ipfs-bitswap/src/decision-engine/task-merger.ts

JavaScript implementation of the Bitswap data exchange protocol used by IPFS

import type { Task } from './index.js'

export const DefaultTaskMerger = {
  /**
   * Indicates whether the given task has newer information than the active
   * tasks with the same topic.
   *
   * @param {Task} task
   * @param {Task[]} tasksWithTopic
   * @returns {boolean}
   */
  hasNewInfo (task: Task, tasksWithTopic: Task[]): boolean {
    let haveBlock = false
    let isWantBlock = false
    for (const existing of tasksWithTopic) {
      if (existing.data.haveBlock) {
        haveBlock = true
      }

      if (existing.data.isWantBlock) {
        isWantBlock = true
      }
    }

    // If there is no active want-block and the new task is a want-block,
    // the new task is better
    if (!isWantBlock && task.data.isWantBlock) {
      return true
    }

    // If we didn't have the block, and the new task indicates that we now
    // do have the block, then we must also have size information for the
    // block, so the new task has new information.
    if (!haveBlock && task.data.haveBlock) {
      return true
    }

    return false
  },

  /**
   * Merge the information from the given task into the existing task (with the
   * same topic)
   */
  merge (newTask: Task, existingTask: Task) {
    // The merge function ignores the topic and priority as these don't change.
    //
    // We may receive new information about a want before the want has been
    // popped from the queue in the following scenarios:
    //
    // - Replace want type:
    //   1. Client sends want-have CID1
    //   2. Client sends want-block CID1
    //   In this case we should replace want-have with want-block, including
    //   updating the task size to be the block size.
    //
    // - Replace DONT_HAVE with want:
    //   1. Client sends want-have CID1 or want-block CID1
    //   2. Local node doesn't have block for CID1
    //   3. Local node receives block for CID1 from peer
    //   In this case we should replace DONT_HAVE with the want, including
    //   updating the task size and block size.
    const taskData = newTask.data
    const existingData = existingTask.data

    // If we didn't have block size information (because we didn't have the
    // block) and we receive the block from a peer, update the task with the
    // new block size
    if (!existingData.haveBlock && taskData.haveBlock) {
      existingData.haveBlock = taskData.haveBlock
      existingData.blockSize = taskData.blockSize
    }

    // If replacing a want-have with a want-block
    if (!existingData.isWantBlock && taskData.isWantBlock) {
      // Change the type from want-have to want-block
      existingData.isWantBlock = true
      // If the want-have was a DONT_HAVE, or the want-block has a size
      if (!existingData.haveBlock || taskData.haveBlock) {
        // Update the entry size
        existingData.haveBlock = taskData.haveBlock
        existingTask.size = newTask.size
      }
    }

    // If the task is a want-block, make sure the entry size is equal
    // to the block size (because we will send the whole block)
    if (existingData.isWantBlock && existingData.haveBlock) {
      existingTask.size = existingData.blockSize
    }
  }
}