/
18.go
209 lines (190 loc) · 3.74 KB
/
18.go
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package main
import (
"bytes"
"slices"
"strconv"
"strings"
"golang.org/x/exp/maps"
)
func init() {
addSolutions(18, problem18)
}
func problem18(ctx *problemContext) {
var insts []digInstruction
scanner := ctx.scanner()
for scanner.scan() {
inst := parseDigInstruction(scanner.text())
insts = append(insts, inst)
}
ctx.reportLoad()
t := newTrench(insts)
t.flood()
ctx.reportPart1(t.countArea())
for i, inst := range insts {
insts[i] = inst.fix()
}
t = newTrench(insts)
t.flood()
ctx.reportPart2(t.countArea())
}
type digInstruction struct {
v vec2
n int64
color string
}
func (d digInstruction) fix() digInstruction {
var inst digInstruction
switch d.color[5] {
case '3':
inst.v = northv
case '0':
inst.v = eastv
case '1':
inst.v = southv
case '2':
inst.v = westv
default:
panic("bad")
}
n, err := strconv.ParseInt(d.color[:5], 16, 64)
if err != nil {
panic(err)
}
inst.n = n
return inst
}
func parseDigInstruction(s string) digInstruction {
var inst digInstruction
parts := strings.Fields(s)
switch parts[0][0] {
case 'U':
inst.v = northv
case 'R':
inst.v = eastv
case 'D':
inst.v = southv
case 'L':
inst.v = westv
default:
panic("bad")
}
inst.n = parseInt(parts[1])
inst.color = strings.TrimSuffix(strings.TrimPrefix(parts[2], "(#"), ")")
return inst
}
type trench struct {
// All vectors are indexes into gridx/gridy.
gridx []int64
gridy []int64
g grid[byte] // in grid coords
}
func newTrench(insts []digInstruction) *trench {
setx := map[int64]struct{}{0: {}}
sety := map[int64]struct{}{0: {}}
var v vec2
var vecs []vec2
for _, inst := range insts {
vecs = append(vecs, v)
v = v.add(inst.v.mul(inst.n))
setx[v.x] = struct{}{}
sety[v.y] = struct{}{}
}
if v != vecs[0] {
panic("not a loop")
}
xvals := maps.Keys(setx)
slices.Sort(xvals)
yvals := maps.Keys(sety)
slices.Sort(yvals)
var t trench
for i, x := range xvals {
t.gridx = append(t.gridx, x)
if i == len(xvals)-1 || xvals[i+1] > x+1 {
t.gridx = append(t.gridx, x+1)
}
}
for i, y := range yvals {
t.gridy = append(t.gridy, y)
if i == len(yvals)-1 || yvals[i+1] > y+1 {
t.gridy = append(t.gridy, y+1)
}
}
for i := 0; i < len(t.gridy); i++ {
t.g.addRow(bytes.Repeat([]byte("."), len(t.gridx)))
}
for i, inst := range insts {
v := vecs[i]
cur := t.mapToGrid(v)
if i < len(vecs)-1 {
v = vecs[i+1]
} else {
v = vecs[0]
}
next := t.mapToGrid(v)
for cur != next {
t.g.set(cur, '#')
cur = cur.add(inst.v)
}
}
return &t
}
func (t *trench) mapToGrid(v vec2) vec2 {
ix, _ := slices.BinarySearch(t.gridx, v.x)
iy, _ := slices.BinarySearch(t.gridy, v.y)
return vec2{int64(ix), int64(iy)}
}
func (t *trench) gridToMap(v vec2) (start, end vec2) {
start = vec2{t.gridx[v.x], t.gridy[v.y]}
end = vec2{t.gridx[v.x+1], t.gridy[v.y+1]}
return start, end
}
func (t *trench) flood() {
outside := make(map[vec2]struct{})
t.g.forEach(func(v vec2, c byte) bool {
if c != '.' {
return true
}
if _, ok := outside[v]; ok {
return true
}
region := make(map[vec2]struct{})
out := t.floodRec(v, region)
for v1 := range region {
if out {
outside[v1] = struct{}{}
} else {
t.g.set(v1, '#')
}
}
return true
})
}
func (t *trench) floodRec(v vec2, region map[vec2]struct{}) (outside bool) {
if !t.g.contains(v) {
return true
}
if t.g.at(v) != '.' {
return false
}
if _, ok := region[v]; ok {
return false
}
region[v] = struct{}{}
for _, n := range v.neighbors4() {
outside = outside || t.floodRec(n, region)
}
return outside
}
func (t *trench) countArea() int64 {
var n int64
t.g.forEach(func(v vec2, c byte) bool {
if c == '.' {
return true
}
start, end := t.gridToMap(v)
d := end.sub(start)
n += d.x * d.y
return true
})
return n
}