vim/runtime/indent/make.vim

" Vim indent file
" Language:         Makefile
" Maintainer:       Nikolai Weibull <now@bitwi.se>
" Latest Revision:  2006-04-26

if exists("b:did_indent")
  finish
endif
let b:did_indent = 1

setlocal indentexpr=GetMakeIndent()
setlocal indentkeys=!^F,o,O
setlocal nosmartindent

if exists("*GetMakeIndent")
  finish
endif

let s:rule_rx = '^[^ \t#:][^#:]*:\{1,2}\%([^=:]\|$\)'
let s:continuation_rx = '\\$'
let s:assignment_rx = '^\s*\h\w*\s*+\==\s*\zs.*\\$'

" TODO: Deal with comments, string, and all kinds of other crap, e.g., defines.
" TODO: Unwrap the whole logic of this function into something that requires a
" lot less “return”s.
function GetMakeIndent()
  let lnum = v:lnum - 1
  if lnum == 0
    return 0
  endif

  " Figure out if the previous line is part of a rule or not.  If it is, then
  " we more or less just indent by a ‘tabstop’, the previous’ lines indent, or
  " remove all indent if the current line is itself a rule.  Also, if the line
  " in question is part of a continuation-line set constituting the rule line
  " itself, we indent by either a ‘shiftwidth’, if the line is the first in the
  " continuation, or use the indent of the previous line, if not.
  while lnum > 0
    let line = getline(lnum)
    if line[0] != "\t"
      " We found a non-shell-command line, i.e., one that doesn’t have a
      " leading tab.
      if line =~ s:rule_rx
        " The line looks like a rule line, so we must therefore either be inside a
        " rule or we are a continuation line to that rule line.
        if line =~ s:continuation_rx
          " Ah, the rule line was continued, so look up the last continuation
          " line that’s above the current line.
          while line =~ s:continuation_rx && lnum < v:lnum
            let lnum += 1
            let line = getline(lnum)
          endwhile
          let lnum -= 1
          let line = getline(lnum)
        endif

        " If the line that we’ve found is right above the current line, deal
        " with it specifically.
        if lnum == v:lnum - 1
          " If it was continued, indent the current line by a shiftwidth, as it
          " is the first to follow it.  Otherwise, depending on if the current
          " line is a rule line, i.e, a rule line following another rule line,
          " then indent to the left margin.  Otherwise, the current line is the
          " first shell-command line in the rule, so indent by a ‘tabstop’
          if line =~ s:continuation_rx
            return &sw
          else
            return getline(v:lnum) =~ s:rule_rx ? 0 : &ts
          endif
        else
          " If the previous line was a continuation line, then unless it was
          " itself a part of a continuation line, add a ‘shiftwidth’’s worth of
          " indent.  Otherwise, just use the indent of the previous line.
          " Otherwise, if the previous line wasn’t a continuation line, check
          " if the one above it was.  If it was then indent to whatever level
          " the “owning” line had.  Otherwise, indent to the previous line’s
          " level.
          let lnum = v:lnum - 1
          let line = getline(lnum)
          if line =~ s:continuation_rx
            let pnum = v:lnum - 2
            let pine = getline(pnum)
            if pine =~ s:continuation_rx
              return indent(lnum)
            else
              return indent(lnum) + &sw
            endif
          else
            let lnum = v:lnum - 2
            let line = getline(lnum)
            if line =~ s:continuation_rx
              while lnum > 0
                if line !~ s:continuation_rx
                  let lnum += 1
                  let line = getline(lnum)
                  break
                endif
                let lnum -= 1
                let line = getline(lnum)
              endwhile
              " We’ve found the owning line.  Indent to it’s level.
              return indent(lnum)
            else
              return indent(v:lnum - 1)
            endif
          endif
        endif
      endif

      " The line wasn’t a rule line, so the current line is part of a series
      " of tab-indented lines that don’t belong to any rule.
      break
    endif
    let lnum -= 1
  endwhile

  " If the line before the one we are currently indenting ended with a
  " continuation, then try to figure out what “owns” that line and indent
  " appropriately.
  let lnum = v:lnum - 1
  let line = getline(lnum)
  if line =~ s:continuation_rx
    let indent = indent(lnum)
    if line =~ s:assignment_rx
      " The previous line is a continuation line that begins a variable-
      " assignment expression, so set the indent to just beyond the whitespace
      " following the assignment operator (‘=’).
      call cursor(lnum, 1)
      if search(s:assignment_rx, 'W') != 0
        let indent = virtcol('.') - 1
      endif
    endif
    
    " The previous line didn’t constitute an assignment, so just indent to
    " whatever level it had.
    return indent
  endif

  " If the line above the line above the current line ended was continued,
  " then the line above the current line was part of a continued line.  Find
  " the “owning” line and indent to its level.
  let lnum = v:lnum - 2
  let line = getline(lnum)
  if line =~ s:continuation_rx
    while lnum > 0
      if line !~ s:continuation_rx
        let lnum += 1
        let line = getline(lnum)
        break
      endif
      let lnum -= 1
      let line = getline(lnum)
    endwhile
    " We’ve found the owning line.  Indent to it’s level.
    return indent(lnum)
  endif

  " If nothing else caught on, then check if this line is a rule line.  If it
  " is, indent it to the left margin.  Otherwise, simply use the indent of the
  " previous line.
  let line = getline(v:lnum)
  if line =~ s:rule_rx
    return 0
  else
    return indent(v:lnum - 1)
  endif
endfunction