1 files changed, 83 insertions, 0 deletions
diff --git a/src/engine/evaluate.c b/src/engine/evaluate.c
new file mode 100644
index 0000000..1722da2
--- /dev/null
+++ b/src/engine/evaluate.c
@@ -0,0 +1,83 @@
+#include "engine/evaluate.h"
+
+#include <stdlib.h>
+
+enum {
+  EVAL_PAWN   = 100,
+  EVAL_KNIGHT = 320,
+  EVAL_BISHOP = 320,
+  EVAL_ROOK   = 500,
+  EVAL_QUEEN  = 900,
+};
+
+static const i32 MATERIAL[PIECE_TYPE_COUNT] = {
+  [WHITE_KING]   = 0,
+  [WHITE_QUEEN]  = EVAL_QUEEN,
+  [WHITE_ROOK]   = EVAL_ROOK,
+  [WHITE_BISHOP] = EVAL_BISHOP,
+  [WHITE_KNIGHT] = EVAL_KNIGHT,
+  [WHITE_PAWN]   = EVAL_PAWN,
+  [BLACK_KING]   = 0,
+  [BLACK_QUEEN]  = EVAL_QUEEN,
+  [BLACK_ROOK]   = EVAL_ROOK,
+  [BLACK_BISHOP] = EVAL_BISHOP,
+  [BLACK_KNIGHT] = EVAL_KNIGHT,
+  [BLACK_PAWN]   = EVAL_PAWN,
+};
+
+// simple positional bonus (a light "pst"): pawns like to advance, knights like
+// the centre. everything else is just material.
+static i32 pst_bonus(int piece_type, int square) {
+  int rank = square / 8;
+  int file = square % 8;
+  if (piece_type == WHITE_PAWN) return (rank - 1) * 6;
+  if (piece_type == BLACK_PAWN) return (6 - rank) * 6;
+  if (piece_type == WHITE_KNIGHT || piece_type == BLACK_KNIGHT) {
+    bool central = (file == 3 || file == 4) && (rank == 3 || rank == 4);
+    bool nearcentre = (file >= 2 && file <= 5) && (rank >= 2 && rank <= 5);
+    return central ? 16 : (nearcentre ? 4 : -4);
+  }
+  return 0;
+}
+
+i32 evaluate(position_t position) {
+  i32 white = 0;
+  i32 black = 0;
+  for (int piece = 0; piece < PIECE_TYPE_COUNT; piece++) {
+    bitboard_t bb = position.bitboards[piece];
+    while (bb) {
+      int sq = __builtin_ctzll(bb);
+      bb &= bb - 1;
+      if (piece < BLACK_KING) {
+        white += MATERIAL[piece] + pst_bonus(piece, sq);
+      } else {
+        black += MATERIAL[piece] + pst_bonus(piece, sq);
+      }
+    }
+  }
+  i32 score = white - black;
+  return (position.turn == WHITE_TURN) ? score : -score;
+}
+
+#ifdef TEST_MOD
+#include <stdbool.h>
+
+bool test_eval_startpos_is_balanced() {
+  position_t start = position_starting();
+  // symmetric starting position -> static eval should be ~0.
+  return abs(evaluate(start)) < 100;
+}
+
+bool test_eval_scales_with_material() {
+  position_t pos = {0};
+  pos.turn = WHITE_TURN;
+  // give white a queen on a1, black only a king on h8
+  pos.bitboards[WHITE_QUEEN] = 1ULL;
+  pos.bitboards[BLACK_KING] = (1ULL << 63);
+  pos.bitboards[WHITE_KING] = (1ULL << 4);   // a5, far from the action
+  pos.bitboards[BLACK_PAWN] = 0;
+  i32 score = evaluate(pos);
+  // white is up a queen (~900) plus a little; should be comfortably positive.
+  return score > 800;
+}
+#endif