;;;; The falling-sand simulation, with no raylib in it.
;;;;
;;;; It is a package of its own because milestone 4 asks for sand to be tested
;;;; twice — interactive at 120 fps, and headless over N frames with the grid
;;;; hashed (plan.org, Build sequence). The headless run is the one CI does on
;;;; wasm32, and a program that imports the raylib package links the raylib
;;;; shared library on *every* target, whatever its main does. So the headless
;;;; artifact cannot import raylib at all, and the only way to have both
;;;; without two copies of the simulation is for both to import this.
;;;;
;;;; The directory is the package (plan.org, Modules): sand.flan imports it as
;;;; sim/, test/programs/sand-headless.flan imports it as sim/ too.

(defconst screen-width  1400)
(defconst screen-height 1000)
(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] [0xE6B800FF 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 empty-at? [row i32  col i32] bool
  (= 0 (at grid row col)))

(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))
                     (empty-at? r c)
                     (< (rand-f32) 0.5))
            (set (at grid r c)     (nth colors current-color))
            (set (at velocity r c) 1.0)))))))

(defn move-grain [from-row i32  from-col i32
                  to-row   i32  to-col   i32
                  vel      f32]
  (set (at grid to-row to-col)         (at grid from-row from-col))
  (set (at grid from-row from-col)     0)
  (set (at velocity to-row to-col)     vel)
  (set (at velocity from-row from-col) 0.0))

;; 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 (empty-at? y col)
        (move-grain row col y col vel)
        (return))
      (let [left?  (and (> col 0)          (empty-at? y (- col 1)))
            right? (and (< col (- cols 1)) (empty-at? y (+ col 1)))]
        (when (or left? right?)
          (let [side (cond
                       (not left?)  1
                       (not right?) -1
                       :else        (if (< (rand-f32) 0.5) 1 -1))]
            (move-grain row col y (+ col side) vel)
            (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 (empty-at? 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))