engine/src/bitboard.c

@@ -135,6 +135,214 @@ static void push_move(struct Move *out, int *count, int from, int to, uint8_t pi
     (*count)++;
 }
 
+/**
+    PAWN struct Movement Psuedo Moves
+
+
+    Separate the white and black pawn logic. Although the logic is similar,
+    by separating the logic, we can isolate the different cases and have clearer parent functions.
+    These pieces are also the only ones that move in opposite ways, meaning we only need
+    to separate out seemingly related code for this case. Additionally, pawns have the
+    most complicated movement patterns and behaviors in the game. Separating out the logic should
+    hopefully make debugging easier.
+*/
+static void gen_white_pawn_quiet_pushes(const struct Board *b, struct Move *out, int *n) {
+    const uint64_t occ   = b->occ[BOTH];
+    const uint64_t empty = ~occ;
+
+    const uint64_t pawns = b->pieces[P];
+
+    // One-step pushes: shift pawns up by 8 onto empty squares
+    uint64_t one_step = (pawns << 8) & empty;
+
+    // Exclude promotions here (dest on rank 8) — handled by a separate promo-push function
+    uint64_t one_step_no_promo = one_step & ~RANK_8;
+
+    // Two-step pushes: those one-step pawns that landed on rank 3 can go one more if still empty
+    // (means they originally stood on rank 2)
+    uint64_t two_step = ((one_step & RANK_3) << 8) & empty;
+
+    uint64_t bb = one_step_no_promo;
+    while (bb) {
+        int to = pop_lsb_index(&bb);
+        int from = to - 8;
+        push_move(out, n, from, to, P, 0, MF_NONE);
+    }
+
+    bb = two_step;
+    while (bb) {
+        int to = pop_lsb_index(&bb);
+        int from = to - 16;
+        push_move(out, n, from, to, P, 0, MF_DOUBLE_PUSH);
+    }
+}
+
+static void gen_black_pawn_quiet_pushes(const struct Board *b, struct Move *out, int *n) {
+    const uint64_t occ   = b->occ[BOTH];
+    const uint64_t empty = ~occ;
+
+    const uint64_t pawns = b->pieces[p];
+
+    // One-step pushes: shift pawns down by 8 onto empty squares
+    uint64_t one_step = (pawns >> 8) & empty;
+
+    // Exclude promotions here (dest on rank 1) — handle in a promo-push function
+    uint64_t one_step_no_promo = one_step & ~RANK_1;
+
+    uint64_t two_step = ((one_step & RANK_6) >> 8) & empty;
+
+    uint64_t bb = one_step_no_promo;
+    while (bb) {
+        int to   = pop_lsb_index(&bb);
+        int from = to + 8;
+        push_move(out, n, from, to, p, 0, MF_NONE);
+    }
+
+    bb = two_step;
+    while (bb) {
+        int to   = pop_lsb_index(&bb);
+        int from = to + 16;
+        push_move(out, n, from, to, p, 0, MF_DOUBLE_PUSH);
+    }
+}
+
+// We will only allow pawns to promote to queen. Technically, they should be allowed
+// to promote to any of the following: Rook, Bishop, Knight, Queen.
+static void gen_white_pawn_push_promotions(const struct Board *b, struct Move *out, int *n) {
+    const uint64_t occ   = b->occ[BOTH];
+    const uint64_t empty = ~occ;
+    const uint64_t pawns = b->pieces[P];
+
+    // destinations on rank 8 reachable by a single push
+    uint64_t promos = ((pawns << 8) & empty) & RANK_8;
+
+    while (promos) {
+        int to   = pop_lsb_index(&promos);
+        int from = to - 8;
+        push_move(out, n, from, to, P, Q, MF_PROMO);
+    }
+}
+
+static void gen_black_pawn_push_promotions(const struct Board *b, struct Move *out, int *n) {
+    const uint64_t occ   = b->occ[BOTH];
+    const uint64_t empty = ~occ;
+    const uint64_t pawns = b->pieces[p];
+
+    // destinations on rank 1 reachable by a single push
+    uint64_t promos = ((pawns >> 8) & empty) & RANK_1;
+
+    while (promos) {
+        int to   = pop_lsb_index(&promos);
+        int from = to + 8;
+        push_move(out, n, from, to, p, q, MF_PROMO);
+    }
+}
+
+static void gen_white_pawn_capture_promotions(const struct Board *b, struct Move *out, int *n) {
+    const uint64_t opp = b->occ[BLACK];
+    // left capture (from white view): +7, mask off file A
+    uint64_t left  = ((b->pieces[P] & ~FILE_A) << 7) & opp & RANK_8;
+    while (left) {
+        int to = pop_lsb_index(&left);
+        int from = to - 7;
+        push_move(out, n, from, to, P, Q, MF_CAPTURE | MF_PROMO);
+    }
+    // right capture: +9, mask off file H
+    uint64_t right = ((b->pieces[P] & ~FILE_H) << 9) & opp & RANK_8;
+    while (right) {
+        int to = pop_lsb_index(&right);
+        int from = to - 9;
+        push_move(out, n, from, to, P, Q, MF_CAPTURE | MF_PROMO);
+    }
+}
+
+static void gen_black_pawn_capture_promotions(const struct Board *b, struct Move *out, int *n) {
+    const uint64_t opp = b->occ[WHITE];
+    // from black view, “left” is -7 (mask off file H before shifting)
+    uint64_t left  = ((b->pieces[p] & ~FILE_H) >> 7) & opp & RANK_1;
+    while (left) {
+        int to = pop_lsb_index(&left);
+        int from = to + 7;
+        push_move(out, n, from, to, p, q, MF_CAPTURE | MF_PROMO);
+    }
+    // “right” is -9 (mask off file A)
+    uint64_t right = ((b->pieces[p] & ~FILE_A) >> 9) & opp & RANK_1;
+    while (right) {
+        int to = pop_lsb_index(&right);
+        int from = to + 9;
+        push_move(out, n, from, to, p, q, MF_CAPTURE | MF_PROMO);
+    }
+}
+
+static void gen_white_pawn_captures(const struct Board *b, struct Move *out, int *n) {
+    const uint64_t pawns  = b->pieces[P];
+    const uint64_t opp    = b->occ[BLACK];
+
+    // Normal captures (exclude promotion rank)
+    uint64_t left_caps  = ((pawns & ~FILE_A) << 7) & opp & ~RANK_8;
+    uint64_t right_caps = ((pawns & ~FILE_H) << 9) & opp & ~RANK_8;
+
+    // Emit normal captures
+    while (left_caps) {
+        int to   = pop_lsb_index(&left_caps);
+        int from = to - 7;
+        push_move(out, n, from, to, P, 0, MF_CAPTURE);
+    }
+    while (right_caps) {
+        int to   = pop_lsb_index(&right_caps);
+        int from = to - 9;
+        push_move(out, n, from, to, P, 0, MF_CAPTURE);
+    }
+
+    // En passant (destination is ep_square)
+    if (b->ep_square >= 0) {
+        uint64_t ep = 1ULL << b->ep_square;
+
+        // From-squares that can capture onto ep (reverse of +7/+9)
+        uint64_t ep_from = (((ep & ~FILE_H) >> 7) | ((ep & ~FILE_A) >> 9)) & pawns;
+
+        while (ep_from) {
+            int from = pop_lsb_index(&ep_from);
+            int to   = b->ep_square;
+            // EP never promotes, so no promo piece; still a capture
+            push_move(out, n, from, to, P, 0, MF_CAPTURE | MF_ENPASSANT);
+        }
+    }
+}
+
+static void gen_black_pawn_captures(const struct Board *b, struct Move *out, int *n) {
+    const uint64_t pawns  = b->pieces[p];
+    const uint64_t opp    = b->occ[WHITE];
+
+    // Normal captures (exclude promotion rank)
+    uint64_t left_caps  = ((pawns & ~FILE_H) >> 7) & opp & ~RANK_1; // from black POV, "left" is -7
+    uint64_t right_caps = ((pawns & ~FILE_A) >> 9) & opp & ~RANK_1; // "right" is -9
+
+    while (left_caps) {
+        int to   = pop_lsb_index(&left_caps);
+        int from = to + 7;
+        push_move(out, n, from, to, p, 0, MF_CAPTURE);
+    }
+    while (right_caps) {
+        int to   = pop_lsb_index(&right_caps);
+        int from = to + 9;
+        push_move(out, n, from, to, p, 0, MF_CAPTURE);
+    }
+
+    // En passant
+    if (b->ep_square >= 0) {
+        uint64_t ep = 1ULL << b->ep_square;
+
+        // From-squares that can capture onto ep (reverse of -7/-9)
+        uint64_t ep_from = (((ep & ~FILE_A) << 7) | ((ep & ~FILE_H) << 9)) & pawns;
+
+        while (ep_from) {
+            int from = pop_lsb_index(&ep_from);
+            int to   = b->ep_square;
+            push_move(out, n, from, to, p, 0, MF_CAPTURE | MF_ENPASSANT);
+        }
+    }
+}
 
 /**