Index: aocutils.c
==================================================================
--- aocutils.c
+++ aocutils.c
@@ -1,9 +1,21 @@
+#include <stdbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include "aocutils.h"
 
+bool TGvalid(struct TextGrid *tg, unsigned row, unsigned col) {
+    if (row >= tg->rows) return false;
+    if (col >= tg->cols) return false;
+    return true;
+}
+char *TGcharptr(struct TextGrid *tg, unsigned row, unsigned col) {
+    if (!TGvalid(tg, row, col)) return NULL;
+    return tg->data + (row * tg->cols) + col;
+}
+
+// TODO: rewrite this shit
 size_t linearize2d(unsigned width, unsigned row, unsigned col) {
     return (row * width) + col;
 }
 
 size_t text2array(unsigned **dst, const char *r) {

Index: aocutils.h
==================================================================
--- aocutils.h
+++ aocutils.h
@@ -1,9 +1,17 @@
 #ifndef AOCUTILS_H_INCLUDED
 #define AOCUTILS_H_INCLUDED
 
-size_t linearize2d(unsigned width, unsigned row, unsigned col);
+struct TextGrid {
+    unsigned rows, cols;
+    char *data; // may have '\n' at end of rows
+};
+
+bool TGvalid(struct TextGrid *tg, unsigned row, unsigned col);
+char *TGcharptr(struct TextGrid *tg, unsigned row, unsigned col);
+
+size_t linearize2d(unsigned width, unsigned row, unsigned col); // TODO
 size_t text2array(unsigned **dst, const char *txt);
 size_t slurp(char **dst, const char *filename);
 unsigned distance(unsigned a, unsigned b);
 unsigned max3u(unsigned a, unsigned b, unsigned c);
 unsigned min3u(unsigned a, unsigned b, unsigned c);