Rosetta Stone

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Revision as of 12:41, 22 October 2012 by Chronos (talk | contribs) (fix category)

Introduction

Often, people new to a programming language/library have trouble mapping the things they know how to do in other environments to the new environments.

It can be handy to have a sort of "Rosetta stone" that roughly translates tasks from one environment/language into another.

Programming languages

If you have experience in other programming languages (such as C++, Visual Basic), there are some Rosetta stone web pages that might help you. As FreePascal/Lazarus syntax and library is fairly consistent with Delphi's, sites that "translate" to Delphi are also a valuable resource.

Some useful pages:

Operating systems

If you know how to do something in an operating system, chances are you can do the same using FPC/Lazarus.

The table below gives a rough mapping of some tasks:

Windows/DOS Unix/Linux Description FreePascal/Lazarus
copy cp Copy file Lazarus fileutil.copyfile; roll your own with filestreams
date /t date Show current date sysutils.now (date/time); sysutils.date (date); sysutils.time (time)
del, erase rm, rm -f Delete file sysutils.deletefile, erase
deltree, rmdir /s rm -r,rm -rf Remove directories with subdirectories Lazarus fileutil.deletedirectory
dir ls Find files in a directory Lazarus FindAllFiles
diskpart list volume mount Show partitions/volumes/drives on a computer Windows: see Windows_Programming_Tips#Listing_all_available_drives

Linux: see Linux_Programming_Tips#Get_the_list_of_mounted_partitions

Unix: you'll have to parse some files (e.g. /etc/mtab) yourself, depending on Unix flavour; see Forum post

find grep Search for a text/string Several options, including the pos, ansipos. Regular expression support is provided by the RegExpr unit.
ldifde ldapsearch Search through Active Directory/LDAP Use Synapse ldapsend unit to work with LDAP queries; see e.g. example code on Leonardo Ramé's blog
md5sum md5sum Calculate hash of file contents You can use the MDFile function in the md5 FPC unit.
mkdir mkdir Create directory mkdir; forcedirectories (creates entire path, if necessary)
move mv Move file to another directory Within partition/disk: sysutils.renamefile; otherwise copy and delete original
rename, ren mv Rename file sysutils.renamefile (can also "move" a file between directories, but only on same partition/disk
sc query <servicename> Show status for service See ServiceManager
? sed -i 's|Delphi|FreePascal|g' readme.txt Search and replace text in file. See example code below.
shutdown shutdown Shut down computer ?
? touch Create a new empty file sysutils.filecreate(filename)
wget, curl wget, curl Download file from HTTP/FTP Multiple options, e.g. use synapse httpsend or ftpsend units: Synapse#Downloading_files
? which Search for executable in path Lazarus fileutil.FindDefaultExecutablePath

Finding all occurrences of some bytes in a file

A sort of grep. From forum post by KpjComp: [1]

The block search class:

unit block_search;

{$mode objfpc}{$H+}

interface

uses
  Classes, SysUtils, fgl;

type

  { TBlockSearch }
  TBlockSearchResults = specialize TFPGList<int64>;

  TBlockSearch = class
  private
    src:TStream;
    fresults:TBlockSearchResults;
    block:array of byte;
  public
    procedure SearchFor(a:array of byte);
    constructor Create(_Src:TStream; blocksize:integer = 1024*1024);
    destructor Destroy; override;
    property Results:TBlockSearchResults read fResults;
  end;

implementation

{ TBlockSearch }

procedure TBlockSearch.SearchFor(a: array of byte);
var
  readsize:integer;
  fPos:Int64;
  fifoBuff:array of byte;
  fifoSt,fifoEn,searchLen,lpbyte:integer;

  //
  procedure CheckPos;
  var
    l,p:integer;
  begin
    p := fifoST;
    for l := 0 to pred(SearchLen) do
    begin
      if a[l] <> fifoBuff[p] then exit;
      //p := (p+1) mod SearchLen,   the if seems quicker
      inc(p); if p >= SearchLen then p := 0;
    end;
    fresults.Add(fpos-searchLen);
  end;
  //
begin
  fresults.clear;
  src.Position:=0;
  readsize := src.Read(block[0],Length(block));
  searchLen := length(a);
  if searchLen > length(block) then
    raise Exception.Create('Search term larger than blocksize');
  if readsize < searchLen then exit;
  setlength(fifoBuff,searchLen);
  move(block[0],fifoBuff[0],searchLen);
  fPos:=0;
  fifoSt:=0;
  fifoEn:=SearchLen-1;
  CheckPos;
  while readsize > 0 do
  begin
    for lpByte := 0 to pred(readsize) do
    begin
      inc(fifoSt); if fifoSt>=SearchLen then fifoST := 0;
      inc(fifoEn); if fifoEn>=SearchLen then fifoEn := 0;
      fifoBuff[fifoEn] := block[lpByte];
      inc(fPos);
      CheckPos;
    end;
    readsize := src.Read(block[0],Length(block));
  end;
end;

constructor TBlockSearch.Create(_Src: TStream; blocksize: integer);
begin
  inherited Create;
  setlength(block,blocksize);
  src := _src;
  fresults := TBlockSearchResults.Create;
end;

destructor TBlockSearch.Destroy;
begin
  freeAndNil(fresults);
  inherited Destroy;
end;

end.

Example of usage:

var
  bs:TBlockSearch;
..
..
  bs := TBlockSearch.Create(SourceFile);
  try
    bs.SearchFor(WhatIAmSearchingFor);
    for i := 0 to bs.results.count-1 do
    begin
      // Example: we need to do something at an offset after the result
      SourceFile.Position := bs.Results.Items[i] + 5;
      SourceFile.ReadBuffer(RecordToParse, SizeOf(RecordToParse)); 
      // And now I work on the data in the second packed record of arrays and output to memo or whatever
    end;
  finally
    bs.free;
  end;

Search and replace in a file

This will search and replace by loading the entire file into the stringlist in memory and replacing there, then saving the strignlist again. While fine for small files, performance will likely suffer for large files.

procedure FindAndReplace(SearchFor, ReplaceWith, FileName: string);
var
  FileContents: TStringList;
begin
  FileContents:=TStringList.Create;
  try
    FileContents.LoadFromFile(FileName);
    FileContents.Text:=StringReplace(FileContents.Text, SearchFor, ReplaceWith, [rfReplaceAll,rfIgnoreCase]);
    FileContents.SaveToFile(FileName);
  finally
    FileContents.Free;
  end;
end;