day 08

day-08/input.txt

@@ -0,0 +1,50 @@
+................................y.................
+............9.....Q................y..............
+..................................................
+..................................................
+..........Q.......................x..N..1.........
+.....9......6.e......................x.......j....
+............e..x6Q9...............................
+...........6..................................j...
+...e....................................j.........
+.............Q8.......................x..1........
+.....w.......8...................y................
+..n.......................y...................s...
+.n................................................
+.........n...............e........................
+8..C..............r.....F......................j..
+.......C......................................1..s
+................n.u..................W...t........
+......w..........r.........W..5J..................
+.....p..............................J.............
+.....T.................................d..........
+......prw................uW.....Z.....t..6........
+....p.r.....f............7........................
+........C.f...q..................3.Y..............
+.......w.f..........M.....C..5.......t............
+....S..f.q.........................5..............
+.............p......J........c................Z...
+......................5...........................
+....T...........u........D.....8........R.........
+..0....T.............7M..........J....RZ..........
+......t.Iu......................P.......W..Z......
+...............D.......M.....i.......z..........s.
+...........F..DM..q............R..................
+........T....0............................c....s..
+.E........0..............N........................
+.......................................1........2X
+..........Y....0.q....F..................X........
+...............F.I..........................X.....
+....U......z..............7i.........S..c.........
+E.D..S...............................4.....2......
+..S.........z..I......i.........m.............2...
+.......E............I.....i..................R....
+..................N...............................
+....................................m.............
+...Y...............P.............m...2............
+................N...z................c............
+.......................................4..........
+........U.........P...............7..d..........4.
+........................X....3....d...............
+Y................P.U..........3...........d.......
+...U..................................3...........

day-08/main.c

@@ -0,0 +1,109 @@
+#include "aoc.h"
+#include <stdlib.h>
+
+typedef struct {
+    i16 x, y;
+    u8 letter;
+} Antenna;
+
+static int
+cmp_antennas(const void *a, const void *b) {
+    Antenna *aa = (Antenna *)a;
+    Antenna *bb = (Antenna *)b;
+    return aa->letter - bb->letter;
+}
+
+static i32
+sum(const u8 *restrict values, isize len) {
+    i32 sum = 0;
+    for (isize i = 0; i < len; i++) {
+        sum += values[i];
+    }
+    return sum;
+}
+
+int main(int argc, char **argv) {
+    Arena *arena = make_arena(Megabytes(1));
+    str input = read_file(arena, argv[1]);
+    Grid grid = parse_grid(input, arena);
+
+    struct {
+        Antenna *data;
+        isize len, cap;
+    } antennas = {0};
+    for (isize y = 0; y < grid.height; y++) {
+        for (isize x = 0; x < grid.width; x++) {
+            u8 letter;
+            if ((letter = grid.grid[y * grid.width + x]) != '.') {
+                *push(&antennas, arena) = (Antenna){x, y, letter};
+            }
+        }
+    }
+
+    // sort/group antennas by frequency (i.e. letter)
+    qsort(antennas.data, antennas.len, sizeof(Antenna), cmp_antennas);
+
+    u8 *antinodes = ARENA_ALLOC_ARRAY(arena, u8, grid.width * grid.height);
+
+    isize group_start = 0;
+    for (isize i = 0; i < antennas.len; i++) {
+        if (antennas.data[i].letter != antennas.data[group_start].letter) {
+            group_start = i;
+        }
+        for (isize j = group_start; j < i; j++) {
+            ASSERT(antennas.data[i].letter == antennas.data[j].letter);
+            i32 dx = antennas.data[i].x - antennas.data[j].x;
+            i32 dy = antennas.data[i].y - antennas.data[j].y;
+
+            // first antinode
+            i32 antinode_x = antennas.data[i].x + dx;
+            i32 antinode_y = antennas.data[i].y + dy;
+            if (antinode_x >= 0 && antinode_y >= 0 && antinode_x < grid.width && antinode_y < grid.height) {
+                antinodes[antinode_y * grid.width + antinode_x] = 1;
+            }
+
+            // second antinode
+            antinode_x = antennas.data[j].x - dx;
+            antinode_y = antennas.data[j].y - dy;
+            if (antinode_x >= 0 && antinode_y >= 0 && antinode_x < grid.width && antinode_y < grid.height) {
+                antinodes[antinode_y * grid.width + antinode_x] = 1;
+            }
+        }
+    }
+
+    i32 part_1 = sum(antinodes, grid.width * grid.height);
+    printf("%d\n", part_1);
+
+    // Part 2
+    group_start = 0;
+    for (isize i = 0; i < antennas.len; i++) {
+        if (antennas.data[i].letter != antennas.data[group_start].letter) {
+            group_start = i;
+        }
+        for (isize j = group_start; j < i; j++) {
+            ASSERT(antennas.data[i].letter == antennas.data[j].letter);
+            i32 dx = antennas.data[i].x - antennas.data[j].x;
+            i32 dy = antennas.data[i].y - antennas.data[j].y;
+
+            i32 x = antennas.data[i].x;
+            i32 y = antennas.data[i].y;
+
+            while (x >= 0 && y >= 0 && x < grid.width && y < grid.height) {
+                antinodes[y * grid.width + x] = 1;
+                x += dx;
+                y += dy;
+            }
+
+            x = antennas.data[i].x;
+            y = antennas.data[i].y;
+            while (x >= 0 && y >= 0 && x < grid.width && y < grid.height) {
+                antinodes[y * grid.width + x] = 1;
+                x -= dx;
+                y -= dy;
+            }
+        }
+    }
+
+    i32 part_2 = sum(antinodes, grid.width * grid.height);
+    printf("%d\n", part_2);
+}

day-08/test.txt

@@ -0,0 +1,12 @@
+............
+........0...
+.....0......
+.......0....
+....0.......
+......A.....
+............
+............
+........A...
+.........A..
+............
+............