Index: aoc2024.c
==================================================================
--- aoc2024.c
+++ aoc2024.c
@@ -75,10 +75,11 @@
    Hmmm, I've done something like this already this year... on day 4
    I need rcmap() moved to aocutils ... done; now it's called `linearize2d`
 Part Two: brute force? It likely is faster than coming up with an algo
    Let's go: save map, for every available position put in an obstacle,
    loop around until some place is visited 4? times or area is exited
+TODO remove linearize2d
 ===================================================================== */
 
 static bool validpos(unsigned s, int r, int c) {
     if (r < 0) return false;
     if (c < 0) return false;
@@ -232,10 +233,11 @@
     printf("The sum of middle update numbers is {%u}.\n", accumsum);
     printf("The sum of middle update numbers for newly ordered updates is {%u}.\n", accumsum2);
 }
 
 /* === aoc202404 =======================================================
+TODO remove linearize2d
 ===================================================================== */
 
 static bool masat(char *data, unsigned size, unsigned row, unsigned col, int drow, int dcol) {
     int maxrow = (int)row + 3*drow;
     int maxcol = (int)col + 3*dcol;

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);

ADDED   test.txt
Index: test.txt
==================================================================
--- /dev/null
+++ test.txt
@@ -0,0 +1,12 @@
+............
+........0...
+.....0......
+.......0....
+....0.......
+......A.....
+............
+............
+........A...
+.........A..
+............
+............

ADDED   testtg.c
Index: testtg.c
==================================================================
--- /dev/null
+++ testtg.c
@@ -0,0 +1,26 @@
+#include <stdbool.h>
+#include <stdio.h>
+#include <string.h>
+
+#include "aocutils.h"
+
+int main(void) {
+    char *input = NULL;
+    size_t ilen = slurp(&input, "test.txt");
+
+    struct TextGrid tg;
+    tg.data = input;
+    tg.cols = strchr(input, '\n') - input + 1;
+    tg.rows = tg.cols - 1; // rows includes the '\n'
+
+    if (*TGcharptr(&tg, 11, 11) == '.') puts("1");
+    if (TGcharptr(&tg, 12, 11) == NULL) puts("2");
+    if (TGcharptr(&tg, 11, 12) == NULL) puts("3");
+    if (*TGcharptr(&tg, 11, 12) == '\n') puts("33");
+    if (TGcharptr(&tg, -1, -2) == NULL) puts("4");
+
+    for (int k = 0; k < 12; k++) {
+        *TGcharptr(&tg, k, k) = '#';
+    }
+    printf("\n\n%s\n", tg.data);
+}