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Source code

import std.stdio;
import core.sync.rwmutex : ReadWriteMutex;

enum PAGE_SIZE = 4096;
alias PageId = uint;

ubyte[] toBytes(uint value)
{
    ubyte[] bytes = new ubyte[4];
    bytes[0] = cast(ubyte)(value >> 24);
    bytes[1] = cast(ubyte)(value >> 16);
    bytes[2] = cast(ubyte)(value >> 8);
    bytes[3] = cast(ubyte)(value);
    return bytes;
}

class DiskManager
{
    File dbFile;
    __gshared ReadWriteMutex dbIOMutex;

shared static this()
    {
        this.dbIOMutex = new ReadWriteMutex(ReadWriteMutex.Policy.PREFER_WRITERS);
    }

this(string dbFileName)
    {
        this.dbFile = File(dbFileName, "ab+");
    }

void removeIfExists()
    {
        static import std.file;

if (std.file.exists(dbFile.name))
        {
            std.file.remove(dbFile.name);
        }
    }

void readPage(PageId pageId, out ubyte[PAGE_SIZE] pageData)
    {
        synchronized (dbIOMutex.reader)
        {
            dbFile.seek(pageId * PAGE_SIZE);
            dbFile.rawRead(pageData);
        }
    }

void writePage(PageId pageId, ubyte[PAGE_SIZE] pageData)
    {
        synchronized (dbIOMutex.writer)
        {
            dbFile.seek(pageId * PAGE_SIZE);
            dbFile.rawWrite(pageData);
        }
    }
}

void singleReadWrite()
{
     // Scenario: write a page to disk and read it back.
    DiskManager diskManager = new DiskManager("test.db");
    diskManager.removeIfExists();

PageId pageId = 0;

ubyte[PAGE_SIZE] pageData = new ubyte[PAGE_SIZE];
    pageData[0..4] = [1,2,3,4];
    diskManager.writePage(pageId, pageData);

ubyte[PAGE_SIZE] readData = new ubyte[PAGE_SIZE];
    diskManager.readPage(pageId, readData);

writeln("pageData[0..4] = ", pageData[0 .. 4]);
    writeln("readData[0..4] = ", readData[0 .. 4]);

assert(pageData[0..4] == readData[0..4]);
}

void multiReadWrite()
{
    // Scenario: successively read and write to the same page.
    DiskManager diskManager = new DiskManager("test.db");
    diskManager.removeIfExists();

PageId pageId = 0;

foreach (i; 0 .. 10)
    {
        ubyte[PAGE_SIZE] pageData = new ubyte[PAGE_SIZE];

pageData[0 .. 4] = toBytes(i);
        diskManager.writePage(pageId, pageData);

ubyte[PAGE_SIZE] readData = new ubyte[PAGE_SIZE];
        diskManager.readPage(pageId, readData);

writeln("pageData[0..4] = ", pageData[0 .. 4]);
        writeln("readData[0..4] = ", readData[0 .. 4]);

assert(pageData[0] == readData[0]);
        assert(pageData[1] == readData[1]);
        assert(pageData[2] == readData[2]);
    }
}

void main()
{

singleReadWrite();
    writeln("~~~~~~");
    multiReadWrite();
}