Checking in Day 17 even though I haven't figured out part 2 yet. I\'m coming back to it

day17/main.exs

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+defmodule Day17 do
+  def run do
+    [registers, program] =
+      File.read!("data.txt")
+      |> String.trim()
+      |> String.split("\n\n")
+      |> Enum.with_index()
+      |> Enum.map(fn {lines, idx} ->
+        if idx == 0, do: parseRegisters(lines), else: parseProgram(lines)
+      end)
+
+    # registers = %{A: 2024, B: 0, C: 0}
+    # program = [0, 1, 5, 4, 3, 0]
+
+    initial_state = %{
+      program: program,
+      registers: registers,
+      pointer: 0,
+      output: []
+    }
+
+    dbg(initial_state)
+
+    state = run(initial_state)
+    output = Enum.reverse(state.output)
+    dbg(output)
+
+    # part 2
+    # target is [2, 4, 1, 6, 7, 5, 4, 4, 1, 7, 0, 3, 5, 5, 3, 0]
+    # noticing every power of 8 it adds a digit
+    # the new big digit goes [1], 0, 3, 2, 4, 4, 1, 5
+    # when there's 1 digit, the biggest digit changes every 1
+    # when there are 2 digits, the biggest digit changes every 8
+    # when there are 3 digits, the biggest digit changes every 8 * 8
+    # etc.
+    # when there are 16 digits, the biggest digit changes every 8 ^ 15
+    # if the first digit is 2, that's the 3rd in the sequence, so 3x that
+    #
+    #    1 0 3 2 4 4 1 5
+    # when 2 digits, the smaller digit goes:
+    # 0: 1 0 3 2 6 5 1 1
+    # 3: 1 0 2 2 0 6 1 5
+    # 2: 1 0 2 2 2 7 1 1
+    # 4: 1 0 1 3 4 0 1 5
+    # 4: 1 0 1 3 6 1 1 1
+    # 1: 1 0 0 3 0 2 1 5
+    # 5: 1 0 0 3 2 3 1 1
+    #
+    # when 3 digits, smallest digit goes
+    # 0 0 7 0 4 4 1 5
+    #
+    # starter = 3 * 8 ** 15 + 6 * 8 ** 14 + 6 * 8 ** 13 # interesting, all ones
+    starter = 2 * 8 ** 15
+    increment = 8 ** 10
+
+    try(starter, initial_state, increment)
+  end
+
+  defp try(a, state, increment) do
+    # IO.gets("")
+    new_state = %{state | registers: %{state.registers | A: a}}
+    val = run(new_state)
+
+    # if Integer.mod(a, 1000) == 0 do
+    IO.puts(a)
+    dbg(a)
+    dbg(val)
+    # end
+
+    if Enum.reverse(val.output) == state.program do
+      a
+    else
+      try(a + increment, state, increment)
+    end
+  end
+
+  defp run(
+         %{
+           program: program,
+           registers: registers,
+           pointer: pointer,
+           output: output
+         } = state
+       ) do
+    opcode = Enum.at(program, pointer)
+    operand = Enum.at(program, pointer + 1)
+    # dbg({"run", state})
+
+    if is_nil(opcode) do
+      state
+    else
+      new_state =
+        case opcode do
+          # adv
+          0 ->
+            num = registers[:A]
+            den = Integer.pow(2, getComboOperandValue(operand, registers))
+            new_regs = %{registers | A: trunc(num / den)}
+
+            %{state | registers: new_regs, pointer: pointer + 2}
+
+          # bxl
+          1 ->
+            new_regs = %{registers | B: Bitwise.bxor(registers[:B], operand)}
+            %{state | registers: new_regs, pointer: pointer + 2}
+
+          # bst
+          2 ->
+            new_regs = %{
+              registers
+              | B: Integer.mod(getComboOperandValue(operand, registers), 8)
+            }
+
+            %{state | registers: new_regs, pointer: pointer + 2}
+
+          # jnz
+          3 ->
+            if registers[:A] == 0 do
+              %{state | pointer: pointer + 2}
+            else
+              %{state | pointer: operand}
+            end
+
+          # bxc
+          4 ->
+            new_regs = %{registers | B: Bitwise.bxor(registers[:B], registers[:C])}
+            %{state | registers: new_regs, pointer: pointer + 2}
+
+          # out
+          5 ->
+            val = Integer.mod(getComboOperandValue(operand, registers), 8)
+            %{state | pointer: pointer + 2, output: [val | output]}
+
+          # bdv
+          6 ->
+            num = registers[:A]
+            den = Integer.pow(2, getComboOperandValue(operand, registers))
+            new_regs = %{registers | B: trunc(num / den)}
+
+            %{state | registers: new_regs, pointer: pointer + 2}
+
+          # cdv
+          7 ->
+            num = registers[:A]
+            den = Integer.pow(2, getComboOperandValue(operand, registers))
+            new_regs = %{registers | C: trunc(num / den)}
+
+            %{state | registers: new_regs, pointer: pointer + 2}
+        end
+
+      run(new_state)
+    end
+  end
+
+  defp parseProgram(line) do
+    [_header, digits] = String.split(line, " ")
+    digits |> String.split(",") |> Enum.map(&String.to_integer/1)
+  end
+
+  defp getComboOperandValue(op, registers) do
+    case op do
+      o when o <= 3 -> op
+      4 -> registers[:A]
+      5 -> registers[:B]
+      6 -> registers[:C]
+    end
+  end
+
+  defp parseRegisters(lines) do
+    lines
+    |> String.split("\n")
+    |> Enum.map(&String.split(&1, " "))
+    |> Enum.reduce(%{}, fn [_header, label, value], acc ->
+      atom = String.trim(label, ":") |> String.to_atom()
+      Map.put(acc, atom, String.to_integer(value))
+    end)
+  end
+end
+
+Day17.run()