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reduce.go
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reduce.go
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package mapreduce
import (
"errors"
"reflect"
)
// Reducer Error Collection
var (
ErrNilReducer = errors.New("reducer cannot be nil")
ErrReducerNotFunc = errors.New("reducer must be a function")
ErrMissingInitialValue = errors.New("missing initial value")
ErrInvalidReducer = errors.New("invalid reducer signature, must be \"reducer(TypeA, TypeB, int) TypeA\" or \"reducer(TypeA, TypeB) TypeA\"")
)
// Reduce reduces iteratible objects into a scalar value.
// The reducer can be reducer(TypeA, TypeB, int) TypeA or reducer(TypeA, TypeB) TypeA
// The source accept array, slice, channel or Iterator.
func Reduce(reducer, source interface{}, initials ...interface{}) (interface{}, error) {
var initialValue interface{}
if len(initials) > 0 {
initialValue = initials[0]
}
// Normalize source as iterator
iterator, ok := source.(Iterator)
if !ok {
var err error
iterator, err = NewIterator(source)
if err != nil {
return initialValue, err
}
}
// Validate reducer
if reducer == nil {
return initialValue, ErrNilReducer
}
rk := reflect.TypeOf(reducer)
if rk.Kind() != reflect.Func {
return initialValue, ErrReducerNotFunc
} else if rk.NumIn() < 2 || rk.NumIn() > 3 || rk.NumOut() != 1 || rk.In(0) != rk.Out(0) ||
(rk.NumIn() == 3 && rk.In(2) != reflect.TypeOf(0)) ||
(initialValue != nil && rk.Out(0) != reflect.TypeOf(initialValue)) {
return initialValue, ErrInvalidReducer
}
// Try generate initial value
if len(initials) == 0 {
outT := rk.Out(0)
if outT.Kind() == reflect.Ptr {
initialValue = reflect.New(outT.Elem()).Interface()
} else {
initialValue = reflect.Zero(outT).Interface()
}
}
accumulator := reflect.ValueOf(initialValue)
rv := reflect.ValueOf(reducer)
for iterator.Next() {
i, entry := iterator.Value()
// call reducer via reflection
if rk.NumIn() > 2 {
accumulator = rv.Call([]reflect.Value{
accumulator, // send accumulator value
reflect.ValueOf(entry), // send current source entry
reflect.ValueOf(i), // send current loop index
})[0]
} else {
accumulator = rv.Call([]reflect.Value{
accumulator, // send accumulator value
reflect.ValueOf(entry), // send current source entry
})[0]
}
}
return accumulator.Interface(), nil
}