m3te/tests/refill.sx

// Refill golden: run the full resolution pipeline clear -> collapse -> refill
// over a HAND-CRAFTED seeded board and snapshot every stage. The starting board
// is the run-free O/G checkerboard from match_detect carrying three disjoint
// matches (RRR row 1, PPP row 5, YYY col 6); clearing punches 9 holes, gravity
// floats them to the top of their columns, and refill drops fresh gems in.
//
// Determinism is shown three independent ways, all locked by the snapshot and by
// asserts:
//   * stream-continuation — each refilled hole holds exactly the NEXT draw of the
//     board's own RNG, replayed from the pre-refill state. A reseed-from-scratch
//     or a constant fill both fail this.
//   * reproducibility — the same start + seed refills to a byte-identical board.
//   * threading across refills — re-opening the just-filled holes and refilling
//     again yields DIFFERENT gems, proving the RNG advances rather than reseeding.
#import "modules/std.sx";
#import "board.sx";
t :: #import "test.sx";

SEED :: 1337;

// Inverse of `gem_char`: map a board character back to its Gem so the starting
// board can be written as a human-readable grid. The hole glyph maps to `.empty`.
char_to_gem :: (c: u8) -> Gem {
    if c == EMPTY_CHAR { return .empty; }
    for 0..GEM_COUNT (i) {
        if GEM_CHARS[i] == c { return cast(Gem) i; }
    }
    .red
}

// Load an 8x8 board from `rows` (top row first, each exactly BOARD_COLS chars).
// The RNG is left unseeded — callers seed it before drawing.
load_board :: (rows: []string) -> Board {
    b : Board = ---;
    for 0..BOARD_ROWS (row) {
        line := rows[row];
        for 0..BOARD_COLS (col) {
            b.set(col, row, char_to_gem(line[col]));
        }
    }
    b
}

count_empties :: (b: *Board) -> i64 {
    n : i64 = 0;
    for 0..BOARD_CELLS (i) { if b.cells[i] == .empty { n += 1; } }
    n
}

boards_equal :: (a: *Board, b: *Board) -> bool {
    for 0..BOARD_CELLS (i) { if a.cells[i] != b.cells[i] { return false; } }
    true
}

// A fresh starting board with its RNG seeded from SEED. Because the RNG lives on
// the board, the refill it later performs is reproducible for SEED. The board is
// a run-free checkerboard carrying three disjoint matches — a horizontal RRR run
// (row 1, cols 0-2), a horizontal PPP run (row 5, cols 2-4) and a vertical YYY
// run (col 6, rows 4-6) — so clearing them punches 9 holes across six columns.
fresh_board :: () -> Board {
    b := load_board(.[
        "OGOGOGOG",
        "RRROGOGO",
        "OGOGOGOG",
        "GOGOGOGO",
        "OGOGOGYG",
        "GOPPPOYO",
        "OGOGOGYG",
        "GOGOGOGO",
    ]);
    b.rng = rng_seeded(SEED);
    b
}

main :: () -> i32 {
    print("== refill (seeded) ==\n");

    // Pipeline, snapshotting each stage.
    b := fresh_board();
    out("start:\n");          out(board_dump(@b));
    clear_matches(@b);
    out("after clear:\n");    out(board_dump(@b));
    collapse(@b);
    out("after collapse:\n"); out(board_dump(@b));

    // Snapshot the holed board (cells + RNG state) BEFORE refill, so we can both
    // check which cells refill touched and replay the stream from the same state.
    pre := b;
    filled := refill(@b);
    out("after refill:\n");   out(board_dump(@b));
    print("filled {} holes\n", filled);

    // (1) No empty cells remain.
    t.expect(count_empties(@b) == 0, "refill: board has zero empty cells");

    // (2) Stream continuation + not-a-constant: every refilled cell holds exactly
    // the next draw of the board's RNG, taken row-major from the pre-refill state,
    // and the drawn gems are not all identical.
    v := Rng.{ state = pre.rng.state };
    stream_ok := true;
    distinct := false;
    have_first := false;
    first : Gem = .empty;
    for 0..BOARD_CELLS (i) {
        if pre.cells[i] == .empty {
            want := cast(Gem) v.next_range(GEM_COUNT);
            if b.cells[i] != want { stream_ok = false; }
            if !have_first { first = b.cells[i]; have_first = true; }
            else if b.cells[i] != first { distinct = true; }
        }
    }
    t.expect(stream_ok, "refill: each hole holds the next seeded-stream gem");
    t.expect(distinct, "refill: drawn gems vary (not a fixed constant)");

    // (3) Reproducibility: same start + seed refills to a byte-identical board.
    b2 := fresh_board();
    clear_matches(@b2);
    collapse(@b2);
    refill(@b2);
    t.expect(boards_equal(@b, @b2), "refill: same start + seed -> identical board");

    // (4) Threading across refills: re-open exactly the cells the first refill
    // filled, then refill again. The board's RNG has advanced past the first
    // fill, so the second fill draws new gems — proof it does NOT reseed per call.
    holes_n := 0;
    hole_idx : [BOARD_CELLS]i64 = ---;
    fill1 : [BOARD_CELLS]Gem = ---;
    for 0..BOARD_CELLS (i) {
        if pre.cells[i] == .empty {
            hole_idx[holes_n] = i;
            fill1[holes_n] = b.cells[i];
            holes_n += 1;
        }
    }
    for 0..holes_n (k) { b.cells[hole_idx[k]] = .empty; }
    refill(@b);
    differs := false;
    for 0..holes_n (k) {
        if b.cells[hole_idx[k]] != fill1[k] { differs = true; }
    }
    t.expect(differs, "refill: a second refill of the same holes draws new gems (RNG threads, no reseed)");

    print("ok: refill fills every hole from the seeded stream\n");
    return 0;
}