;;;; Falling sand — Flan port of the Odin/Janet/Lisp/jank versions in ~/Development/fnm.
;;;;
;;;; Kept at parity with lisp/sand.lisp, clojure/src/fnm/sand.clj and
;;;; src/fnm/sand.jank: the same constants, the same eight functions, and
;;;; nothing else. Anything a reference version does not have does not belong
;;;; here — this file is the acceptance program for the language, not a
;;;; showcase for raylib bindings.
;;;;
;;;; It is tested twice: headless (N frames, hash the grid — the version CI runs
;;;; on native and wasm32) and interactive at 120 fps. Both halves are one file
;;;; now. The link follows what the program reaches, so
;;;; test/programs/sand-headless.flan imports *this file* as a package, calls
;;;; the simulation directly, and pulls in neither a window nor libraylib. The
;;;; main below is not exported: an entry point is not something a package
;;;; offers.
;;;;
;;;; Note what it deliberately does not use: no Vec, no Map, no generics, no
;;;; user-written macros, no allocator other than the stack and static storage.
;;;;
;;;; Notation reminders (see plan.org and spec-memory.md):
;;;;   [n T]        fixed array, length n, element T — a VALUE, copies
;;;;   [T]          slice, ptr+len, non-owning     (Vec T)  owning, move-only
;;;;   (Ptr T)      pointer      (Handle T)  generational handle
;;;;   types are inline name/type pairs, as in `let` and `defstruct`
;;;;   a return type of () means the function returns nothing

(import rl  "vendor:raylib")   ; directory = package; declaration optional
(import agent "vendor:agent")  ; the dev agent: redefinitions, installed below

;;;; ── The simulation ───────────────────────────────────────────
;;;;
;;;; No raylib between here and the next banner, which is what the headless
;;;; driver imports this file for. The hash is over exactly this.

(defconst screen-width  900)
(defconst screen-height 600)
(defconst cell-size     5)
;; f32: velocity is [f32], and there is no implicit widening.
(defconst gravity       f32 0.05)
(defconst rows          (/ screen-height cell-size))
(defconst cols          (/ screen-width  cell-size))
(defconst brush-size    10)

;; Packed 0xRRGGBBAA. A cell of 0 means empty, so no Option and no tag word.
(defconst colors [4 u32] [0xFFF00FFF 0x3B6E8CFF 0xA83232FF 0xCC6B1FFF])

;; Flat, unboxed, statically sized. No headers, so these are exactly
;; rows*cols*4 bytes each — the same memory the Odin port has. Fixed arrays are
;; values, so `(set grid (zeroed))` overwrites in place rather than reallocating.
;; No initialiser means all-bytes-zero (plan.org, zero values), so these are
;; BSS and cost nothing to start. `(zeroed)` below is the explicit spelling for
;; re-zeroing later — a memset, not an allocation.
(defvar grid     [rows [cols u32]])
(defvar velocity [rows [cols f32]])
;; An index into colors, not a colour.
(defvar current-color i32)

(defn clear-grid [] ()
  (set grid     (zeroed))
  (set velocity (zeroed)))

(defn next-color [] ()
  (set current-color (% (+ current-color 1) (len colors))))

;; Drop a brush-sized cloud of grains centred on [row col]. This is what the
;; mouse drives interactively and what the headless run calls directly — the
;; only difference between the two is where the centre comes from.
(defn paint-at [row i32  col i32] ()
  (let [half (/ brush-size 2)]
    (dotimes [x brush-size]
      (dotimes [y brush-size]
        (let [r (+ y (- row half))
              c (+ x (- col half))]
          (when (and (>= r 0) (< r (- rows 1))
                     (>= c 0) (< c (- cols 1))
                     (= 0 (at grid r c))
                     (< (rand-f32) 0.5))
            (set (at grid r c)     (at colors current-color))
            (set (at velocity r c) 1.0)))))))

;; Move the grain at [row col] as far down as it can, sliding to a free
;; diagonal neighbour when the cell below is taken.
;;
;; Imperative `while` with early `return`, not loop/recur — see plan.org
;; "Loop story". The recur version read as a tail call but was a countdown
;; over a mutable scan position, which is what a while loop is.
(defn settle [row i32  col i32] ()
  (let [vel (+ gravity (at velocity row col))
        y   (min (- rows 1) (+ row (i32 vel)))]
    (while (> y row)
      (when (= 0 (at grid y col))
        (set (at grid y col)         (at grid row col))
        (set (at grid row col)       0)
        (set (at velocity y col)     vel)
        (set (at velocity row col)   0.0)
        (return))
      (let [left?  (and (> col 0)          (= 0 (at grid y (- col 1))))
            right? (and (< col (- cols 1)) (= 0 (at grid y (+ col 1))))]
        (when (or left? right?)
          (let [side (cond
                       (not left?)  1
                       (not right?) -1
                       :else        (if (< (rand-f32) 0.5) 1 -1))]
            (set (at grid y (+ col side))       (at grid row col))
            (set (at grid row col)              0)
            (set (at velocity y (+ col side))   vel)
            (set (at velocity row col)          0.0)
            (return))))
      (set y (- y 1)))
    ;; Nowhere to fall: reset the accumulated velocity and stay put.
    (set (at velocity row col) 0.0)))

;; One frame of physics. Bottom-up, so a grain settles at most once per frame.
(defn step [] ()
  (let [row (- rows 2)]
    (while (>= row 0)
      (dotimes [col cols]
        (unless (= 0 (at grid row col))
          (settle row col)))
      (set row (- row 1)))))

;; FNV-1a over the grid, so the headless run has one number to compare. It has
;; to be identical on native and wasm32, which is the whole reason rand-f32 is
;; a seeded PRNG written in Flan rather than libc's (plan.org, RNG is ours).
;; Named because a let binding takes no type annotation, and 0xcbf29ce484222325
;; does not fit the i32 an unannotated integer literal would default to.
(defconst fnv-offset u64 0xcbf29ce484222325)
(defconst fnv-prime  u64 1099511628211)

(defn hash-grid [] u64
  (let [h fnv-offset]
    (dotimes [row rows]
      (dotimes [col cols]
        (let [c (at grid row col)]
          (dotimes [b 4]
            (set h (bit-xor h (u64 (bit-and (>> c (u32 (* b 8))) 255))))
            (set h (* h fnv-prime))))))
    h))

;;;; ── The raylib front-end ──────────────────────────────────
;;;;
;;;; Everything from here on needs a window, and nothing headless reaches any
;;;; of it — which is why importing this file costs a headless build nothing.

;; Every cross-function call in a dev build routes through an indirection cell,
;; so redefining this from the REPL reaches the running loop on the next frame.
;; No `varfn` (Janet), no `let update = ref` (OCaml), no var-routing (jank).
;; Release builds compile the same source to direct calls.
;;
;; A cell holds an (Fn ...) — a plain function pointer, no captured environment;
;; this one is (Fn [] ()), `settle`'s is (Fn [i32 i32] ()). Redefining
;; `settle` while `game-update` is mid-frame is safe because old code is never
;; unloaded; changing its SIGNATURE is not, and the reload rejects it. See
;; plan.org "What redefinition cannot do".
(defn game-update [] ()
  (when (rl/key-pressed? :r) (clear-grid))
  (when (rl/mouse-button-down? :left)
    (let [m (rl/get-mouse-position)]
      (paint-at (/ (i32 (.y m)) cell-size)
                (/ (i32 (.x m)) cell-size))))
  (when (rl/mouse-button-released? :left) (next-color))
  (step))

(defn game-draw [] ()
  (rl/clear-background rl/black)
  (dotimes [row rows]
    (dotimes [col cols]
      (let [c (at grid row col)]
        (unless (= 0 c)
          (rl/draw-rectangle (i32 (* col cell-size))
                             (i32 (* row cell-size))
                             cell-size cell-size
                             (rl/get-color c))))))
  (rl/draw-fps 20 20))

(defn main [] ()
  (rl/set-trace-log-level :warning)
  (rl/init-window screen-width screen-height "SAND")
  (defer (rl/close-window))
  (rl/set-target-fps 120)
  ;; The dev agent listens on a socket for redefinitions and hands them over;
  ;; (agent/poll) below is where they are installed. Building without --dev is
  ;; fine — nothing has cells to install into, so a module is refused on the
  ;; listener thread and the loop never notices.
  (agent/start "/tmp/flan-sand.sock")
  ;; Bare (defn main []) — argv and the i32 status are both optional.
  ;; Nothing in this loop allocates, so context/temp is never even touched.
  (until (rl/window-should-close?)
    ;; The frame boundary, and the only place a redefinition becomes visible.
    ;; An error while installing/evaluating a dev form, or during the update,
    ;; leaves one CL-style escape hatch: choose `continue' in the editor to
    ;; abandon this frame and return to the next one with the game still live.
    ;; Keep drawing outside it.  Skipping between BeginDrawing and EndDrawing
    ;; would leave raylib's frame unbalanced.
    (restart-case
      (do (agent/poll)
          (game-update))
      (continue [] (do)))
    (rl/begin-drawing)
    (game-draw)
    (rl/end-drawing)))