From 537891a43e88407719dc290e6725180139053a55 Mon Sep 17 00:00:00 2001
From: "andpal@linlap" <andrepalmborg@gmail.com>
Date: Sun, 27 Feb 2022 22:14:16 +0100
Subject: [PATCH] Odd operator overload composing functions and vectors

---
 f-composition/Makefile         |  26 ++++++++
 f-composition/functional.hpp   | 116 +++++++++++++++++++++++++++++++++
 f-composition/main.cpp         |  29 +++++++++
 f-composition/pretty_print.hpp |  26 ++++++++
 4 files changed, 197 insertions(+)
 create mode 100755 f-composition/Makefile
 create mode 100644 f-composition/functional.hpp
 create mode 100644 f-composition/main.cpp
 create mode 100644 f-composition/pretty_print.hpp

diff --git a/f-composition/Makefile b/f-composition/Makefile
new file mode 100755
index 0000000..bc2273b
--- /dev/null
+++ b/f-composition/Makefile
@@ -0,0 +1,26 @@
+EXE=a.out
+CC=clang++
+DEBUG?=0
+CFLAGS=-O2 -std=c++20 -Wall -Wextra -D DEBUG=$(DEBUG)
+
+.PHONY: all clean test
+test:
+	- @rm $(EXE)
+	$(CC) $(CFLAGS) -o $(EXE) main.cpp
+	./$(EXE)
+
+all: $(EXE)
+
+clean:
+	rm -rf $(EXE) ./*.out ./*.gch
+
+
+$(EXE): main.cpp pretty_print.hpp.gch functional.hpp.gch
+	$(CC) $(CFLAGS) -o $(EXE) main.cpp
+
+functional.hpp.gch: functional.hpp
+	$(CC) $(CFLAGS) -c functional.hpp
+
+pretty_print.hpp.gch: pretty_print.hpp
+	$(CC) $(CFLAGS) -c pretty_print.hpp
+
diff --git a/f-composition/functional.hpp b/f-composition/functional.hpp
new file mode 100644
index 0000000..2f0fd11
--- /dev/null
+++ b/f-composition/functional.hpp
@@ -0,0 +1,116 @@
+#ifndef FUNCTIONAL_H
+#define FUNCTIONAL_H
+
+
+#include <iostream>
+#include <vector>
+#include <utility>
+
+/*
+ * Composition of functions through operator*
+ *
+ * Immediate evaluation of <lambda> * <rvalue>
+ */
+template<typename F, typename G>
+auto composition_h(F&& f, G&& g, std::true_type) {
+#if DEBUG==1
+    std::cout << "<Lambda> * <Lambda> -> <Lambda>" << std::endl; // DEBLURG
+#endif
+    return [f, g](auto&& t){
+        return f( g( std::forward<decltype(t)>(t) ) );
+    };
+}
+
+template<typename F, typename G>
+auto composition_h(F&& f, G&& g, std::false_type) {
+#if DEBUG==1
+    std::cout << "<Lambda> * T -> T" << std::endl; // DEBLURG
+#endif
+    return f( g );
+}
+
+template<typename F, typename G>
+auto operator*(F&& f, G&& g) {
+#if DEBUG==1
+    std::cout << "f * g -> ";
+#endif
+    return composition_h(f, g, std::is_lvalue_reference<G&&>());
+}
+
+
+/*
+ * Following templates handle mapping of transforming function
+ * onto vector<T> types
+ */
+template<typename Transform, typename T>
+auto operator*(Transform&& transform, std::vector<T> const& vec) {
+#if DEBUG==1
+    std::cout << "<Lambda> * vec<T> -> vec<T> \t[const]" << std::endl; // DEBLURG
+#endif
+    std::vector<T> transformed_vec;
+    transformed_vec.reserve( vec.size() );
+
+    for (auto const& v : vec) {
+        transformed_vec.push_back( transform(v) );
+    }
+
+    return transformed_vec;
+}
+
+template<typename Transform, typename T>
+auto operator*(Transform&& transform, std::vector<T>&& vec) {
+#if DEBUG==1
+    std::cout << "<Lambda> * vec<T> -> vec<T> \t[rvalue]" << std::endl; // DEBLURG
+#endif
+    std::vector<T> transformed_vec;
+    transformed_vec.reserve( vec.size() );
+
+    for (auto const& v : vec) {
+        transformed_vec.push_back( transform(v) );
+    }
+
+    return transformed_vec;
+}
+
+template<typename Transform, typename T>
+auto operator*(Transform&& transform, std::vector<T>& vec) {
+#if DEBUG==1
+    std::cout << "<Lambda> * vec<T> -> vec<T> \t[non-const]" << std::endl;
+#endif
+
+    std::vector<T> transformed_vec;
+    transformed_vec.reserve( vec.size() );
+
+    for (auto const& v : vec) {
+        transformed_vec.push_back( transform(v) );
+    }
+
+    return transformed_vec;
+}
+
+
+auto const id = [](auto&& t) -> decltype(t)&& {
+    return std::forward<decltype(t)>(t);
+};
+
+
+
+/*
+ * Maps a transform function onto each element in vector<T>
+ *
+ * Obsolete
+ */
+template<typename F, typename T>
+auto map = [](F&& transform) {
+    return [transform](std::vector<T> const& vec) {
+        std::vector<T> transformed_vec;
+        transformed_vec.reserve( vec.size() );
+        for (auto const& v : vec) {
+            transformed_vec.push_back( transform(v) );
+        }
+        return transformed_vec;
+    };
+};
+
+
+#endif
diff --git a/f-composition/main.cpp b/f-composition/main.cpp
new file mode 100644
index 0000000..ab400aa
--- /dev/null
+++ b/f-composition/main.cpp
@@ -0,0 +1,29 @@
+#include <iostream>
+#include <typeinfo>
+#include <stdint.h>
+#include <vector>
+
+
+#include "functional.hpp"   // (f * g)(t) == f(g(t))
+#include "pretty_print.hpp" // ostream << vector
+
+
+int main()
+{
+    std::vector<int> const vec{1,2,3,4,5};
+
+    auto const dec = [](int const& v) {
+        return v-1;
+    };
+    auto const sq = [](int const& v) {
+        return v*v;
+    };
+    
+    std::vector<int> const transformed_vec{ dec * sq * vec };
+
+    std::cout << "Orig: \t" << vec              << '\n';
+    std::cout << "Sq:   \t" << sq * vec         << '\n';
+    std::cout << "Sq-1: \t" << transformed_vec  << "\n\n";
+
+    std::cout << "Const:\t" << sq*dec*7         << std::endl;
+}
diff --git a/f-composition/pretty_print.hpp b/f-composition/pretty_print.hpp
new file mode 100644
index 0000000..9975927
--- /dev/null
+++ b/f-composition/pretty_print.hpp
@@ -0,0 +1,26 @@
+#ifndef PRETTY_PRINT_H
+#define PRETTY_PRINT_H
+
+#include <iostream>
+#include <iomanip>
+#include <vector>
+
+template<typename T>
+std::ostream& operator<<(std::ostream& os, std::vector<T> const& vec)
+{
+    if (vec.size() == 0) { return os; } // Empty
+    os << "[" << vec[0];                // First
+
+    if (1 < vec.size()) {               // Rest
+        for (auto it = ++vec.cbegin(); it != vec.cend(); ++it) {
+            os << ", " << std::setw(2) << *it;
+        }
+    }
+
+    os << "]";
+
+    return os;
+}
+
+
+#endif