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day8.go
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day8.go
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package day8
import (
"errors"
"fmt"
"github.com/chigley/advent2020"
"github.com/chigley/advent2020/machine"
)
type outcome int
const (
infiniteLoop outcome = iota
halt
)
func Part1(in []string) (int, error) {
program, err := machine.ParseProgram(in)
if err != nil {
return 0, fmt.Errorf("day8: parsing program: %w", err)
}
m := machine.New(program)
outcome, err := runUntilOutcome(m)
if err != nil {
return 0, fmt.Errorf("day8: running machine: %w", err)
}
if outcome != infiniteLoop {
return 0, errors.New("day8: expected infinite loop")
}
return m.Acc, nil
}
func Part2(in []string) (int, error) {
program, err := machine.ParseProgram(in)
if err != nil {
return 0, fmt.Errorf("day8: parsing program: %w", err)
}
for i, instr := range program {
if instr.Opcode != machine.OpJmp && instr.Opcode != machine.OpNop {
continue
}
clonedProgram := machine.CloneProgram(program)
if instr.Opcode == machine.OpJmp {
clonedProgram[i].Opcode = machine.OpNop
} else {
// instr.Opcode == machine.OpNop
clonedProgram[i].Opcode = machine.OpJmp
}
m := machine.New(clonedProgram)
outcome, err := runUntilOutcome(m)
if err != nil {
return 0, fmt.Errorf("day8: running machine: %w", err)
}
if outcome == halt {
return m.Acc, nil
}
}
return 0, advent2020.ErrNoResult
}
func runUntilOutcome(m *machine.Machine) (outcome, error) {
seen := make(map[int]struct{})
for {
if _, ok := seen[m.PC]; ok {
return infiniteLoop, nil
}
seen[m.PC] = struct{}{}
if err := m.Step(); errors.Is(err, machine.ErrHalt) {
return halt, nil
} else if err != nil {
return 0, fmt.Errorf("day8: stepping machine: %w", err)
}
}
}