From 1ee34b9d35a1ad6083967f858988180a425e43ec Mon Sep 17 00:00:00 2001
From: eug-vs <eugene@eug-vs.xyz>
Date: Wed, 31 Aug 2022 07:37:27 +0300
Subject: feat: discourage bad pawn structure

---
 src/main.c | 40 ++++++++++++++++++++++++++++++++++++++--
 1 file changed, 38 insertions(+), 2 deletions(-)

(limited to 'src/main.c')

diff --git a/src/main.c b/src/main.c
index 39346c2..c68c62e 100644
--- a/src/main.c
+++ b/src/main.c
@@ -432,6 +432,41 @@ int list_available_moves(Move* moves, int* board, int color) {
   return moves_count;
 }
 
+// Count doubled, blocked and isolated pawns
+int count_bad_pawns(int* board, int color) {
+  int bad_pawns = 0;
+  long occupied_files = 0b00000000;
+
+  for (int file = 0; file < 8; file++) {
+    int file_pawns = 0;
+    for (int rank = 7; rank >= 0; rank--) {
+      int piece = board[rank * 16 + file];
+      if ((piece & NO_COLOR) == PAWN && piece % 2 == color) {
+        file_pawns++;
+        occupied_files |= 1 << file;
+
+        // If pawn can't advance, it's blocked
+        int advance_rank = rank + (color == WHITE ? 1 : -1);
+        if (board[advance_rank * 16 + file] != EMPTY) bad_pawns++;
+      }
+    }
+
+    // Doubled or tripled pawns will get punished
+    bad_pawns += file_pawns - 1;
+  }
+
+  // Shift everything to the left to get some space around right-most bit
+  occupied_files = occupied_files << 1;
+
+  // Count isolated pawns based on bits in occupied files
+  for (int bit = 1; bit < 9; bit++) {
+    long bit_mask = (1 << (bit + 1)) | (1 << (bit - 1));
+    if (!(occupied_files & bit_mask)) bad_pawns++;
+  }
+
+  return bad_pawns;
+}
+
 /*
  * Return a total board evaluation symbolizing advantage
  * for WHITE, meaining WHITE tries to maximize this
@@ -448,7 +483,9 @@ int evaluate_position(int* board, int precomputed_mobility, int mobility_color)
 
   int white_mobility_advantage = (mobility - opponent_mobility) * (mobility_color == WHITE ? 1 : -1);
 
-  return white_material_advantage + white_mobility_advantage;
+  int pawn_structure_advantage = count_bad_pawns(board, BLACK) - count_bad_pawns(board, WHITE);
+
+  return white_material_advantage + white_mobility_advantage + 5 * pawn_structure_advantage;
 }
 
 int get_piece_raw_value(int piece) {
@@ -562,7 +599,6 @@ int main() {
     int color = ply % 2;
     printf("##### Ply %i #####\n", ply);
     printf("Current evaluation: %i\n", evaluate_position(board, 0, WHITE));
-    printf("Zobrist hash: %lo\n", hash);
 
     if (color == PLAYER) {
       printf("Enter a move for %s:\n", COLORS[color]);
-- 
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