nim-codex/codex/indexingstrategy.nim

import ./errors
import ./utils
import ./utils/asynciter

{.push raises: [].}

type
  StrategyType* = enum
    # Simplest approach:
    # 0 => 0, 1, 2
    # 1 => 3, 4, 5
    # 2 => 6, 7, 8
    LinearStrategy,

    # Stepped indexing:
    # 0 => 0, 3, 6
    # 1 => 1, 4, 7
    # 2 => 2, 5, 8
    SteppedStrategy

  # Representing a strategy for grouping indices (of blocks usually)
  # Given an interation-count as input, will produce a seq of
  # selected indices.

  IndexingError* = object of CodexError
  IndexingWrongIndexError* = object of IndexingError
  IndexingWrongIterationsError* = object of IndexingError

  IndexingStrategy* = object
    strategyType*: StrategyType
    firstIndex*: int             # Lowest index that can be returned
    lastIndex*: int              # Highest index that can be returned
    iterations*: int             # getIndices(iteration) will run from 0 ..< iterations
    step*: int

func checkIteration(self: IndexingStrategy, iteration: int): void {.raises: [IndexingError].} =
  if iteration >= self.iterations:
    raise newException(
      IndexingError,
      "Indexing iteration can't be greater than or equal to iterations.")

proc getIter(first, last, step: int): Iter[int] =
  var
    finish = false
    cur = first

  func get(): int =
    result = cur
    cur += step

    if cur > last:
      finish = true

  func isFinished(): bool =
    finish

  Iter.new(get, isFinished)

func getLinearIndicies(
  self: IndexingStrategy,
  iteration: int): Iter[int] {.raises: [IndexingError].} =
  self.checkIteration(iteration)

  let
    first = self.firstIndex + iteration * (self.step + 1)
    last = min(first + self.step, self.lastIndex)

  getIter(first, last, 1)

func getSteppedIndicies(
  self: IndexingStrategy,
  iteration: int): Iter[int] {.raises: [IndexingError].} =
  self.checkIteration(iteration)

  let
    first = self.firstIndex + iteration
    last = self.lastIndex

  getIter(first, last, self.iterations)

func getIndicies*(
  self: IndexingStrategy,
  iteration: int): Iter[int] {.raises: [IndexingError].} =

  case self.strategyType
  of StrategyType.LinearStrategy:
    self.getLinearIndicies(iteration)
  of StrategyType.SteppedStrategy:
    self.getSteppedIndicies(iteration)

func init*(
  strategy: StrategyType,
  firstIndex, lastIndex, iterations: int): IndexingStrategy {.raises: [IndexingError].} =

  if firstIndex > lastIndex:
    raise newException(
      IndexingWrongIndexError,
      "firstIndex (" & $firstIndex & ") can't be greater than lastIndex (" & $lastIndex & ")")

  if iterations <= 0:
    raise newException(
      IndexingWrongIterationsError,
      "iterations (" & $iterations & ") must be greater than zero.")

  IndexingStrategy(
    strategyType: strategy,
    firstIndex: firstIndex,
    lastIndex: lastIndex,
    iterations: iterations,
    step: divUp((lastIndex - firstIndex), iterations))