fix bug in capstone -> wall logicHEADmain

From: ~devp

Previously, the cap flag checked stone 0 of a stack rather than stone
n-1, so it did not take into account a capstone moved as part of a
stack. I additionally added logic to check that no other stones were
dropped with the capstones, per Tak rules.

2 files changed, 14 insertions, 2 deletions

diff --git a/README.md b/README.md
index a01db3f..4830c94 100644
--- a/README.md
+++ b/README.md
@@ -49,3 +49,9 @@ for this purpose.
 Note that the engine is expected to respect the `[Variant XXX]` line
 in the PTN game-state, so expect breakage with anything but the
 default.
+
+## Additional contributors
+
+With thanks to
+
+- ~devp for fixing a capstone->wall flattening bug
diff --git a/src/game.rs b/src/game.rs
index 4a91755..6bc43dc 100644
--- a/src/game.rs
+++ b/src/game.rs
@@ -424,7 +424,7 @@ impl GameState {
                 }
 
                 let mut steps: usize = drops.len();
-                let cap: bool = stack[0].stone == Stone::Capstone;
+                let cap: bool = stack[stack.len() - 1].stone == Stone::Capstone;
                 let mut pos_new = Position { x: pos.x, y: pos.y };
 
                 while steps > 0 {
@@ -453,7 +453,13 @@ impl GameState {
                                         ));
                                     }
                                     Stone::Standing => {
-                                        if (steps > 1) || (!cap) {
+                                        if cap {
+                                            if (steps > 1) || (drops[drops.len() - 1] > 1) {
+                                                return Err(String::from(
+                                                    "A capstone move may not flatten a standing stone unless dropped on its own.",
+                                                ));
+                                            }
+                                        } else {
                                             return Err(String::from(
                                                 "A move may not cover a standing stone.",
                                             ));