transform raw string positions with text ops

transformJSPosition.js

const {strPosToUni, uniToStrPos} = require('unicount')

// The cursor position (input and output) are specified in JS string
// offsets here.
function transformJSPosition(cursor /*: number*/, doc /*: string*/, op /*: TextOp*/) {
  let prePos = 0, postPos = 0 // string index in doc.

  for (let i = 0; i < op.length && cursor > postPos; i++) {
    const c = op[i]

    // I could actually use the op_iter stuff above - but I think its simpler
    // like this.
    switch (typeof c) {
      case 'number': { // skip
        const offset = uniToStrPos(doc.slice(prePos), c)
        prePos += offset
        postPos += offset
        break
      }

      case 'string': // insert
        // Its safe to use c.length here because they're both utf16 offsets.
        // Ignoring pos because the doc doesn't know about the insert yet.
        postPos += c.length
        cursor += c.length
        break

      case 'object': // delete
        cursor -= Math.min(uniToStrPos(doc.slice(prePos), c.d), cursor - postPos)
        prePos += c.d
        break
    }
  }
  return cursor
}

usage.js

const {type} = require('ot-text-unicode')
let doc = '...'
let cursors = [{user: 'jane', pos: 5}, {user: 'fred', pos: 100}]

function onRemoteOp(op) {
  for (let user of cursors) {
    // Update user positions based on operation. doc must be the document as a
    // string *before* the operation has been applied!
    transformJSPosition(user.pos, op)
  }

  doc = type.apply(doc, op)
}