1 files changed, 159 insertions, 0 deletions
diff --git a/src/engine/fen.c b/src/engine/fen.c
new file mode 100644
index 0000000..187271e
--- /dev/null
+++ b/src/engine/fen.c
@@ -0,0 +1,159 @@
+#include "fen.h"
+#include "bitboard.h"
+#include <assert.h>
+#include <stdio.h>
+
+int get_castling(char c) {
+  if (c == 'K') return WHITE_SHORT_CASTLE;
+  if (c == 'Q') return WHITE_LONG_CASTLE;
+  if (c == 'k') return BLACK_SHORT_CASTLE;
+  if (c == 'q') return BLACK_LONG_CASTLE;
+  assert(0);
+}
+
+int get_turn(char c) {
+  if (c == 'w') return WHITE_TURN;
+  if (c == 'b') return BLACK_TURN;
+  assert(0);
+}
+
+int get_piece_enum_item(char c) {
+  if (c == 'P') return WHITE_PAWN;
+  if (c == 'N') return WHITE_KNIGHT;
+  if (c == 'B') return WHITE_BISHOP;
+  if (c == 'R') return WHITE_ROOK;
+  if (c == 'Q') return WHITE_QUEEN;
+  if (c == 'K') return WHITE_KING;
+  
+  if (c == 'p') return BLACK_PAWN;
+  if (c == 'n') return BLACK_KNIGHT;
+  if (c == 'b') return BLACK_BISHOP;
+  if (c == 'r') return BLACK_ROOK;
+  if (c == 'q') return BLACK_QUEEN;
+  if (c == 'k') return BLACK_KING;
+
+  assert(0);
+}
+
+/*
+ * from Wikipedia
+ * Piece placement data
+ *    Each rank is described, starting with rank 8 and ending with rank 1,
+ *    with a "/" between each one; within each rank, the contents of the
+ *    squares are described in order from the a-file to the h-file.
+ *    Each piece is identified by a single letter taken from the standard
+ *    English names in algebraic notation
+ *    (pawn = "P", knight = "N", bishop = "B",
+ *    rook = "R", queen = "Q", and king = "K").
+ *    White pieces are designated using uppercase letters ("PNBRQK"),
+ *    while black pieces use lowercase letters ("pnbrqk").
+ *    A set of one or more consecutive empty squares within a rank is denoted
+ *    by a digit from "1" to "8", corresponding to the number of squares.
+ * Active color
+ *    "w" means that White is to move; "b" means that Black is to move.
+ * Castling availability
+ *    If neither side has the ability to castle, this field uses the character
+ *    "-". Otherwise, this field contains one or more letters:
+ *    "K" if White can castle kingside,
+ *    "Q" if White can castle queenside,
+ *    "k" if Black can castle kingside,
+ *    and "q" if Black can castle queenside.
+ *    A situation that temporarily prevents castling does not
+ *    prevent the use of this notation.
+ * En passant target square
+ *    This is a square over which a pawn has just passed while moving two
+ *    squares; it is given in algebraic notation.
+ *    If there is no en passant target square, this field uses the character "-".
+ *    This is recorded regardless of whether there is a pawn in position to
+ *    capture en passant. An updated version of the spec has since made it
+ *    so the target square is recorded only if a legal en passant capture is
+ *    possible, but the old version of the standard
+ *    is the one most commonly used.
+ * Halfmove clock
+ *    The number of halfmoves since the last capture or pawn advance,
+ *    used for the fifty-move rule.
+ * Fullmove number
+ *    The number of the full moves.
+ *    It starts at 1 and is incremented after Black's move.
+ *
+ *  e.g. 1B6/2n5/p1N1P2R/P1K3N1/4Pk2/1Q2p2p/6nP/1B4R1 w - - 0 1
+ */
+position_t load_fen(const char* fen) {
+  position_t position = {0};
+
+  int square = 56;
+  int i = -1;
+  char c;
+  while ((c = fen[++i]) != ' ') {
+    if (c >= '0' && c <= '9') {
+      assert(c != '9');
+      square += c - '0';
+      continue;
+    }
+    if (c == '/') {
+      square -= 16;
+      continue;
+    }
+
+    int piece_type = get_piece_enum_item(c);
+    position.bitboards[piece_type] |= (bitboard_t)1 << square;
+  }
+
+  c = fen[++i];
+  position.turn = get_turn(c);
+  assert(fen[++i] == ' ');
+
+  while ((c = fen[++i]) != ' ') {
+    if (c == '-') continue;
+    position.castling |= get_castling(c);
+  }
+
+  c = fen[++i];
+  if (c >= 'a' && c <= 'z') {
+    assert(c <= 'h');
+    position.passantable_file = c - 'a' + 1;
+    c = fen[++i];
+    assert(c >= '1' && c <= '8');
+  } else {
+    position.passantable_file = 0;
+  }
+  assert(fen[++i] == ' ');
+
+  while ((c = fen[++i]) != ' ') {
+    assert(c >= '0' && c <= '9');
+    position.halfmove_clock *= 10;
+    position.halfmove_clock += c - '0';
+  }
+
+  while ((c = fen[++i]) != 0) {
+    assert(c >= '0' && c <= '9');
+    position.fullmove_clock *= 10;
+    position.fullmove_clock += c - '0';
+  }
+
+  assert_valid_position(position);
+  return position;
+}
+
+#ifdef TEST_MOD
+#include "bitboard.c"
+#include <stdbool.h>
+
+bool test_fen_no_passant() {
+  position_t p = load_fen("1B6/2n5/p1N1P2R/P1K3N1/4Pk2/1Q2p2p/6nP/1B4R1 w - - 0 1");
+  if (p.castling != 0) return false;
+  if (p.passantable_file != 0) return false;
+  return true;
+}
+bool test_fen_passant() {
+  position_t p = load_fen("rnbqkbnr/ppp1p1pp/8/3pPp2/8/8/PPPP1PPP/RNBQKBNR w KQkq f6 0 3");
+  if (
+    p.castling != (
+      WHITE_LONG_CASTLE | WHITE_SHORT_CASTLE |
+      BLACK_LONG_CASTLE | BLACK_SHORT_CASTLE
+    )
+  ) return false;
+  if (p.passantable_file != 6) return false;
+  return true;
+}
+#endif