Text Diff

// Line-by-line diff using the LCS algorithm
function diffLines(a, b) {
  const linesA = a.split('\n')
  const linesB = b.split('\n')

  // Build LCS table
  const m = linesA.length, n = linesB.length
  const dp = Array.from({ length: m + 1 }, () => new Array(n + 1).fill(0))
  for (let i = 1; i <= m; i++)
    for (let j = 1; j <= n; j++)
      dp[i][j] = linesA[i-1] === linesB[j-1]
        ? dp[i-1][j-1] + 1
        : Math.max(dp[i-1][j], dp[i][j-1])

  // Backtrack to produce diff
  const result = []
  let i = m, j = n
  while (i > 0 || j > 0) {
    if (i > 0 && j > 0 && linesA[i-1] === linesB[j-1]) {
      result.unshift({ type: 'equal', text: linesA[i-1] }); i--; j--
    } else if (j > 0 && (i === 0 || dp[i][j-1] >= dp[i-1][j])) {
      result.unshift({ type: 'add', text: linesB[j-1] }); j--
    } else {
      result.unshift({ type: 'remove', text: linesA[i-1] }); i--
    }
  }
  return result
}

const diff = diffLines("alpha\nbeta\ngamma", "alpha\nbeta changed\ngamma\ndelta")
// → [
//   { type: 'equal',  text: 'alpha' },
//   { type: 'remove', text: 'beta' },
//   { type: 'add',    text: 'beta changed' },
//   { type: 'equal',  text: 'gamma' },
//   { type: 'add',    text: 'delta' }
// ]

import difflib

text_a = """alpha
beta
gamma""".splitlines()

text_b = """alpha
beta changed
gamma
delta""".splitlines()

# Unified diff (same format as git diff)
for line in difflib.unified_diff(text_a, text_b, fromfile='a.txt', tofile='b.txt', lineterm=''):
    print(line)
# --- a.txt
# +++ b.txt
# @@ -1,3 +1,4 @@
#  alpha
# -beta
# +beta changed
#  gamma
# +delta

# HTML side-by-side diff
d = difflib.HtmlDiff()
html = d.make_file(text_a, text_b, fromdesc='Original', todesc='Modified')

package main

import (
	"fmt"
	"strings"
)

// Minimal LCS-based line diff
func diffLines(a, b string) {
	la := strings.Split(a, "\n")
	lb := strings.Split(b, "\n")
	m, n := len(la), len(lb)

	dp := make([][]int, m+1)
	for i := range dp {
		dp[i] = make([]int, n+1)
	}
	for i := 1; i <= m; i++ {
		for j := 1; j <= n; j++ {
			if la[i-1] == lb[j-1] {
				dp[i][j] = dp[i-1][j-1] + 1
			} else if dp[i-1][j] >= dp[i][j-1] {
				dp[i][j] = dp[i-1][j]
			} else {
				dp[i][j] = dp[i][j-1]
			}
		}
	}

	var result []string
	i, j := m, n
	for i > 0 || j > 0 {
		if i > 0 && j > 0 && la[i-1] == lb[j-1] {
			result = append([]string{" " + la[i-1]}, result...)
			i--; j--
		} else if j > 0 && (i == 0 || dp[i][j-1] >= dp[i-1][j]) {
			result = append([]string{"+" + lb[j-1]}, result...)
			j--
		} else {
			result = append([]string{"-" + la[i-1]}, result...)
			i--
		}
	}

	for _, line := range result {
		fmt.Println(line)
	}
}

// Output:
//  alpha
// -beta
// +beta changed
//  gamma
// +delta

# Compare two files with unified diff (3 lines of context)
diff -u original.txt modified.txt

# Git diff between working tree and last commit
git diff HEAD -- file.txt

# Git diff between two branches
git diff main..feature -- src/

# Side-by-side diff in the terminal
diff -y --width=120 original.txt modified.txt

# Color-coded diff (requires colordiff)
diff -u original.txt modified.txt | colordiff