Add change-header-lines action to dynamically change --header-lines

All input lines now enter the chunklist with sequential indices, and
header lines are excluded from matching via Pattern.startIndex and
PassMerger offset. This allows the number of header lines to be changed
at runtime with change-header-lines(N), transform-header-lines, and
bg-transform-header-lines actions.

- Remove EvtHeader event; header items are read directly from chunks
- Add startIndex to Pattern and PassMerger for skipping header items
- Add targetIndex field to Terminal for cursor repositioning across
  header-lines changes

Close #4659

src/pattern.go

@@ -64,6 +64,7 @@ type Pattern struct {
 	procFun       map[termType]algo.Algo
 	cache         *ChunkCache
 	denylist      map[int32]struct{}
+	startIndex    int32
 }
 
 var _splitRegex *regexp.Regexp
@@ -74,7 +75,7 @@ func init() {
 
 // BuildPattern builds Pattern object from the given arguments
 func BuildPattern(cache *ChunkCache, patternCache map[string]*Pattern, fuzzy bool, fuzzyAlgo algo.Algo, extended bool, caseMode Case, normalize bool, forward bool,
-	withPos bool, cacheable bool, nth []Range, delimiter Delimiter, revision revision, runes []rune, denylist map[int32]struct{}) *Pattern {
+	withPos bool, cacheable bool, nth []Range, delimiter Delimiter, revision revision, runes []rune, denylist map[int32]struct{}, startIndex int32) *Pattern {
 
 	var asString string
 	if extended {
@@ -146,6 +147,7 @@ func BuildPattern(cache *ChunkCache, patternCache map[string]*Pattern, fuzzy boo
 		delimiter:     delimiter,
 		cache:         cache,
 		denylist:      denylist,
+		startIndex:    startIndex,
 		procFun:       make(map[termType]algo.Algo)}
 
 	ptr.cacheKey = ptr.buildCacheKey()
@@ -301,10 +303,19 @@ func (p *Pattern) Match(chunk *Chunk, slab *util.Slab) []Result {
 func (p *Pattern) matchChunk(chunk *Chunk, space []Result, slab *util.Slab) []Result {
 	matches := []Result{}
 
+	// Skip header items in chunks that contain them
+	startIdx := 0
+	if p.startIndex > 0 && chunk.count > 0 && chunk.items[0].Index() < p.startIndex {
+		startIdx = int(p.startIndex - chunk.items[0].Index())
+		if startIdx >= chunk.count {
+			return matches
+		}
+	}
+
 	if len(p.denylist) == 0 {
 		// Huge code duplication for minimizing unnecessary map lookups
 		if space == nil {
-			for idx := 0; idx < chunk.count; idx++ {
+			for idx := startIdx; idx < chunk.count; idx++ {
 				if match, _, _ := p.MatchItem(&chunk.items[idx], p.withPos, slab); match != nil {
 					matches = append(matches, *match)
 				}
@@ -320,7 +331,7 @@ func (p *Pattern) matchChunk(chunk *Chunk, space []Result, slab *util.Slab) []Re
 	}
 
 	if space == nil {
-		for idx := 0; idx < chunk.count; idx++ {
+		for idx := startIdx; idx < chunk.count; idx++ {
 			if _, prs := p.denylist[chunk.items[idx].Index()]; prs {
 				continue
 			}