internal/lsp: fix incorrect line and start of semantic tokens

Current implementation returns incorrect result when some of the items are skipped, because positions could be relative to a skipped item. Instead, each position must be relative to the previous item added to the result.

Change-Id: I3c1a68d37bf0c9cfc1bccfe6f76c25b536224293
GitHub-Last-Rev: 6eedc7c
GitHub-Pull-Request: #376
Reviewed-on: https://go-review.googlesource.com/c/tools/+/396715
Run-TryBot: Peter Weinberger <pjw@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Peter Weinberger <pjw@google.com>
Trust: Peter Weinberger <pjw@google.com>
Trust: Suzy Mueller <suzmue@golang.org>

internal/lsp/semantic.go

@@ -829,19 +829,20 @@ func (e *encoded) Data() []uint32 {
 	// (see Integer Encoding for Tokens in the LSP spec)
 	x := make([]uint32, 5*len(e.items))
 	var j int
+	var last semItem
 	for i := 0; i < len(e.items); i++ {
 		typ, ok := typeMap[e.items[i].typeStr]
 		if !ok {
 			continue // client doesn't want typeStr
 		}
-		if i == 0 {
+		if j == 0 {
 			x[0] = e.items[0].line
 		} else {
-			x[j] = e.items[i].line - e.items[i-1].line
+			x[j] = e.items[i].line - last.line
 		}
 		x[j+1] = e.items[i].start
-		if i > 0 && e.items[i].line == e.items[i-1].line {
-			x[j+1] = e.items[i].start - e.items[i-1].start
+		if j > 0 && x[j] == 0 {
+			x[j+1] = e.items[i].start - last.start
 		}
 		x[j+2] = e.items[i].len
 		x[j+3] = uint32(typ)
@@ -852,6 +853,7 @@ func (e *encoded) Data() []uint32 {
 		}
 		x[j+4] = uint32(mask)
 		j += 5
+		last = e.items[i]
 	}
 	return x[:j]
 }