From 7af3baa2e0afd06f5edb8d9646228b8466de0dc5 Mon Sep 17 00:00:00 2001
From: Rokas Puzonas <rokas.puz@gmail.com>
Date: Sun, 2 Jul 2023 21:03:22 +0300
Subject: [PATCH] solve day 18

---
 day18.c | 153 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 main.c  |   1 +
 2 files changed, 154 insertions(+)
 create mode 100644 day18.c

diff --git a/day18.c b/day18.c
new file mode 100644
index 0000000..9b4612c
--- /dev/null
+++ b/day18.c
@@ -0,0 +1,153 @@
+#include <string.h>
+#include <assert.h>
+#include <stdio.h>
+#include <sys/param.h>
+
+#include "types.h"
+#include "aoc.h"
+
+#define DAY18_WORLD_SIZE 32768 // 2^15
+
+i8 g_day18_ox[] = { 1, -1, 0,  0, 0,  0 };
+i8 g_day18_oy[] = { 0,  0, 1, -1, 0,  0 };
+i8 g_day18_oz[] = { 0,  0, 0,  0, 1, -1 };
+static_assert(ARRAY_LEN(g_day18_ox) == ARRAY_LEN(g_day18_oy) && ARRAY_LEN(g_day18_oy) == ARRAY_LEN(g_day18_oz), "Number of elements between ox, oy, oz don't match");
+
+typedef struct {
+	u8 x, y, z;
+} day18_droplet;
+
+typedef struct {
+	day18_droplet *droplets;
+	size_t count;
+} day18_data;
+
+static void* day18_parse(char** lines, int line_count)
+{
+	day18_data *data = malloc(sizeof(day18_data));
+	data->count = line_count;
+	data->droplets = malloc(sizeof(day18_droplet)*line_count);
+
+	for (int i = 0; i < line_count; i++) {
+		char *comma1 = strchr(lines[i], ',');
+		char *comma2 = strchr(comma1+1, ',');
+
+		day18_droplet *droplet = &data->droplets[i];
+		droplet->x = strtol(lines[i], NULL, 10);
+		droplet->y = strtol(comma1+1, NULL, 10);
+		droplet->z = strtol(comma2+1, NULL, 10);
+	}
+
+	return data;
+}
+
+// Assumes that x, y, z each are at most 5 bits
+static u16 day18_get_key(u8 x, u8 y, u8 z)
+{
+	return (z & 0b11111) << 10 | (y & 0b11111) << 5 | (x & 0b11111) << 0;
+}
+
+static bool day18_is_droplet(bool world[], i8 x, i8 y, i8 z)
+{
+	if (x < 0 || y < 0 || z < 0) return false;
+	if (x > 32 || y > 32 || z > 32) return false;
+
+	u16 key = day18_get_key(x, y, z);
+	return world[key];
+}
+
+static void day18_place_droplets(bool world[], day18_droplet *droplets, size_t count, u16 ox, u16 oy, u16 oz)
+{
+	// Ensure that all droplet coordinates don't go over 5 bits
+	for (int i = 0; i < count; i++) {
+		day18_droplet *droplet = &droplets[i];
+		assert(droplet->x + ox < 32);
+		assert(droplet->y + oy < 32);
+		assert(droplet->z + oz < 32);
+	}
+
+	for (int i = 0; i < count; i++) {
+		day18_droplet *droplet = &droplets[i];
+		u16 key = day18_get_key(droplet->x + ox, droplet->y + oy, droplet->z + oz);
+		world[key] = true;
+	}
+}
+
+static void day18_part1(void *p)
+{
+	day18_data *data = (day18_data*)p;
+
+	bool world[DAY18_WORLD_SIZE] = { false };
+	day18_place_droplets(world, data->droplets, data->count, 0, 0, 0);
+
+
+	u32 area = 0;
+	for (int i = 0; i < data->count; i++) {
+		day18_droplet *droplet = &data->droplets[i];
+
+		for (int j = 0; j < ARRAY_LEN(g_day18_ox); j++) {
+			i8 x = droplet->x + g_day18_ox[j];
+			i8 y = droplet->y + g_day18_oy[j];
+			i8 z = droplet->z + g_day18_oz[j];
+			area += !day18_is_droplet(world, x, y, z);
+		}
+	}
+	printf("%d\n", area);
+}
+
+static void day18_part2(void *p)
+{
+	day18_data *data = (day18_data*)p;
+
+	bool world[DAY18_WORLD_SIZE] = { false };
+	day18_place_droplets(world, data->droplets, data->count, 1, 1, 1);
+
+	u32 area = 0;
+
+	bool visited[DAY18_WORLD_SIZE] = { false };
+	u32 stack_capacity = DAY18_WORLD_SIZE;
+	u8 stack_x[stack_capacity];
+	u8 stack_y[stack_capacity];
+	u8 stack_z[stack_capacity];
+	u32 stack_size = 0;
+
+	stack_x[0] = 29;
+	stack_y[0] = 29;
+	stack_z[0] = 29;
+	visited[day18_get_key(31, 31, 31)] = true;
+	stack_size = 1;
+
+	while (stack_size > 0) {
+		u8 x = stack_x[stack_size-1];
+		u8 y = stack_y[stack_size-1];
+		u8 z = stack_z[stack_size-1];
+		stack_size--;
+
+		for (int j = 0; j < ARRAY_LEN(g_day18_ox); j++) {
+			i8 new_x = x + g_day18_ox[j];
+			i8 new_y = y + g_day18_oy[j];
+			i8 new_z = z + g_day18_oz[j];
+			if (new_x < 0 || new_y < 0 || new_z < 0) continue;
+			if (new_x > 32 || new_y > 32 || new_z > 32) continue;
+
+			u16 key = day18_get_key(new_x, new_y, new_z);
+
+			if (day18_is_droplet(world, new_x, new_y, new_z)) {
+				area++;
+			} else {
+				if (visited[key]) continue;
+				visited[key] = true;
+
+				assert(stack_size < stack_capacity);
+				stack_x[stack_size] = new_x;
+				stack_y[stack_size] = new_y;
+				stack_z[stack_size] = new_z;
+				stack_size++;
+			}
+		}
+	}
+
+	printf("%d\n", area);
+}
+
+ADD_SOLUTION(18, day18_parse, day18_part1, day18_part2);
diff --git a/main.c b/main.c
index 740fe7e..0974339 100644
--- a/main.c
+++ b/main.c
@@ -26,6 +26,7 @@
 #include "day15.c"
 #include "day16.c"
 #include "day17.c"
+#include "day18.c"
 
 Solution *find_solution(int day)
 {