66 lines
2.3 KiB
Markdown
66 lines
2.3 KiB
Markdown
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# Table file structure (v1)
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## Table file version changelog
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### v1.0
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+ Added previous page offset in page.
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## v1.x specification
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1. `TBL!` file signature (4 bytes) **could be `TBL?` if an operation on a table is incompleted**
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2. Table file version (2 bytes)
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3. The offset to the first page in a table. (8 bytes)
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4. The offset to the last page in a table. (8 bytes)
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5. The offset to the last available *free* page (8 bytes)
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6. Pages (64 KiB each)
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1. Page flags (1 byte)
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2. Next page offset (8 bytes) **could be 0 if last page**
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3. Previous page offset (8 bytes) **could be 0 if first page**
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4. Row count (2 bytes)
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5. Rows
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1. Row flags (1 byte)
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2. Row data
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## Table file version specification
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In the further time, it's planned to rewrite the file structure, so it's useful to have a version marker.
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| 15-8 | 7-0 |
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|---------------|---------------|
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| Major version | Minor version |
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That means that the first byte defines major version, and the second one defines minor version.
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If tables with different versions are binary-compatible with each other, their major version must be the same.
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## Page flags specification
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| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
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|-----|-----|-----|-----|-----|-----|-----|-----|
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| RSV | RSV | RSV | RSV | RSV | RSV | RSV | DEL |
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- **RSV** -- reserved for further usage.
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- **DEL** -- free page flag.
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## Row flags specification
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See "Page flags specification" above.
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## Free pages
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A page can be called *free* iff all its rows are deleted.
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If there is a free page, there actions are being done:
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1. Set `DEL` page flag ((6.1) |= FLAG_DEL)
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2. If a page is **not** the first one, replace next page offset in the previous page with a value in current page
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((previous 6.2) = (6.2))
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3. If a page is **not** the last one, replace previous page offset in the next page with a value in current page
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((previous 6.3) = (6.3))
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4. Put last available free page as the next page ((6.2) = 5)
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5. Set current page offset as the offset to the last available free page ((5) = current_page_offset)
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## File signature
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*Since v0.2* a file signature could be `TBL?`, which signals for incomplete table write operation.
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If that signature is detected, the state of a table should be reverted to that it was before failed
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transaction.
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