Day 14: Add C solution

14/input.txt

@@ -0,0 +1,102 @@
+FSKBVOSKPCPPHVOPVFPC
+
+BV -> O
+OS -> P
+KP -> P
+VK -> S
+FS -> C
+OK -> P
+KC -> S
+HV -> F
+HC -> K
+PF -> N
+NK -> F
+SC -> V
+CO -> K
+PO -> F
+FB -> P
+CN -> K
+KF -> N
+NH -> S
+SF -> P
+HP -> P
+NP -> F
+OV -> O
+OP -> P
+HH -> C
+FP -> P
+CS -> O
+SK -> O
+NS -> F
+SN -> S
+SP -> H
+BH -> B
+NO -> O
+CB -> N
+FO -> N
+NC -> C
+VF -> N
+CK -> C
+PC -> H
+BP -> B
+NF -> O
+BB -> C
+VN -> K
+OH -> K
+CH -> F
+VB -> N
+HO -> P
+FH -> K
+PK -> H
+CC -> B
+VH -> B
+BF -> N
+KS -> V
+PV -> B
+CP -> N
+PB -> S
+VP -> V
+BO -> B
+HS -> H
+BS -> F
+ON -> B
+HB -> K
+KH -> B
+PP -> H
+BN -> C
+BC -> F
+KV -> K
+VO -> P
+SO -> V
+OF -> O
+BK -> S
+PH -> V
+SV -> F
+CV -> H
+OB -> N
+SS -> H
+VV -> B
+OO -> V
+CF -> H
+KB -> F
+NV -> B
+FV -> V
+HK -> P
+VS -> P
+FF -> P
+HN -> N
+FN -> F
+OC -> K
+SH -> V
+KO -> C
+HF -> B
+PN -> N
+SB -> F
+VC -> B
+FK -> S
+KK -> N
+FC -> F
+NN -> P
+NB -> V
+PS -> S
+KN -> S

14/solution.c

@@ -0,0 +1,68 @@
+#include <stdio.h>
+#include <string.h>
+
+/* we can pack the pairs into integers… */
+#define P(a,b) (( (a-'A')<<6 )| (b-'A'))
+
+/* …and use them as keys for a lookup table */
+enum {maxk=P('Z','Z')+1};
+size_t counts[maxk] = {0}, tcounts[maxk];
+
+void pcounts(const char *s) {
+    size_t min=-1,max=0,k,n,i;
+    for(i='A';i<'Z'+1;i++) {
+        k=P(i,'z');
+        if ((n=counts[k])){
+            if (n<min) min=n;
+            else if (n>max) max=n;
+        }
+    }
+    printf("%s: %lu\n", s, max-min);
+}
+
+int main(void) {
+    char tmpl[128], rules[128][3]={0};
+    size_t i,j,n,k;
+
+    scanf("%127[A-Z]\n", tmpl);
+    for(i=0;tmpl[i];i++){
+        /* set initial counts for pairs and single chars */
+        if(i) counts[P(tmpl[i-1],tmpl[i])]++;
+        /* use the same LUT for both */
+        counts[P(tmpl[i],'z')]++;
+    }
+
+    for(i=0;scanf("%c%c -> %c\n",
+                  &rules[i][0], &rules[i][1], &rules[i][2])==3;i++);
+
+    for(i=0;i<40;i++) {
+        /* reset temp LUT */
+        memset(tcounts, 0, maxk*sizeof(size_t));
+
+        /* apply each rule */
+        for(j=0;rules[j][0];j++) {
+            k=P(rules[j][0],rules[j][1]);
+
+            if ((n=counts[k])) {
+                counts[k]=0;
+                /* single chars can be updated here */
+                counts[P(rules[j][2],'z')]+=n;
+                /* pairs must be counted independently */
+                tcounts[P(rules[j][0],rules[j][2])]+=n;
+                tcounts[P(rules[j][2],rules[j][1])]+=n;
+            }
+        }
+
+        /* commit temp results */
+        for(j=0;rules[j][0];j++) {
+            k=P(rules[j][0],rules[j][2]);
+            counts[k] = tcounts[k];
+            k=P(rules[j][2],rules[j][1]);
+            counts[k] = tcounts[k];
+        }
+
+        if (i==9)
+            pcounts("Silver");
+    }
+    pcounts("Gold");
+}