Claudenw commented on a change in pull request #140: Bloomfilter updates2
URL:
https://github.com/apache/commons-collections/pull/140#discussion_r392085172
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File path:
src/main/java/org/apache/commons/collections4/bloomfilter/hasher/Shape.java
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@@ -81,34 +83,36 @@
private final HashFunctionIdentity hashFunctionIdentity;
/**
- * Constructs a filter configuration with the specified number of items and
- * probability.
+ * Constructs a filter configuration with a desired false-positive
probability ({@code p}) and the
+ * specified number of bits ({@code m}) and hash functions ({@code k}).
*
- * @param hashFunctionIdentity The HashFunctionIdentity of the hash
function this shape uses.
- * @param probability The probability of duplicates. Must be in the range
- * (0.0,1.0).
- * @param numberOfBits The number of bits in the filter.
- * @param numberOfHashFunctions The number of hash functions in the filter.
+ * <p>The number of items ({@code n}) to be stored in the filter is
computed.
+ * <pre>n = ceil(m / (-k / log(1 - exp(log(p) / k))))</pre>
+ *
+ * <p>The actual probability will be approximately equal to the
+ * desired probability but will be dependent upon the calculated Bloom
filter capacity
+ * (number of items). An exception is raised if this is greater than or
equal to 1 (i.e. the
+ * shape is invalid for use as a Bloom filter).
+ *
+ * @param hashFunctionIdentity The identity of the hash function this
shape uses
+ * @param probability The desired false-positive probability in the range
{@code (0, 1)}
+ * @param numberOfBits The number of bits in the filter
+ * @param numberOfHashFunctions The number of hash functions in the filter
+ * @throws NullPointerException if the hash function identity is null
+ * @throws IllegalArgumentException if the desired probability is not in
the range {@code (0, 1)}
+ * @throws IllegalArgumentException if the number of bits is not above 8
Review comment:
I like the change in conditions statement.
8 bits is really just a recommendation. I guess we can remove the checks.
The issue arises when we calculate the other fields. The actual minimum is 4
(if you have 1 item and 7 if you have 2). It falls out in the hash
calculations, or the probability calculations
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