From d16656919753438a57a32f9ad6ba972c25d66b11 Mon Sep 17 00:00:00 2001
From: Sebastiano Tronto <sebastiano@tronto.net>
Date: Sun, 9 Feb 2025 19:38:26 +0100
Subject: Added random number generator for bigint

---
 README.md     |  4 +++-
 bigint.h      | 13 +++++++++++++
 random_bigint | 34 ++++++++++++++++++++++++++++++++++
 3 files changed, 50 insertions(+), 1 deletion(-)
 create mode 100755 random_bigint

diff --git a/README.md b/README.md
index 043b626..1d4a3f9 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,9 @@ Usage:
 ## BigInt
 
 This repo contains also a class for big integer. Performance are not great,
-but you have a look at it for educational purposes.
+but you can have a look at it for educational purposes.
+
+The script `random_bigint` can be used to generate random big integers.
 
 ## Development
 
diff --git a/bigint.h b/bigint.h
index 9287fda..096ed9c 100644
--- a/bigint.h
+++ b/bigint.h
@@ -3,6 +3,7 @@
 
 #include <cstdint>
 #include <iostream>
+#include <random>
 #include <string_view>
 
 constexpr uint64_t abs64(int64_t);
@@ -123,6 +124,18 @@ public:
 	constexpr BigInt operator/=(const BigInt& z) { return *this = *this / z; }
 	constexpr BigInt operator%=(const BigInt& z) { return *this = *this % z; }
 
+	static BigInt random(BigInt r) {
+		std::random_device rd;
+		std::default_random_engine rng(rd());
+		std::uniform_int_distribution<int> distribution(0, M-1);
+
+		BigInt ret;
+		for (uint64_t i = 0; i < D; i++)
+			ret.digits[i] = distribution(rng);
+
+		return ret % r;
+	}
+
 	friend std::ostream& operator<<(std::ostream& os, const BigInt<N, E>& z) {
 		if (z == 0) {
 			os << "0";
diff --git a/random_bigint b/random_bigint
new file mode 100755
index 0000000..250de64
--- /dev/null
+++ b/random_bigint
@@ -0,0 +1,34 @@
+#if 0
+
+cc=${CC:-g++}
+bin="$(mktemp)"
+
+if [ -z "$1" ] || [ -z "$2" ]; then
+	echo "Usage: $0 m n"
+	echo "Example: to generate 5 random 25-digits number use"
+	echo "  $0 \\\"10000000000000000000000000\\\" 5"
+	exit 1
+fi
+
+${cc} -x c++ -std=c++20 -o "$bin" -g -O0 "$(realpath $0)" \
+	-DMAX_RANDOM="$1" -DHOW_MANY="$2"
+echo "Running $bin"
+"$bin"
+
+exit 0
+#endif
+
+#include "bigint.h"
+
+#include <iostream>
+
+int main() {
+	// Read compile-time variables
+	int n = HOW_MANY;
+	BigInt<100> r(MAX_RANDOM);
+
+	for (int i = 0; i < n; i++)
+		std::cout << BigInt<100>::random(r) << std::endl;
+
+	return 0;
+}
-- 
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