#!/usr/bin/env zsh
#
# Usage: -z4h-replace-buf START END REPLACEMENT
#
# Replaces BUFFER[START,END] with REPLACEMENT while keeping the cursor
# pointing to the same content as before.

emulate -L zsh

local -i start=$1 end=$2
local orig_word=$BUFFER[start,end] new_word=$3

local reply
if (( $#orig_word * $#new_word <= 10000 )); then
  -z4h-string-diff "$orig_word" "$new_word"
else
  reply=("$orig_word" "$new_word")
fi

local -i orig_cur='CURSOR - start + 1' cur new_cur
local src dst
for src dst in "${reply[@]}"; do
  (( cur += $#src ))
  if (( cur < orig_cur )); then
    (( new_cur += $#dst - $#src ))
  elif (( cur == orig_cur )); then
    (( cur == $#orig_word )) && break
    (( new_cur += $#dst - $#src ))
  else
    if (( orig_cur - (cur - $#src) > $#dst )); then
      (( new_cur -= orig_cur - (cur - $#src) - $#dst ))
    fi
    break
  fi
done

(( new_cur += CURSOR ))
BUFFER[start,end]=$new_word
(( CURSOR = new_cur ))
return 0