day 10

day-10/input.txt

@@ -0,0 +1,45 @@
+676781023010121078756541010565410126589652103
+787692014523134569687238921076321087676543012
+896543210674013278798107831089980896567122107
+654100134985329143237356540123678925498033498
+783210325676438050123445443294541012321044589
+694309018984567267034576356789032008769655678
+345678567823432178125689219878102109678724369
+456969430214563089068701008765210234569013212
+327854321005478873879010123674377654354321001
+218901232876569912968123294589988912210132432
+107650343985567803451054387487676903432101561
+210545674783498712589965236596567876543650170
+323432185692105605678876145645430967858743289
+214981092185434104987986001034321458969801001
+105670123076393213098887632125010321578932102
+789889874101284332106798540136521230432840013
+876776965692379876087034567287650145521051224
+965460150789561045696129898398010676670569345
+234321041276432038765408765498723487989678496
+165432132345987129932317454389654395432310987
+074540122187656087801326761230101276211001236
+783458043090345196540410890121210989303456545
+892169834901210105432589789032367893456327896
+701078985810012234501678776543456302587410187
+667654856798943107657578905494543211693217896
+578983012367874038748765412387687600784506787
+457832343455465129889854307898990521099615690
+300761567854321012934781212387121434988794321
+211650434969482103245690101236012345670188760
+672349123478091014132386789845621012873279454
+589678012562182365001675632736790123964560303
+432547001601276478976543541345887654456781212
+321232118762345569885012310212994569323998800
+210321129098710378894322343200123478010878901
+300410030123601256765011056123430765430765432
+321567542034510349810782987001521894321045645
+434788601945654878725693965432676541013236012
+095699717898783965434344876501987034901107823
+187659826500192854303239884567698127872787934
+234549834410201601214138793298012016543396543
+067812765325360519871025682105623456671230123
+154908901876451456789014871234702965580145674
+233217010945962359874323960189811874495432985
+945606321034876543265221054076320432356781876
+876545432321089012100100123165410321065690165

day-10/main.c

@@ -0,0 +1,83 @@
+#include "aoc.h"
+
+typedef struct CellBitMap {
+    u64 *bits;
+    int size;
+} CellBitMap;
+
+static void
+bitmap_set_bit(CellBitMap *bitmap, int index) {
+    bitmap->bits[index / 64] |= 1ULL << (index % 64);
+}
+
+static int
+bitmap_popcount(CellBitMap *bitmap) {
+    int count = 0;
+    for (int i = 0; i < bitmap->size; i++) {
+        count += __builtin_popcountll(bitmap->bits[i]);
+    }
+    return count;
+}
+
+static void
+bitmap_clear(CellBitMap *bitmap) {
+    memset(bitmap->bits, 0, bitmap->size * sizeof(u64));
+}
+
+static const int OFFSETS[][2] = {
+    { 0, -1},
+    { 1,  0},
+    { 0,  1},
+    {-1,  0},
+};
+
+static int 
+find_trails_and_compute_rating(Grid *grid, int x, int y, char current_number, CellBitMap *bitmap) {
+    ASSERT(grid->grid[y * grid->width + x] == current_number);
+    int result = 0;
+    if (current_number == '9') {
+        int index = y * grid->width + x;
+        bitmap_set_bit(bitmap, index);
+        result = 1;
+    }
+    else {
+        for (int i = 0; i < 4; i++) {
+            int dx = OFFSETS[i][0];
+            int dy = OFFSETS[i][1];
+            int nx = x + dx;
+            int ny = y + dy;
+            if (nx >= 0 && nx < grid->width && ny >= 0 && ny < grid->height) {
+                char neighbor = grid->grid[ny * grid->width + nx];
+                if (neighbor == current_number + 1) {
+                    result += find_trails_and_compute_rating(grid, nx, ny, neighbor, bitmap);
+                }
+            }
+        }
+    }
+    return result;
+}
+
+int main(int argc, char **argv) {
+    Arena *arena = make_arena(Megabytes(1));
+    str input = read_file(arena, argv[1]);
+    Grid grid = parse_grid(str_trim(input), arena);
+
+    CellBitMap bitmap = {0};
+    bitmap.size = (grid.width * grid.height + 63) / 64;
+    bitmap.bits = ARENA_ALLOC_ARRAY(arena, u64, bitmap.size);
+
+    i32 part_1 = 0;
+    i32 part_2 = 0;
+    for (int y = 0; y < grid.height; y++) {
+        for (int x = 0; x < grid.width; x++) {
+            if (grid.grid[y * grid.width + x] == '0') {
+                part_2 += find_trails_and_compute_rating(&grid, x, y, '0', &bitmap);
+                int score = bitmap_popcount(&bitmap);
+                bitmap_clear(&bitmap);
+                part_1 += score;
+            }
+        }
+    }
+    printf("%d\n", part_1);
+    printf("%d\n", part_2);
+}

day-10/test.txt

@@ -0,0 +1,4 @@
+0123
+1234
+8765
+9876

day-10/test2.txt

@@ -0,0 +1,8 @@
+89010123
+78121874
+87430965
+96549874
+45678903
+32019012
+01329801
+10456732