highlight.go

package diffx

// highlight represents a byte range [start, end) within a line that should
// be visually highlighted (the part that actually changed).
type highlight struct {
	start, end int
}

// inlineDiff computes character-level differences between two lines. It
// returns whether the lines are "similar enough" to warrant inline
// highlighting (common=true), plus the highlighted byte ranges for each line.
//
// Lines are considered similar if more than 50% of characters are shared.
func inlineDiff(a, b string) (common bool, aHighlights, bHighlights []highlight) {
	// Find common prefix.
	prefixLen := commonPrefix(a, b)
	// Find common suffix (on the remaining part after the prefix).
	suffixLen := commonSuffix(a[prefixLen:], b[prefixLen:])

	// The differing middle sections.
	aMid := a[prefixLen : len(a)-suffixLen]
	bMid := b[prefixLen : len(b)-suffixLen]

	if len(aMid) == 0 && len(bMid) == 0 {
		// Lines are identical, no highlights needed.
		return true, nil, nil
	}

	// Check similarity: common chars vs total.
	commonChars := prefixLen + suffixLen
	totalChars := max(len(a), len(b))
	if totalChars > 0 && float64(commonChars)/float64(totalChars) < 0.5 {
		return false, nil, nil
	}

	if len(aMid) > 0 {
		aHighlights = []highlight{{start: prefixLen, end: prefixLen + len(aMid)}}
	}
	if len(bMid) > 0 {
		bHighlights = []highlight{{start: prefixLen, end: prefixLen + len(bMid)}}
	}

	return true, aHighlights, bHighlights
}

func commonPrefix(a, b string) int {
	n := min(len(a), len(b))
	for i := range n {
		if a[i] != b[i] {
			return i
		}
	}
	return n
}

func commonSuffix(a, b string) int {
	la, lb := len(a), len(b)
	n := min(la, lb)
	for i := range n {
		if a[la-1-i] != b[lb-1-i] {
			return i
		}
	}
	return n
}