vim9script

def GetMatch(base: string, line: string): list<any>
    var s = base
    while stridx(line, s) == -1
        s = strpart(s, 1)
        if strlen(s) == 0
            return [0, ""]
        endif
    endwhile
    const idx = stridx(line, s)
    const slen = strlen(s)
    return [slen, base .. strpart(line, idx + slen)]
enddef

def LineSuffixCompl(findstart: bool, base: string): any
    if findstart
        return 0
    endif
    return  getbufinfo()
        ->map( (_, buf) => getbufline(buf["bufnr"], 1, "$"))
        ->flattennew()
        ->sort()
        ->uniq()
        ->map( (_, v) => GetMatch(base, v) )
        ->sort( (x, y) => y[0] - x[0] )
        ->map( (_, v) => v[1] )
enddef

setlocal completefunc=LineSuffixCompl