From 583f9ba34610d959db25e9e11f6db9599bc104fe Mon Sep 17 00:00:00 2001
From: Artsvi Manukyan <42891870+Manukyanq@users.noreply.github.com>
Date: Sun, 28 May 2023 16:59:51 +0300
Subject: [PATCH] Add check for invalid pawn placement in validateFen (#412)

---
 __tests__/validate_fen.test.ts | 10 ++++++++++
 src/chess.ts                   | 12 ++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/__tests__/validate_fen.test.ts b/__tests__/validate_fen.test.ts
index 17316f5c..79c9b24a 100644
--- a/__tests__/validate_fen.test.ts
+++ b/__tests__/validate_fen.test.ts
@@ -26,6 +26,16 @@ test.each([
     fen: 'kk6/8/8/8/8/8/8/7K w - - 0 1',
     ok: false,
   },
+  {
+    // white pawn on 8th row
+    fen: '3P4/1k2K3/8/8/8/8/8/8 w - - 0 1',
+    ok: false,
+  },
+  {
+    // black pawn on 1st row
+    fen: '8/8/8/8/8/8/1k3K2/3p4 w - - 0 1',
+    ok: false,
+  },
   {
     fen: 'rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNRw KQkq - 0 1',
     ok: false,
diff --git a/src/chess.ts b/src/chess.ts
index 178e3850..b01c6bdd 100644
--- a/src/chess.ts
+++ b/src/chess.ts
@@ -379,6 +379,7 @@ export function validateFen(fen: string) {
     }
   }
 
+  // 9th criterion: is en-passant square legal?
   if (
     (tokens[3][1] == '3' && tokens[1] == 'w') ||
     (tokens[3][1] == '6' && tokens[1] == 'b')
@@ -386,6 +387,7 @@ export function validateFen(fen: string) {
     return { ok: false, error: 'Invalid FEN: illegal en-passant square' }
   }
 
+  // 10th criterion: does chess position contain exact two kings?
   const kings = [
     { color: 'white', regex: /K/g },
     { color: 'black', regex: /k/g },
@@ -401,6 +403,16 @@ export function validateFen(fen: string) {
     }
   }
 
+  // 11th criterion: are any pawns on the first or eighth rows?
+  if (
+    Array.from(rows[0] + rows[7]).some((char) => char.toUpperCase() === 'P')
+  ) {
+    return {
+      ok: false,
+      error: 'Invalid FEN: some pawns are on the edge rows',
+    }
+  }
+
   return { ok: true }
 }