From c5fcc8b353fcd019ca89536e54aeba87e38323d1 Mon Sep 17 00:00:00 2001
From: Kharec <sandro@cazzaniga.fr>
Date: Sat, 6 Dec 2025 23:34:48 +0100
Subject: [PATCH] feat: solve part one

---
 internal/2021/DayFour/code.go | 134 ++++++++++++++++++++++++++++++++++
 1 file changed, 134 insertions(+)
 create mode 100644 internal/2021/DayFour/code.go

diff --git a/internal/2021/DayFour/code.go b/internal/2021/DayFour/code.go
new file mode 100644
index 0000000..cdb471a
--- /dev/null
+++ b/internal/2021/DayFour/code.go
@@ -0,0 +1,134 @@
+package dayfour
+
+import (
+	"os"
+	"strconv"
+	"strings"
+
+	"advent-of-code/internal/registry"
+)
+
+const boardSize = 5
+
+func init() {
+	registry.Register("2021D4", ParseInput, PartOne, PartTwo)
+}
+
+func ParseInput(filepath string) string {
+	content, _ := os.ReadFile(filepath)
+	return string(content)
+}
+
+type board struct {
+	numbers      [boardSize][boardSize]int
+	positionMap  map[int][2]int
+	rowCounts    [boardSize]int
+	columnCounts [boardSize]int
+	marked       map[int]bool
+	won          bool
+}
+
+func newBoard() board {
+	return board{
+		positionMap: make(map[int][2]int, boardSize*boardSize),
+		marked:      make(map[int]bool),
+	}
+}
+
+func parseBoards(lines []string) []board {
+	var boards []board
+	current := newBoard()
+	row := 0
+
+	for _, line := range lines {
+		line = strings.TrimSpace(line)
+		if line == "" {
+			if row == boardSize {
+				boards = append(boards, current)
+				current = newBoard()
+				row = 0
+			}
+			continue
+		}
+
+		fields := strings.Fields(line)
+		for column, field := range fields {
+			number, _ := strconv.Atoi(field)
+			current.numbers[row][column] = number
+			current.positionMap[number] = [2]int{row, column}
+		}
+		row++
+	}
+
+	if row == boardSize {
+		boards = append(boards, current)
+	}
+
+	return boards
+}
+
+func (b *board) mark(number int) bool {
+	if b.won {
+		return false
+	}
+
+	position, exists := b.positionMap[number]
+	if !exists {
+		return false
+	}
+
+	b.marked[number] = true
+	row, column := position[0], position[1]
+	b.rowCounts[row]++
+	b.columnCounts[column]++
+
+	if b.rowCounts[row] == boardSize || b.columnCounts[column] == boardSize {
+		b.won = true
+		return true
+	}
+
+	return false
+}
+
+func (b *board) sumUnmarked() int {
+	sum := 0
+	for row := range boardSize {
+		for column := range boardSize {
+			number := b.numbers[row][column]
+			if !b.marked[number] {
+				sum += number
+			}
+		}
+	}
+	return sum
+}
+
+func parseNumbers(line string) []int {
+	numbersStr := strings.Split(line, ",")
+	numbers := make([]int, 0, len(numbersStr))
+	for _, numStr := range numbersStr {
+		num, _ := strconv.Atoi(numStr)
+		numbers = append(numbers, num)
+	}
+	return numbers
+}
+
+func PartOne(input string) int {
+	lines := strings.Split(strings.TrimSpace(input), "\n")
+	numbers := parseNumbers(lines[0])
+	boards := parseBoards(lines[1:])
+
+	for _, number := range numbers {
+		for idx := range boards {
+			if boards[idx].mark(number) {
+				return boards[idx].sumUnmarked() * number
+			}
+		}
+	}
+
+	return 0
+}
+
+func PartTwo(input string) int {
+	return 0
+}