day 05

day-05/main.c

@@ -0,0 +1,83 @@
+#include "aoc.h"
+#include <stdlib.h>
+
+static const u8 PRED_BIT = 0x01;
+static const u8 SUCC_BIT = 0x02;
+// matrix[a][b] & PRED_BIT != 0 means that a must precede b.
+// matrix[a][b] & SUCC_BIT != 0 means that a must succeed b.
+// by symmetry, (matrix[a][b] & PRED_BIT) == (matrix[b][a] & SUCC_BIT).
+// matrix[a][b] == 0 means that there is no constraint between a and b.
+// by inspection, the inputs are all >= 0 and < 100.
+static u8 matrix[100][100] = {0};
+
+static int
+compare(const void *pa, const void *pb) {
+    i32 a = *(const i32 *)pa;
+    i32 b = *(const i32 *)pb;
+    if (matrix[a][b] & PRED_BIT) return -1;
+    if (matrix[a][b] & SUCC_BIT) return 1;
+    return 0;
+}
+
+int main(int argc, char **argv) {
+    Arena *arena = make_arena(Megabytes(1));
+    str input = read_file(arena, argv[1]);
+
+    str line;
+    while ((line = str_next_token(&input, STR("\n"))).len > 0) {
+        str fst, snd;
+        str_split(line, STR("|"), &fst, &snd);
+        i32 a = parse_i64(fst, *arena);
+        i32 b = parse_i64(snd, *arena);
+
+        matrix[a][b] |= PRED_BIT;
+        matrix[b][a] |= SUCC_BIT;
+    }
+
+    i32 part_1 = 0;
+    i32 part_2 = 0;
+
+    while ((line = str_next_token(&input, STR("\n"))).len > 0) {
+        str value_str;
+        struct {
+            i32 *data;
+            isize len, cap;
+        } values = {0};
+
+        void *watermark = arena->top;
+        while ((value_str = str_next_token(&line, STR(","))).len > 0) {
+            i32 value = parse_i64(value_str, *arena);
+            *push(&values, arena) = value;
+        }
+        arena->top = watermark;
+
+        bool row_ok = true;
+        for (isize i = 0; i < values.len && row_ok; i++) {
+            i32 value = values.data[i];
+            for (isize j = 0; j < values.len; j++) {
+                if (i == j) continue;
+                i32 other = values.data[j];
+                if (matrix[value][other]) {
+                    if (i < j && !(matrix[value][other] & PRED_BIT)) {
+                        row_ok = false;
+                        break;
+                    }
+                    if (i > j && !(matrix[value][other] & SUCC_BIT)) {
+                        row_ok = false;
+                        break;
+                    }
+                }
+            }
+        }
+        if (row_ok) {
+            part_1 += values.data[values.len / 2];
+        }
+        else {
+            qsort(values.data, values.len, sizeof(i32), compare);
+            part_2 += values.data[values.len / 2];
+        }
+    }
+
+    printf("%d\n", part_1);
+    printf("%d\n", part_2);
+}

day-05/test.txt

@@ -0,0 +1,28 @@
+47|53
+97|13
+97|61
+97|47
+75|29
+61|13
+75|53
+29|13
+97|29
+53|29
+61|53
+97|53
+61|29
+47|13
+75|47
+97|75
+47|61
+75|61
+47|29
+75|13
+53|13
+
+75,47,61,53,29
+97,61,53,29,13
+75,29,13
+75,97,47,61,53
+61,13,29
+97,13,75,29,47