Day 8: Part 1 and 2

Signed-off-by: James Griffin <james@unsupervised.ca>

eight/day_eight.go

@@ -0,0 +1,129 @@
+package eight
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+)
+
+type program struct {
+	instructions []instruction
+	accumulator  int
+	preempted    bool
+}
+
+type instruction struct {
+	operation string
+	argument  int
+	executed  bool
+}
+
+func (p *program) load(filename string) error {
+	file, err := os.Open(filename)
+	if err != nil {
+		return err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		var op string
+		var value int
+
+		count, err := fmt.Sscanf(scanner.Text(), "%s %d", &op, &value)
+		if count != 2 || err != nil {
+			return fmt.Errorf("Unable to parse %s: %w", scanner.Text(), err)
+		}
+
+		p.instructions = append(p.instructions, instruction{
+			operation: op,
+			argument:  value,
+			executed:  false,
+		})
+	}
+
+	return nil
+}
+
+func (p *program) exec() int {
+	i := 0
+	for {
+		if i >= len(p.instructions) {
+			p.preempted = false
+			return p.accumulator
+		} else if i < 0 {
+			return p.accumulator
+		}
+
+		if p.instructions[i].executed {
+			break
+		}
+
+		p.instructions[i].executed = true
+		switch p.instructions[i].operation {
+		case "acc":
+			p.accumulator += p.instructions[i].argument
+			i++
+		case "jmp":
+			i += p.instructions[i].argument
+		default:
+			i++
+		}
+	}
+
+	return p.accumulator
+}
+
+func (p *program) reset() {
+	p.accumulator = 0
+	p.preempted = true
+	for i := 0; i < len(p.instructions); i++ {
+		p.instructions[i].executed = false
+	}
+}
+
+func (p *program) flipJmpNop(index int) {
+	if index < 0 || index > len(p.instructions) {
+		return
+	}
+
+	switch p.instructions[index].operation {
+	case "nop":
+		p.instructions[index].operation = "jmp"
+	case "jmp":
+		p.instructions[index].operation = "nop"
+	}
+}
+
+func (p *program) heal() int {
+	for i := 0; i < len(p.instructions); i++ {
+		p.reset()
+		p.flipJmpNop(i)
+		p.exec()
+
+		if !p.preempted {
+			return i
+		}
+		p.flipJmpNop(i)
+	}
+
+	return -1
+}
+
+// PartOne Find the value of the accumulator before any instruction is executed twice
+func PartOne() string {
+	p := program{}
+	p.load("eight/handheld.code")
+
+	return fmt.Sprintf("The accumulator is at %d before the loop begins", p.exec())
+}
+
+// PartTwo Find the corrupt instruction, flip it, and execute to termination
+func PartTwo() string {
+	p := program{}
+	p.load("eight/handheld.code")
+	p.exec()
+	index := p.heal()
+
+	return fmt.Sprintf("The accumulator is at %d after termination by correcting instruction %d", p.accumulator, index+1)
+}