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  1. # @file pendulum.rb
  2. # Question-8 from some problem set.
  3. # @date 03/28/2026
  4.  
  5. # Rearrange the word so the character with the smallest ASCII value
  6. # is placed at the center, the second smallest to its right and the
  7. # third to its left and so on.
  8. #
  9. # Example-1: In : COMPUTER
  10. #            Out: TPMCEORU
  11. #
  12. # Example-2: In : SCIENCE
  13. #            Out: SIECCEN
  14. #
  15. # The problem specifies characters should be rearranged by invoking
  16. # the function 'int minimumCharIndex(String str)'.
  17.  
  18. def pendulum_0(s)
  19.   def minimum_char_index(s)
  20.     s.index(s.each_byte.min.chr) # faster than each_char.min
  21.   end
  22.  
  23.   s = s.dup
  24.   n = s.size
  25.   m = (n+1) / 2
  26.   r = []
  27.   n.times do |i|
  28.     j = i / 2
  29.     k = minimum_char_index(s)
  30.     r[i.even? ? m-j-1 : m+j] = s.slice!(k)
  31.   end
  32.   r.join
  33. end
  34.  
  35. # Breaking the rules a bit.
  36.  
  37. def pendulum_0_1(s)
  38.   def minimum_index(a, first, last)
  39.     subarray = a[first...last]
  40.     subarray.index(subarray.min) + first
  41.   end
  42.  
  43.   s = s.bytes
  44.   n = s.size
  45.   m = (n+1) / 2
  46.   r = []
  47.   n.times do |i|
  48.     j = i / 2
  49.     k = minimum_index(s, i, n)
  50.     r[i.even? ? m-j-1 : m+j] = s[k]
  51.     s[k] = s[i]
  52.   end
  53.   r.map(&:chr).join
  54. end
  55.  
  56. def pendulum_0_2(s)
  57.   def swap(a, i, j)
  58.     a[i], a[j] = a[j], a[i] if i != j
  59.   end
  60.  
  61.   def minimum_index(a, first, last)
  62.     subarray = a[first...last]
  63.     subarray.index(subarray.min) + first
  64.   end
  65.  
  66.   def selectsort!(a)
  67.     (a.size-1).times do |i|
  68.       swap(a, i, minimum_index(a, i, a.size))
  69.     end
  70.     a
  71.   end
  72.  
  73.   n = s.size
  74.   m = (n+1) / 2
  75.   r = []
  76.   selectsort!(s.bytes).each_with_index do |x, i|
  77.     j = i / 2
  78.     r[i.even? ? m-j-1 : m+j] = x
  79.   end
  80.   r.map(&:chr).join
  81. end
  82.  
  83. # Breaking the rules a bit more.
  84.  
  85. def pendulum_1(s)
  86.   n = s.size
  87.   m = (n+1) / 2
  88.   r = []
  89.   s.chars.sort!.each_with_index do |x, i|
  90.     j = i / 2
  91.     r[i.even? ? m-j-1 : m+j] = x
  92.   end
  93.   r.join
  94. end
  95.  
  96. def pendulum_2(s)
  97.   r = []
  98.   s.chars.sort!.each_with_index do |x, i|
  99.     r.public_send(i.even? ? :unshift : :push, x)
  100.   end
  101.   r.join
  102. end
  103.  
  104. def pendulum_3(s)
  105.   t = s.chars.sort!
  106.   l, r = t.drop(1).partition.each_with_index{|_, i| i.odd?}
  107.   (l.reverse + t.take(1) + r).join
  108. end
  109.  
  110. def pendulum_4(s)
  111.   l, r = (1...s.size).partition{|i| i.even?}
  112.   s.chars.sort!.values_at(*l.reverse, 0, *r).join
  113. end
  114.  
  115. # Main.
  116.  
  117. def test(f)
  118.   ss = ["", "a", "bca", "computer", "science"]
  119.   rs = ["", "a", "cab", "tpmceoru", "sieccen"]
  120.  
  121.   ss.zip(rs) do |s, r|
  122.     s = f.call(s)
  123.     if s != r
  124.       puts ".Failed: expected `#{r}' got `#{s}'"
  125.       return
  126.     end
  127.   end
  128.   puts ".Passed"
  129. end
  130.  
  131. def time(f)
  132.   def choices(pool, n)
  133.     n.times.map{pool.sample}
  134.   end
  135.  
  136.   symbols = ('a'..'z').to_a
  137.   elapsed = 0
  138.  
  139.   12.times do |i|
  140.     s = choices(symbols, 2**i).join
  141.     t = Time.now
  142.     f.call(s)
  143.     elapsed += Time.now - t
  144.   end
  145.   printf(".Elapsed: %.6fs\n", elapsed)
  146. end
  147.  
  148. fs = [
  149.   :pendulum_0,
  150.   :pendulum_0_1,
  151.   :pendulum_0_2,
  152.   :pendulum_1,
  153.   :pendulum_2,
  154.   :pendulum_3,
  155.   :pendulum_4
  156. ]
  157.  
  158. fs.map(&method(:method)).each do |f|
  159.   puts f.name
  160.   test(f)
  161.   time(f)
  162. end
Success #stdin #stdout 0.17s 38624KB
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pendulum_0
.Passed
.Elapsed: 0.072929s
pendulum_0_1
.Passed
.Elapsed: 0.035879s
pendulum_0_2
.Passed
.Elapsed: 0.035047s
pendulum_1
.Passed
.Elapsed: 0.002061s
pendulum_2
.Passed
.Elapsed: 0.003582s
pendulum_3
.Passed
.Elapsed: 0.001762s
pendulum_4
.Passed
.Elapsed: 0.001986s
https://ideone.com/yZJRJj
language:
Ruby (ruby 2.5.5)
created:
2 days ago
visibility:
public

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