From badea839f93c0b9c482982ac0a3e2772253beb7a Mon Sep 17 00:00:00 2001
From: Tommy Thorn <tommy-github2@thorn.ws>
Date: Sun, 17 Sep 2023 17:44:21 -0700
Subject: [PATCH] Fix sieve.c and make it faster

---
 examples/sieve.c | 50 ++++++++++++++++++++++++++++++------------------
 1 file changed, 31 insertions(+), 19 deletions(-)

diff --git a/examples/sieve.c b/examples/sieve.c
index 15e001b..b251a7d 100644
--- a/examples/sieve.c
+++ b/examples/sieve.c
@@ -5,28 +5,40 @@
 //  for (..)
 
 #include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
 
 
 int main(int argc, char **argv) {
-    int N, p, j;
-    char *excluded, *excluded_end, *cp;
-
-    N = 10000;
-    excluded = malloc(N);
-    excluded_end = excluded + N;
-    memset(excluded,  0, N);
-
-    p = 2;
-    while (p < N) {
-        if (!excluded[p]) {
-            printf(" %d", p);
-            j = p*2;
-            cp = excluded + j;
-            while (cp < excluded_end) {
-                *cp = 1;
-                cp = cp + j;
-            }
+    int N, j, p;
+    char *primes, *primes_end, *cp, *pi;
+
+    N = 1000;
+    // 3 5 7 9 11 13 .. i*2+3
+    // primes[N] is a sential 0; this enables us to scan without
+    // having to test against the array limit
+    primes = malloc(N+1);
+    primes_end = primes + N;
+    memset(primes, 1, N + 1);
+
+    printf("2");
+
+    pi = primes;
+    while (pi < primes_end) {
+        p = 3 + 2*(pi - primes);
+        printf(" %d", p);
+        cp = pi + p;
+        while (cp < primes_end) {
+            *cp = 0; // Sieve out multiples of p
+            cp = cp + p;
         }
-        ++p;
+
+        ++pi;
+        while (!*pi)
+            ++pi;
     }
+
+    printf("\n");
+
+    return 0;
 }