Fix tank shell spawn position

Shell now spawns 16px past the tank's leading edge rather than at the
tank's origin, matching the corrected spec (8.2.2).

Author: morozov

pkg/logic/enemy_ai.go

@@ -185,7 +185,7 @@ func moveTank(obj *state.ViewportObject, tick int, ts *state.TankShell, bridgeDe
 			// River edge reached (or already at it): fire.
 			// FireTankShell is a no-op while the shell is flying or exploding,
 			// so this naturally implements the fire/wait cycle.
-			FireTankShell(ts, obj.X, obj.Y, tick, obj.Orientation)
+			FireTankShell(ts, shellSpawnX(obj.X, obj.Orientation), obj.Y, tick, obj.Orientation)
 		}
 	}
 }

pkg/logic/tank_shell.go

@@ -1,11 +1,27 @@
 package logic
 
 import (
+	"github.com/morozov/river-raid-ebiten/pkg/assets"
 	"github.com/morozov/river-raid-ebiten/pkg/domain"
 	"github.com/morozov/river-raid-ebiten/pkg/platform"
 	"github.com/morozov/river-raid-ebiten/pkg/state"
 )
 
+// shellSpawnOffset is the distance (px) from the tank's leading edge to the shell
+// spawn point, in the direction the tank faces.
+const shellSpawnOffset = 16
+
+// ShellSpawnX returns the shell's initial X coordinate for a tank at the given
+// origin X and orientation.
+func shellSpawnX(tankX domain.SP, orient domain.Orientation) domain.SP {
+	offset := domain.SP((assets.SpriteTankWidth + shellSpawnOffset) * domain.SubpixelScale)
+	if orient == domain.OrientationLeft {
+		return tankX - domain.SP(shellSpawnOffset*domain.SubpixelScale)
+	}
+
+	return tankX + offset
+}
+
 // Physical tank shell speeds (px/sec).
 const (
 	shellHorizSpeedPxSec = 24 // px/sec per speed unit (multiplier 1–4)

pkg/logic/tank_shell_test.go

@@ -0,0 +1,44 @@
+package logic
+
+import (
+	"testing"
+
+	"github.com/morozov/river-raid-ebiten/pkg/assets"
+	"github.com/morozov/river-raid-ebiten/pkg/domain"
+)
+
+func TestShellSpawnX(t *testing.T) {
+	t.Parallel()
+
+	const tankX domain.SP = 100 * domain.SubpixelScale
+
+	tests := []struct {
+		name   string
+		orient domain.Orientation
+		wantPx domain.Px
+	}{
+		{
+			name:   "left-facing: 16px left of left edge",
+			orient: domain.OrientationLeft,
+			wantPx: 100 - shellSpawnOffset,
+		},
+		{
+			name:   "right-facing: 16px right of right edge",
+			orient: domain.OrientationRight,
+			wantPx: 100 + assets.SpriteTankWidth + shellSpawnOffset,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			got := shellSpawnX(tankX, tc.orient)
+			want := tc.wantPx.ToSP()
+
+			if got != want {
+				t.Errorf("ShellSpawnX(%d, %v) = %d, want %d", tankX, tc.orient, got, want)
+			}
+		})
+	}
+}