@ethereumjs/evm/src/stack.ts

JavaScript Ethereum Virtual Machine (EVM) implementation

import { EVMError } from './errors.ts'

/**
 * Implementation of the stack used in evm.
 */
export class Stack {
  // This array is initialized as an empty array. Once values are pushed, the array size will never decrease.
  private _store: bigint[]
  private _maxHeight: number

  private _len: number = 0

  constructor(maxHeight?: number) {
    // It is possible to initialize the array with `maxHeight` items. However,
    // this makes the constructor 10x slower and there do not seem to be any observable performance gains
    this._store = []
    this._maxHeight = maxHeight ?? 1024
  }

  get length() {
    return this._len
  }

  /**
   * Pushes a new bigint onto the stack.
   * @param value - Value to push (must fit within the configured max height)
   */
  push(value: bigint) {
    if (this._len >= this._maxHeight) {
      throw new EVMError(EVMError.errorMessages.STACK_OVERFLOW)
    }

    // Read current length, set `_store` to value, and then increase the length
    this._store[this._len++] = value
  }

  /**
   * Pops the top value from the stack.
   * @returns The value removed from the stack
   */
  pop(): bigint {
    if (this._len < 1) {
      throw new EVMError(EVMError.errorMessages.STACK_UNDERFLOW)
    }

    // Length is checked above, so pop shouldn't return undefined
    // First decrease current length, then read the item and return it
    // Note: this does thus not delete the item from the internal array
    // However, the length is decreased, so it is not accessible to external observers
    return this._store[--this._len]
  }

  /**
   * Pops multiple items from the stack with the top-most item returned first.
   * @param num - Number of items to pop (defaults to 1)
   * @returns Array containing the popped values
   */
  popN(num: number = 1): bigint[] {
    if (this._len < num) {
      throw new EVMError(EVMError.errorMessages.STACK_UNDERFLOW)
    }

    if (num === 0) {
      return []
    }

    const arr = Array(num)
    const cache = this._store

    for (let pop = 0; pop < num; pop++) {
      // Note: this thus also (correctly) reduces the length of the internal array (without deleting items)
      arr[pop] = cache[--this._len]
    }

    return arr
  }

  /**
   * Returns items from the stack without removing them.
   * @param num - Number of items to return (defaults to 1)
   * @returns Array of items, with index 0 representing the top of the stack
   * @throws {@link EVMError} with code STACK_UNDERFLOW if there are not enough items on the stack
   */
  peek(num: number = 1): bigint[] {
    const peekArray: bigint[] = Array(num)
    let start = this._len

    for (let peek = 0; peek < num; peek++) {
      const index = --start
      if (index < 0) {
        throw new EVMError(EVMError.errorMessages.STACK_UNDERFLOW)
      }
      peekArray[peek] = this._store[index]
    }
    return peekArray
  }

  /**
   * Swaps the top of the stack with another item.
   * @param position - Zero-based index from the top of the stack (0 swaps with the top itself)
   */
  swap(position: number) {
    if (this._len <= position) {
      throw new EVMError(EVMError.errorMessages.STACK_UNDERFLOW)
    }

    const head = this._len - 1
    const i = head - position
    const storageCached = this._store

    const tmp = storageCached[head]
    storageCached[head] = storageCached[i]
    storageCached[i] = tmp
  }

  // I would say that we do not need this method any more
  // since you can't copy a primitive data type
  // Nevertheless not sure if we "loose" something here?
  // Will keep commented out for now
  /**
   * Pushes a copy of an item deeper in the stack.
   * @param position - One-based index of the item to duplicate
   */
  dup(position: number) {
    const len = this._len
    if (len < position) {
      throw new EVMError(EVMError.errorMessages.STACK_UNDERFLOW)
    }

    // Note: this code is borrowed from `push()` (avoids a call)
    if (len >= this._maxHeight) {
      throw new EVMError(EVMError.errorMessages.STACK_OVERFLOW)
    }

    const i = len - position
    this._store[this._len++] = this._store[i]
  }

  /**
   * Swaps two arbitrary entries relative to the top of the stack.
   * @param swap1 - Distance from the top (0 = top element) for the first entry
   * @param swap2 - Distance from the top for the second entry
   */
  exchange(swap1: number, swap2: number) {
    const headIndex = this._len - 1
    const exchangeIndex1 = headIndex - swap1
    const exchangeIndex2 = headIndex - swap2

    // Stack underflow is not possible in EOF
    if (exchangeIndex1 < 0 || exchangeIndex2 < 0) {
      throw new EVMError(EVMError.errorMessages.STACK_UNDERFLOW)
    }

    const cache = this._store[exchangeIndex2]
    this._store[exchangeIndex2] = this._store[exchangeIndex1]
    this._store[exchangeIndex1] = cache
  }

  /**
   * Returns a copy of the current stack. This represents the actual state of the stack
   * (not the internal state of the stack, which might have unreachable elements in it)
   */
  getStack() {
    return this._store.slice(0, this._len)
  }
}