profile result of https://gist.github.com/itchyny/8e5f88f9e5c4700ff1d3

profile.log

FUNCTION  <SNR>4_new_strwidthpart_reverse()
Called 3843 times
Total time:  10.562034
 Self time:   9.863578

count  total (s)   self (s)
 3843              0.028234   if a:width <= 0
   63              0.001507     return ''
                              endif
                              " Split into array
 3780              4.381157   let strarr = split(a:str, '\zs')
 3780   0.270479   0.083609   let width = s:wcswidth(a:str)
 3780              0.040699   let strlen = len(strarr)
                              " Start binary search
 3780              0.020375   let diff = strlen / 2
 3780              0.027083   let leftindex = 0
 3780              0.013445   let index = -1
32886              0.184495   while width > a:width && diff
29106              0.411092     let index = min([leftindex + diff, strlen]) - 1
29106   2.996260   2.484674     let partwidth = s:wcswidth(join(strarr[(leftindex):(index)], ''))
29106              0.271477     if width - partwidth >= a:width || diff <= 1
19656              0.210977       let width -= partwidth
19656              0.127308       let leftindex = index + 1
19656              0.047250     endif
29106              0.091725     if diff > 1
23751              0.105325       let diff = diff / 2
23751              0.157759     endif
29106              0.088385   endwhile
 3780              0.587081   return index < strlen ? join(strarr[(index + 1):], '') : ''

FUNCTION  <SNR>4_strwidthpart_reverse()
Called 3843 times
Total time:  59.031721
 Self time:  50.336910

count  total (s)   self (s)
 3843              0.019939   if a:width <= 0
   63              0.000168     return ''
                              endif
 3780              0.021779   let ret = a:str
 3780   0.259746   0.060382   let width = s:wcswidth(a:str)
772380              3.781111   while width > a:width
768600             16.278451     let char = matchstr(ret, '^.')
768600             11.799622     let ret = ret[len(char) :]
768600  22.698868  14.203421     let width -= s:wcswidth(char)
768600              2.735721   endwhile
                            
 3780              0.015197   return ret

FUNCTION  <SNR>4_new_strwidthpart()
Called 3843 times
Total time:  10.272793
 Self time:   9.622317

count  total (s)   self (s)
 3843              0.023118   if a:width <= 0
   63              0.000167     return ''
                              endif
                              " Split into array
 3780              4.262276   let strarr = split(a:str, '\zs')
 3780   0.275712   0.080953   let width = s:wcswidth(a:str)
                              " Start binary search
 3780              0.031338   let index = len(strarr)
 3780              0.019846   let diff = index / 2
 3780              0.033331   let rightindex = index - 1
32508              0.163413   while width > a:width && diff
28728              0.378663     let index = max([rightindex - diff + 1, 0])
28728   2.985590   2.529873     let partwidth = s:wcswidth(join(strarr[(index):(rightindex)], ''))
28728              0.268011     if width - partwidth >= a:width || diff <= 1
19782              0.117992       let width -= partwidth
19782              0.141968       let rightindex = index - 1
19782              0.048681     endif
28728              0.093494     if diff > 1
23562              0.100147       let diff = diff / 2
23562              0.049855     endif
28728              0.070989   endwhile
 3780              0.651851   return index ? join(strarr[:index - 1], '') : ''

FUNCTION  Test_strwidthpart()
Called 1 time
Total time: 195.972491
 Self time:   1.765382

count  total (s)   self (s)
    1              0.000006   let strmin = 0
    1              0.000004   let strmax = 300
    1              0.000004   let strstep = 5
    1              0.000003   let outmin = -10
    1              0.000003   let outmax = 300
    1              0.000003   let outstep = 5
   62              0.000256   for i in range(strmin, strmax, strstep)
   61              0.000671     let str = repeat('あa', i)
 3904              0.022893     for j in range(outmin, outmax, outstep)
 3843 114.442991   0.102430       let out1 = s:P.strwidthpart(str, i)
 3843  10.961122   0.688329       let out2 = s:P.new_strwidthpart(str, i)
 3843              0.032616       if out1 !=# out2
                                    call Error_strwidthpart()
                                  endif
 3843  59.126255   0.094534       let out1 = s:P.strwidthpart_reverse(str, i)
 3843  11.257802   0.695768       let out2 = s:P.new_strwidthpart_reverse(str, i)
 3843              0.029949       if out1 !=# out2
                                    call Error_strwidthpart()
                                  endif
 3843              0.009382     endfor
   61              0.000112   endfor

FUNCTION  <SNR>4_strwidthpart()
Called 3843 times
Total time: 114.340561
 Self time: 105.334140

count  total (s)   self (s)
 3843              0.021707   if a:width <= 0
   63              0.000162     return ''
                              endif
 3780              0.022978   let ret = a:str
 3780   0.288444   0.061628   let width = s:wcswidth(a:str)
773640              3.924480   while width > a:width
769860             69.921682     let char = matchstr(ret, '.$')
769860             12.728451     let ret = ret[: -1 - len(char)]
769860  22.997894  14.218289     let width -= s:wcswidth(char)
769860              2.938713   endwhile
                            
 3780              0.056247   return ret

FUNCTIONS SORTED ON TOTAL TIME
count  total (s)   self (s)  function
    1 195.972491   1.765382  Test_strwidthpart()
 3843 114.340561 105.334140  <SNR>4_strwidthpart()
 3843  59.031721  50.336910  <SNR>4_strwidthpart_reverse()
 3843  10.562034   9.863578  <SNR>4_new_strwidthpart_reverse()
 3843  10.272793   9.622317  <SNR>4_new_strwidthpart()

FUNCTIONS SORTED ON SELF TIME
count  total (s)   self (s)  function
 3843 114.340561 105.334140  <SNR>4_strwidthpart()
 3843  59.031721  50.336910  <SNR>4_strwidthpart_reverse()
 3843  10.562034   9.863578  <SNR>4_new_strwidthpart_reverse()
 3843  10.272793   9.622317  <SNR>4_new_strwidthpart()
    1 195.972491   1.765382  Test_strwidthpart()