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@@ -10,27 +10,28 @@
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 struct array_comparison {
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   long intersection_size;
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   long union_size;
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+  long a_size;
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+  long b_size;
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 };
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-static struct array_comparison compare_arrays(VALUE a, VALUE b) {
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-  size_t a_size = RARRAY(a)->len;
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-  size_t b_size = RARRAY(b)->len;
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-  struct array_comparison result;
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+typedef double (* fun_array_cmp)(const struct array_comparison);
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+
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+static VALUE compare_arrays(VALUE a, VALUE b, fun_array_cmp cmp) {
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+  struct array_comparison result = { 0, 0, RARRAY(a)->len, RARRAY(b)->len };
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   long * long_a;
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   long * long_b;
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   int i, j;
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-  result.intersection_size = 0;
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-  result.union_size = a_size + b_size;
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+  result.union_size = result.a_size + result.b_size;
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-  if((a_size > 0) && (b_size > 0))
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+  if((result.a_size > 0) && (result.b_size > 0))
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   {
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     COPYRUBYHASHARRAY(a, long_a);
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     COPYRUBYHASHARRAY(b, long_b);
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-    for(i = 0; i < a_size; ++i)
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+    for(i = 0; i < result.a_size; ++i)
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     {
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-      for(j = 0; j < b_size; ++j)
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+      for(j = 0; j < result.b_size; ++j)
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       {
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         if(long_a[i] == long_b[j])
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         {
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@@ -41,27 +42,45 @@ static struct array_comparison compare_arrays(VALUE a, VALUE b) {
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   }
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-  return result;
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+  return rb_float_new((*cmp)(result));
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 }
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-// Calculates the Tanimoto coefficient between two sets.
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-static VALUE Similarity_tanimoto_coefficient(VALUE self, VALUE data1, VALUE data2) {
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-  struct array_comparison cmp = compare_arrays(data1, data2);
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-  double result = 0.0;
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-  
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+double tanimoto_coefficient(const struct array_comparison cmp) {
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   if(cmp.union_size > 0)
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   {
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-    result = cmp.intersection_size / (double)(cmp.union_size - cmp.intersection_size);
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+    return (cmp.intersection_size / (double)(cmp.union_size - cmp.intersection_size));
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+  }
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+  else
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+  {
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+    return 0.0;
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   }
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-  
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-  return rb_float_new(result);
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 }
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+double dice_coefficient(const struct array_comparison cmp) {
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+  if(cmp.union_size > 0)
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+  {
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+    return (2 * cmp.intersection_size) / (double)(cmp.a_size + cmp.b_size);
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+  }
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+  else
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+  {
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+    return 0.0;
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+  }
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+}
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+// Calculates the Tanimoto coefficient between two sets.
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+static VALUE Similarity_tanimoto_coefficient(VALUE self, VALUE data1, VALUE data2) {
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+  return compare_arrays(data1, data2, tanimoto_coefficient);
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+}
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+
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+
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+static VALUE Similarity_dice_coefficient(VALUE self, VALUE data1, VALUE data2) {
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+  return compare_arrays(data1, data2, dice_coefficient);
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+}
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 VALUE rbgsl_mSimilarity;
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 void Init_Similarity() {
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   rbgsl_mSimilarity = rb_define_module_under(rbgsl_mGSL, "Similarity");
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   rb_define_module_function(rbgsl_mSimilarity, "tanimoto_coefficient", Similarity_tanimoto_coefficient, 2);
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+  rb_define_module_function(rbgsl_mSimilarity, "dice_coefficient", Similarity_dice_coefficient, 2);
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 }
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\ No newline at end of file

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@@ -2,7 +2,7 @@ require File.join(File.dirname(__FILE__), "..", "spec_helper")
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 describe "Similarity" do
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-  describe "calculating the Tanimoto difference" do
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+  describe "calculating the Tanimoto coefficient" do
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     it "should return 0.0 if either of the sets are empty" do
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       GSL::Similarity.tanimoto_coefficient(%w{ shirt shoes pants socks}, []).should be_close(0.0, 0.0001)
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       GSL::Similarity.tanimoto_coefficient([], %w{ shirt skirt shoes }).should be_close(0.0, 0.0001)
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@@ -10,7 +10,19 @@ describe "Similarity" do
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     end
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     it "should return a float" do
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-      GSL::Similarity.tanimoto_coefficient(%w{ shirt shoes pants socks}, %w{ shirt skirt shoes }).should be_close(0.4, 0.0001)
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+      GSL::Similarity.tanimoto_coefficient(%w{ shirt shoes pants socks }, %w{ shirt skirt shoes }).should be_close(0.4, 0.0001)
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+    end
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+  end
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+  
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+  describe "calculating Dice's coefficient" do
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+    it "should return 0.0 if either of the sets are empty" do
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+      GSL::Similarity.dice_coefficient(%w{ shirt shoes pants socks}, []).should be_close(0.0, 0.0001)
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+      GSL::Similarity.dice_coefficient([], %w{ shirt skirt shoes }).should be_close(0.0, 0.0001)
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+      GSL::Similarity.dice_coefficient([], []).should be_close(0.0, 0.0001)
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+    end
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+    
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+    it "should return a float" do
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+      GSL::Similarity.dice_coefficient(%w{ ni ig gh ht }, %w{ na ac ch ht }).should be_close(0.25, 0.0001)
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     end
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   end
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 end
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\ No newline at end of file