Negative OP_ROLL and OP_PICK
There is a need to insert items into the middle of the stack (as opposed to overwriting the item via OP_PLACE). It was observed that OP_ROLL can be thought of as rotating a number of stack items. Insertion is effectively rotation in the opposite direction. Capturing this opposite direction idea with negative numbers efficiently uses the available operation space.
Original Definition
OP_ROLL and OP_PICK were previously defined as follows:
| Word | Value | Hex | Input | Output | Description |
|---|---|---|---|---|---|
| OP_PICK | 121 | 0x79 | xn ... x2 x1 x0 n | xn ... x2 x1 x0 xn | The item n back in the stack is copied to the top. |
| OP_ROLL | 122 | 0x7a | xn ... x2 x1 x0 n | x(n-1) ... x2 x1 x0 xn | The item n back in the stack is moved to the top. |
n was limited to 0 or positive numbers.
New Definition
| Word | Value | Hex | Input | Output | Description |
|---|---|---|---|---|---|
| OP_PICK | 121 | 0x79 | xn ... x2 x1 x0 n | xn ... x2 x1 x0 xn | n is popped. If n is positive, the item n back in the stack is copied to the top. If n is negative, the top item is inserted to the position n back in the stack. |
| OP_ROLL | 122 | 0x7a | xn ... x2 x1 x0 n | x(n-1) ... x2 x1 x0 xn | n is popped. If n is positive, the item n back in the stack is moved to the top. If n is negative, the top item is moved to the position n back in the stack. Note that since the top item is itself on the stack -1 OP_ROLL is a no-op |
Examples to clarify behavior
- Moving/copying to last index for both OP_ROLL and OP_PICK would be OP_DEPTH OP_NEGATE OP_ROLL.
- x -1 OP_PICK is equivalent to x OP_DUP. It produces a stack of x x.