Contents

# frozen_string_literal: true

# Original author: {Marc Siegel}[https://github.com/ms-ati].

require "scan_left/enumerable_with_scan_left"
require "scan_left/version"

# Wraps any +Enumerable+ to provide the {#scan_left} operation.
#
# Please see {file:README.md} for details, examples, background, and further
# reading about this operation.
#
# *Note:* if you'd prefer to use the {#scan_left} method directly on any
# +Enumerable+ instance, please see the optional refinement
# {EnumerableWithScanLeft}.
#
# @example
#   ScanLeft.new([1, 2, 3]).scan_left(0) { |s, x| s + x } == [0, 1, 3, 6]
#
# @see EnumerableWithScanLeft
class ScanLeft
  # @return [Enumerable]  Enumerable to transform via {#scan_left}
  attr_reader :enumerable

  # @param enumerable [Enumerable]  Enumerable to transform via {#scan_left}
  def initialize(enumerable)
    @enumerable = enumerable
  end

  # Map the enumerable to the incremental state of a calculation by
  # applying the given block, in turn, to each element and the
  # previous state of the calculation, resulting in an enumerable of
  # the state after each iteration.
  #
  # @return [Enumerable] Generate a stream of intermediate states
  #   resulting from applying a binary operator. Equivalent to a
  #   stream of +#inject+ calls on first N elements, increasing N from
  #   zero to the size of the stream. NOTE: Preserves laziness if
  #   +enumerable+ is lazy.
  #
  # @param initial [Object] Initial state value to yield.
  #
  # @yield [memo, obj] Invokes given block with previous state value
  #   +memo+ and next element of the stream +obj+.
  #
  # @yieldreturn [Object] The next state value for +memo+.
  def scan_left(initial)
    memo = initial
    outs = enumerable.map { |*obj| memo = yield(memo, *obj) }
    prepend_preserving_laziness(value: initial, enum: outs)
  end

  private

  def prepend_preserving_laziness(value:, enum:)
    case enum
    when Enumerator::Lazy
      [[value], enum].lazy.flat_map { |x| x }
    else
      [value] + enum
    end
  end
end

Version data entries

3 entries across 3 versions & 1 rubygems

Version	Path
scan_left-0.3.1	lib/scan_left.rb
scan_left-0.3.0	lib/scan_left.rb
scan_left-0.2.1	lib/scan_left.rb