vm.Prog[vm.ptr], vm.GetVal(vm.Prog[vm.ptr+1]))
		}
		vm.DetectLoop()
		pPtr := vm.ptr
		do(vm.Prog[vm.ptr], vm.Prog[vm.ptr+1])
		if vm.ptr == pPtr {
			vm.ptr = vm.ptr + 2
		}
		if vm.Debug {
			fmt.Println()
			fmt.Println(vm)
			time.Sleep(time.Second)
		}
	}
}

func (vm *VM) GetVal(cmb int) int {
	switch cmb {
	case 4:
		return vm.RegA
	case 5:
		return vm.RegB
	case 6:
		return vm.RegC
	}
	return cmb
}

func (vm *VM) PrintDebug(op, b int) {
	if !vm.Debug {
		return
	}
	switch op {
	case 0:
		fmt.Printf("adv (0): %d / int(math.Pow(2, %d)) (A)\n", vm.RegA, vm.GetVal(b))
	case 1:
		fmt.Printf("bxl (1): %d ^ %d = %d (B)\n", vm.RegB, vm.GetVal(b), (vm.RegB ^ b))
	case 2:
		fmt.Printf("bst (2): %d %% 8 = %d (B)\n", vm.GetVal(b), vm.GetVal(b)%8)
	case 3:
		fmt.Printf("jnz (3) if %d != 0; ptr = %d\n", vm.RegA, b)
	case 4:
		fmt.Printf("bxc (4): %d ^ %d = %d (B)\n", vm.RegB, vm.RegC, (vm.RegB ^ vm.RegC))
	case 5:
		fmt.Printf("out (5): Output %d\n", (vm.GetVal(b) % 8))
	case 6:
		fmt.Printf("bdv (6): %d / int(math.Pow(2, %d)) (B)\n", vm.RegA, vm.GetVal(b))
	case 7:
		fmt.Printf("cdv (7): %d / int(math.Pow(2, %d)) (C)\n", vm.RegA, vm.GetVal(b))
	}
}

func (vm VM) String() string {
	var ret string
	ret = fmt.Sprintf("Register A: %d", vm.RegA)
	ret = fmt.Sprintf("%s\nRegister B: %d", ret, vm.RegB)
	ret = fmt.Sprintf("%s\nRegister C: %d", ret, vm.RegC)
	ret = fmt.Sprintf("%s\n\nProgram: ", ret)
	for i := range vm.Prog {
		if vm.ptr == int(i) {
			ret = fmt.Sprintf("%s[%d],", ret, vm.Prog[i])
		} else {
			ret = fmt.Sprintf("%s%d,", ret, vm.Prog[i])
		}
	}
	return ret[:len(ret)-1]
}