Data commit

Task/Farey-sequence/C/farey-sequence.c

@@ -0,0 +1,61 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+void farey(int n)
+{
+	typedef struct { int d, n; } frac;
+	frac f1 = {0, 1}, f2 = {1, n}, t;
+	int k;
+
+	printf("%d/%d %d/%d", 0, 1, 1, n);
+	while (f2.n > 1) {
+		k = (n + f1.n) / f2.n;
+		t = f1, f1 = f2, f2 = (frac) { f2.d * k - t.d, f2.n * k - t.n };
+		printf(" %d/%d", f2.d, f2.n);
+	}
+
+	putchar('\n');
+}
+
+typedef unsigned long long ull;
+ull *cache;
+size_t ccap;
+
+ull farey_len(int n)
+{
+	if (n >= ccap) {
+		size_t old = ccap;
+		if (!ccap) ccap = 16;
+		while (ccap <= n) ccap *= 2;
+		cache = realloc(cache, sizeof(ull) * ccap);
+		memset(cache + old, 0, sizeof(ull) * (ccap - old));
+	} else if (cache[n])
+		return cache[n];
+
+	ull len = (ull)n*(n + 3) / 2;
+	int p, q = 0;
+	for (p = 2; p <= n; p = q) {
+		q = n/(n/p) + 1;
+		len -= farey_len(n/p) * (q - p);
+	}
+
+	cache[n] = len;
+	return len;
+}
+
+int main(void)
+{
+	int n;
+	for (n = 1; n <= 11; n++) {
+		printf("%d: ", n);
+		farey(n);
+	}
+
+	for (n = 100; n <= 1000; n += 100)
+		printf("%d: %llu items\n", n, farey_len(n));
+
+	n = 10000000;
+	printf("\n%d: %llu items\n", n, farey_len(n));
+	return 0;
+}