From 0f454c5d1fa55f7095e6f8dfd8920e3b855a1b69 Mon Sep 17 00:00:00 2001
From: Jeremy Kaplan <jdkaplan@metagram.net>
Date: Mon, 6 Dec 2021 21:42:15 -0800
Subject: [PATCH] 2021 Day 7

---
 2021/aoc/aoc.go   |   7 +++
 2021/day7/main.go | 114 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 121 insertions(+)
 create mode 100644 2021/day7/main.go

diff --git a/2021/aoc/aoc.go b/2021/aoc/aoc.go
index fc9e325..fec2efe 100644
--- a/2021/aoc/aoc.go
+++ b/2021/aoc/aoc.go
@@ -84,3 +84,10 @@ func Cut(s, sep string) (before, after string) {
 	}
 	panic("Separator was not found in string")
 }
+
+func Ints(text string) (ns []int) {
+	for _, n := range strings.Split(text, ",") {
+		ns = append(ns, MustInt(n))
+	}
+	return
+}
diff --git a/2021/day7/main.go b/2021/day7/main.go
new file mode 100644
index 0000000..bf81dd3
--- /dev/null
+++ b/2021/day7/main.go
@@ -0,0 +1,114 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/jdkaplan/advent-of-code/aoc"
+)
+
+func main() {
+	text := aoc.Input().ReadFile("day7.txt")
+	fmt.Println(part1(text))
+	fmt.Println(part2(text))
+}
+
+func part1(text string) int {
+	ps := aoc.Ints(text)
+	argmin := ps[0]
+	min := cost1(argmin, ps)
+	for _, p := range ps[1:] {
+		c := cost1(p, ps)
+		if c < min {
+			argmin, min = p, c
+		}
+	}
+	fmt.Println("Pos:", argmin)
+	return min
+}
+
+func part2(text string) int {
+	ps := aoc.Ints(text)
+	lo, hi := minmax(ps)
+	cost, pos := bisect(func(p int) int { return cost2(p, ps) }, lo, hi)
+	fmt.Println("Pos:", pos)
+	return cost
+}
+
+func cost1(x int, ns []int) (cost int) {
+	for _, n := range ns {
+		cost += abs(n - x)
+	}
+	return
+}
+
+func cost2(x int, ns []int) (cost int) {
+	for _, n := range ns {
+		d := abs(n - x)
+		cost += tri(d)
+	}
+	return
+}
+
+func abs(n int) int {
+	if n < 0 {
+		return -n
+	}
+	return n
+}
+
+func tri(n int) int {
+	return n * (n + 1) / 2
+}
+
+func bisect(f func(int) int, lo, hi int) (min int, argmin int) {
+	if lo == hi {
+		return f(lo), lo
+	}
+	if hi-lo == 1 {
+		flo, fhi := f(lo), f(hi)
+		if flo < fhi {
+			return flo, lo
+		}
+		return fhi, hi
+	}
+	md := (hi + lo) / 2
+	switch md {
+	case lo:
+		return f(lo), lo
+	case hi:
+		return f(hi), hi
+	default:
+		s := slope(f, md)
+		if s == 0 {
+			return f(md), md
+		}
+		if s < 0 {
+			return bisect(f, md, hi)
+		}
+		return bisect(f, lo, md)
+	}
+}
+
+func slope(f func(int) int, n int) int {
+	var (
+		lo  = n - 1
+		hi  = n + 1
+		flo = f(lo)
+		fhi = f(hi)
+	)
+	return fhi - flo
+}
+
+func minmax(ns []int) (min int, max int) {
+	min = ns[0]
+	max = ns[0]
+	for _, n := range ns[1:] {
+		if n < min {
+			min = n
+		}
+		if n > max {
+			max = n
+		}
+	}
+	return
+}