#lang racket/base

(require racket/path
         "config.rkt")

(provide find-mergetool
         find-mergetool-path
         configured-mergetool
         set-mergetool!)

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;; Supporting functions
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; goal   : Build a path below an environment variable when it is defined.
; pre    : variable is an environment variable name.
; post   : The environment has only been inspected.
; result : The constructed path, or #f.
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(define (environment-path variable . parts)
  (let ((base (getenv variable)))
    (if base
        (apply build-path base parts)
        #f)))

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; goal   : Find one merge tool candidate on PATH or at a well-known path.
; pre    : candidate contains tool name, executable name and zero or more paths.
; post   : PATH and the filesystem have only been inspected.
; result : A list containing tool name and executable path, or #f.
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(define (find-candidate candidate)
  (let* ((tool (car candidate))
         (executable (cadr candidate))
         (path-executable (find-executable-path executable)))
    (cond
      (path-executable
       (list tool path-executable))
      (else
       (let loop ((paths (cddr candidate)))
         (cond
           ((null? paths) #f)
           ((and (car paths)
                 (file-exists? (car paths)))
            (list tool (car paths)))
           (else
            (loop (cdr paths)))))))))

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; goal   : Find the first usable merge tool candidate.
; pre    : candidates contains merge tool candidate descriptions.
; post   : PATH and the filesystem have only been inspected.
; result : A list containing tool name and executable path, or #f.
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(define (find-candidates candidates)
  (cond
    ((null? candidates) #f)
    (else
     (let ((candidate (find-candidate (car candidates))))
       (if candidate
           candidate
           (find-candidates (cdr candidates)))))))

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; goal   : Return well-known graphical merge tool candidates for Windows.
; pre    : Windows environment variables may or may not be defined.
; post   : The environment has only been inspected.
; result : Merge tool candidate descriptions in preference order.
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(define (windows-mergetool-candidates)
  (list
   (list "winmerge" "WinMergeU.exe"
         (environment-path "ProgramFiles" "WinMerge" "WinMergeU.exe")
         (environment-path "ProgramFiles(x86)" "WinMerge" "WinMergeU.exe"))
   (list "vscode" "code.cmd"
         (environment-path "LOCALAPPDATA" "Programs" "Microsoft VS Code" "bin" "code.cmd")
         (environment-path "ProgramFiles" "Microsoft VS Code" "bin" "code.cmd")
         (environment-path "ProgramFiles(x86)" "Microsoft VS Code" "bin" "code.cmd"))
   (list "vscode" "code.exe"
         (environment-path "LOCALAPPDATA" "Programs" "Microsoft VS Code" "Code.exe")
         (environment-path "ProgramFiles" "Microsoft VS Code" "Code.exe")
         (environment-path "ProgramFiles(x86)" "Microsoft VS Code" "Code.exe"))
   (list "kdiff3" "kdiff3.exe"
         (environment-path "ProgramFiles" "KDiff3" "kdiff3.exe")
         (environment-path "ProgramFiles(x86)" "KDiff3" "kdiff3.exe"))
   (list "meld" "meld.exe"
         (environment-path "LOCALAPPDATA" "Programs" "Meld" "Meld.exe")
         (environment-path "ProgramFiles" "Meld" "Meld.exe")
         (environment-path "ProgramFiles(x86)" "Meld" "Meld.exe"))
   (list "tortoisemerge" "TortoiseMerge.exe"
         (environment-path "ProgramFiles" "TortoiseSVN" "bin" "TortoiseMerge.exe")
         (environment-path "ProgramFiles(x86)" "TortoiseSVN" "bin" "TortoiseMerge.exe"))))

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; goal   : Return well-known graphical merge tool candidates for macOS.
; pre    : The current platform is macOS.
; post   : No program has been started.
; result : Merge tool candidate descriptions in preference order.
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(define (macos-mergetool-candidates)
  (list
   (list "opendiff" "opendiff"
         (string->path "/usr/bin/opendiff"))
   (list "vscode" "code"
         (string->path
          "/Applications/Visual Studio Code.app/Contents/Resources/app/bin/code"))
   (list "kdiff3" "kdiff3"
         (string->path "/Applications/kdiff3.app/Contents/MacOS/kdiff3"))
   (list "meld" "meld")))

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; goal   : Return well-known graphical merge tool candidates for Unix/Linux.
; pre    : The current platform is Unix.
; post   : No program has been started.
; result : Merge tool candidate descriptions in preference order.
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(define (unix-mergetool-candidates)
  (list
   (list "meld" "meld"
         (string->path "/usr/bin/meld")
         (string->path "/usr/local/bin/meld"))
   (list "kdiff3" "kdiff3"
         (string->path "/usr/bin/kdiff3")
         (string->path "/usr/local/bin/kdiff3"))
   (list "vscode" "code"
         (string->path "/snap/bin/code")
         (string->path "/usr/bin/code")
         (string->path "/usr/local/bin/code"))))

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; goal   : Return merge tool candidates for the current platform.
; pre    : The current platform is supported by Racket.
; post   : No program has been started.
; result : Merge tool candidate descriptions.
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(define (mergetool-candidates)
  (case (system-type 'os)
    ((windows) (windows-mergetool-candidates))
    ((macosx) (macos-mergetool-candidates))
    ((unix) (unix-mergetool-candidates))
    (else '())))

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; goal   : Find the automatically detected merge tool and executable path.
; pre    : Platform paths are accessible.
; post   : No merge tool has been started.
; result : A list containing tool name and path, or #f.
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(define (detected-mergetool)
  (find-candidates (mergetool-candidates)))

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;; Provided functions
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; goal   : Read the merge tool explicitly configured for git-cli.
; pre    : The git-cli configuration is readable.
; post   : The configuration has not been changed.
; result : The configured Git merge tool name, or #f.
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(define (configured-mergetool)
  (cfg-get 'git 'mergetool #f))

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; goal   : Store the Git merge tool name used by git-cli.
; pre    : tool is a Git mergetool name such as "winmerge" or "meld".
; post   : The tool name has been stored in the git-cli configuration.
; result : The result returned by the configuration layer.
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(define (set-mergetool! tool)
  (cfg-set! 'git 'mergetool tool))

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; goal   : Find the graphical merge tool that git-cli should prefer.
; pre    : The platform and git-cli configuration are available.
; post   : No merge tool has been started.
; result : A configured or detected Git merge tool name, or #f.
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(define (find-mergetool)
  (or (configured-mergetool)
      (let ((detected (detected-mergetool)))
        (if detected
            (car detected)
            #f))))

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; goal   : Find the executable path belonging to the automatically detected merge tool.
; pre    : Platform paths are accessible.
; post   : No merge tool has been started.
; result : The executable path, or #f.
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(define (find-mergetool-path)
  (let ((configured (configured-mergetool))
        (detected (detected-mergetool)))
    (if configured
        #f
        (if detected
            (cadr detected)
            #f))))