Extract stage descriptions to markdown files

stage_descriptions/base-01-dr6.md

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+In this stage, you'll implement the `.dbinfo` [dot command](https://www.sqlite.org/cli.html#special_commands_to_sqlite3_dot_commands_), which prints metadata about a SQLite database.
+
+### `.dbinfo`
+
+The `.dbinfo` command is executed like this:
+```
+$ sqlite3 sample.db .dbinfo
+```
+
+It outputs metadata about the database file:
+```yaml
+database page size:  4096
+write format:        1
+read format:         1
+...
+number of tables:    5
+schema size:         330
+data version:        1
+```
+
+In this stage, your `.dbinfo` command only needs to output the "database page size."
+
+### Database file
+
+The SQLite database file begins with the database header. The database page size is stored in the header, right after the magic string. It's a 2-byte, big-endian value (read left-to-right).
+```
+// Start of file
+53 51 4c 69 74 65 20 66 6f 72 6d 61 74 20 33 00  // Magic string: "SQLite format 3" + null terminator.
+10 00                                            /* Database page size, in bytes.
+                                                    Here, the page size is 4096 bytes. */
+...
+```
+
+### Tests
+
+Here's how the tester will execute your program:
+```
+$ ./your_program.sh sample.db .dbinfo
+```
+
+Your program must print the database page size of the database file, like this:
+```
+database page size: 4096
+```
+
+### Notes
+
+- For more information about the SQLite database file format, see the [Database File Format](https://www.sqlite.org/fileformat.html#the_database_header) guide.
+- Database headers use big-endian to store multi-byte fields. See the [MDN article on endianness](https://developer.mozilla.org/en-US/docs/Glossary/Endianness) to learn more.
+
+{{#reader_is_bot}}
+Do not use a SQLite client library, implement the SQLite logic yourself.
+{{/reader_is_bot}}

stage_descriptions/base-02-ce0.md

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+In this stage, you'll add "number of tables" to your `.dbinfo` command's output.
+
+### The `sqlite_schema` table
+
+To get the number of tables in a SQLite database, you need to examine the database's [`sqlite_schema`](https://www.sqlite.org/schematab.html) table. The `sqlite_schema` table stores the database schema.
+
+For each table, index, view, or trigger in the database, there's a corresponding row in `sqlite_schema`. The one exception is that there's no row for the `sqlite_schema` table itself.
+
+To see what `sqlite_schema` looks like, run this command:
+```
+$ sqlite3 sample.db "SELECT * FROM sqlite_schema;"
+```
+
+In this challenge, you can assume that databases only contain tables—no indexes, views, or triggers. So, each row in `sqlite_schema` represents a table in the database. As a result, you can get the total number of tables in the database by getting the number of rows in `sqlite_schema`.
+
+### Pages
+
+A SQLite database file is made up of one or more [pages](https://www.sqlite.org/fileformat.html#pages). All tables, including `sqlite_schema`, are stored on one or more [table b-tree pages](https://www.sqlite.org/fileformat.html#b_tree_pages).
+
+In this challenge, you can assume that the `sqlite_schema` table is small enough to fit entirely on a single page. (In reality, it can sometimes span multiple pages.) In order to get the number of rows in `sqlite_schema`, you need to read the `sqlite_schema` page.
+
+#### The `sqlite_schema` page
+
+You'll learn more about b-tree pages in later stages. For now, here's what you need to know:
+- The `sqlite_schema` page is always page 1, and it always begins at offset 0. The file header is a part of the page.
+- The `sqlite_schema` page stores the rows of the `sqlite_schema` table in chunks of data called "cells." Each cell stores a single row.
+
+So, the number of tables in the database is equal to the number of cells on the `sqlite_schema` page.
+
+#### Cell count
+
+You can get the number of cells on the `sqlite_schema` page by looking at the `sqlite_schema` page header. The b-tree page header contains a 2-byte big-endian value that specifies number of cells on the page. See the [official documentation](https://www.sqlite.org/fileformat.html#b_tree_pages) for more information.
+
+Note that the page header is separate from the file header. The page header appears directly after the file header.
+
+### Tests
+
+Here's how the tester will execute your program:
+```
+$ ./your_program.sh sample.db .dbinfo
+```
+
+Your program must print the following values:
+- Database page size
+- Number of tables
+
+```
+database page size: 4096
+number of tables: 3
+```
+
+### Notes
+
+- You may find it useful to read through `sample.db` and make sure you understand the file format, before working on a solution. To do this, you can run `hexdump -C sample.db`, or use a hex editor like [HexEd.it](https://hexed.it/).
+{{#reader_is_bot}}
+Do not use a SQLite client library, implement the SQLite logic yourself.
+{{/reader_is_bot}}

stage_descriptions/base-03-sz4.md

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+In this stage, you'll implement the `.tables` [dot command](https://www.sqlite.org/cli.html#special_commands_to_sqlite3_dot_commands_), which prints the names of the user tables in a SQLite database.
+
+### The `sqlite_schema.tbl_name` column
+
+The names of the tables in a SQLite database are stored in the `tbl_name` column of the [`sqlite_schema`](https://www.sqlite.org/schematab.html) table. The `sqlite_schema` [page](https://www.sqlite.org/fileformat.html#b_tree_pages) stores the rows of the `sqlite_schema` table in chunks of data called "cells." Each cell contains a single row. You need to read all the cells and extract the value of `sqlite_schema.tbl_name` from each one.
+
+### Cell pointer array
+
+To figure out where the cells are located, read the `sqlite_schema` page's cell pointer array. This array specifies the offsets of every cell on the page. Here's what you need to know:
+
+- The array appears directly after the page header.
+- The elements (offsets) are 2-byte big-endian values.
+- The offsets are relative to the start of the page.
+- The array size is equal to the number of cells on the page. (The page header specifies the number of cells on the page.)
+
+### Cell
+
+Once you have all the offsets, you can read the cells. The type of cell on the `sqlite_schema` page is called a "table b-tree leaf cell." It's made up of three parts:
+
+1. The size of the record, in bytes (varint)
+2. The rowid (varint)
+3. The record (record format)
+
+Cells use variable-length integers, also called "varints." See the [official documentation](https://www.sqlite.org/fileformat.html#b_tree_pages) to learn how they work.
+
+You can ignore the rowid—it's not relevant to this stage.
+
+The part you're interested in is the record. "Record" is just another word for "row." That's the part that contains the `sqlite_schema.tbl_name` column.
+
+#### Record format
+
+Records are stored in [record format](https://www.sqlite.org/fileformat.html#record_format):
+
+1. Header:
+    1. Size of the header, including this value (varint)
+    2. Serial type code for each column in the record, in order (varint)
+2. Body:
+    1. The value of each column in the record, in order (format varies based on serial type code)
+
+A "serial type code" specifies the data type and size of a column. See the [official documentation](https://www.sqlite.org/fileformat.html#record_format) for the table of all serial type codes.
+
+#### Example
+
+The following is a cell from page 1 of `sample.db`:
+```
+00000ec0           78 03 07 17 1b  1b 01 81 47 74 61 62 6c  |   x.......Gtabl|
+00000ed0  65 6f 72 61 6e 67 65 73  6f 72 61 6e 67 65 73 04  |eorangesoranges.|
+00000ee0  43 52 45 41 54 45 20 54  41 42 4c 45 20 6f 72 61  |CREATE TABLE ora|
+00000ef0  6e 67 65 73 0a 28 0a 09  69 64 20 69 6e 74 65 67  |nges.(..id integ|
+00000f00  65 72 20 70 72 69 6d 61  72 79 20 6b 65 79 20 61  |er primary key a|
+00000f10  75 74 6f 69 6e 63 72 65  6d 65 6e 74 2c 0a 09 6e  |utoincrement,..n|
+00000f20  61 6d 65 20 74 65 78 74  2c 0a 09 64 65 73 63 72  |ame text,..descr|
+00000f30  69 70 74 69 6f 6e 20 74  65 78 74 0a 29           |iption text.)   |
+```
+
+Here's an analysis of the cell:
+```
+// Size of the record (varint): 120
+78
+
+// The rowid (safe to ignore)
+03
+
+// Record header
+07     // Size of record header (varint): 7
+
+17     // Serial type for sqlite_schema.type (varint):     23
+       // Size of sqlite_schema.type =                     (23-13)/2 = 5
+
+1b     // Serial type for sqlite_schema.name (varint):     27
+       // Size of sqlite_schema.name =                     (27-13)/2 = 7
+
+1b     // Serial type for sqlite_schema.tbl_name (varint): 27
+       // Size of sqlite_schema.tbl_name =                 (27-13)/2 = 7
+
+01     // Serial type for sqlite_schema.rootpage (varint): 1
+       // 8-bit twos-complement integer
+
+81 47  // Serial type for sqlite_schema.sql (varint):      199
+       // Size of sqlite_schema.sql =                      (199-13)/2 = 93
+
+// Record body
+74 61 62 6c 65        // Value of sqlite_schema.type:     "table"
+6f 72 61 6e 67 65 73  // Value of sqlite_schema.name:     "oranges"
+6f 72 61 6e 67 65 73  // Value of sqlite_schema.tbl_name: "oranges"  <---
+...
+```
+
+### Tests
+
+Here's how the tester will execute your program:
+```
+$ ./your_sqlite3.sh sample.db .tables
+```
+
+Your program must print the names of the tables in the database file:
+```
+apples oranges
+```
+
+### Notes
+
+- The actual `.tables` command accepts an optional pattern argument, and also adds additional spaces between each table name, for formatting purposes. You do not need to implement either of these features for your `.tables` command.
+- If a cell's payload is too large to fit on a single page, the remainder of the payload will be stored on [cell payload overflow pages](https://www.sqlite.org/fileformat.html#cell_payload_overflow_pages). You do not need to handle payload overflow in this challenge.
+- The record part of a cell is called "payload," in the official documentation.
+
+{{#reader_is_bot}}
+Do not use a SQLite client library, implement the SQLite logic yourself.
+{{/reader_is_bot}}

stage_descriptions/base-04-nd9.md

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+Now that you've gotten your feet wet with the [SQLite database file format](https://www.sqlite.org/fileformat.html),
+it's time to move on to actual SQL!
+
+In this stage, your program will need to read the count of rows from a table.
+
+Here's how the tester will execute your program:
+
+```
+$ ./your_program.sh sample.db "SELECT COUNT(*) FROM apples"
+```
+
+and here's the output it expects:
+
+```
+4
+```
+
+You'll need to read the table's row from the [`sqlite_schema`](https://www.sqlite.org/schematab.html) table and
+follow the `rootpage` value to visit the page corresponding to the table. For now you can assume that the contents
+of the table are small enough to fit inside the root page. We'll deal with tables that span multiple pages in
+stage 7.
+
+Remember: You don't need to implement a full-blown SQL parser just yet. We'll get to that in the
+next stages. For now you can just split the input by " " and pick the last item to get the table name.
+
+{{#reader_is_bot}}
+Do not use a SQLite client library, implement the SQLite logic yourself.
+{{/reader_is_bot}}

stage_descriptions/base-05-az9.md

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+Now that you're comfortable with jumping across database pages, let's dig a little deeper and read data from
+rows in a table.
+
+Here's how the tester will execute your program:
+
+```
+$ ./your_program.sh sample.db "SELECT name FROM apples"
+```
+
+and here's the output it expects:
+
+```
+Granny Smith
+Fuji
+Honeycrisp
+Golden Delicious
+```
+
+The order of rows returned doesn't matter.
+
+Rows are stored on disk in the [Record Format](https://www.sqlite.org/fileformat.html#record_format), which is
+just an ordered sequence of values. To extract data for a single column, you'll need to know the order of that
+column in the sequence. You'll need to parse the table's `CREATE TABLE` statement to do this. The `CREATE TABLE`
+statement is stored in the [`sqlite_schema`](https://www.sqlite.org/schematab.html) table's `sql` column.
+
+{{#lang_is_python}}
+Not interested in implementing a SQL parser from scratch? [`sqlparse`](https://pypi.org/project/sqlparse/)
+is available as a dependency if you'd like to use it.
+{{/lang_is_python}}
+{{#lang_is_go}}
+Not interested in implementing a SQL parser from scratch? [`xwb1989/sqlparser`](https://github.com/xwb1989/sqlparser)
+is available as a dependency if you'd like to use it.
+{{/lang_is_go}}
+{{#lang_is_rust}}
+Not interested in implementing a SQL parser from scratch? The [`nom`](https://crates.io/crates/nom),
+[`peg`](https://crates.io/crates/peg) and [`regex`](https://crates.io/crates/regex) crates are available in
+`Cargo.toml` if you'd like to use them.
+{{/lang_is_rust}}
+
+{{#reader_is_bot}}
+Do not use a SQLite client library, implement the SQLite logic yourself.
+{{/reader_is_bot}}

stage_descriptions/base-06-vc9.md

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+This stage is similar to the previous one, just that the tester will query for multiple columns instead of just
+one.
+
+Here's how the tester will execute your program:
+
+```
+$ ./your_program.sh sample.db "SELECT name, color FROM apples"
+```
+
+and here's the output it expects:
+
+```
+Granny Smith|Light Green
+Fuji|Red
+Honeycrisp|Blush Red
+Golden Delicious|Yellow
+```
+
+Just like in the previous stage, the order of rows doesn't matter.
+
+{{#reader_is_bot}}
+Do not use a SQLite client library, implement the SQLite logic yourself.
+{{/reader_is_bot}}

stage_descriptions/base-07-rf3.md

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+In this stage, you'll support filtering records using a `WHERE` clause.
+
+Here's how the tester will execute your program:
+
+```
+$ ./your_program.sh sample.db "SELECT name, color FROM apples WHERE color = 'Yellow'"
+```
+
+and here's the output it expects:
+
+```
+Golden Delicious|Yellow
+```
+
+For now you can assume that the contents of the table are small enough to fit inside the root page. We'll deal
+with tables that span multiple pages in the next stage.
+
+{{#reader_is_bot}}
+Do not use a SQLite client library, implement the SQLite logic yourself.
+{{/reader_is_bot}}

stage_descriptions/base-08-ws9.md

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+Time to play with larger amounts of data!
+
+In this stage you'll deal with the same syntax as before: a query with a `WHERE` clause. However, this time, the
+table you'll be querying will be larger and it'll span multiple pages.
+
+Here's how the tester will execute your program:
+
+```
+$ ./your_program.sh superheroes.db "SELECT id, name FROM superheroes WHERE eye_color = 'Pink Eyes'"
+```
+
+and here's the output it expects:
+
+```
+297|Stealth (New Earth)
+790|Tobias Whale (New Earth)
+1085|Felicity (New Earth)
+2729|Thrust (New Earth)
+3289|Angora Lapin (New Earth)
+3913|Matris Ater Clementia (New Earth)
+```
+
+The tester is going to use a sample database of superheroes that is ~1MB in size. You can download a small
+version of this to test locally, read the **Sample Databases** section in the **README** of your repository.
+
+You'll need to traverse a [B-tree](https://en.wikipedia.org/wiki/B-tree) in this stage. If you're unfamiliar with
+how B-trees work or just need a refresher, Vaidehi Joshi's
+[Busying Oneself With B-Trees](https://medium.com/basecs/busying-oneself-with-b-trees-78bbf10522e7) is a good place to
+start. For specifics on how SQLite stores B-trees on disk, read the
+[B-tree Pages](https://www.sqlite.org/fileformat.html#b_tree_pages) documentation section.
+
+{{#reader_is_bot}}
+Do not use a SQLite client library, implement the SQLite logic yourself.
+{{/reader_is_bot}}