Index: aoc2024.c
==================================================================
--- aoc2024.c
+++ aoc2024.c
@@ -4,10 +4,54 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include "aocdailies.h"
 #include "aocutils.h"
+
+/* === aoc202408 =======================================================
+   Oh! another square of text!
+   Idea: for all points p with an antenna find all points q>p with
+a corresponding frequency. For each such pair calculate the 2 antinodes
+and add the resulting points (if not there already) to an array.
+   The answer to Part One is the number of elements in the array
+   TODO: I don't like my `linearize2d` thing! Thinking of a replacement
+===================================================================== */
+
+void aoc202408(char *data, size_t len) {
+    (void)len; // unused argument
+    unsigned anti[256][2]; // locations of antinodes
+    unsigned nanti = 0;
+    unsigned size = 1 + strchr(data, '\n') - data;
+    for (unsigned row = 0; row < size-1; row++) {
+        for (unsigned col = 0; col < size-1; col++) {
+            char *p = data + linearize2d(size, row, col);
+            if (*p != '.') {
+                char *q = strchr(p + 1, *p);
+                while (q) {
+                    printf("found '%c'('%c')\n", *p, *q);
+                    ptrdiff_t delta = q - p;
+                    #if 0
+    int row_pos = (int)(linear_pos / size);
+    int col_pos = (int)(linear_pos % size);
+                    #endif
+                    if (p >= data + delta) {
+                        // antinode
+                        anti[nanti][0] = row(p - delta);
+                        anti[nanti++][1] = col(p - delta);
+                    }
+                    if (q < "end of data" - delta) {
+                        // antinode
+                        anti[nanti][0] = row(q + delta);
+                        anti[nanti++][1] = col(q + delta);
+                    }
+                    q = strchr(q + 1, *p);
+                }
+            }
+        }
+    }
+    printf("There are %u antinodes within the map.\n", nanti);
+}
 
 /* === aoc202407 =======================================================
    Part one looks easy
    Part two also easy
 ===================================================================== */
@@ -75,10 +119,11 @@
    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 +277,11 @@
     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: aocdailies.c
==================================================================
--- aocdailies.c
+++ aocdailies.c
@@ -5,10 +5,11 @@
     aocfunc *p;
     switch (y * 100 + d) {
         default: p = NULL; break;
 
         //   YYYYdd ==>  aocYYYYdd
+        case 202408: p = aoc202408; break;
         case 202407: p = aoc202407; break;
         case 202406: p = aoc202406; break;
         case 202405: p = aoc202405; break;
         case 202404: p = aoc202404; break;
         case 202403: p = aoc202403; break;

Index: aocdailies.h
==================================================================
--- aocdailies.h
+++ aocdailies.h
@@ -4,10 +4,11 @@
 #include <stddef.h>
 
 typedef void aocfunc(char *, size_t);
 aocfunc *aocselect(unsigned, unsigned);
 
+aocfunc aoc202408;
 aocfunc aoc202407;
 aocfunc aoc202406;
 aocfunc aoc202405;
 aocfunc aoc202404;
 aocfunc aoc202403;

Index: aocutils.c
==================================================================
--- aocutils.c
+++ aocutils.c
@@ -1,9 +1,27 @@
+#include <stbool.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include "aocutils.h"
 
+struct TextGrid {
+    unsigned rows, cols;
+    char *data; // may have '\n' at end of rows
+};
+
+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 (row >= tg->rows) return NULL;
+    if (col >= tg->cols) return NULL;
+    return 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);