Custom Calculator"
Line 6: | Line 6: | ||
the context refers to the intermediate result we keep track of while iterating over the operands (accumulation), while summarization refers to the computation from the context to the end result. For example, if we want to compute the '''AVERAGE''' over a list of numbers, we would likely keep track of their '''SUM''' and '''COUNT''', and then retrieve the final result by dividing the two. In the above example, the context would be a data structure holding the sum and the count, while the summarization would be the division. | the context refers to the intermediate result we keep track of while iterating over the operands (accumulation), while summarization refers to the computation from the context to the end result. For example, if we want to compute the '''AVERAGE''' over a list of numbers, we would likely keep track of their '''SUM''' and '''COUNT''', and then retrieve the final result by dividing the two. In the above example, the context would be a data structure holding the sum and the count, while the summarization would be the division. | ||
− | + | ==Define a Custom Calcualtor== | |
+ | |||
+ | To define a custom calculator, you must specify a ContextType so the model knows what to keep track of in accumulation phase. | ||
<source lang="java"> | <source lang="java"> | ||
Line 40: | Line 42: | ||
===Example: Data Range=== | ===Example: Data Range=== | ||
+ | Say, we want to create a custom calculator to compute the number range ('''MAX''' minus '''MIN''') of data, and luckily the StandardContextType.MIN_MAX already carries sufficient information to do so. We just need to define a ContextualCalculator based on the context type: | ||
+ | |||
+ | <source lang="java"> | ||
+ | public class Range implements ContextualCalculator<MinMaxContext> { | ||
+ | |||
+ | public static final Range INSTANCE = new Range(); | ||
+ | private Range() {} | ||
+ | |||
+ | @Override | ||
+ | @SuppressWarnings("unchecked") | ||
+ | public ContextType<MinMaxContext> getContextType() { | ||
+ | return StandardContextType.MIN_MAX; | ||
+ | } | ||
+ | @Override | ||
+ | public Number getResult(MinMaxContext context) { | ||
+ | return context.getMax().doubleValue() - context.getMin().doubleValue(); | ||
+ | } | ||
+ | @Override | ||
+ | public String getLabel() { | ||
+ | return "Range"; | ||
+ | } | ||
+ | @Override | ||
+ | public String getLabelKey() { | ||
+ | return "range"; | ||
+ | } | ||
+ | } | ||
+ | </source> | ||
===Example: Distinct Count=== | ===Example: Distinct Count=== | ||
+ | It may not be too obvious how to implement '''DISTINCT COUNT''' (which calculates the number of distinct items in the collection), but the best approach on context is a Set. As it is no similar built-in context, we have to implement our own: | ||
+ | |||
+ | <source lang="java"> | ||
+ | public class SetContext implements Context<SetContext> { | ||
+ | |||
+ | private final Set<Object> _set = new HashSet<Object>(); | ||
+ | @Override | ||
+ | public void add(Object item) { | ||
+ | _set.add(item); | ||
+ | } | ||
+ | @Override | ||
+ | public void merge(SetContext ctx) { | ||
+ | _set.addAll(ctx._set); | ||
+ | } | ||
+ | |||
+ | public Set<Object> getSet() { | ||
+ | return _set; | ||
+ | } | ||
+ | |||
+ | public static final ContextType<SetContext> CONTEXT_TYPE = | ||
+ | new ContextType<SetContext>() { | ||
+ | @Override | ||
+ | public SetContext create() { | ||
+ | return new SetContext(); | ||
+ | } | ||
+ | }; | ||
+ | } | ||
+ | </source> | ||
+ | |||
+ | Then the summarization part becomes quite obvious: | ||
+ | |||
+ | <source lang="java"> | ||
+ | public class DistinctCount implements ContextualCalculator<SetContext> { | ||
+ | |||
+ | public static final DistinctCount INSTANCE = new DistinctCount(); | ||
+ | private DistinctCount() {} | ||
+ | |||
+ | @Override | ||
+ | public ContextType<SetContext> getContextType() { | ||
+ | return SetContext.CONTEXT_TYPE; | ||
+ | } | ||
+ | @Override | ||
+ | public Number getResult(SetContext context) { | ||
+ | return context.getSet().size(); | ||
+ | } | ||
+ | @Override | ||
+ | public String getLabel() { | ||
+ | return "Distinct Count"; | ||
+ | } | ||
+ | @Override | ||
+ | public String getLabelKey() { | ||
+ | return "distinctCount"; | ||
+ | } | ||
+ | } | ||
+ | </source> | ||
+ | |||
+ | ==Show a Custom Calculator in PivotFieldControl Menu== | ||
==Version History== | ==Version History== |
Revision as of 09:20, 28 May 2012
TabularPivotModel
In TabularPivotModel, Calculator is implemented by two segments: summarization and context. Given a Calculator as an aggregation operation, the context refers to the intermediate result we keep track of while iterating over the operands (accumulation), while summarization refers to the computation from the context to the end result. For example, if we want to compute the AVERAGE over a list of numbers, we would likely keep track of their SUM and COUNT, and then retrieve the final result by dividing the two. In the above example, the context would be a data structure holding the sum and the count, while the summarization would be the division.
Define a Custom Calcualtor
To define a custom calculator, you must specify a ContextType so the model knows what to keep track of in accumulation phase.
public interface ContextualCalculator<C extends Context<C>> extends Calculator {
public Number getResult(C context); // summarize the end result from the context
public ContextType<C> getContextType(); // specify the context type
}
You can either use an existing ContextType, or create your own. The built-in context type are: TODO
It is encouraged to make ContextType as singletons, as Calculators can share Context if they are of the same ContextType.
public interface ContextType<C extends Context<C>> {
public C create(); // ContextType has the responsibility as a Context factory
}
public interface Context<C extends Context<C>> {
public void add(Object item); // what to do when iterating over raw data.
public void merge(C ctx); // what to do when merging from contexts of a partition of its raw data set.
}
Example: Data Range
Say, we want to create a custom calculator to compute the number range (MAX minus MIN) of data, and luckily the StandardContextType.MIN_MAX already carries sufficient information to do so. We just need to define a ContextualCalculator based on the context type:
public class Range implements ContextualCalculator<MinMaxContext> {
public static final Range INSTANCE = new Range();
private Range() {}
@Override
@SuppressWarnings("unchecked")
public ContextType<MinMaxContext> getContextType() {
return StandardContextType.MIN_MAX;
}
@Override
public Number getResult(MinMaxContext context) {
return context.getMax().doubleValue() - context.getMin().doubleValue();
}
@Override
public String getLabel() {
return "Range";
}
@Override
public String getLabelKey() {
return "range";
}
}
Example: Distinct Count
It may not be too obvious how to implement DISTINCT COUNT (which calculates the number of distinct items in the collection), but the best approach on context is a Set. As it is no similar built-in context, we have to implement our own:
public class SetContext implements Context<SetContext> {
private final Set<Object> _set = new HashSet<Object>();
@Override
public void add(Object item) {
_set.add(item);
}
@Override
public void merge(SetContext ctx) {
_set.addAll(ctx._set);
}
public Set<Object> getSet() {
return _set;
}
public static final ContextType<SetContext> CONTEXT_TYPE =
new ContextType<SetContext>() {
@Override
public SetContext create() {
return new SetContext();
}
};
}
Then the summarization part becomes quite obvious:
public class DistinctCount implements ContextualCalculator<SetContext> {
public static final DistinctCount INSTANCE = new DistinctCount();
private DistinctCount() {}
@Override
public ContextType<SetContext> getContextType() {
return SetContext.CONTEXT_TYPE;
}
@Override
public Number getResult(SetContext context) {
return context.getSet().size();
}
@Override
public String getLabel() {
return "Distinct Count";
}
@Override
public String getLabelKey() {
return "distinctCount";
}
}
Show a Custom Calculator in PivotFieldControl Menu
Version History
Version | Date | Content |
---|---|---|
2.0.0 | June 2012 | ContextualCalculator |