// m3te core model — pure, headless match-3 board (Phase 1).
//
// Everything here is deterministic and rendering-free: a fixed seed always
// produces the same board. Later phases build on these primitives —
// P1.2 (match detection), P1.3 (swap legality), P2 (clear/cascade/refill) —
// so the layout favours plain index access (`at` / `idx`) over anything
// rendering-specific.
#import "modules/std.sx";

// ── Gem ──────────────────────────────────────────────────────────────────
// Six distinct gem types. The enum's ordinal (0..5) IS the gem index, so it
// casts cleanly to/from the integers the RNG and the textual dump work in.
GEM_COUNT :: 6;

Gem :: enum {
    red;
    orange;
    yellow;
    green;
    blue;
    purple;
}

// One stable character per gem type, indexed by ordinal — the alphabet the
// board dump (and its golden) is written in.
GEM_CHARS :: "ROYGBP";

gem_char :: (g: Gem) -> u8 {
    GEM_CHARS[cast(s64) g]
}

// ── Deterministic RNG ─────────────────────────────────────────────────────
// A 32-bit linear congruential generator (Numerical Recipes constants),
// carried in an s64 and masked back to 32 bits after every step so the
// stream is identical regardless of host integer width. The state*MUL+ADD
// product stays well under s64 range, so no intermediate overflow. Any seed
// (including 0) yields a valid stream — an LCG has no forbidden state.
RNG_MASK32 :: 0xFFFFFFFF;
RNG_MUL    :: 1664525;
RNG_ADD    :: 1013904223;

Rng :: struct {
    state: s64;

    // Advance and return the next 32-bit value.
    next_u32 :: (self: *Rng) -> s64 {
        self.state = (self.state * RNG_MUL + RNG_ADD) & RNG_MASK32;
        self.state
    }

    // Uniform-ish value in [0, n). Uses the high bits, whose period is far
    // longer than the low bits of an LCG.
    next_range :: (self: *Rng, n: s64) -> s64 {
        (self.next_u32() >> 16) % n
    }
}

rng_seeded :: (seed: s64) -> Rng {
    Rng.{ state = seed & RNG_MASK32 }
}

// ── Board ─────────────────────────────────────────────────────────────────
BOARD_COLS :: 8;
BOARD_ROWS :: 8;
BOARD_CELLS :: BOARD_COLS * BOARD_ROWS;

Board :: struct {
    // Row-major: cell (col, row) lives at row*BOARD_COLS + col.
    cells: [BOARD_CELLS]Gem;

    idx :: (col: s64, row: s64) -> s64 {
        row * BOARD_COLS + col
    }

    at :: (self: *Board, col: s64, row: s64) -> Gem {
        self.cells[Board.idx(col, row)]
    }

    set :: (self: *Board, col: s64, row: s64, g: Gem) {
        self.cells[Board.idx(col, row)] = g;
    }

    // Fill every cell from `seed` so that NO horizontal or vertical run of
    // three same-type gems exists. Cells are placed in row-major order; when
    // placing one, any gem type that would complete a 3-in-a-row with the two
    // already-placed cells to its left or above is excluded, and the gem is
    // drawn from the remaining allowed types. At most two types are ever
    // excluded, so a choice always remains.
    init :: (self: *Board, seed: s64) {
        rng := rng_seeded(seed);
        for 0..BOARD_ROWS: (row) {
            for 0..BOARD_COLS: (col) {
                self.set(col, row, pick_gem(self, @rng, col, row));
            }
        }
    }
}

// Choose a gem for (col, row) that can't extend an existing run leftward or
// upward. Pure given the board's already-placed prefix and the RNG state.
pick_gem :: (board: *Board, rng: *Rng, col: s64, row: s64) -> Gem {
    forbidden : [GEM_COUNT]bool = ---;
    for 0..GEM_COUNT: (t) { forbidden[t] = false; }

    // Two same gems immediately to the left → a third of that type matches.
    if col >= 2 {
        left := board.at(col - 1, row);
        if left == board.at(col - 2, row) {
            forbidden[cast(s64) left] = true;
        }
    }
    // Two same gems immediately above → a third of that type matches.
    if row >= 2 {
        up := board.at(col, row - 1);
        if up == board.at(col, row - 2) {
            forbidden[cast(s64) up] = true;
        }
    }

    allowed := 0;
    for 0..GEM_COUNT: (t) { if !forbidden[t] { allowed += 1; } }

    // Pick the k-th still-allowed type; single RNG draw, always terminates.
    k := rng.next_range(allowed);
    for 0..GEM_COUNT: (t) {
        if !forbidden[t] {
            if k == 0 { return cast(Gem) t; }
            k -= 1;
        }
    }
    .red // unreachable: `allowed` >= GEM_COUNT-2 >= 4, so k is always consumed
}

// Deterministic textual dump: one row per line, top (row 0) to bottom, a
// single gem character per cell. Suitable for snapshotting.
board_dump :: (self: *Board) -> string {
    line_w := BOARD_COLS + 1;            // 8 gem chars + newline
    buf := cstring(BOARD_ROWS * line_w);
    for 0..BOARD_ROWS: (row) {
        base := row * line_w;
        for 0..BOARD_COLS: (col) {
            buf[base + col] = gem_char(self.at(col, row));
        }
        buf[base + BOARD_COLS] = 10;     // '\n'
    }
    buf
}