First Fibonacci with 1000 digits

# See https://stackoverflow.com/q/47953598/1672429 for discussion
# and sources.
# 
# Rehearsal -------------------------------------------------------
# to_s.length           0.297000   0.015000   0.312000 (  0.316113)
# digits.length        17.562000   0.016000  17.578000 ( 17.597551)
# laborious             3.485000   0.000000   3.485000 (  3.495671)
# BigDecimal#exponent   0.046000   0.000000   0.046000 (  0.041986)
# < 10**999             0.032000   0.000000   0.032000 (  0.027199)
# --------------------------------------------- total: 21.453000sec
# 
#                           user     system      total        real
# to_s.length           0.297000   0.016000   0.313000 (  0.305834)
# digits.length        17.640000   0.000000  17.640000 ( 17.694883)
# laborious             3.625000   0.000000   3.625000 (  3.626703)
# BigDecimal#exponent   0.047000   0.000000   0.047000 (  0.054947)
# < 10**999             0.015000   0.000000   0.015000 (  0.025909)

require 'bigdecimal'
require 'benchmark'

Benchmark.bmbm do |x|
  @x = x
  def report(name, &block)
    @x.report(name) {
      @n.times(&block)
    }
  end
  @n = 10
  
  report('to_s.length') {
    number = 3
    a = 1
    b = 2
    while b.to_s.length < 1000
      a, b = b, a + b
      number += 1
    end
    raise "wrong" if number != 4782
  }
  
  report('digits.length') {
    number = 3
    a = 1
    b = 2
    while b.digits.length < 1000
      a, b = b, a + b
      number += 1
    end
    raise "wrong" if number != 4782
  }
  
  report('laborious') {
    number = 3
    a = 1
    b = 2
    while b.to_s.chars.map(&:to_i).reverse.length < 1000
      a, b = b, a + b
      number += 1
    end
    raise "wrong" if number != 4782
  }
  
  report('BigDecimal#exponent') {
    number = 3
    a = BigDecimal.new(1)
    b = BigDecimal.new(2)
    while b.exponent < 1000
      a, b = b, a + b
      number += 1
    end
    raise "wrong" if number != 4782
  }
  
  report('< 10**999') {
    number = 3
    a = 1
    b = 2
    goal = 10**999
    while b < goal
      a, b = b, a + b
      number += 1
    end
    raise "wrong" if number != 4782
  }
  
end