From e0a9c21bdeed5b91de67fe294c0b1055ef3f33b6 Mon Sep 17 00:00:00 2001
From: Nick Krichevsky <nick@ollien.com>
Date: Sun, 3 Dec 2023 10:49:48 -0500
Subject: [PATCH] Add day 3 part 1

---
 day3/main.go | 146 +++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 146 insertions(+)
 create mode 100644 day3/main.go

diff --git a/day3/main.go b/day3/main.go
new file mode 100644
index 0000000..9827a8c
--- /dev/null
+++ b/day3/main.go
@@ -0,0 +1,146 @@
+package main
+
+import (
+	"fmt"
+	"io"
+	"os"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+type Coordinate struct {
+	row int
+	col int
+}
+
+func main() {
+	if len(os.Args) != 2 {
+		fmt.Fprintf(os.Stderr, "Usage: %s inputfile\n", os.Args[0])
+		os.Exit(1)
+	}
+
+	filename := os.Args[1]
+	inputFile, err := os.Open(filename)
+	if err != nil {
+		panic(fmt.Sprintf("failed to open input file: %s", err))
+	}
+
+	inputBytes, err := io.ReadAll(inputFile)
+	if err != nil {
+		panic(fmt.Sprintf("failed to read input file: %s", err))
+	}
+
+	input := string(inputBytes)
+	inputLines := strings.Split(strings.TrimSpace(input), "\n")
+
+	fmt.Printf("Part 1: %d\n", part1(inputLines))
+}
+
+func part1(inputLines []string) int {
+	symbolPositions := findSymbols(inputLines)
+	partNumberCandidates := []Coordinate{}
+	for _, symbolPos := range symbolPositions {
+		candidates := findNumbersAdjacentTo(inputLines, symbolPos)
+		partNumberCandidates = append(partNumberCandidates, candidates...)
+	}
+
+	total := 0
+	scanned := map[Coordinate]struct{}{}
+	for _, candidate := range partNumberCandidates {
+		if _, ok := scanned[candidate]; ok {
+			continue
+		}
+
+		line := inputLines[candidate.row]
+
+		backwardsBuf := strings.Builder{}
+		// Scan backward
+		for i := candidate.col - 1; i >= 0; i-- {
+			char := line[i]
+			if !unicode.IsDigit(rune(char)) {
+				break
+			}
+
+			scanned[Coordinate{row: candidate.row, col: i}] = struct{}{}
+			backwardsBuf.WriteByte(char)
+		}
+
+		forwardsBuf := strings.Builder{}
+		// Scan forward
+		for i := candidate.col; i < len(line); i++ {
+			char := line[i]
+			if !unicode.IsDigit(rune(char)) {
+				break
+			}
+
+			scanned[Coordinate{row: candidate.row, col: i}] = struct{}{}
+			forwardsBuf.WriteByte(char)
+		}
+
+		fullNumber := reverseString(backwardsBuf.String()) + forwardsBuf.String()
+		scannedNumber, err := strconv.Atoi(fullNumber)
+		if err != nil {
+			panic(fmt.Sprintf("%s was not a number", fullNumber))
+		}
+
+		total += scannedNumber
+	}
+
+	return total
+}
+
+func findSymbols(inputLines []string) []Coordinate {
+	coords := []Coordinate{}
+	for row, line := range inputLines {
+		for col, char := range line {
+			if isSymbol(char) {
+				coord := Coordinate{
+					row: row,
+					col: col,
+				}
+
+				coords = append(coords, coord)
+			}
+		}
+	}
+
+	return coords
+}
+
+func findNumbersAdjacentTo(inputLines []string, coordinate Coordinate) []Coordinate {
+	coords := []Coordinate{}
+	for dRow := -1; dRow <= 1; dRow++ {
+		for dCol := -1; dCol <= 1; dCol++ {
+			if dRow == 0 && dCol == 0 {
+				continue
+			}
+
+			row := coordinate.row + dRow
+			col := coordinate.col + dCol
+			if unicode.IsDigit(rune(inputLines[row][col])) {
+				coord := Coordinate{
+					row: row,
+					col: col,
+				}
+
+				coords = append(coords, coord)
+			}
+		}
+	}
+
+	return coords
+}
+
+func isSymbol(r rune) bool {
+	return r != '.' && !unicode.IsDigit(r)
+}
+
+func reverseString(s string) string {
+	buffer := strings.Builder{}
+	for i := len(s) - 1; i >= 0; i-- {
+		buffer.WriteByte(s[i])
+	}
+
+	return buffer.String()
+}