Complete Day 6 Part 1

06/06.c

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+#include <stdio.h>
+#include <stdint.h>
+#include <stdbool.h>
+#include <stdlib.h>
+#include <math.h>
+
+#define DEFAULT_FILE    "input.txt"
+#define STRBUF_LEN      200
+#define ISDIGIT(X)      (X >= '0' && X <= '9')
+
+#define NUM_RACES       4
+
+uint64_t parse_num(char *str)
+{
+    uint64_t num = 0;
+    while (ISDIGIT(*str))
+    {
+        num = (num * 10) + (*str - '0');
+        str++;
+    }
+    return num;
+}
+
+void parse_line(char *str, uint16_t *nums)
+{
+    uint16_t race = 0;
+
+    for (race = 0; race < NUM_RACES; race++)
+    {
+        while (!ISDIGIT(*str)) str++;
+        nums[race] = parse_num(str);
+        while (ISDIGIT(*str)) str++;
+    }
+}
+
+// Solves for the 0-intersection points, placing the results in result1 and result2.
+// Returns true if it can be solved, returns false if it is unsolvable.
+bool solve_quadratic(uint64_t time, uint64_t distance, uint64_t *result1, uint64_t *result2)
+{
+    double doubleres1;
+    double doubleres2;
+    int64_t s = (time * time) - (4 * distance);
+    if (s < 0) return false;
+    doubleres1 = ( time - sqrt(s) ) / 2;
+    doubleres2 = ( time + sqrt(s) ) / 2;
+
+    if (doubleres1 == (double)(ceil(doubleres1))) doubleres1 += 1;
+    *result1 = ceil(doubleres1);
+
+    if (doubleres2 == (double)(floor(doubleres2))) doubleres2 -= 1;
+    *result2 = floor(doubleres2);
+    return true;
+}
+
+int main(int argc, char **argv)
+{
+    // Get filename as an argument, if there is one
+    char *path = (argc > 1) ? argv[1] : DEFAULT_FILE;
+
+    if (!path)
+    {
+        printf("Requires input file.\n");
+        return 1;
+    }
+
+    FILE *file = fopen(path, "r");
+    char strbuf[STRBUF_LEN] = {0};
+
+    if (!file)
+    {
+        printf("Could not find file.");
+        return 1;
+    }
+
+    uint16_t times[NUM_RACES] = {0};
+    uint16_t dists[NUM_RACES] = {0};
+    uint16_t results[NUM_RACES] = {0};
+
+    fgets(strbuf, STRBUF_LEN, file);
+    parse_line(strbuf, times);
+
+    fgets(strbuf, STRBUF_LEN, file);
+    parse_line(strbuf, dists);
+
+    uint16_t i = 0;
+    uint64_t multiplied = 1;
+
+    for (i = 0; i < NUM_RACES; i++)
+    {
+        uint64_t res1;
+        uint64_t res2;
+        if (!solve_quadratic(times[i], dists[i], &res1, &res2))
+        {
+            printf("Error?? Invalid quadratic.\n");
+            return 1;
+        }
+
+        results[i] = res2 - res1 + 1;
+        printf("%d ", results[i]);
+        multiplied *= results[i];
+    }
+
+    printf("\nMultiplied: %lu\n", multiplied);
+
+    return 0;
+}

06/input.txt

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+Time:        50     74     86     85
+Distance:   242   1017   1691   1252

06/input2.txt

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+Time:      7  15   30
+Distance:  9  40  200