Add approximate counting algorithm in C

contents/approximate_counting/approximate_counting.md

@@ -360,6 +360,8 @@ As we do not have any objects to count, we will instead simulate the counting wi
 {% method %}
 {% sample lang="jl" %}
 [import, lang:"julia"](code/julia/approximate_counting.jl)
+{% sample lang="c" %}
+[import, lang:"c"](code/c/approximate_counting.c)
 {% sample lang="cpp" %}
 [import, lang:"cpp"](code/c++/approximate_counting.cpp)
 {% endmethod %}

contents/approximate_counting/code/c/approximate_counting.c

@@ -0,0 +1,82 @@
+#include <assert.h>
+#include <math.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <time.h>
+
+// This function returns a pseudo-random number between 0 and 1
+double drand()
+{
+    return (double)rand() / RAND_MAX;
+}
+
+// This function takes
+//  - v: value in register
+//  - a: a scaling value for the logarithm based on Morris's paper
+// It returns the approximate count
+double n(double v, double a)
+{
+    return a * (pow(1 + 1 / a, v) - 1);
+}
+
+// This function takes
+//  - v: value in register
+//  - a: a scaling value for the logarithm based on Morris's paper
+// It returns a new value for v
+double increment(double v, double a)
+{
+    // delta is the probability of incrementing our counter
+    double delta = 1 / (n(v + 1, a) - n(v, a));
+
+    if (drand() <= delta) {
+        return v + 1;
+    }
+    return v;
+}
+
+// This function simulates counting and takes
+//  - n_items: number of items to count and loop over
+//  - a: a scaling value for the logarithm based on Morris's paper
+// It returns n(v, a), the approximate count
+double approximate_count(size_t n_items, double a)
+{
+    int v = 0;
+    for (size_t i = 0; i < n_items; ++i) {
+        v = increment(v, a);
+    }
+
+    return n(v, a);
+}
+
+// This function takes
+//  - n_trials: the number of counting trials
+//  - n_items: the number off items to count
+//  - a: a scaling value for the logarithm based on Morris's paper
+//  - threshold: the maximum percent error allowed
+// It terminates the program on failure
+void test_approximation_count(size_t n_trials, size_t n_items, double a,
+                              double threshold)
+{
+    double sum = 0.0;
+    for (size_t i = 0; i < n_trials; ++i) {
+        sum += approximate_count(n_items, a);
+    }
+    double avg = sum / n_trials;
+
+    assert(fabs((avg - n_items) / n_items) < threshold);
+}
+
+int main()
+{
+    srand(time(NULL));
+
+    printf("Counting Tests, 100 trials\n");
+    printf("testing 1000, a = 30, 1%% error\n");
+    test_approximation_count(100, 1000, 30, 0.1);
+    printf("testing 12345, a = 10, 1%% error\n");
+    test_approximation_count(100, 12345, 10, 0.1);
+    printf("testing 222222, a = 0.5, 10%% error\n");
+    test_approximation_count(100, 222222, 0.5, 0.2);
+
+    return 0;
+}