type.ts

/* Text OT!
 *
 * This is an OT implementation for text. It is the standard implementation of
 * text used by ShareJS.
 *
 * This type is composable and by default non-invertable (operations do not by
 * default contain enough information to invert them). Its similar to ShareJS's
 * old text-composable type, but its not invertable and its very similar to the
 * text-tp2 implementation but it doesn't support tombstones or purging.
 *
 * Ops are lists of components which iterate over the document. Components are
 * either:
 *
 * - A number N: Skip N characters in the original document
 * - "str": Insert "str" at the current position in the document
 * - {d:N}: Delete N characters at the current position in the document
 * - {d:"str"}: Delete "str" at the current position in the document. This is
 *   equivalent to {d:N} but provides extra information for operation
 *   invertability.
 *
 * Eg: [3, 'hi', 5, {d:8}]
 *
 * The operation does not have to skip the last characters in the document.
 *
 * Snapshots are by default strings.
 *
 * Cursors are either a single number (which is the cursor position) or a pair
 * of [anchor, focus] (aka [start, end]). Be aware that end can be before start.
 *
 * The actual string type is configurable. The OG default exposed text type uses
 * raw javascript strings, but they're not compatible with OT implementations in
 * other languages because string.length returns the wrong value for unicode
 * characters that don't fit in 2 bytes. And JS strings are quite an inefficient
 * data structure for manipulating lines & UTF8 offsets. For this reason, you
 * can use your own data structure underneath the text OT code.
 *
 * Note that insert operations themselves are always raw strings. Its just
 * snapshots that are configurable.
 */

import {strPosToUni, uniToStrPos} from 'unicount'

const debugMode = false

export type TextOpComponent = number | string | {d: number | string}
export type TextOp = TextOpComponent[]

export interface TextType<R> {
  name: string
  uri: string

  create(): R

  trim(op: TextOp): TextOp
  checkOp(op: TextOp): void
  normalize(op: TextOp): TextOp
  apply(doc: R, op: TextOp): R
  transform(op: TextOp, other: TextOp, side: 'left' | 'right'): TextOp
  compose(a: TextOp, b: TextOp): TextOp
  transformPosition(cursor: number, op: TextOp): number
  transformSelection(selection: number | [number, number], op: TextOp): number | [number, number]

  invert?(op: TextOp): TextOp

  [k: string]: any
}

export interface Rope<Snap> {
  /** Create a snapshot from the given javascript string */
  create(s: string): Snap
  /** Convert a snapshot into a javascript string */
  toString(doc: Snap): string
  /** Create or update a document snapshot by walking the document */
  builder(doc: Snap): {
    skip(n: number): void
    append(s: string): void
    del(n: number): void

    build(): Snap
  }
  /** Equivalent to String.slice in javascript, but using UTF8 offsets. */
  slice(doc: Snap, start: number, end?: number): string
}


/** Check the operation is valid. Throws if not valid. */
const checkOp = (op: TextOp) => {
  if (!Array.isArray(op)) throw Error('Op must be an array of components');

  let last = null
  for (let i = 0; i < op.length; i++) {
    const c = op[i]
    switch (typeof c) {
      case 'object':
        // The only valid objects are {d:X} for +ive values of X or non-empty strings.
        if (typeof c.d !== 'number' && typeof c.d !== 'string') throw Error('Delete must be number or string')
        if (dlen(c.d) <= 0) throw Error('Deletes must not be empty')
        break
      case 'string':
        // Strings are inserts.
        if (!(c.length > 0)) throw Error('Inserts cannot be empty')
        break
      case 'number':
        // Numbers must be skips. They have to be +ive numbers.
        if (!(c > 0)) throw Error('Skip components must be >0')
        if (typeof last === 'number') throw Error('Adjacent skip components should be combined')
        break
    }
    last = c
  }

  if (typeof last === 'number') throw Error('Op has a trailing skip')
}

// TODO: Consider exposing this at the library level.
// TODO: Also consider rewriting this to use es iterators instead of callback-passing style.
export function eachOp(op: TextOp, fn: (c: TextOpComponent, prePos: number, postPos: number) => void) {
  let prePos = 0, postPos = 0

  for (let i = 0; i < op.length; i++) {
    const c = op[i]
    fn(c, prePos, postPos)
    switch (typeof c) {
      case 'object': // Delete
        prePos += dlen(c.d)
        break

      case 'string': // Insert
        postPos += strPosToUni(c)
        break
        
      case 'number': // Skip
        prePos += c
        postPos += c
        break
    }
  }
}

function mapOp(op: TextOp, fn: (c: TextOpComponent, prePos: number, postPos: number) => TextOpComponent): TextOp {
  const newOp: TextOp = []
  const append = makeAppend(newOp)
  eachOp(op, (c, prePos, postPos) => {
    append(fn(c, prePos, postPos))
  })
  return trim(newOp)
}

const id = <T>(x: T) => x
const normalize = (op: TextOp) => {
  return mapOp(op, id)
}

/** Check that the given selection range is valid. */
const checkSelection = (selection: [number, number]) => {
  // This may throw from simply inspecting selection[0] / selection[1]. Thats
  // sort of ok, though it'll generate the wrong message.
  if (typeof selection !== 'number'
      && (typeof selection[0] !== 'number' || typeof selection[1] !== 'number')) {
    throw Error('Invalid selection')
  }
}

export const dlen = (d: number | string) => typeof d === 'number' ? d : strPosToUni(d)

/** Make a function that appends to the given operation. */
const makeAppend = (op: TextOp) => (component: TextOpComponent) => {
  if (!component || (component as any).d === 0 || (component as any).d === '') {
    // The component is a no-op. Ignore!

  } else if (op.length === 0) {
    op.push(component)

  } else if (typeof component === typeof op[op.length - 1]) {
    if (typeof component === 'object') {
      // Concatenate deletes. This is annoying because the op or component could
      // contain strings or numbers.
      const last = op[op.length - 1] as {d:number | string}
      last.d = typeof last.d === 'string' && typeof component.d === 'string'
        ? last.d + component.d // Preserve invert information
        : dlen(last.d) + dlen(component.d) // Discard invert information, if any.
      
      // (op[op.length - 1] as {d:number}).d += component.d
    } else {
      // Concat strings / inserts. TSC should be smart enough for this :p
      (op[op.length - 1] as any) += (component as any)
    }
  } else {
    op.push(component)
  }
}

/** Get the length of a component */
const componentPreLength = (c: TextOpComponent): number => (
  typeof c === 'number' ? c
    : typeof c === 'string' ? 0
    : typeof c.d === 'number' ? c.d : strPosToUni(c.d)
)

const componentPostLength = (c: TextOpComponent): number => (
  typeof c === 'number' ? c
    : typeof c === 'string' ? strPosToUni(c)
    : 0
)

// Does not support negative numbers.
export const uniSlice = (s: string, startUni: number, endUni?: number): string => {
  const start = uniToStrPos(s, startUni)
  const end = endUni == null ? Infinity : uniToStrPos(s, endUni)
  return s.slice(start, end)
}

const dslice = (d: number | string, start: number, end?: number): string | number => (
  typeof d === 'number'
    ? (end == null) ? d - start : Math.min(d, end) - start
    : uniSlice(d, start, end)
)

/** Makes and returns utility functions take and peek.
 */
const makeTake = (op: TextOp, isPre: boolean) => {
  // TODO: Rewrite this by passing a context, like the rust code does. Its cleaner that way.

  // The index of the next component to take
  let idx = 0
  // The offset into the component. For strings this is in UCS2 length, not
  // unicode codepoints.
  let offset = 0

  // Take up to length n from the front of op. If n is -1, take the entire next
  // op component. If indivisableField == 'd', delete components won't be separated.
  // If indivisableField == 'i', insert components won't be separated.
  const take = (n: number): TextOpComponent | null => {
    // We're at the end of the operation. The op has skips, forever. Infinity
    // might make more sense than null here.
    if (idx === op.length) return n === -1 ? null : n

    // TODO: Tidy this up using pre / post component length methods.
    const c = op[idx]
    let part
    if (typeof c === 'number') {
      // Skip
      if (n === -1 || c - offset <= n) {
        part = c - offset
        ++idx
        offset = 0
        return part
      } else {
        offset += n
        return n
      }
    } else if (typeof c === 'string') {
      // Insert
      if (n === -1 || isPre || strPosToUni(c.slice(offset)) <= n) {
        part = c.slice(offset)
        ++idx
        offset = 0
        return part
      } else {
        const offset2 = offset + uniToStrPos(c.slice(offset), n)
        part = c.slice(offset, offset2)
        offset = offset2
        return part
      }
    } else {
      // Delete
      //
      // So this is a little weird - the insert case uses UCS2 length offsets
      // directly instead of counting in codepoints. Thats more efficient, but
      // more complicated. It only matters for non-invertable ops with huge
      // deletes being composed / transformed by other very complicated ops.
      // Probably not common enough to optimize for. Esp since this is a little
      // bit of a mess anyway, and the tests should iron out any problems.
      if (n === -1 || !isPre || dlen(c.d) - offset <= n) {
        // Emit the remainder of the delete.
        part = {d: dslice(c.d, offset)}
        // part = {d: dlen(c.d) - offset}
        ++idx
        offset = 0
        return part
      } else {
        // Slice into the delete content
        let result = dslice(c.d, offset, offset + n)
        offset += n
        return {d: result}
      }
    }
  }

  // Peek at the next op that will be returned.
  const peek = () => op[idx]

  return {take, peek}
}


/** Trim any excess skips from the end of an operation.
 *
 * There should only be at most one, because the operation was made with append.
 */
const trim = (op: TextOp) => {
  if (op.length > 0 && typeof op[op.length - 1] === 'number') {
    op.pop()
  }
  return op
}


/** Transform op by otherOp.
 *
 * @param op - The operation to transform
 * @param otherOp - Operation to transform it by
 * @param side - Either 'left' or 'right'
 */
function transform(op1: TextOp, op2: TextOp, side: 'left' | 'right') {
  if (side !== 'left' && side !== 'right') {
    throw Error("side (" + side + ") must be 'left' or 'right'")
  }

  checkOp(op1)
  checkOp(op2)

  const newOp: TextOp = []

  const append = makeAppend(newOp)
  const {take, peek} = makeTake(op1, true)

  for (let i = 0; i < op2.length; i++) {
    const c2 = op2[i]

    let length, c1
    switch (typeof c2) {
      case 'number': // Skip
        length = c2
        while (length > 0) {
          c1 = take(length)!
          append(c1)
          if (typeof c1 !== 'string') {
            length -= componentPreLength(c1)
          }
        }
        break

      case 'string': // Insert
        if (side === 'left') {
          // The left insert should go first.
          if (typeof peek() === 'string') {
            append(take(-1)!)
          }
        }

        // Otherwise skip the inserted text.
        append(strPosToUni(c2))
        break

      case 'object': // Delete
        length = dlen(c2.d)
        while (length > 0) {
          c1 = take(length)!
          switch (typeof c1) {
            case 'number':
              length -= c1
              break
            case 'string':
              append(c1)
              break
            case 'object':
              // The delete is unnecessary now - the text has already been deleted.
              length -= dlen(c1.d)
          }
        }
        break
    }
  }
  
  // Append any extra data in op1.
  let c
  while ((c = take(-1))) append(c)
  
  if (debugMode && isInvertible(op1) && isInvertible(op2) && !isInvertible(newOp)) {
    throw Error('Internal error - composed operation should also be invertible')
  }

  return trim(newOp)
}

/** Compose op1 and op2 together and return the result */
function compose(op1: TextOp, op2: TextOp) {
  checkOp(op1)
  checkOp(op2)

  const result: TextOp = []
  const append = makeAppend(result)
  const {take} = makeTake(op1, false)

  for (let i = 0; i < op2.length; i++) {
    const component = op2[i]
    let length: number, chunk: TextOpComponent
    switch (typeof component) {
      case 'number': { // Skip
        length = component
        while (length > 0) {
          chunk = take(length)!
          append(chunk)
          if (typeof chunk !== 'object') {
            length -= componentPostLength(chunk)
          }
        }
        break
      }

      case 'string': { // Insert
        append(component)
        break
      }

      case 'object': { // Delete
        length = dlen(component.d) // Length of the delete we're doing
        let offset = 0 // Offset into our deleted content

        while (offset < length) {
          chunk = take(length - offset)!

          switch (typeof chunk) {
            case 'number':
              // We're deleting the skipped characters.
              append({d: dslice(component.d, offset, offset + chunk)})
              offset += chunk
              break
            case 'string':
              offset += strPosToUni(chunk)
              break
            case 'object':
              append(chunk)
          }
        }
        break
      }
    }
  }

  let c
  while ((c = take(-1))) append(c)

  if (debugMode && isInvertible(op1) && isInvertible(op2) && !isInvertible(result)) {
    throw Error('Internal error - composed operation should also be invertible')
  }

  return trim(result)
}

// This operates in unicode offsets to make it consistent with the equivalent
// methods in other languages / systems.
const transformPosition = (cursor: number, op: TextOp) => {
  let pos = 0

  for (let i = 0; i < op.length && cursor > pos; 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
        pos += c
        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.
        const offset = strPosToUni(c)
        pos += offset
        cursor += offset
        break

      case 'object': // delete
        cursor -= Math.min(dlen(c.d), cursor - pos)
        break
    }
  }
  return cursor
}

const transformSelection = (selection: number | [number, number], op: TextOp): number | [number, number] => (
  typeof selection === 'number'
    ? transformPosition(selection, op)
    : selection.map(s => transformPosition(s, op)) as [number, number]
)


function makeInvertible<S, R extends Rope<S>>(op: TextOp, doc: S, ropeImpl: R) {
  return mapOp(op, (c, prePos) => (
    (typeof c === 'object' && typeof c.d === 'number') // Delete
      ? {d: ropeImpl.slice(doc, prePos, prePos + c.d)}
      : c
  ))
}

/** Attempt to invert the operation. Operations with {d:N} components cannot be inverted, and this method will throw. */
function invert(op: TextOp) {
  return mapOp(op, c => {
    switch (typeof c) {
      case 'object': // Delete
        if (typeof c.d === 'number') {
          throw Error('Cannot invert text op: Deleted characters missing from operation. makeInvertible must be called first.')
        } else return c.d // delete -> insert

      case 'string': return {d: c} // Insert -> delete
      case 'number': return c // skip -> skip
    }
  })
}

/** Strip extraneous invertibility information from the operation */
function stripInvertible(op: TextOp) {
  return mapOp(op, c => (
    (typeof c === 'object' && typeof c.d === 'string')
      ? {d: strPosToUni(c.d)}
      : c
  ))
}

/** Helper method. returns true if the operation can be successfully inverted. */
function isInvertible(op: TextOp): boolean {
  let invertible = true
  eachOp(op, c => {
    if (typeof c === 'object' && typeof c.d === 'number') invertible = false
  })
  return invertible
}


export default function makeType<Snap>(ropeImpl: Rope<Snap>): TextType<Snap> {
  return {
    name: 'text-unicode',
    uri: 'http://sharejs.org/types/text-unicode',
    trim,
    normalize,
    checkOp,

    /** Create a new text snapshot.
     *
     * @param {string} initial - initial snapshot data. Optional. Defaults to ''.
     * @returns {Snap} Initial document snapshot object
     */
    create(initial: string = '') {
      if (typeof initial !== 'string') {
        throw Error('Initial data must be a string')
      }
      return ropeImpl.create(initial)
    },


    /** Apply an operation to a document snapshot
     */
    apply(str, op) {
      checkOp(op)

      const builder = ropeImpl.builder(str)

      for (let i = 0; i < op.length; i++) {
        const component = op[i]
        switch (typeof component) {
          case 'number': builder.skip(component); break
          case 'string': builder.append(component); break
          case 'object': builder.del(dlen(component.d)); break
        }
      }

      return builder.build()
    },

    transform,
    compose,

    transformPosition,
    transformSelection,

    isInvertible,
    makeInvertible(op: TextOp, doc: Snap) { return makeInvertible(op, doc, ropeImpl) },
    stripInvertible,
    invert, // Only valid to call on operations with invert data.

    invertWithDoc(op: TextOp, doc: Snap) { return invert(makeInvertible(op, doc, ropeImpl)) },

    isNoop: (op: TextOp) => op.length === 0
  }
}