From f8a2e839b9cdde5442455fb72ef9f96971625c76 Mon Sep 17 00:00:00 2001
From: Kharec <sandro@cazzaniga.fr>
Date: Sat, 6 Dec 2025 23:55:34 +0100
Subject: [PATCH] feat: solve part one using maps and regex

---
 internal/2018/DayFour/code.go | 88 +++++++++++++++++++++++++++++++++++
 1 file changed, 88 insertions(+)
 create mode 100644 internal/2018/DayFour/code.go

diff --git a/internal/2018/DayFour/code.go b/internal/2018/DayFour/code.go
new file mode 100644
index 0000000..417fac2
--- /dev/null
+++ b/internal/2018/DayFour/code.go
@@ -0,0 +1,88 @@
+package dayfour
+
+import (
+	"os"
+	"regexp"
+	"slices"
+	"strconv"
+	"strings"
+	"time"
+
+	"advent-of-code/internal/registry"
+)
+
+var guardRegex = regexp.MustCompile(`Guard #(\d+) begins shift`)
+
+func init() {
+	registry.Register("2018D4", ParseInput, PartOne, PartTwo)
+}
+
+func ParseInput(filepath string) []string {
+	content, _ := os.ReadFile(filepath)
+	return strings.Split(string(content), "\n")
+}
+
+type record struct {
+	timestamp time.Time
+	action    string
+	guardID   int
+}
+
+func PartOne(data []string) int {
+	records := make([]record, 0, len(data))
+	timeLayout := "2006-01-02 15:04"
+
+	for _, line := range data {
+		timeStr, action, _ := strings.Cut(line[1:], "] ")
+		timestamp, _ := time.Parse(timeLayout, timeStr)
+		guardID := -1
+		if matches := guardRegex.FindStringSubmatch(action); matches != nil {
+			guardID, _ = strconv.Atoi(matches[1])
+		}
+		records = append(records, record{timestamp, action, guardID})
+	}
+
+	slices.SortFunc(records, func(a, b record) int {
+		return a.timestamp.Compare(b.timestamp)
+	})
+
+	guardSleepMinutes := make(map[int]int)
+	guardMinuteCount := make(map[int]map[int]int)
+	currentGuard := -1
+
+	for i := range records {
+		if records[i].guardID != -1 {
+			currentGuard = records[i].guardID
+			if guardMinuteCount[currentGuard] == nil {
+				guardMinuteCount[currentGuard] = make(map[int]int)
+			}
+		}
+		if records[i].action == "falls asleep" && i+1 < len(records) {
+			start, end := records[i].timestamp.Minute(), records[i+1].timestamp.Minute()
+			guardSleepMinutes[currentGuard] += end - start
+			for m := start; m < end; m++ {
+				guardMinuteCount[currentGuard][m]++
+			}
+		}
+	}
+
+	maxGuard, maxMinutes := -1, 0
+	for id, minutes := range guardSleepMinutes {
+		if minutes > maxMinutes {
+			maxGuard, maxMinutes = id, minutes
+		}
+	}
+
+	maxMinute, maxCount := -1, 0
+	for m, count := range guardMinuteCount[maxGuard] {
+		if count > maxCount {
+			maxMinute, maxCount = m, count
+		}
+	}
+
+	return maxGuard * maxMinute
+}
+
+func PartTwo(data []string) int {
+	return 0
+}