Alternative for nreplace

nreplace_inplace

let nreplace ~str ~sub ~by =                                                                                               
  let strlen = length str in                                                                                               
  let sublen = length sub in                                                                                               
  let bylen  = length by in                                                                                                
  let dlen   = bylen - sublen in                                                                                           
  let rec loop_subst l i = 
    match Exceptionless.rfind_from str (i-1) sub with                                                                      
    | None -> l                                                                                                            
    | Some i' -> loop_subst (l + dlen) i' in                                                                               
  let newlen =                                                                                                             
    if dlen = 0 then strlen else loop_subst strlen strlen in                                                               
  let newstr = create newlen in                                                                                            
  let rec loop_copy i j =                                                                                                  
    match Exceptionless.rfind_from str (i-1) sub with                                                                      
    | None ->                                                                                                              
      (* still need the first chunk *)                                                                                     
      assert (i = j) ;                                                                                                     
      blit str 0 newstr 0 i
    | Some i' ->
      let j' = j - (i - i') - dlen in
      (* newstring.[j .. end] is already inited. Init from j' to (j-1). *)
      blit by 0 newstr j' bylen ;
      blit str (i'+sublen) newstr (j'+bylen) (i-i'-sublen) ;
      loop_copy i' j' in
  loop_copy strlen newlen ;
  newstr

My original thought was to store the offsets when scanning the first time so you don't have to scan the original string twice; i.e. building up a recipe for how to construct the final string.

See my fork: https://gist.github.com/thelema/5639270