Day 3: Add C solution

03/solution_1.c

@@ -0,0 +1,32 @@
+#include <stdio.h>
+#include <string.h>
+
+int main(void) {
+    FILE *fp = fopen("input.txt", "r");
+    char buf[16];
+    unsigned rlen = sizeof(buf) - 1;
+    unsigned bitcnt[2][sizeof(buf) - 1] = {0};
+    size_t gamma = 0, epsilon = 0;
+
+    while (fgets(buf, sizeof(buf), fp) != NULL) {
+        for (size_t i = 0; i < rlen; i++) {
+            if (buf[i] == '0')
+                bitcnt[0][i]++;
+            else if (buf[i] == '1')
+                bitcnt[1][i]++;
+            else {
+                rlen = i;
+                break;
+            }
+        }
+    }
+
+    for (size_t i = 0; i < rlen; i++) {
+        if (bitcnt[0][i] < bitcnt[1][i])
+            gamma ^= 1 << (rlen - i - 1);
+        else
+            epsilon ^= 1 << (rlen - i - 1);
+    }
+
+    printf("Answer: %lu\n", gamma * epsilon);
+}

03/solution_2.c

@@ -0,0 +1,65 @@
+#include <stdio.h>
+#include <string.h>
+#include <stdlib.h>
+
+long long flt(long long *data, size_t n, size_t rlen, char most) {
+    size_t nused = n;
+    char used[n];
+    memset(used, 1, n);
+
+    for (size_t b = 0; b < rlen; b++) {
+        size_t bitcnt[2][rlen];
+        memset(bitcnt[0], 0, rlen * sizeof(size_t));
+        memset(bitcnt[1], 0, rlen * sizeof(size_t));
+
+        for (size_t i = 0; i < n; i++) {
+            if (used[i]) {
+                if ((data[i] >> (rlen - b - 1)) & 1)
+                    bitcnt[1][b]++;
+                else
+                    bitcnt[0][b]++;
+            }
+        }
+
+        if (nused > 1) {
+            char cond;
+            if (bitcnt[0][b] == bitcnt[1][b])
+                cond = most;
+            else
+                cond = (bitcnt[0][b] > bitcnt[1][b]) ? !most : most;
+
+            for (size_t i = 0; i < n; i++) {
+                if (used[i]) {
+                    if (((data[i] >> (rlen - b - 1)) & 1) != cond) {
+                        used[i] = 0;
+                        nused--;
+                    }
+                }
+            }
+        }
+
+        if (nused == 1) {
+            for (size_t i = 0; i < n; i++) {
+                if (used[i])
+                    return data[i];
+            }
+        }
+    }
+
+    return 0;
+}
+
+int main(void) {
+    FILE *fp = fopen("input.txt", "r");
+    char buf[16], *end;
+    size_t n = 0;
+    long long data[1000] = {0};
+
+    while (fgets(buf, sizeof(buf), fp) != NULL) {
+        data[n] = strtoll(buf, &end, 2);
+        n++;
+    }
+
+    printf("Answer: %lld\n",
+           flt(data, n, end - buf, 1) * flt(data, n, end - buf, 0));
+}