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/**

* Object representing a single character in a difference between strings

*/

export type DiffCharacter = {

character: string;

wasAdded: boolean;

wasRemoved: boolean;

};

/**

* Return the longest common subsequence of two strings

* @example

* longestCommonSubsequence("ABCBDAB", "BDCABA") // "BCBA"

* longestCommonSubsequence("abCd", "AbCD", true) // "AbCD"

*/

export function longestCommonSubsequence(

a: string,

b: string,

ignoreCase = false,

): string {

if (a.length === 0 || b.length === 0) return "";

const arr: {

length: number;

character: string | null;

previous: number[];

}[][] = [];

for (let i = 0; i < a.length; i++) {

arr[i] = [];

for (let j = 0; j < b.length; j++) {

arr[i].push({

length: 0,

previous: [-1, -1],

character: null,

});

}

}

const steps = [

[-1, 0],

[0, -1],

];

function inBoundary(i: number, j: number): boolean {

return i >= 0 && i < a.length && j >= 0 && j < b.length;

}

for (let i = 0; i < a.length; i++) {

for (let j = 0; j < b.length; j++) {

const currentObject: {

length: number;

previous: number[];

character: string | null;

} = {

length: 0,

previous: [-1, -1],

character: null,

};

for (const step of steps) {

const h = i + step[0];

const w = j + step[1];

if (inBoundary(h, w)) {

const previousObject = arr[h][w];

if (previousObject.length > currentObject.length) {

currentObject.length = previousObject.length;

currentObject.previous = [h, w];

}

}

}

if (

a[i] === b[j] ||

(ignoreCase && a[i].toLowerCase() === b[j].toLowerCase())

) {

if (

i > 0 &&

j > 0 &&

arr[i - 1][j - 1].length + 1 > currentObject.length

) {

currentObject.length = arr[i - 1][j - 1].length + 1;

currentObject.previous = [i - 1, j - 1];

currentObject.character = b[j];

} else if (i == 0 || j == 0) {

currentObject.character = b[j];

currentObject.length = 1;

currentObject.previous = [-1, -1];

}

}

arr[i][j] = currentObject;

}

}

let currentObject = arr[a.length - 1][b.length - 1];

let result = "";

while (currentObject.length !== 0) {

if (currentObject.character !== null) {

result = currentObject.character + result;

}

if (currentObject.previous[0] == -1) break;

currentObject = arr[currentObject.previous[0]][currentObject.previous[1]];

}

return result;

}

/**

* Returns an array representing how the strings are different

*/

export function diffCharacters(

oldString: string,

newString: string,

ignoreCase = false,

): DiffCharacter[] {

const commonSubsequence = longestCommonSubsequence(

oldString,

newString,

ignoreCase,

);

const result: DiffCharacter[] = [];

let oldStringPointer = 0;

let newStringPointer = 0;

let commonSubsequencePointer = 0;

while (

oldStringPointer < oldString.length ||

newStringPointer < newString.length ||

commonSubsequencePointer < commonSubsequence.length

) {

if (

oldStringPointer < oldString.length &&

(!ignoreCase && oldString[oldStringPointer] !==

commonSubsequence[commonSubsequencePointer] ||

(ignoreCase &&

oldString[oldStringPointer].toLowerCase() !==

commonSubsequence[commonSubsequencePointer].toLowerCase()))

) {

result.push({

character: oldString[oldStringPointer],

wasAdded: false,

wasRemoved: true,

});

oldStringPointer++;

} else if (

newStringPointer < newString.length &&

(!ignoreCase && newString[newStringPointer] !==

commonSubsequence[commonSubsequencePointer] ||

(ignoreCase &&

newString[newStringPointer].toLowerCase() !==

commonSubsequence[commonSubsequencePointer].toLowerCase()))

) {

result.push({

character: newString[newStringPointer],

wasAdded: true,

wasRemoved: false,

});

newStringPointer++;

} else {

result.push({

character: commonSubsequence[commonSubsequencePointer],

wasAdded: false,

wasRemoved: false,

});

oldStringPointer++;

newStringPointer++;

commonSubsequencePointer++;

}

}

return result;

}