1 files changed, 84 insertions, 0 deletions
diff --git a/firmware/sieve.c b/firmware/sieve.c
new file mode 100644
index 0000000..ff945eb
--- /dev/null
+++ b/firmware/sieve.c
@@ -0,0 +1,84 @@
+// This is free and unencumbered software released into the public domain.
+//
+// Anyone is free to copy, modify, publish, use, compile, sell, or
+// distribute this software, either in source code form or as a compiled
+// binary, for any purpose, commercial or non-commercial, and by any
+// means.
+
+// A simple Sieve of Eratosthenes
+
+#include "firmware.h"
+
+#define BITMAP_SIZE 64
+
+static uint32_t bitmap[BITMAP_SIZE/32];
+static uint32_t hash;
+
+static uint32_t mkhash(uint32_t a, uint32_t b)
+{
+	// The XOR version of DJB2
+	return ((a << 5) + a) ^ b;
+}
+
+static void bitmap_set(int idx)
+{
+	bitmap[idx/32] |= 1 << (idx % 32);
+}
+
+static bool bitmap_get(int idx)
+{
+	return (bitmap[idx/32] & (1 << (idx % 32))) != 0;
+}
+
+static void print_prime(int idx, int val)
+{
+	if (idx < 10)
+		print_str(" ");
+	print_dec(idx);
+	if (idx / 10 == 1)
+		goto force_th;
+	switch (idx % 10) {
+		case 1: print_str("st"); break;
+		case 2: print_str("nd"); break;
+		case 3: print_str("rd"); break;
+	force_th:
+		default: print_str("th"); break;
+	}
+	print_str(" prime is ");
+	print_dec(val);
+	print_str(".\n");
+
+	hash = mkhash(hash, idx);
+	hash = mkhash(hash, val);
+}
+
+void sieve(void)
+{
+	int idx = 1;
+	hash = 5381;
+	print_prime(idx++, 2);
+	for (int i = 0; i < BITMAP_SIZE; i++) {
+		if (bitmap_get(i))
+			continue;
+		print_prime(idx++, 3+2*i);
+		for (int j = 2*(3+2*i);; j += 3+2*i) {
+			if (j%2 == 0)
+				continue;
+			int k = (j-3)/2;
+			if (k >= BITMAP_SIZE)
+				break;
+			bitmap_set(k);
+		}
+	}
+
+	print_str("checksum: ");
+	print_hex(hash, 8);
+
+	if (hash == 0x1772A48F) {
+		print_str(" OK\n");
+	} else {
+		print_str(" ERROR\n");
+		__asm__ volatile ("ebreak");
+	}
+}
+