overleaf/services/project-history/app/js/DiffGenerator.js

import _ from 'lodash'
import OError from '@overleaf/o-error'

export class ConsistencyError extends OError {}

/**
 * Container for functions that need to be mocked in tests
 *
 * TODO: Rewrite tests in terms of exported functions only
 */
export const _mocks = {}

export function buildDiff(initialContent, updates) {
  let diff = [{ u: initialContent }]
  for (const update of updates) {
    diff = applyUpdateToDiff(diff, update)
  }
  diff = compressDiff(diff)
  return diff
}

_mocks.compressDiff = diff => {
  const newDiff = []
  for (const part of diff) {
    const users = part.meta?.users ?? []

    if (part.meta?.origin?.kind === 'history-resync') {
      // Skip history resync updates. Inserts are converted to unchanged text
      // and deletes are skipped, so that they effectively don't appear in the
      // diff.
      if (part.u != null) {
        newDiff.push(part)
      } else if (part.i != null) {
        newDiff.push({ u: part.i })
      }
      continue
    }

    if (newDiff.length === 0) {
      // If we haven't seen other parts yet, we have nothing to merge.
      newDiff.push(part)
      continue
    }

    const lastPart = newDiff[newDiff.length - 1]
    const lastUsers = lastPart.meta?.users ?? []
    const usersNotInBothParts = _.xor(users, lastUsers)

    if (usersNotInBothParts.length > 0) {
      // If the set of users in the last part and this part are not the same, we
      // can't merge.
      newDiff.push(part)
      continue
    }

    if (lastPart.i != null && part.i != null) {
      // Merge two inserts
      lastPart.i += part.i
      lastPart.meta.start_ts = Math.min(
        lastPart.meta.start_ts,
        part.meta.start_ts
      )
      lastPart.meta.end_ts = Math.max(lastPart.meta.end_ts, part.meta.end_ts)
    } else if (lastPart.d != null && part.d != null) {
      // Merge two deletes
      lastPart.d += part.d
      lastPart.meta.start_ts = Math.min(
        lastPart.meta.start_ts,
        part.meta.start_ts
      )
      lastPart.meta.end_ts = Math.max(lastPart.meta.end_ts, part.meta.end_ts)
    } else {
      newDiff.push(part)
    }
  }
  return newDiff
}

export function compressDiff(...args) {
  return _mocks.compressDiff(...args)
}

export function applyOpToDiff(diff, op, meta) {
  let consumedDiff

  let remainingDiff = diff.slice()
  ;({ consumedDiff, remainingDiff } = _consumeToOffset(remainingDiff, op.p))
  const newDiff = consumedDiff

  if (op.i != null) {
    newDiff.push({
      i: op.i,
      meta,
    })
  } else if (op.d != null) {
    ;({ consumedDiff, remainingDiff } = _consumeDiffAffectedByDeleteOp(
      remainingDiff,
      op,
      meta
    ))
    newDiff.push(...(consumedDiff || []))
  }

  newDiff.push(...(remainingDiff || []))

  return newDiff
}

_mocks.applyUpdateToDiff = (diff, update) => {
  for (const op of update.op) {
    if (op.broken !== true) {
      diff = applyOpToDiff(diff, op, update.meta)
    }
  }
  return diff
}

export function applyUpdateToDiff(...args) {
  return _mocks.applyUpdateToDiff(...args)
}

function _consumeToOffset(remainingDiff, totalOffset) {
  let part
  const consumedDiff = []
  let position = 0
  while ((part = remainingDiff.shift())) {
    const length = _getLengthOfDiffPart(part)
    if (part.d != null) {
      consumedDiff.push(part)
    } else if (position + length >= totalOffset) {
      const partOffset = totalOffset - position
      if (partOffset > 0) {
        consumedDiff.push(_slicePart(part, 0, partOffset))
      }
      if (partOffset < length) {
        remainingDiff.unshift(_slicePart(part, partOffset))
      }
      break
    } else {
      position += length
      consumedDiff.push(part)
    }
  }

  return {
    consumedDiff,
    remainingDiff,
  }
}

function _consumeDiffAffectedByDeleteOp(remainingDiff, deleteOp, meta) {
  const consumedDiff = []
  let remainingOp = deleteOp
  while (remainingOp && remainingDiff.length > 0) {
    let newPart
    ;({ newPart, remainingDiff, remainingOp } = _consumeDeletedPart(
      remainingDiff,
      remainingOp,
      meta
    ))
    if (newPart != null) {
      consumedDiff.push(newPart)
    }
  }
  return {
    consumedDiff,
    remainingDiff,
  }
}

function _consumeDeletedPart(remainingDiff, op, meta) {
  let deletedContent, newPart, remainingOp
  const part = remainingDiff.shift()
  const partLength = _getLengthOfDiffPart(part)

  if (part.d != null) {
    // Skip existing deletes
    remainingOp = op
    newPart = part
  } else if (partLength > op.d.length) {
    // Only the first bit of the part has been deleted
    const remainingPart = _slicePart(part, op.d.length)
    remainingDiff.unshift(remainingPart)

    deletedContent = _getContentOfPart(part).slice(0, op.d.length)
    if (deletedContent !== op.d) {
      throw new ConsistencyError(
        `deleted content, '${deletedContent}', does not match delete op, '${op.d}'`
      )
    }

    if (part.u != null) {
      newPart = {
        d: op.d,
        meta,
      }
    } else if (part.i != null) {
      newPart = null
    }

    remainingOp = null
  } else if (partLength === op.d.length) {
    // The entire part has been deleted, but it is the last part

    deletedContent = _getContentOfPart(part)
    if (deletedContent !== op.d) {
      throw new ConsistencyError(
        `deleted content, '${deletedContent}', does not match delete op, '${op.d}'`
      )
    }

    if (part.u != null) {
      newPart = {
        d: op.d,
        meta,
      }
    } else if (part.i != null) {
      newPart = null
    }

    remainingOp = null
  } else if (partLength < op.d.length) {
    // The entire part has been deleted and there is more

    deletedContent = _getContentOfPart(part)
    const opContent = op.d.slice(0, deletedContent.length)
    if (deletedContent !== opContent) {
      throw new ConsistencyError(
        `deleted content, '${deletedContent}', does not match delete op, '${opContent}'`
      )
    }

    if (part.u) {
      newPart = {
        d: part.u,
        meta,
      }
    } else if (part.i != null) {
      newPart = null
    }

    remainingOp = {
      p: op.p,
      d: op.d.slice(_getLengthOfDiffPart(part)),
    }
  }

  return {
    newPart,
    remainingDiff,
    remainingOp,
  }
}

function _slicePart(basePart, from, to) {
  let part
  if (basePart.u != null) {
    part = { u: basePart.u.slice(from, to) }
  } else if (basePart.i != null) {
    part = { i: basePart.i.slice(from, to) }
  }
  if (basePart.meta != null) {
    part.meta = basePart.meta
  }
  return part
}

function _getLengthOfDiffPart(part) {
  return (part.u || part.d || part.i || '').length
}

function _getContentOfPart(part) {
  return part.u || part.d || part.i || ''
}