Set the random number seed. (#87)

It's used in a non-crypto safe RNG in MicroPython so just seed it with a
Math.random-derived value.

Closes #85

Author: microbit-matt-hillsdon

src/demo.html

@@ -93,6 +93,7 @@ <h1>MicroPython-micro:bit simulator example embedding</h1>
               <option value="music">Music</option>
               <option value="pin_logo">Pin logo</option>
               <option value="radio">Radio</option>
+              <option value="random">Random</option>
               <option value="sensors">Sensors</option>
               <option value="sound_effects_builtin">
                 Sound effects (builtin)

src/examples/random.py

@@ -0,0 +1,13 @@
+from microbit import display, sleep
+import random
+import machine
+
+
+n = random.randrange(0, 100)
+print("Default seed ", n)
+random.seed(1)
+n = random.randrange(0, 100)
+print("Fixed seed ", n)
+print()
+sleep(2000)
+machine.reset()

src/jshal.h

@@ -27,6 +27,8 @@
 void mp_js_hal_init(void);
 void mp_js_hal_deinit(void);
 
+uint32_t mp_js_rng_generate_random_word();
+
 uint32_t mp_js_hal_ticks_ms(void);
 void mp_js_hal_stdout_tx_strn(const char *ptr, size_t len);
 int mp_js_hal_stdin_pop_char(void);

src/jshal.js

@@ -36,6 +36,10 @@ mergeInto(LibraryManager.library, {
     Module.board.stopComponents();
   },
 
+  mp_js_rng_generate_random_word: function () {
+    return (Math.random() * 0x100000000) >>> 0;
+  },
+
   mp_js_hal_ticks_ms: function () {
     return Module.board.ticksMilliseconds();
   },
@@ -131,7 +135,10 @@ mergeInto(LibraryManager.library, {
     return Module.board.pins[pin].getAnalogPeriodUs();
   },
 
-  mp_js_hal_pin_set_analog_period_us: function (/** @type {number} */ pin, /** @type {number} */ period) {
+  mp_js_hal_pin_set_analog_period_us: function (
+    /** @type {number} */ pin,
+    /** @type {number} */ period
+  ) {
     return Module.board.pins[pin].setAnalogPeriodUs(period);
   },
 

src/microbithal_js.c

@@ -389,10 +389,9 @@ int microbit_hal_log_data(const char *key, const char *value) {
     return mp_js_hal_log_data(key, value);
 }
 
-// This is needed by the microbitfs implementation.
+// This is used to seed the random number generator.
 uint32_t rng_generate_random_word(void) {
-    //return uBit.random(65536) << 16 | uBit.random(65536);
-    return 0;
+    return mp_js_rng_generate_random_word();
 }
 
 void microbit_hal_audio_select_pin(int pin) {