Finding difference between strings in Javascript -

i'd compare 2 strings (a before , after) , detect , changed between them.

for change, want know:

1. starting position of change (inclusive, starting @ 0)
2. ending position of change (inclusive, starting @ 0) relative previous text
3. the "change"

assume strings change in 1 place @ time (for example, never "bill" -> "kiln").

additionally, need start , end positions reflect type of change:

• if deletion, start , end position should start , end positions of deleted text, respectively
• if replacement, start , end position should start , end positions of "deleted" text, respectively (the change "added" text)
• if insertion, start , end positions should same; entry point of text
• if no change, let start , end positions remain zero, empty change

for example:

"0123456789" -> "03456789"   start: 1, end: 2, change: "" (deletion)  "03456789" -> "0123456789"   start: 1, end: 1, change: "12" (insertion)  "hello world!" -> "hello aliens!"   start: 6, end: 10, change: "aliens" (replacement)  "hi" -> "hi"   start: 0, end: 0, change: "" (no change) 

i able detect positions of changed text, doesn't work in cases because in order accurately, need know kind of change made.

var oldtext = "my edited string!"; var newtext = "my first string!";  var changestart = 0; var newchangeend = 0; var oldchangeend = 0; console.log("comparing start:"); (var = 0; < newtext.length; i++) {     console.log(i + ": " + newtext[i] + " -> " + oldtext[i]);     if (newtext[i] != oldtext[i]) {         changestart = i;         break;     } } console.log("comparing end:"); // "addition"? if (newtext.length > oldtext.length) {     (var = 1; < newtext.length; i++) {         console.log(i + "(n: " + (newtext.length - i) + " o: " + (oldtext.length - i) + ": " + newtext.substring(newtext.length - i, newtext.length - + 1) + " -> " + oldtext.substring(oldtext.length - i, oldtext.length - + 1));         if (newtext.substring(newtext.length - i, newtext.length - + 1) != oldtext.substring(oldtext.length - i, oldtext.length - + 1)) {             newchangeend = newtext.length - i;             oldchangeend = oldtext.length - i;             break;         }     } // "deletion"? } else if (newtext.length < oldtext.length) {     (var = 1; < oldtext.length; i++) {         console.log(i + "(n: " + (newtext.length - i) + " o: " + (oldtext.length - i) + ": " + newtext.substring(newtext.length - i, newtext.length - + 1) + " -> " + oldtext.substring(oldtext.length - i, oldtext.length - + 1));         if (newtext.substring(newtext.length - i, newtext.length - + 1) != oldtext.substring(oldtext.length - i, oldtext.length - + 1)) {             newchangeend = newtext.length - i;             oldchangeend = oldtext.length - i;             break;         }     } // same length... } else {     // } console.log("change start: " + changestart); console.log("nchange end : " + newchangeend); console.log("ochange end : " + oldchangeend); console.log("change: " + oldtext.substring(changestart, oldchangeend + 1)); 

how tell whether or not insertion, deletion, or replacement took place?

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i've searched , came few other similar questions, don't seem help.

i have gone through code , logic matching string makes sense me. logs changestart, newchangeend , oldchangeend correctly , algorithm flows alright. want know if insertion, deletion or replacement took place. here's how go it.

first of all, need make sure after have got first point of mis-match i.e. changestart when traverse strings end, index shouldn't cross changestart.

i'll give example. consider following strings:

 var newtext = "hello worllolds!";  var oldtext = "hello worlds!";   changestart -> 10 //makes sense  oldchangeend -> 8  newchangeend -> 11   console.log("change: " + newtext.substring(changestart, newchangeend + 1));   //ouputs "lo" 

the problem in case when starts matching back, flow this:

 comparing end:    1(n: 12 o: 12: ! -> !)    2(n: 11 o: 11: s -> s)    3(n: 10 o: 10: d -> d)  -> need stop here!   //although there not mismatch, have reached changestart ,   //we have established characters 0 -> changestart-1 match  //that why outputs "lo" instead of "lol" 

assuming, said makes sense, need modify for loops so:

 if (newtext.length > oldtext.length) {  (var = 1; < newtext.length && ((oldtext.length-i)>=changestart); i++) {   ...      newchangeend = newtext.length - -1;     oldchangeend = oldtext.length - -1;   if(//mismatch condition reached){          //break..that code fine.     }  } 

this condition -> (oldtext.length-i)>=changestart takes care of anomaly mentioned , therefore for loop automatically terminates if condition reached. however, mentioned there might situations condition reached before mis-match encountered demonstrated. need update values of newchangeend , oldchangeend 1 less matched value. in case of mis-match, store values appropriately.

instead of else -if wrap 2 conditions in situation know newtext.length > oldtext.length not true i.e. either replacement or deletion. again newtext.length > oldtext.length means replacement or insertion per examples, makes sense. else like:

else { (var = 1; < oldtext.length && ((oldtext.length-i)>=changestart); i++) {       ...     newchangeend = newtext.length - -1;     oldchangeend = oldtext.length - -1;   if(//mismatch condition reached){          //break..that code fine.     }  } 

if have understood minor changes far, identifying specific cases simple:

1. deletion - condition -> changestart > newchangeend. deleted string changestart -> oldchangeend.

deleted text -> oldtext.substring(changestart, oldchangeend + 1);

2. insertion - condition -> changestart > oldchangeend. inserted string @ changestart.

inserted text -> newtext.substring(changestart, newchangeend + 1);

3. replacement - if newtext != oldtext , above 2 conditions not met, is replacement.

text in old string got replaced -> oldtext.substring(changestart, oldchangeend + 1);

the replacement text -> newtext.substring(changestart, newchangeend + 1);

start , end positions in oldtext got replaced -> changestart -> oldchangeend

i have created jsfiddle incorporating changes have mentioned in code. might want check out. hope gets started in right direction.