some cheap implementations of mergesort and quicksort

Author: bomatson

week_3/mergesort.rb

@@ -1,49 +1,40 @@
 require 'byebug'
 
 class MergeSort
-  # def sort(aux, lo, high)
-  #   return if high <= lo
-
-  #   mid = lo + (high - lo) / 2
-
-  #   sort(aux, lo, mid)
-  #   sort(aux, mid + 1, high)
-
-  #   merge(aux, lo, mid, high)
-  # end
-
-  # def merge(aux, lo, mid, high)
-  #   high.times do |i|
-  #     aux[i] = @actual[i]
-  #   end
-
-  #   j = mid + 1
-  #   i = lo
-
-  #   high.times do |k|
-  #     if i > mid
-  #       @actual[k] = aux[j]
-  #       j += 1
-  #     elsif j > high
-  #       # this should only need to find j > high
-  #       @actual[k] = aux[i]
-  #       i += 1
-  #     elsif less(aux[j], aux[i])
-  #       @actual[k] = aux[j]
-  #       j += 1
-  #     else
-  #       @actual[k] = aux[i]
-  #       i += 1
-  #     end
-  #   end
-  # end
+  def bottom_up_merge_sort(a)
+    sz = 1
+    n = a.count
+    while sz < n
+      lo = 0
+
+      # Ensure there are at least sz numbers to sort
+      while lo < (n - sz)
+
+        # a[lo..hi] has at most 2*sz numbers or
+        # at least n-1-lo numbers
+        hi = [n-1, lo+sz+sz-1].min
+
+        # a[lo..mid] has sz numbers
+        mid = lo+sz-1
+
+        merge_it(a, lo, mid, hi)
+
+        p a.inspect
+
+        lo = hi + 1
+      end
+
+      # Ensure that sz is a power of 2
+      sz = sz + sz
+    end
+    a
+  end
 
   def merge_sort(array)
     return array if array.size <= 1
     left = merge_sort array[0, array.size / 2]
     right = merge_sort array[array.size / 2, array.size]
 
-    # p left, right
     merging(left, right)
   end
 
@@ -59,8 +50,51 @@ def merging(left, right)
       end
     end
 
-    something = result.concat(left).concat(right)
-    p something
+    result.concat(left).concat(right)
+  end
+
+  def merge_it(a, lo, mid, hi)
+    # a[lo..mid] and a[mid+1..hi] must be sorted
+    dup = a.clone
+
+    # index for the left sub-array: lo <= i <= mid
+    i = lo
+
+    # index for the right sub-array: mid+1 <= j <= hi
+    j = mid+1
+
+    # index for the sorted sub-array: lo <= k <= hi
+    k = lo
+
+    while k <= hi
+      # All numbers in left sub-array have been compared
+      # so copy number from the right sub-array.
+      if i > mid
+        a[k] = dup[j]
+        j += 1
+
+      # All numbers in right sub-array have been compared
+      # so copy number from the left sub-array
+      elsif j > hi
+        a[k] = dup[i]
+        i += 1
+
+      # Number in right sub-array is less than the number in
+      # left sub-array. So copy number from the right sub-array
+      elsif dup[j] < dup[i]
+        a[k] = dup[j]
+        j += 1
+
+      # Number in left sub-array is less than the number in
+      # right sub-array. So copy number from left sub-array
+      else
+        a[k] = dup[i]
+        i += 1
+      end
+      k += 1
+    end
+
+    # The array `a[lo..hi]` is sorted.
   end
 
   private

week_3/mergesort_spec.rb

@@ -15,14 +15,25 @@
     end
   end
 
-  describe '#sort' do
+  xdescribe '#merge_sort' do
     context 'given an unsorted array' do
-      let(:actual) { [42, 28, 65, 49, 24, 43, 94, 33, 79, 57, 86, 56 ] }
+      let(:actual) { [42, 60, 59, 40, 75, 65, 57, 43, 30, 29, 10, 53] }
 
       it 'returns a sorted array' do
         p subject.merge_sort(actual)
         # expect(subject.actual).to eq [2,4,5,6,7,8,8,10]
       end
-   end
+    end
+  end
+
+  describe '#bottom_up' do
+    context 'given an unsorted array' do
+      let(:actual) { [17, 11, 68, 79, 36, 13, 33, 35, 99, 91] }
+
+      it 'returns a sorted array' do
+        p subject.bottom_up_merge_sort(actual)
+        # expect(subject.actual).to eq [2,4,5,6,7,8,8,10]
+      end
+    end
   end
 end