Add hold box

main.go

@@ -89,13 +89,15 @@ func main() {
 		sscore := fmt.Sprintf("score %d", score)
 		slines := fmt.Sprintf("lines %d", linescleared)
 		if !harddrop {
-			fpos(0, 0, f.Add(p))
+			ppos(0, 0, "Hold (c)")
+			npos(3, 0, tris.HoldBox)
+			fpos(0, 10, f.Add(p))
 			var next tris.Field
 			b, next = b.Next(5)
-			npos(0, 24, next)
-			ppos(1, 32, sscore)
-			ppos(3, 32, slevel)
-			ppos(5, 32, slines)
+			npos(0, 34, next)
+			ppos(1, 42, sscore)
+			ppos(3, 42, slevel)
+			ppos(5, 42, slines)
 			key = GetKey(t)
 		}
 		switch key[0] {
@@ -119,6 +121,8 @@ func main() {
 			rot = (p.Rot + 1) % 4
 		case 'z':
 			rot = (p.Rot + 3) % 4
+		case 'c':
+			b, p = b.Swap(p)
 		case ' ':
 			if !harddrop {
 				dropfrom = p.Y

tris/core.go

@@ -20,6 +20,7 @@ type Piece [4]uint16
 // A table that represents each piece from https://tetris.fandom.com/wiki/SRS
 // in binary starting with 1 top left going per row, here in hexadecimal shorthand.
 var (
+	EmptyPiece = Piece{}
 	IPiece = Piece{0x0F00, 0x4444, 0x00F0, 0x2222}
 	JPiece = Piece{0x1700, 0x6220, 0x0740, 0x2230}
 	LPiece = Piece{0x4700, 0x2260, 0x0710, 0x3220}
@@ -130,6 +131,7 @@ func (b Bag) Randomize() Bag {
 }
 
 func (b Bag) Pick() (Bag, Placement) {
+	Swapped = false
 	if len(b) == 0 {
 		b = NewBag()
 	}

tris/hold.go

@@ -0,0 +1,23 @@
+package tris
+
+var HoldPiece Piece
+
+var HoldBox Field
+
+var Swapped bool
+
+func (b Bag) Swap(p Placement) (Bag, Placement) {
+	if Swapped {
+		return b, p
+	}
+	var tempp Placement
+	if HoldPiece == EmptyPiece {
+		b, tempp = b.Pick()
+		HoldPiece = tempp.piece
+	}
+	p.piece, HoldPiece = HoldPiece, p.piece
+	Swapped = true
+	HoldBox, _ = Placement{piece: HoldPiece, X: 0, Y:0}.Field(2,4)
+	p.Rot, p.X, p.Y = 0, 3, -2
+	return b, p
+}