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added exclusive reduce algorithm

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Ahmet Inan 2018-10-13 10:34:42 +02:00
commit eabdff7209
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17
README.md
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@ -48,3 +48,20 @@ Implemented are the following Encoders and Decoders:
* [bose_chaudhuri_hocquenghem_encoder.hh](bose_chaudhuri_hocquenghem_encoder.hh)
* [bose_chaudhuri_hocquenghem_decoder.hh](bose_chaudhuri_hocquenghem_decoder.hh)
### [exclusive_reduce.hh](exclusive_reduce.hh)
Reduce N times while excluding ith input element
It computes the following, but having only O(N) complexity and using O(1) extra storage:
```
output[0] = input[1];
output[1] = input[0];
for (int i = 2; i < N; ++i)
output[i] = op(input[0], input[1]);
for (int i = 0; i < N; ++i)
for (int j = 2; j < N; ++j)
if (i != j)
output[i] = op(output[i], input[j]);
```

32
exclusive_reduce.hh Normal file
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@ -0,0 +1,32 @@
/*
Reduce N times while excluding ith input element
Copyright 2018 Ahmet Inan <inan@aicodix.de>
*/
#ifndef EXCLUSIVE_REDUCE_HH
#define EXCLUSIVE_REDUCE_HH
namespace CODE {
template <typename TYPE, typename OPERATOR>
void exclusive_reduce(const TYPE *in, TYPE *out, int N, OPERATOR op)
{
TYPE pre = in[0];
for (int i = 1; i < N-1; ++i) {
out[i] = pre;
pre = op(pre, in[i]);
}
out[N-1] = pre;
TYPE suf = in[N-1];
for (int i = N-2; i > 0; --i) {
out[i] = op(out[i], suf);
suf = op(suf, in[i]);
}
out[0] = suf;
}
}
#endif