random: randint() no longer rejects the largest upper bound

shared-bindings/random/__init__.c

@@ -116,10 +116,7 @@ static mp_obj_t random_randint(mp_obj_t a_in, mp_obj_t b_in) {
     if (a > b) {
         mp_raise_ValueError(NULL);
     }
-    if (b == (mp_int_t)(~(mp_uint_t)0 >> 1)) {
-        mp_raise_ValueError(NULL);
-    }
-    return mp_obj_new_int(shared_modules_random_randrange(a, b + 1, 1));
+    return mp_obj_new_int(shared_modules_random_randint(a, b));
 }
 static MP_DEFINE_CONST_FUN_OBJ_2(random_randint_obj, random_randint);
 

shared-bindings/random/__init__.h

@@ -13,5 +13,7 @@
 void shared_modules_random_seed(mp_uint_t seed);
 mp_uint_t shared_modules_random_getrandbits(uint8_t n);
 mp_int_t shared_modules_random_randrange(mp_int_t start, mp_int_t stop, mp_int_t step);
+// Returns a random integer in [a, b] inclusive. The caller must ensure a <= b.
+mp_int_t shared_modules_random_randint(mp_int_t a, mp_int_t b);
 mp_float_t shared_modules_random_random(void);
 mp_float_t shared_modules_random_uniform(mp_float_t a, mp_float_t b);

shared-module/random/__init__.c

@@ -82,6 +82,16 @@ mp_int_t shared_modules_random_randrange(mp_int_t start, mp_int_t stop, mp_int_t
     return start + step * yasmarang_randbelow(n);
 }
 
+mp_int_t shared_modules_random_randint(mp_int_t a, mp_int_t b) {
+    // Compute the span unsigned so that b - a + 1 cannot overflow for any a <= b.
+    mp_uint_t n = (mp_uint_t)b - (mp_uint_t)a + 1;
+    if (n == 0) {
+        // a and b cover the full mp_int_t range: any value is in range.
+        return a + (mp_int_t)yasmarang();
+    }
+    return a + yasmarang_randbelow(n);
+}
+
 // returns a number in the range [0..1) using Yasmarang to fill in the fraction bits
 static mp_float_t yasmarang_float(void) {
     #if MICROPY_FLOAT_IMPL == MICROPY_FLOAT_IMPL_DOUBLE

tests/extmod/random_extra.py

@@ -48,6 +48,11 @@
     assert 2 <= random.randint(2, 6) <= 6
     assert -2 <= random.randint(-2, 2) <= 2
 
+# CIRCUITPY-CHANGE: upper bound at the largest 32-bit mp_int_t must not overflow
+for i in range(50):
+    assert 0 <= random.randint(0, 0x7FFFFFFF) <= 0x7FFFFFFF
+    assert 1 <= random.randint(1, 0x7FFFFFFF) <= 0x7FFFFFFF
+
 # empty range
 try:
     random.randint(2, 1)