[CmdletBinding(DefaultParameterSetName='Generate')]
param(
    # File(s) or wildcard pattern to compute checksum for
    [Parameter(Position = 0, ParameterSetName = 'Generate', ValueFromPipeline = $true)]
    [Parameter(Position = 0, ParameterSetName = 'Match', ValueFromPipeline = $true)]
    [string[]]$FilePath,

    # Output file switch (-o) - auto generates file based on rules
    [Parameter(Mandatory = $false, ParameterSetName = 'Generate')]
    [Alias('o', 'out')]
    [switch]$SaveToFile,

    # Custom Output file name (-f)
    [Parameter(Mandatory = $false, ParameterSetName = 'Generate')]
    [Alias('f', 'outfile')]
    [string]$OutFile,

    # File containing checksums to verify (-c mode)
    [Parameter(Mandatory = $false, ParameterSetName = 'Verify')]
    [Alias('c', 'check')]
    [string]$Check,

    # Direct checksum hash string to match against (-m mode)
    [Parameter(Mandatory = $true, ParameterSetName = 'Match')]
    [Alias('m', 'match')]
    [string]$Match,

    # Hashing algorithm to use (Defaults to SHA256)
    [Parameter(Mandatory = $false)]
    [Alias('a', 'algo', 'algorithm')]
    [ValidateSet('SHA1', 'SHA256', 'SHA384', 'SHA512', 'MD5')]
    [string]$Algorithm,

    # Recursive flag (-r)
    [Parameter(Mandatory = $false)]
    [Alias('r', 'recursive')]
    [switch]$Recursive,

    # Help flag (-h)
    [Parameter(Mandatory = $false)]
    [Alias('h', 'help')]
    [switch]$Help
)

begin {
    function Show-Help {
        @'
NAME
    checksum - Compute and verify message digests / file checksums

SYNOPSIS
    .\checksum.ps1 [-FilePath] <string[]> [-o] [-OutFile <string>] [-Algorithm <string>] [-r]
    .\checksum.ps1 [-FilePath] <string[]> -Match <string> [-Algorithm <string>] [-r]
    .\checksum.ps1 -Check <string> [-Algorithm <string>] [-r]
    .\checksum.ps1 -Help

DESCRIPTION
    Computes or verifies checksums using various cryptographic algorithms. 
    Outputs hash values in standard Linux utility format (<hash>  <filename>).
    Ignores checksum files (.md5, .sha1, .sha256, .sha384, .sha512) unless explicitly passed.

PARAMETERS
    -FilePath <string[]>
        Specifies the path to one or more files to hash. Accepts wildcards (*).

    -o, -Out, --Out <switch>
        Enables writing checksum results to an auto-named file.
        - If input has a wildcard (*), outputs to 'checksums.<algo>'
        - If no wildcard is used, outputs to '<filename>.<algo>' per file.
        When combined with -r and *, it auto-generates checksum files 
        inside the current working directory and each subdirectory recursively.

    -f, -OutFile, --OutFile <string>
        Specifies an explicit output file name to save results to.

    -r, -Recursive, --Recursive <switch>
        Recursively generate or verify checksums in subdirectories.

    -c, -Check, --Check <string>
        Read checksums from the specified file and verify them against files on disk.
        If -r is used, searches for the specified filename recursively in subdirectories.

    -m, -Match, --Match <string>
        Compare the computed hash of the target file(s) against a specific hash string.

    -a, -Algorithm, --Algorithm <string>
        Specifies the cryptographic hash algorithm to use.
        Supported values: SHA1, SHA256 (default), SHA384, SHA512, MD5.
        If omitted in -c or -m mode, the algorithm is auto-detected by hash length.

    -h, -Help, --Help <switch>
        Display this help message.

EXAMPLES
    1. Compute SHA256 checksum for a single file:
       .\checksum.ps1 myfile.iso

    2. Compute checksums for all files and auto-write to files (-o flag):
       .\checksum.ps1 * -r -o

    3. Verify checksums from verification files recursively:
       .\checksum.ps1 -c checksums.sha256 -r
'@
    };

    function Get-AlgorithmFromHashLength {
        param([string]$HashString);

        switch ($HashString.Trim().Length) {
            32  { return 'MD5'; }
            40  { return 'SHA1'; }
            64  { return 'SHA256'; }
            96  { return 'SHA384'; }
            128 { return 'SHA512'; }
            default { 
                Write-Error "checksum: Could not auto-detect algorithm for hash length $($HashString.Length). Please specify -Algorithm.";
                return $null;
            }
        };
    };
}

process {
    $helpInPath =$false;
    if ($FilePath) {
        foreach ($p in ($FilePath)) {
            if ($p -match '^(?i)[-/]{1,2}(h\vert{}help)$') {
                $helpInPath =$true;
                break;
            }
        };
    };

    if ($Help.IsPresent -or$helpInPath) {
        Show-Help;
        return;
    };

    if (-not [string]::IsNullOrWhiteSpace($Algorithm)) {
        $Algorithm =$Algorithm.ToUpper();
    };

    # --- ITEM GATHERING (Generate & Match Modes) ---
    if ($PSCmdlet.ParameterSetName -in @('Generate', 'Match')) {
        if (-not $FilePath -or$FilePath.Count -eq 0) {
            Write-Error "checksum: Missing file path parameter. Use -Help for usage.";
            return;
        };

        $allItems = @();
        $isWildcardGlobal =$false;
        $skipExtensions = @('.md5', '.sha1', '.sha256', '.sha384', '.sha512');

        foreach ($pathEntry in ($FilePath)) {
            $isWildcard = ($pathEntry -match '\*');
            if ($isWildcard) { $isWildcardGlobal =$true; };

            $resolvedItems = @();
            if ($Recursive) {
                $resolvedItems = Get-ChildItem -Path ($pathEntry) -Recurse -File -ErrorAction 'SilentlyContinue';
            } else {
                if ($isWildcard -or ($pathEntry -match '\?')) {
                    $resolvedItems = Get-ChildItem -Path ($pathEntry) -File -ErrorAction 'SilentlyContinue';
                } else {
                    $resolvedItems = Get-Item -Path ($pathEntry) -ErrorAction 'SilentlyContinue';
                }
            };

            if (-not $resolvedItems) {
                Write-Error "checksum: $($pathEntry)`: No such file or directory";
                continue;
            };

            $explicitFileFound = $false;
            if ((-not $isWildcard) -and (Test-Path -Path ($pathEntry) -PathType 'Leaf')) {
                $explicitFileFound = $true;
            };

            foreach ($item in ($resolvedItems)) {
                if ($item.PSIsContainer) { continue; };
                
                if (-not $explicitFileFound) {
                    if ($skipExtensions -contains $item.Extension.ToLower()) {
                        continue;
                    }
                };
                $allItems += $item;
            };
        };
    };

    # --- DIRECT MATCH MODE (-m / -Match) ---
    if ($PSCmdlet.ParameterSetName -eq 'Match') {
        if ([string]::IsNullOrWhiteSpace($Algorithm)) {
            $Algorithm = Get-AlgorithmFromHashLength -HashString ($Match);
            if (-not $Algorithm) { return; };
        };

        $expectedHash = $Match.Trim().ToLower();

        foreach ($item in ($allItems)) {
            $actualHash = (Get-FileHash -Path ($item.FullName) -Algorithm ($Algorithm)).Hash.ToLower();
            $targetFile = Resolve-Path -Path ($item.FullName) -Relative -ErrorAction 'SilentlyContinue';
            if (-not $targetFile) { $targetFile = $item.Name; };
            if ($targetFile.StartsWith(".\")) { $targetFile = $targetFile.Substring(2); };

            if ($actualHash -eq $expectedHash) {
                Write-Host "${targetFile}: " -NoNewline;
                Write-Host "OK" -ForegroundColor Green;
            } else {
                Write-Host "${targetFile}: " -NoNewline;
                Write-Host "FAILED" -ForegroundColor Red;
            };
        };
    }
    # --- VERIFICATION MODE (-c / -Check) ---
    elseif ($PSCmdlet.ParameterSetName -eq 'Verify') {
        $checkFiles = @();
        if ($Recursive) {
            $checkFileName = Split-Path -Path ($Check) -Leaf;
            $checkFiles = Get-ChildItem -Path '.\' -Filter ($checkFileName) -Recurse -File -ErrorAction 'SilentlyContinue';
        } else {
            if (Test-Path -Path ($Check) -PathType 'Leaf') {
                $checkFiles = @(Get-Item -Path ($Check));
            }
        };

        if (-not $checkFiles -or $checkFiles.Count -eq 0) {
            Write-Error "checksum: $($Check)`: No such file or directory";
            return;
        };

        foreach ($cf in ($checkFiles)) {
            $lines = Get-Content -Path ($cf.FullName);
            $baseDir =$cf.DirectoryName;
            
            $fileAlgo =$Algorithm;
            if ([string]::IsNullOrWhiteSpace($fileAlgo)) {
                foreach ($line in ($lines)) {
                    if ([string]::IsNullOrWhiteSpace($line) -or$line.StartsWith("#")) { continue; };
                    if ($line -match '^([a-fA-F0-9]+)\s+') {
                        $fileAlgo = Get-AlgorithmFromHashLength -HashString ($Matches[1]);
                        break;
                    }
                };
                if ([string]::IsNullOrWhiteSpace($fileAlgo)) {$fileAlgo = 'SHA256'; };
            };

            foreach ($line in ($lines)) {
                if ([string]::IsNullOrWhiteSpace($line) -or$line.StartsWith("#")) { continue; };

                if ($line -match '^([a-fA-F0-9]+)\s+[* ]?(.*)$') {$expectedHash = $Matches[1].ToLower();$targetFile   = $Matches[2].Trim().TrimStart('*');$targetFullPath = Join-Path -Path ($baseDir) -ChildPath ($targetFile);

                    if (Test-Path -Path ($targetFullPath) -PathType 'Leaf') {$actualHash = (Get-FileHash -Path ($targetFullPath) -Algorithm ($fileAlgo)).Hash.ToLower();
                        
                        $displayPath = Resolve-Path -Path ($targetFullPath) -Relative -ErrorAction 'SilentlyContinue';
                        if (-not $displayPath) { $displayPath =$targetFile; };
                        if ($displayPath.StartsWith(".\")) { $displayPath =$displayPath.Substring(2); };

                        if ($actualHash -eq$expectedHash) {
                            Write-Host "${displayPath}: " -NoNewline;
                            Write-Host "OK" -ForegroundColor Green;
                        } else {
                            Write-Host "${displayPath}: " -NoNewline;
                            Write-Host "FAILED" -ForegroundColor Red;
                        };
                    } else {
                        $displayPath =$targetFile;
                        if ($baseDir -ne (Get-Location).Path) {$displayPath = Join-Path -Path (Resolve-Path -Path ($baseDir) -Relative -ErrorAction 'SilentlyContinue') -ChildPath ($targetFile);
                            if ($displayPath.StartsWith(".\")) { $displayPath =$displayPath.Substring(2); };
                        };
                        Write-Host "${displayPath}: " -NoNewline;
                        Write-Host "FAILED open or read" -ForegroundColor Red;
                    };
                };
            };
        };
    }
    # --- GENERATION MODE ---
    else {
        if ([string]::IsNullOrWhiteSpace($Algorithm)) {$Algorithm = 'SHA256';
        };

        $wantOutFile = $SaveToFile.IsPresent -or (-not [string]::IsNullOrWhiteSpace($OutFile));

        if ($wantOutFile) {
            if ($Recursive -and$isWildcardGlobal) {
                $groupedItems =$allItems | Group-Object DirectoryName;

                foreach ($group in ($groupedItems)) {
                    $dirPath =$group.Name;
                    $filesInDir =$group.Group;
                    
                    if ([string]::IsNullOrWhiteSpace($OutFile)) {
                        $currentOutFile = Join-Path -Path ($dirPath) -ChildPath "checksums.$($Algorithm.ToLower())";
                    } else {
                        $currentOutFile = Join-Path -Path ($dirPath) -ChildPath ($OutFile);
                    };

                    $null = New-Item -Path ($currentOutFile) -ItemType 'File' -Force;

                    foreach ($item in ($filesInDir)) {$hash = (Get-FileHash -Path ($item.FullName) -Algorithm ($Algorithm)).Hash.ToLower();
                        $relativePath =$item.Name;
                        
                        $outputLine = "$hash$relativePath";
                        Add-Content -Path ($currentOutFile) -Value ($outputLine);
                        Write-Output $outputLine;
                    };
                };
            } 
            else {
                $singleOutFile =$null;
                
                if ([string]::IsNullOrWhiteSpace($OutFile)) {
                    if ($isWildcardGlobal) {$singleOutFile = "checksums.$($Algorithm.ToLower())";
                    }
                } else {
                    $singleOutFile =$OutFile;
                };

                if ($singleOutFile) {
                    $null = New-Item -Path ($singleOutFile) -ItemType 'File' -Force;
                };

                foreach ($item in ($allItems)) {$hash = (Get-FileHash -Path ($item.FullName) -Algorithm ($Algorithm)).Hash.ToLower();
                    
                    $relativePath = Resolve-Path -Path ($item.FullName) -Relative -ErrorAction 'SilentlyContinue';
                    if (-not $relativePath) { $relativePath =$item.Name; };
                    if ($relativePath.StartsWith(".\")) {
                        $relativePath =$relativePath.Substring(2);
                    };

                    $outputLine = "$hash$relativePath";

                    if ($singleOutFile) {
                        Add-Content -Path ($singleOutFile) -Value ($outputLine);
                    } else {
                        $currentOutFile = "$($item.FullName).$($Algorithm.ToLower())";
                        $null = New-Item -Path ($currentOutFile) -ItemType 'File' -Force;
                        Add-Content -Path ($currentOutFile) -Value ($outputLine);
                    };

                    Write-Output $outputLine;
                };
            };
        } 
        else {
            foreach ($item in ($allItems)) {$hash = (Get-FileHash -Path ($item.FullName) -Algorithm ($Algorithm)).Hash.ToLower();
                
                $relativePath = Resolve-Path -Path ($item.FullName) -Relative -ErrorAction 'SilentlyContinue';
                if (-not $relativePath) { $relativePath =$item.Name; };
                if ($relativePath.StartsWith(".\")) {
                    $relativePath =$relativePath.Substring(2);
                };

                $outputLine = "$hash$relativePath";
                Write-Output $outputLine;
            };
        };
    }
}

end {}