day15

15/.gitignore

@@ -0,0 +1,2 @@
+.vimspector.json
+day15

15/beacons.txt

@@ -0,0 +1,23 @@
+Sensor at x=3907621, y=2895218: closest beacon is at x=3790542, y=2949630
+Sensor at x=1701067, y=3075142: closest beacon is at x=2275951, y=3717327
+Sensor at x=3532369, y=884718: closest beacon is at x=2733699, y=2000000
+Sensor at x=2362427, y=41763: closest beacon is at x=2999439, y=-958188
+Sensor at x=398408, y=3688691: closest beacon is at x=2275951, y=3717327
+Sensor at x=1727615, y=1744968: closest beacon is at x=2733699, y=2000000
+Sensor at x=2778183, y=3611924: closest beacon is at x=2275951, y=3717327
+Sensor at x=2452818, y=2533012: closest beacon is at x=2733699, y=2000000
+Sensor at x=88162, y=2057063: closest beacon is at x=-109096, y=390805
+Sensor at x=2985370, y=2315046: closest beacon is at x=2733699, y=2000000
+Sensor at x=2758780, y=3000106: closest beacon is at x=3279264, y=2775610
+Sensor at x=3501114, y=3193710: closest beacon is at x=3790542, y=2949630
+Sensor at x=313171, y=1016326: closest beacon is at x=-109096, y=390805
+Sensor at x=3997998, y=3576392: closest beacon is at x=3691556, y=3980872
+Sensor at x=84142, y=102550: closest beacon is at x=-109096, y=390805
+Sensor at x=3768533, y=3985372: closest beacon is at x=3691556, y=3980872
+Sensor at x=2999744, y=3998031: closest beacon is at x=3691556, y=3980872
+Sensor at x=3380504, y=2720962: closest beacon is at x=3279264, y=2775610
+Sensor at x=3357940, y=3730208: closest beacon is at x=3691556, y=3980872
+Sensor at x=1242851, y=838744: closest beacon is at x=-109096, y=390805
+Sensor at x=3991401, y=2367688: closest beacon is at x=3790542, y=2949630
+Sensor at x=3292286, y=2624894: closest beacon is at x=3279264, y=2775610
+Sensor at x=2194423, y=3990859: closest beacon is at x=2275951, y=3717327

15/day15.c

@@ -0,0 +1,117 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <math.h>
+
+#define MAX_SENSORS 30
+#define MAX_X 4000000
+#define MAX_Y 4000000
+
+struct Sensor {
+    int x, y;
+    unsigned range;
+};
+struct Beacon {
+    int x, y;
+};
+struct Area {
+    int left, right, top, bottom;
+};
+
+unsigned distance(int x1, int  y1, int x2, int y2) {
+    int x_dist = abs(x1 - x2);
+    int y_dist = abs(y1 - y2);
+    return x_dist + y_dist;
+}
+int max(int a, int b) {
+    if (a > b) { return a; }
+    return b;
+}
+int min(int a, int b) {
+    if (a < b) { return a; }
+    return b;
+}
+
+int isPositionImpossible(int x, int y, struct Beacon* beacons, struct Sensor* sensors, unsigned sensorCount, unsigned* blockingSensor) {
+    for (unsigned i = 0; i < sensorCount; i++) {
+        if (x == beacons[i].x && y == beacons[i].y) { continue; }  // if beacon
+        if (distance(x, y, sensors[i].x, sensors[i].y) <= sensors[i].range) {
+            *blockingSensor = i;
+            return 1;
+        }
+    }
+    return 0;
+}
+
+int main(int argc, const char** argv) {
+    const char* filename = NULL;
+    if (argc != 2) {
+        filename = "beacons.txt";
+    }
+    else {
+        filename = argv[1];
+    }
+    FILE* p_file;
+    p_file = fopen(filename, "r");
+    if (p_file == NULL) {
+        printf("Could not open file\n");
+        exit(1);
+    }
+    struct Sensor sensors[MAX_SENSORS];
+    struct Beacon beacons[MAX_SENSORS];
+    unsigned sensorCount = 0;
+    struct Area area;
+
+    char* p_linebuffer = NULL;
+    size_t linebufferLength = 0;
+    ssize_t lineLength = getline(&p_linebuffer, &linebufferLength, p_file);
+
+    while (lineLength > 0) {
+        sscanf(p_linebuffer, "Sensor at x=%d, y=%d: closest beacon is at x=%d, y=%d", &sensors[sensorCount].x, &sensors[sensorCount].y, &beacons[sensorCount].x, &beacons[sensorCount].y);
+        sensors[sensorCount].range = distance(sensors[sensorCount].x, sensors[sensorCount].y, beacons[sensorCount].x, beacons[sensorCount].y);
+        lineLength = getline(&p_linebuffer, &linebufferLength, p_file);
+        sensorCount++;
+    }
+    free(p_linebuffer);
+    p_linebuffer = NULL;
+    fclose(p_file);
+
+    area.left   = sensors[0].x - sensors[0].range;
+    area.right  = sensors[0].x + sensors[0].range;
+    area.top    = sensors[0].y - sensors[0].range;
+    area.bottom = sensors[0].y + sensors[0].range;
+    printf("Sensor: x0%d, y=%d, range=%d\n", sensors[0].x, sensors[0].y, sensors[0].range);
+    for (unsigned i = 1; i < sensorCount; i++) {
+        area.left   = min(sensors[i].x - sensors[i].range, area.left);
+        area.right  = max(sensors[i].x + sensors[i].range, area.right);
+        area.top    = min(sensors[i].y - sensors[i].range, area.top);
+        area.bottom = max(sensors[i].y + sensors[i].range, area.bottom);
+        printf("Sensor: x=%d, y=%d, range=%d\n", sensors[i].x, sensors[i].y, sensors[i].range);
+    }
+    printf("Area: l-r: %d-%d, t-b: %d-%d\n", area.left, area.right, area.top, area.bottom);
+    int no_beacons_for_sure_mate = 0;
+    /* int y = 10; */
+    int y = 2000000;
+    // this could be optmized 50000000%
+    unsigned blockingSensor;
+    for (int x = area.left; x <= area.right; x++) {
+        no_beacons_for_sure_mate += isPositionImpossible(x, y, beacons, sensors, sensorCount, &blockingSensor);
+    }
+    printf("In row %d, there are %d positions where no beacons are possible\n", y, no_beacons_for_sure_mate);
+
+    for (int x = 0; x < MAX_X; x++) {
+        for (int y = 0; y < MAX_Y; y++) {
+            if (!isPositionImpossible(x, y, beacons, sensors, sensorCount, &blockingSensor)) {
+                unsigned long frequency = (unsigned long) x * 4000000 + y;
+                printf("Tuning frequency from beacon at (%d, %d): %luHz\n", x, y, frequency);
+                goto done;
+            }
+            // skip the until the end of range for the blocking sensor
+            unsigned y_range = sensors[blockingSensor].range - abs(sensors[blockingSensor].x - x);
+            // last y in range
+            y = sensors[blockingSensor].y + y_range;
+        }
+    }
+    printf("Could not find distress beacon :(\n");
+done:
+    return 0;
+}

README.md

@@ -22,7 +22,7 @@ Also, check out the repositories of my friends who do stuff in go, Visual Basic,
 - **Awk**: day 1, 7
 - **Assembly, x86-64**: day 4
 - **Bash**: day 1, 7
-- **C**: day 3, 4
+- **C**: day 3, 4, 15
 - **C++**: day 2, 8, 12, 14
 - **HolyC:** day 13
 - **Java**: day 10