add-fen-loader (#5)

Reviewed-on: #5
Co-authored-by: Josh <josh@joshuaschuett.com>
Co-committed-by: Josh <josh@joshuaschuett.com>

engine/src/fen.c

@@ -0,0 +1,120 @@
+#include <stdio.h>
+#include <ctype.h>
+#include <stdlib.h>
+#include <string.h>
+#include <stdbool.h>
+#include "bitboard.h"
+
+
+int square_index(int file, int rank) {
+    return rank*8 + file;
+}
+
+int char_to_piece_index(char c) {
+    switch (c) {
+        case 'P': return P; case 'N': return N; case 'B': return B;
+        case 'R': return R; case 'Q': return Q; case 'K': return K;
+        case 'p': return p; case 'n': return n; case 'b': return b;
+        case 'r': return r; case 'q': return q; case 'k': return k;
+        default:  return -1;
+    }
+}
+
+int load_fen(struct Board *board, const char *fen) {
+    // Reset the board first. Weird behavior when not zeroed.
+    memset(board, 0, sizeof *board);
+    board->ep_square = -1;
+
+    int rank = 7, file = 0;
+    const char *ptr = fen;
+
+    // Place pieces into the board.
+    while (*ptr && *ptr != ' ') {
+        if (isdigit(*ptr)) {
+            file += *ptr - '0';
+            ptr++;
+        } else if (isalpha(*ptr)) {
+            if (file >= 8) return -1;
+            
+            int piece = char_to_piece_index(*ptr);
+            
+            if (piece == -1) return -1;
+            
+            SET_BIT(board->pieces[piece], rank * 8 + file);
+            file++;
+            ptr++;
+        } else if (*ptr == '/') {
+            // Each rank must have exactly 8 files
+            if (file != 8) return -1;
+            rank--;
+            file = 0;
+            ptr++;
+        } else {
+            // Unexpected character
+            return -1;
+        }
+    }
+
+    if (rank != 0 || file != 8) return -1;
+    ptr++;
+
+    // Side to move.  
+    if (*ptr != 'w' && *ptr != 'b') return -1;
+
+    board->side_to_move = (*ptr == 'w') ? WHITE : BLACK;
+    ptr += 2;
+
+    // Castling rights.
+    board->castling_rights = 0;
+    while (*ptr && *ptr != ' ') {
+        switch (*ptr) {
+            case 'K': board->castling_rights |= CASTLE_WK; break;
+            case 'Q': board->castling_rights |= CASTLE_WQ; break;
+            case 'k': board->castling_rights |= CASTLE_BK; break;
+            case 'q': board->castling_rights |= CASTLE_BQ; break;
+            case '-': break;
+            default: return -1;
+        }
+        ptr++;
+    }
+
+    if (*ptr != ' ') return -1;
+    ptr++;
+
+    // En Passant square.
+    if (*ptr == '-') {
+        board->ep_square = -1;
+        ptr += 2;
+    } else {
+        if (ptr[0] < 'a' || ptr[0] > 'h' || ptr[1] < '1' || ptr[1] > '8') return -1;
+        
+        // Strict FEN: Rank must be 3 or 6.
+        char rank_char = ptr[1];
+        if (rank_char != '3' && rank_char != '6') return -1;
+        
+        int file = ptr[0] - 'a';
+        int rank = ptr[1] - '1';
+
+
+
+        board->ep_square = square_index(file, rank);
+        ptr += 3;
+    }
+
+    // Parse halfmove clock for the game.
+    board->halfmove_clock = atoi(ptr);
+    while (*ptr && *ptr != ' ') ptr++;
+    if (*ptr == ' ') ptr++;
+
+    // Parse the fullmove number.
+    board->fullmove_number = atoi(ptr);
+
+    int num_pieces = 6;
+    for (int i = 0; i < num_pieces; i++) {
+        board->occ[WHITE] |= board->pieces[i];      // white
+        board->occ[BLACK] |= board->pieces[i + 6];  // black
+    }
+    board->occ[BOTH] = board->occ[WHITE] | board->occ[BLACK];
+
+    return 0;
+}