tally: implement tally_total()

Not as trivial as you might think, with overflow and underflow.

ccan/tally/tally.c

@@ -8,7 +8,7 @@
 #include <stdio.h>
 #include <assert.h>
 
-#define MAX_STEP_BITS (sizeof(size_t)*CHAR_BIT)
+#define SIZET_BITS (sizeof(size_t)*CHAR_BIT)
 
 /* We use power of 2 steps.  I tried being tricky, but it got buggy. */
 struct tally {
@@ -55,9 +55,9 @@ struct tally *tally_new(size_t buckets)
 static unsigned bucket_of(ssize_t min, unsigned step_bits, ssize_t val)
 {
 	/* Don't over-shift. */
-	if (step_bits == MAX_STEP_BITS)
+	if (step_bits == SIZET_BITS)
 		return 0;
-	assert(step_bits < MAX_STEP_BITS);
+	assert(step_bits < SIZET_BITS);
 	return (size_t)(val - min) >> step_bits;
 }
 
@@ -65,9 +65,9 @@ static unsigned bucket_of(ssize_t min, unsigned step_bits, ssize_t val)
 static ssize_t bucket_min(ssize_t min, unsigned step_bits, unsigned b)
 {
 	/* Don't over-shift. */
-	if (step_bits == MAX_STEP_BITS)
+	if (step_bits == SIZET_BITS)
 		return min;
-	assert(step_bits < MAX_STEP_BITS);
+	assert(step_bits < SIZET_BITS);
 	return min + ((ssize_t)b << step_bits);
 }
 
@@ -324,6 +324,33 @@ ssize_t tally_mean(const struct tally *tally)
 	return divls64(tally->total[1], tally->total[0], count);
 }
 
+ssize_t tally_total(const struct tally *tally, ssize_t *overflow)
+{
+	if (overflow) {
+		*overflow = tally->total[1];
+		return tally->total[0];
+	}
+
+	/* If result is negative, make sure we can represent it. */
+	if (tally->total[1] & (1 << (SIZET_BITS-1))) {
+		/* Must have only underflowed once, and must be able to
+		 * represent result at ssize_t. */
+		if ((~tally->total[1])+1 != 0
+		    || (ssize_t)tally->total[0] >= 0) {
+			/* Underflow, return minimum. */
+			return (ssize_t)((size_t)1 << (SIZET_BITS - 1));
+		}
+	} else {
+		/* Result is positive, must not have overflowed, and must be
+		 * able to represent as ssize_t. */
+		if (tally->total[1] || (ssize_t)tally->total[0] < 0) {
+			/* Overflow.  Return maximum. */
+			return (ssize_t)~((size_t)1 << (SIZET_BITS - 1));
+		}
+	}
+	return tally->total[0];
+}
+
 static ssize_t bucket_range(const struct tally *tally, unsigned b, size_t *err)
 {
 	ssize_t min, max;

ccan/tally/tally.h

@@ -51,6 +51,17 @@ ssize_t tally_max(const struct tally *tally);
  */
 ssize_t tally_mean(const struct tally *tally);
 
+/**
+ * tally_total - the total value passed to tally_add.
+ * @tally: the tally structure.
+ * @overflow: the overflow value (or NULL).
+ *
+ * If your total can't overflow a ssize_t, you don't need @overflow.
+ * Otherwise, @overflow is the upper ssize_t, and the return value should
+ * be treated as the lower size_t (ie. the sign bit is in @overflow).
+ */
+ssize_t tally_total(const struct tally *tally, ssize_t *overflow);
+
 /**
  * tally_approx_median - the approximate median value passed to tally_add.
  * @tally: the tally structure.

ccan/tally/test/run-total.c

+#include <ccan/tally/tally.c>
+#include <ccan/tap/tap.h>
+
+int main(void)
+{
+	struct tally *tally;
+	ssize_t total, overflow;
+	ssize_t min, max;
+
+	max = (ssize_t)~(1ULL << (sizeof(max)*CHAR_BIT - 1));
+	min = (ssize_t)(1ULL << (sizeof(max)*CHAR_BIT - 1));
+
+	plan_tests(15);
+
+	/* Simple case. */
+	tally = tally_new(0);
+	tally_add(tally, min);
+	ok1(tally_total(tally, NULL) == min);
+	ok1(tally_total(tally, &overflow) == min);
+	ok1(overflow == -1);
+
+	/* Underflow. */
+	tally_add(tally, min);
+	total = tally_total(tally, &overflow);
+	ok1(overflow == -1);
+	ok1((size_t)total == 0);
+	ok1(tally_total(tally, NULL) == min);
+	free(tally);
+
+	/* Simple case. */
+	tally = tally_new(0);
+	tally_add(tally, max);
+	ok1(tally_total(tally, NULL) == max);
+	ok1(tally_total(tally, &overflow) == max);
+	ok1(overflow == 0);
+
+	/* Overflow into sign bit... */
+	tally_add(tally, max);
+	total = tally_total(tally, &overflow);
+	ok1(overflow == 0);
+	ok1((size_t)total == 0xFFFFFFFE);
+	ok1(tally_total(tally, NULL) == max);
+
+	/* Overflow into upper size_t. */
+	tally_add(tally, max);
+	total = tally_total(tally, &overflow);
+	ok1(overflow == 1);
+	ok1((size_t)total == 0x7FFFFFFD);
+	ok1(tally_total(tally, NULL) == max);
+	free(tally);
+
+	return exit_status();
+}