added prime search function

1 files changed, 20 insertions, 0 deletions

diff --git a/src/main.rs b/src/main.rs
index 135a081..c027b11 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -31,6 +31,23 @@ use rp_pico::hal::pac;
 // higher-level drivers.
 use rp_pico::hal;
 
+fn is_prime (a : u64) -> bool {
+    if a % 2 == 0 {return false};
+    for i in (3..a.isqrt()).step_by(2) {
+        if a % i == 0 {
+            return false;
+        }
+    }
+    return true;
+}
+
+fn find_next_smaller_prime(mut x : u64) -> u64 {
+    if x <= 1 {return x};
+    if x % 2 == 0 {x=x-1};
+    while !is_prime(x) {x = x - 2};
+    return x;
+}
+
 /// Entry point to our bare-metal application.
 ///
 /// The `#[entry]` macro ensures the Cortex-M start-up code calls this function
@@ -84,6 +101,9 @@ fn main() -> ! {
     loop {
         led_pin.set_high().unwrap();
         delay.delay_ms(500);
+        if find_next_smaller_prime(1<<36) > 5 {
+            led_pin.set_high().unwrap();
+        }
         led_pin.set_low().unwrap();
         delay.delay_ms(500);
     }