2 files changed, 155 insertions, 0 deletions
diff --git a/18.c b/18.c
new file mode 100644
index 0000000..05c7940
--- /dev/null
+++ b/18.c
@@ -0,0 +1,154 @@
+
+//#define _GNU_SOURCE
+
+#include <regex.h>
+#include <search.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+
+#define arrlen(x) (sizeof(x)/sizeof((x)[0]))
+
+
+struct face {
+	int x;
+	int y;
+	int z;
+};
+
+
+int
+face_cmp(struct face const * a, struct face const * b)
+{
+	if (a->x != b->x)
+		return a->x - b->x;
+	if (a->y != b->y)
+		return a->y - b->y;
+	return a->z - b->z;
+}
+
+
+int
+face_cmp_t(void const * a, void const * b)
+{
+	return face_cmp(a, b);
+}
+
+
+// Get one of the faces of a cube
+//
+// x = left-right
+// y = up-down
+// z = front-back
+//
+// For a cube, we scale it by a factor of two, and then identify each face by
+// the point at its center.
+struct face *
+cube_face(int x, int y, int z, int face)
+{
+	struct face * f;
+
+	x *= 2;
+	y *= 2;
+	z *= 2;
+
+	switch (face) {
+	case 0: // Bottom face
+		x += 1;
+		z += 1;
+		break;
+
+	case 1: // Top face
+		x += 1;
+		y += 2;
+		z += 1;
+		break;
+
+	case 2: // Left face
+		y += 1;
+		z += 1;
+		break;
+
+	case 3: // Right face
+		x += 2;
+		y += 1;
+		z += 1;
+		break;
+
+	case 4: // Front face
+		x += 1;
+		y += 1;
+		break;
+
+	case 5: // Back face
+		x += 1;
+		y += 1;
+		z += 2;
+		break;
+
+	default:
+		return NULL;
+	}
+
+	if ((f = malloc(sizeof(struct face))) == NULL)
+		return NULL;
+
+	f->x = x;
+	f->y = y;
+	f->z = z;
+	return f;
+}
+
+
+int
+main()
+{
+	char buf[BUFSIZ];
+	regex_t cube;
+	regmatch_t coords[4];
+	void * faces = NULL;
+	int nfaces = 0;
+
+	if (regcomp(&cube, "(-?[[:digit:]]+),(-?[[:digit:]]+),(-?[[:digit:]]+)", REG_EXTENDED) != 0) {
+		fprintf(stderr, "Bad regex\n");
+		return -1;
+	}
+
+	while (fgets(buf, sizeof(buf), stdin)
+			&& regexec(&cube, buf, arrlen(coords), coords, 0) == 0)
+	{
+		int x, y, z, f;
+
+		x = atoi(buf + coords[1].rm_so);
+		y = atoi(buf + coords[2].rm_so);
+		z = atoi(buf + coords[3].rm_so);
+
+		for (f = 0; f < 6; ++f) {
+			struct face * face = cube_face(x, y, z, f);
+
+			struct face * exists = *((struct face **) tsearch(face, &faces, face_cmp_t));
+
+			if (exists == face) {
+				// Inserted new face
+				++nfaces;
+			}
+			else {
+				// Face already existed, so delete it
+				tdelete(face, &faces, face_cmp_t);
+				--nfaces;
+				free(face);
+				free(exists);
+			}
+		}
+	}
+
+	printf("Faces: %d\n", nfaces);
+
+#ifdef _GNU_SOURCE
+	tdestroy(faces, free);
+#endif
+	regfree(&cube);
+
+	return 0;
+}
+
diff --git a/makefile b/makefile
index b4f98af..2a45609 100644
--- a/makefile
+++ b/makefile
@@ -19,6 +19,7 @@ all: bin \
 	bin/15 \
 	bin/16 \
 	bin/17 \
+	bin/18 \
 
 bin:
 	mkdir -p $@