Added two new compile flags, TE_RAND_SEED_TIME and TE_RAND_SEED

TE_RAND_SEED_TIME uses time to see the mersenne twiser (ESP32 microcontrollers don't support entropy devices).

docs/manual/compile-time-options.qmd

@@ -18,6 +18,18 @@ Compile with `TE_LONG_DOUBLE` defined to use `long double` as the default data t
 Depending on the compiler, this may provide support for handling `uint64_t` values. Call `te_parser::supports_64bit()`
 (or `SUPPORTS64BIT()` in an expression at runtime) to confirm this.
 
+## `TE_RAND_SEED` {-}
+
+Define this as an unsigned integer to seed the random number generator with.
+This will affect the function `RAND()`, causing it to produce a deterministic series of numbers.
+
+## `TE_RAND_SEED_TIME` {-}
+
+Compile with `TE_RAND_SEED_TIME` to seed the random number generator with the current time.
+This is useful if compiling for microcontrollers that do not support entropy devices.
+
+The default is to seed the random number generator with `std::random_device`.
+
 ## `TE_BITWISE_OPERATORS` {-#te-bitwise-ops}
 
 By default, the operators `&`, `|`, and `^` represent logical AND, logical OR, and exponentiation. If `TE_BITWISE_OPERATORS` is defined,

docs/manual/embedded-programming.qmd

@@ -21,6 +21,11 @@ case insensitivity, and bookkeeping operations.
 
 Refer to [compile-time options](#compile-time-options) for flags that can provide optimization.
 
+## Device Compatibility
+
+By default, *TinyExpr++* uses `std::random_device` to seed its random number generator, which may cause compatibility issues with some microcontrollers.
+To instead seed it with the current time, compile with `TE_RAND_SEED_TIME`.
+
 ## Volatility {-}
 
 If needing to use a `te_parser` object as `volatile`

tests/tetests.cpp

@@ -2202,8 +2202,11 @@ TEST_CASE("Random", "[random]")
     {
     te_parser tep;
     // can't have reproducible results for rand, so just run it and make
-    // sure it doesn't crash
+    // sure it doesn't crash...
     CHECK_NOTHROW(tep.evaluate("rand()"));
+    // ...and within 0-1
+    auto res = tep.evaluate("rand()");
+    CHECK((res >= 0 && res <= 1));
     }
 
 TEST_CASE("Available functions", "[available]")

tinyexpr.cpp

@@ -360,8 +360,15 @@ namespace te_builtins
     [[nodiscard]]
     static te_type te_random()
         {
-        std::random_device rdev;
+#ifdef TE_RAND_SEED
+        std::mt19937 gen(static_cast<unsigned int>(RAND_SEED));
+#elif defined(TE_RAND_SEED_TIME)
+        std::mt19937 gen(static_cast<unsigned int>(time(nullptr)));
+#else
+        static std::random_device rdev;
         std::mt19937 gen(rdev());
+#endif
+
         std::uniform_real_distribution<te_type> distr(0, 1);
         return distr(gen);
         }

tinyexpr.h

@@ -59,6 +59,7 @@
 #include <cstdio>
 #include <cstdlib>
 #include <cstring>
+#include <ctime>
 #include <functional>
 #include <initializer_list>
 #include <limits>
@@ -76,6 +77,10 @@
 
 class te_parser;
 
+#if defined(TE_RAND_SEED) && defined(TE_RAND_SEED_TIME)
+    #error TE_RAND_SEED and TE_RAND_SEED_TIME compile options cannot be combined. Only one random number generator seeding method can be specified.
+#endif
+
 #if defined(TE_FLOAT) && defined(TE_LONG_DOUBLE)
     #error TE_FLOAT and TE_LONG_DOUBLE compile options cannot be combined. Only one data type can be specified.
 #endif