REPLACEMENT SELECTION SORT,適用於記憶體較小的情況(在高德納提出該演算法的年代,記憶體比起現在少了N個數量級).
該演算法在PG 9.x或以下版本在使用,在高版本已被廢棄.其演算法實現如下:

Let m be the number of records to be sorted which can be kept in the main memory. Imagine that these memory locations are registers and assume we can mark them as “on” or “off”. Replacement selection can overlap reading, sorting and writing.
```
Step1: The m registers are filled with records from the input to be sorted.
Step2: All registers are put into the “on” state.
Step3: Select the register which has the smallest of all “on” registers.
Step4: Transfer the contents of the selected register to the output (call it as key Y).
Step5: Replace the contents of the selected register by the next input record.
            If new record key > Y
                        Go to step 3
            If new record key = =Y
                        Go to step 4
            If new record key < Y
                        Go to step 6
Step6: Turn the selected key register  “off”.
            If all registers are now  “off”
                        We have completed a sorted substring(run).
                        We start a new substring group and go to step 2.
            Else
                        Go to step 3
```

下面是該演算法的一個例子:
```
Example: Suppose number of registers is 3 and input keys are 5, 2, 9, 7, 0, 8, 1, 6, 3, 4 ...

Registers                               Output
            5 2 9                                       2
            5 7 9                                       5
*0 7 9                                     7
*0 8 9                                     8
*0*1 9                                    9
*0*1*6
0 1 6                                       0
3 1 6                                       1
3 4 6                                       3
- 4 6                                        4
- - 6                                        6
- - -
```
相關的資料結構:兩個陣列,一個儲存讀取的實際資料,一個標記陣列;一個外存,用於儲存已完成排序的一個run.
如標記陣列全為F(非活動狀態),則產生一個run,週而復始,直至所有輸入被耗盡為止.
透過這個方法,可以生成N個runs(每個run對應的是已完成排序的部分資料).

[CS 351 -Data Organization and Management ( Section: 02)](http://www.cs.bilkent.edu.tr/~canf/CS351Fall2009/cs351lecturenotes/week4/index.html)
[External_sorting](https://en.wikipedia.org/wiki/External_sorting)
[replacement-selection-sort vs selection-sort](https://stackoverflow.com/questions/16326689/replacement-selection-sort-v-selection-sort)

REPLACEMENT SELECTION SORT

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