PeterAlfreadLee opened a new pull request #95: COMPRESS-505 : bug fix for random access of 7z URL: https://github.com/apache/commons-compress/pull/95 There are some problems in my PR about random access of 7z [#83](COMPRESS-342 random access of 7z files) : 1. I was thinking that the `currentFolderInputStream` can be repositioned by changing the position of the `channel`, which turns out to be impossible. This PR fixesit by reopening the `currentFolderInputStream`. 2. There are 2 ways to access the content of a 7z archive now : by sequential access(getNextEntry) and by random access(getInputStream). They may be used one after another. So there're some conditions we need to deal with : 2.1 In a random access, if `currentEntryIndex` == `entryIndex` && the entry has not been read yet : This means the input stream of the entry we want has already been put in the `deferredBlockStreams` as the last array member. We SHOULD NOT build a new input stream for the entry again, because this will make same the existed stream in `deferredBlockStreams` be skipped. We should just do nothing cause the input stream is already in the `deferredBlockStreams`. 2.2 In a random access, if `currentEntryIndex` == `entryIndex` && the entry has already been read : This means the entry we want has been read(maybe some of entry or all of the entry has been read, it does not matter). Then we should reopen the `currentFolderInputStream` and skip all the entries before the entry we want. BTW : we could determine if the file has been read or not by comparing the `bytesRemaining` of the input stream(as a `CRC32VerifyingInputStream`) and the actual size of the file. 2.3 In a random access, if `currentEntryIndex` < `entryIndex` && the last entry in `deferredBlockStreams` has not been read : The input streams whose index equals or less than `currentEntryIndex` has already been put into the `deferredBlockStreams`. We could just add the remaining entries to the `deferredBlockStreams`. 2.4 In a random access, if `currentEntryIndex` < `entryIndex` && the last entry in `deferredBlockStreams` has already been read : Like 2.2, we have no other choices but reopen the `currentFolderInputStream` and skip all the entries again. 2.5 In a random access, if `currentEntryIndex` > `entryIndex` : This means the entry we want has already been read or skipped beforehand. We could only reopen the `currentFolderInputStream` and skip all the entries again. In short, we should do nothing in 2.1, skip the remaining entries in 2.3, and reopen the `currentFolderInputStream` in 2.2/2.4/2.5. I have to admit this is a bit complicated, but I didn't find any other better ideas building the logic. :( I made some refactoring and added some new comments to make the code more clear. The corresponding testcases are also included in this PR.
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