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+// primes below 60000: an o3 sieve over a fixed array, the primes
+// gathered into a growable one, then a few questions asked of them.
+// with ZYTRACE=1 the o3 lines show each region being lifted and compiled.
+
+// marks the composites; f[i] ends up 1 for every prime i
+o3 int sieve(fixed []int f) {
+	int n = len(f), c = 0;
+	for (int i = 2; i < n; i++) f[i] = 1;
+	for (int i = 2; i * i < n; i++)
+		if (f[i]) for (int j = i * i; j < n; j += i) f[j] = 0;
+	for (int i = 2; i < n; i++) c += f[i];
+	return c;
+}
+// the largest gap between neighbours, and where it starts
+o3 int gap(fixed []int f, int* at) {
+	int n = len(f), last = 2, best = 0, where = 0;
+	for (int i = 3; i < n; i++)
+		if (f[i]) { if (i - last > best) { best = i - last; where = last; } last = i; }
+	*at = where;
+	return best;
+}
+fn main() {
+	fixed []int f = new(fixed []int, 60000);
+	[]int ps = new([]int);
+	int c = sieve(f), at = 0, twins = 0;
+	for (int i = 2; i < len(f); i++) if (f[i]) push(ps, i);
+	o3 { for (int k = 1; k < len(ps); k++) if (ps[k] - ps[k - 1] == 2) twins++; }
+	put_s("primes below 60000: "); put_n(c); put_c(10);
+	put_s("the last five:"); for (int k = len(ps) - 5; k < len(ps); k++) { put_c(' '); put_n(ps[k]); } put_c(10);
+	put_s("twin pairs: "); put_n(twins); put_c(10);
+	int g = gap(f, &at);
+	put_s("widest gap: "); put_n(g); put_s(", after "); put_n(at); put_c(10);
+	free(d*f); free(d*ps);
+	return 0;
+}