Added example Markov

Example Markov : simulation of a Markov chain.

Use of a Random module, external call to C rand() function.

examples/random/README

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+Simple Markov chain simulation by external call to random() C function :
+
+- random.epi : Heptagon interface containing the declaration of
+  random() function
+
+- random.h, random.c : "glue" between actual random() C function and
+  function calls generated by Heptagon
+
+- markov.ept : contains node "process", simulating a two-state Markov
+  chain, with probability p (constant) at each step to go from one
+  state to the other
+
+Function "random" and node "process" are declared unsafe to avoid
+optimization removing calls to "random" ("random" has side effects).
+
+Compilation and simulation :
+
+heptc random.epi
+heptc -target c -s process -hepts markov.ept
+gcc -I . -I markov_c -I /usr/local/lib/heptagon/c random.c markov_c/_main.c markov_c/markov.c markov_c/markov_types.c -o markov
+hepts -mod Markov -node process -exec ./markov

examples/random/markov.ept

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+open Random
+
+const p : float = 0.3
+
+unsafe node process() = (o:bool)
+let
+  automaton
+    state A
+      var c : bool;
+      do o = false; c = random() <. p; 
+      until c then B
+          | not c then C
+    state B
+      var c : bool;
+      do o = true; c = random() <. p
+      until c then A
+    state C
+      do o = false
+  end
+tel

examples/random/mathext.c

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+
+#include <stdlib.h>
+#include "mathext.h"
+
+void Mathext__power_step(float x, int n, Mathext__power_out *o) {
+  float r;
+  int i;
+
+  r = 1.0;
+  for (i = 1; i <= n; i++) {
+    r = r * x;
+  }
+  o->y = r;
+}
+

examples/random/mathext.epi

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+
+(* output : y = x^n *)
+val fun power(x:float; n:int) = (y:float)

examples/random/mathext.h

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+
+#ifndef MATHEXT_H
+#define MATHEXT_H
+
+
+/* Example of a combinatorial function */
+typedef struct Mathext__power_out {
+  float y;
+} Mathext__power_out;
+
+void Mathext__power_step(float x, int n, Mathext__power_out *o);
+
+#endif
+

examples/random/random.c

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+
+#include <stdlib.h>
+#include "random.h"
+
+void Random__random_step(Random__random_out *o) {
+  o->z = ((double)random())/((double)RAND_MAX);
+}
+

examples/random/random.epi

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+
+(* output : random float in interval [0,1] *)
+unsafe val fun random() = (z:float)

examples/random/random.h

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+
+#ifndef RANDOM_H
+#define RANDOM_H
+
+
+/* Example of a combinatorial function */
+typedef struct Random__random_out {
+  float z;
+} Random__random_out;
+
+void Random__random_step(Random__random_out *o);
+
+#endif
+