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book/01-знакомство-с-языком-d/src/chapter-1-6-1/app.d

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+import std.stdio : writeln, stdin;
+import std.exception : enforce;
+import stats;
+
+void main(string[] args)
+{
+    Stat[] stats;
+    foreach (arg; args[1 .. $])
+    {
+        auto newStat = cast(Stat) Object.factory("stats." ~ arg);
+        enforce(newStat, "Invalid statistics function: " ~ arg);
+        stats ~= newStat;
+    }
+    for (double x; stdin.readf(" %s ", &x) == 1;)
+    {
+        foreach (s; stats)
+        {
+            s.accumulate(x);
+        }
+    }
+    foreach (s; stats)
+    {
+        s.postprocess();
+        writeln(s.result());
+    }
+}

book/01-знакомство-с-языком-d/src/chapter-1-6-1/stats.d

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+module stats;
+
+interface Stat
+{
+    void accumulate(double x);
+    void postprocess();
+    double result();
+}
+
+class IncrementalStat : Stat
+{
+    protected double _result;
+    abstract void accumulate(double x);
+    void postprocess() {}
+
+    double result()
+    {
+        return _result;
+    }
+}
+
+class Min : IncrementalStat
+{
+    this()
+    {
+        _result = double.max;
+    }
+
+    override void accumulate(double x)
+    {
+        if (x < _result)
+        {
+            _result = x;
+        }
+    }
+}
+
+class Max : IncrementalStat
+{
+    this()
+    {
+        _result = double.min_normal;
+    }
+
+    override void accumulate(double x)
+    {
+        if (x > _result)
+        {
+            _result = x;
+        }
+    }
+}
+
+class Average : IncrementalStat
+{
+    private uint items = 0;
+
+    this()
+    {
+        _result = 0;
+    }
+
+    override void accumulate(double x)
+    {
+        _result += x;
+        ++items;
+    }
+
+    override void postprocess()
+    {
+        if (items)
+        {
+            _result /= items;
+        }
+    }
+}