Subtract until reach desired value - delphi

Question

Good morning all.
I'm currently trying to figure out something that i'm confident is simple enough but is proving to be a task and a half to actually work out.
I'm working on a project that's designed to minimize drive usage by relocating various files elsewhere. I've got an array (0..12) of int64 values that contains the file sizes of the files i might potentially want to move. The array is ordered in a way that's predicted largest file size down to predicted smallest file size. I've also got the names of these files stored in a different array (known as WoWData, also [0..12]). I've then got an "installation size", and a "desired size".
My task is to calculate which files i need to move in order to bring the "installation size" down to the "desired size" by going through the array of file sizes, and taking the value away from the Installation size until i reach <= desired size.
Here's some sample code (Delphi/Firemonkey) i've been trying to work with. It's confusing me trying to figure out how to go about such a task and so there'll no doubt be a lot of issues with it;
Global Vars;
_WoWDataFileSize : Array [0..12] of Int64;
// "TBWoWDir" is a TTrackBar (Firemonkey)
var
TotalSize, ReqSize, DiffSize, CurDiff : Int64;
i : Integer;
begin
// Set up initial values to work with
ReqSize := Round(TBWoWDir.Value); // Requested Size
TotalSize := Round(TBWoWDir.Max); // Actual installation size
CurDiff := 0; // Assume as "Current Difference in size"
// Calculate difference between install and requested size
DiffSize := TotalSize - ReqSize; // This calculates correctly
// The below is what i'm struggling with
repeat
for i := Low(_WoWDataFileSize) to High(_WoWDataFileSize) do
begin
CurDiff := ReqSize - _WoWDataFileSize[i];
end;
until CurDiff <= ReqSize;
end;
I did try using just a repeat .. until loop without the for loop, but again, i'm getting far too confused while trying to figure it out.
Let me provide an example. Let's assume that _WoWDataFileSize[0] is 200, and _WoWDataFileSize[1] through to _WoWDataFileSize[12] are the same value as their array index (e.g. _WoWDataFileSize[6] = 6, _WoWDataFileSize[8] = 8, etc).
If i wanted to calculate the value of 150 (which would be 200 - 12 - 11 - 10 - 9 - 8, or Array[0] - Array[12] - Array[11] - Array[10] - Array[9] - Array[8] according to the array), and get a list of files i need to move to meet this requirement from the WoWData array, how would i write the routine?
150 could be replaced by any number as i'm working towards a dynamic user-requested size specified by TBWoWDir.Value.
I'm thinking i might need to do a While loop and use i := i+1 setup. Realistically, i could go through and hardcode it so it takes away one value in the array at a time and check each time to see if i'm <= desired value-- it'd be 2-3 lines for each item (so a total of 24-36 lines), but this is both messy to maintain and not optimal. I'm interested to see how it would be done in a loop. I typically don't have trouble with loops, but this is hardly a standard one for me.

Answer 1

curdiff:= 0;
i:= Low(_WoWDataFileSize) - 1;
while (curdiff <= reqsize) and (i < High(_WoWDataFileSize)) do
begin
inc (i);
curdiff:= curdiff + _WoWDataFileSize[i];
end;
At the end of the loop, either you've attained the required reduction in size or you've iterated through the entire array.

Answer 2

It is IMHO just two line missing in your code :o)
CurDiff := ReqSize;
// repeat
for i := Low(_WoWDataFileSize) to High(_WoWDataFileSize) do
begin
CurDiff := CurrDiff - _WoWDataFileSize[i];
if CurDiff <= ReqSize then break; // breaks the for..to loop
end;
// until CurDiff <= ReqSize;
EDIT No need for the repeat...until loop
But IMHO it is not very useful only to count the sizes without storing which files match.
So using a CustomObject and Lists (thanx to Generics) it will be very simple:
type
TFileObject = class
private
FName : string;
FSize : Int64;
public
constructor Create( AName : string; ASize : Int64 );
published
property Name : string read FName;
property Size : Int64 read FSize;
end;
procedure MoveFileObject(AMaxSize : Int64; ASrcList, ATarList : TList<TFileObject> );
var
LItem : TFileObject;
LSize : Int64;
begin
LSize := 0;
for LItem in ASrcList do
begin
if LSize + LItem.Size <= AMaxSize then
begin
LSize := LSize + LItem.Size;
ATarList.Add( LItem );
end;
end;
end;

Answer 3

Thanks to everyone for their answers, i figured out where i was going wrong. When i was calculating in my initial question, i'd forgotten to account for my division on the values (for the sake of showing MB instead of Bytes as TBWoWDir.Value was livebound to a TLabel.text, but the actual size was being divided before assigning TBWoWDir.Max).
Thanks to a few tweaks from an answer by No'am Newman, i managed to figure this out for myself. Here's how i got the result i was after (or much closer to it);
Global Vars;
_WoWDataFileSize : Array [0..12] of Int64;
Global Const;
_WoWData : Array [0..12] of String;
// "TBWoWDir" is a TTrackBar (Firemonkey)
[...]
var
ReqSize : int64;
DiffSize, CurDiff : Int64;
i, ii : Integer;
FilesTot : Integer;
FILESMSG : String;
begin
// Set up initial values to work with
ReqSize := Round(TBWoWDir.Value) * 1024 * 1024; // Requested Size - Multiplied from formatting
TotalSize := Round(TBWoWDir.Max) * 1024 * 1024; // Actual installation size - Multiplied from formatting
DiffSize := TotalSize - ReqSize; // Calculate Difference
CurDiff := 0; // Reset Current Difference
i := -1; // Reset i
repeat
inc (i); // Increment i
CurDiff := CurDiff + _WoWDataFileSize[i]; // Add current array item file size to CurDiff
until (CurDiff >= (DiffSize)) or (i >= 12); // Repeat until we reach ideal size or the end of the array
// Calculate which array item we stopped at
for ii := 0 to i do // use i from previous loop as the max
begin
FILESMSG := FILESMSG + 'File: ' + WoWData[ii] +
' | Size: ' + IntToStr(_WoWDataFileSize[ii])+' '#13#10;
FilesTot := FilesTot + _WoWDataFileSize[ii];
end;
// Show Message providing details
ShowMessage('CurDiff:' + IntToStr(CurDiff div 1024 div 1024) +
' | DiffSize: ' + IntToStr(DiffSize div 1024 div 1024) +
' | Array i: ' +
IntToStr(i) +#13#10+
'Difference between CurDiff and DiffSize: '+ IntToStr(((DiffSize div 1024 div 1024) - (CurDiff div 1024 div 1024)))+#13#10#13#10+
'File Details' +#13#10#13#10+
FilesMsg +#13#10#13#10+
'Total Size: ' + IntToStr(FilesTot));
end;
The code is there to tell me which files i need to copy (so modifying it to copy the files now isn't too difficult), and the whole ShowMessage is there for self-proof (as i use ShowMessage during development when i need to verify a value is returning correctly, as i'm sure many others do as well).