The offset the walk never reaches, and a stat that was running sixty times a second

PORTING.md

@@ -791,6 +791,14 @@ pointer-and-count apart itself, and uses a zeroed `defvar` as the null pointer t
 "the default ASCII set". It was written for this call before anything called it, and the
 call fit it exactly.
 
+**`and` short-circuits, which `step-back` depends on.** The backward walk's loop condition
+is `(and (> i 0) (continuation? (at b i)))`, and at the start of the text `i` is -1 — so if
+`and` were a strict function rather than a form, the guard would not save the `(at b -1)`
+beside it and the first press of LEFT would signal `BoundsError`. It is a form and it does
+short-circuit; `test/programs/raylib-codepoints.flan` has a row for it, because the walk
+itself never reaches that offset and the claim is about the language rather than about the
+example.
+
 **Fixed arrays were the right shape for both.** A codepoint table with a count, a
 `[9 Rectangle]` of toggle buttons, a `[9 string]` of labels. The §3 finding from the
 siam-farmer report — a global cannot hold a `Vec`, and fixed arrays with counts are usually

examples/text-codepoints-loading.flan

@@ -173,6 +173,11 @@
     (if (< i 0) 0 i)))
 
 (defvar font rl/Font)
+;; Whether the TTF was there, asked once. A `defvar` and not the call itself
+;; in the draw loop: path-file? is a stat, and a syscall per frame to answer a
+;; question whose answer cannot change while the program runs is exactly what
+;; the per-frame rule in PORTING.md is about.
+(defvar font-present bool)
 (defvar show-font-atlas bool)
 (defvar cursor i32)
 (defvar codepoint-count i32)
@@ -204,6 +209,7 @@
   ;; nothing else in this program does.
   (rl/set-text-line-spacing 20)
 
+  (set font-present (rl/path-file? font-path))
   (set show-font-atlas false)
   (set cursor 0)
 
@@ -249,7 +255,7 @@
       ;; The line the C does not have, and the reason it is here is in the
       ;; header comment: without the TTF this program draws boxes, and a
       ;; program that draws boxes without saying why reads as broken.
-      (when (not (rl/path-file? font-path))
+      (when (not font-present)
         (d/draw-piece "No font at examples/resources/ — glyphs will be boxes."
                       10 (- screen-height 55) 20 rl/maroon))
 

test/programs/raylib-codepoints.flan

@@ -108,6 +108,14 @@
           (set mirrored false)))
       (print "walks mirror ") (println (yes-no mirrored)))
 
+    ;; The one call walk-backward never makes: step-back at the very start.
+    ;; It is where the example goes the moment anybody presses LEFT before
+    ;; pressing RIGHT, and it is the one offset where the continuation-byte
+    ;; loop is asked to step off the front of the buffer. `and` short-circuits,
+    ;; so the (> i 0) guard is what keeps (at b -1) from being evaluated at
+    ;; all — which is a claim about the language and so is worth a row.
+    (show "back at start" (cp/step-back 0))
+
     ;; Item 3: and the forward walk is what raylib said the text contains.
     (let [agrees (= forward-n total)
           all (slice-from-ptr raw total)]

test/test_acceptance.ml

@@ -1065,6 +1065,7 @@ let () =
        forward 54\n\
        backward 54\n\
        walks mirror yes\n\
+       back at start 0\n\
        walk matches raylib yes\n"
     in
     if Sys.command "ldconfig -p 2>/dev/null | grep -q libraylib" = 0 then begin