Peano.cpp: Pattern Matching and Type Arithmetic in C++

README.md

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 # cpp_samples
 
 A repository of samples in C++
+
+- peano.cpp: Pattern Matching and Type Arithmetic in C++

peano.cpp

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+// pattern matching and type arithmetic in C++
+// see https://en.wikipedia.org/wiki/Peano_axioms
+// see https://en.wikipedia.org/wiki/Successor_function
+
+// compile and test: "c++ peano.cpp -o peano && ./peano"
+
+//
+// zero is our first natural number
+//
+
+struct zero {};
+
+//
+// all other natural numbers are either a successor of zero
+// or a successor of another successor
+//
+
+template<class> // let's forget that type (see ::sum below)
+struct successor {};
+
+//
+// sum = first + second
+//
+
+// non-empty declaration: this is the end of the recursion
+// (second is zero)
+template<class first, class second>
+struct sum {
+    using type = first;
+};
+
+// recursive specialization
+// the pattern "successor<second>" isn't matching zero
+// and is giving us the type of the predecessor we
+// forgot in the declaration of ::successor
+template<class first, class second>
+struct sum<first, successor<second>> {
+    // second is decremented by pattern matching
+    using recursion = typename sum<first, second>::type;
+    // and the result is incremented
+    using type = successor<recursion>;
+};
+
+//
+// product = multiplicand * multiplier
+//
+
+// non-empty declaration: this is the end of the recursion
+// (multiplier is zero)
+template<class multiplicand, class multiplier>
+struct product {
+    using type = zero;
+};
+
+// recursive specialization when multiplier isn't zero
+template<class multiplicand, class multiplier>
+struct product<multiplicand, successor<multiplier>> {
+    using recursion = typename product<multiplicand, multiplier>::type;
+    using type = typename sum<multiplicand, recursion>::type;
+};
+
+//
+// difference = minuend - subtrahend
+//
+
+// empty declaration: compilation error if the result is negative
+template<class minuend, class subtrahend>
+struct difference {
+};
+
+// end of the recursion
+template<class minuend>
+struct difference<minuend, zero> {
+    using type = minuend;
+};
+
+// recursive specialization
+template<class minuend, class subtrahend>
+struct difference<successor<minuend>, successor<subtrahend>> {
+    using recursion = difference<minuend, subtrahend>;
+    using type = typename recursion::type;
+};
+
+//
+// quotient, remainder = dividend / divisor
+//
+
+namespace impl {
+
+    // NOTE: for the division, we decrement divisor and
+    // increment _remainder until divisor is zero, at
+    // this point we increment the result, swap _remainder
+    // with divisor to restore its value, until dividend is
+    // zero and _remainder is the final remainder
+
+    // empty declaration: compilation error when dividing by zero
+    template<class dividend, class divisor, class _remainder>
+    struct quotient {
+    };
+
+    // specialization when the dividend is zero, this is the end
+    // of the recursion when the remainder isn't zero
+    template<class divisor, class _remainder>
+    struct quotient<zero, divisor, _remainder> {
+        using type = zero;
+        using remainder = _remainder;
+    };
+
+    // specialization when both dividend and divisor are zero,
+    // the result is incremented and this is the end of the
+    // recursion when the remainder is zero
+    template<class _remainder>
+    struct quotient<zero, zero, successor<_remainder>> {
+        using type = successor<zero>;
+        using remainder = zero;
+    };
+
+    // recursive specialization when divisor is zero
+    // the result is incremented and divisor is restored
+    // from _remainder
+    template<class dividend, class _remainder>
+    struct quotient<dividend, zero, successor<_remainder>> {
+        using recursion = quotient<dividend, successor<_remainder>, zero>;
+        using type = successor<typename recursion::type>;
+        using remainder = typename recursion::remainder;
+    };
+
+    // recursive specialization
+    template<class dividend, class divisor, class _remainder>
+    struct quotient<successor<dividend>, successor<divisor>, _remainder> {
+        using recursion = quotient<dividend, divisor, successor<_remainder>>;
+        using type = typename recursion::type;
+        using remainder = typename recursion::remainder;
+    };
+
+} // namespace impl
+
+// let's hide the third parameter
+template<class dividend, class divisor>
+using quotient = impl::quotient<dividend, divisor, zero>;
+
+//
+// natural_to_peano: convert from unsigned integer
+//
+
+template<unsigned natural>
+struct natural_to_peano {
+    using recursion = typename natural_to_peano<natural - 1u>::type;
+    using type = successor<recursion>;
+};
+
+template<>
+struct natural_to_peano<0u> {
+    using type = zero;
+};
+
+//
+// peano_to_natural: convert to unsigned integer
+//
+
+template<class>
+struct peano_to_natural {
+    enum : unsigned { value = 0u };
+};
+
+template<class type>
+struct peano_to_natural<successor<type>> {
+    enum : unsigned {
+        recursion = peano_to_natural<type>::value,
+        value = recursion + 1u
+    };
+};
+
+//
+// now let's test!
+//
+
+#include <cstdio>
+
+int main () {
+    using four = natural_to_peano<4u>::type;
+    using six = natural_to_peano<6u>::type;
+    using ten = sum<six, four>::type;
+    using fourty = product<ten, four>::type;
+    using thirty_four = difference<fourty, six>::type;
+
+    // output: thirty_four = 34
+    printf("thirty_four = %u\n",
+        peano_to_natural<thirty_four>::value);
+
+ 	// compilation error
+    // using negative_difference = difference<four, six>::type;
+
+    using fourty_over_ten = quotient<fourty, ten>;
+    using thirty_four_over_ten = quotient<thirty_four, ten>;
+
+    // compilation errors:
+    // using divide_by_zero = quotient<four, zero>;
+    // printf("divide_by_zero = %u, %u\n",
+    // 	peano_to_natural<typename divide_by_zero::type>::value,
+    // 	peano_to_natural<typename divide_by_zero::remainder>::value);
+
+    // output: fourty_over_ten = 4, 0
+    printf("fourty_over_ten = %u, %u\n",
+        peano_to_natural<typename fourty_over_ten::type>::value,
+        peano_to_natural<typename fourty_over_ten::remainder>::value);
+
+    // output: thirty_four_over_ten = 3, 4
+    printf("thirty_four_over_ten = %u, %u\n",
+        peano_to_natural<typename thirty_four_over_ten::type>::value,
+        peano_to_natural<typename thirty_four_over_ten::remainder>::value);
+
+    return 0;
+}