Return correct byte offset from IndexIn

According to the [LSP Spec](https://microsoft.github.io/language-server-protocol/specification#textDocuments) Character Offsets are based on UTF-16 string representations. The current IndexIn assumes the Character is a byte offset leading to incorrect state in the Language Server.
tliron · Aug 18, 2021 · 69f9b44 · 69f9b44
1 parent 98d1a22
commit 69f9b44
Showing 1 changed file with 49 additions and 1 deletion.
diff --git a/protocol_3_16/base-structures.go b/protocol_3_16/base-structures.go
@@ -3,6 +3,7 @@ package protocol
 import (
 	"encoding/json"
 	"strings"
+	"unicode/utf8"
 )
 
 // https://microsoft.github.io/language-server-protocol/specification#uri
@@ -52,6 +53,15 @@ type Position struct {
 }
 
 func (self Position) IndexIn(content string) int {
+	// This code is modified from the gopls implementation found:
+	// https://cs.opensource.google/go/x/tools/+/refs/tags/v0.1.5:internal/span/utf16.go;l=70
+
+	// In accordance with the LSP Spec:
+	// https://microsoft.github.io/language-server-protocol/specification#textDocuments
+	// self.Character represents utf-16 code units, not bytes and so we need to
+	// convert utf-16 code units to a byte offset.
+
+	// Find the byte offset for the line
 	index := 0
 	for row := UInteger(0); row < self.Line; row++ {
 		content_ := content[index:]
@@ -61,7 +71,45 @@ func (self Position) IndexIn(content string) int {
 			return 0
 		}
 	}
-	return index + int(self.Character)
+
+	// The index represents the byte offset from the beginning of the line
+	// count self.Character utf-16 code units from the index byte offset.
+
+	byteOffset := index
+	remains := content[index:]
+	chr := int(self.Character)
+
+	for count := 1; count <= chr; count++ {
+
+		if len(remains) <= 0 {
+			// char goes past content
+			// this a error
+			return 0
+		}
+
+		r, w := utf8.DecodeRuneInString(remains)
+		if r == '\n' {
+			// Per the LSP spec:
+			//
+			// > If the character value is greater than the line length it
+			// > defaults back to the line length.
+			break
+		}
+
+		remains = remains[w:]
+		if r >= 0x10000 {
+			// a two point rune
+			count++
+			// if we finished in a two point rune, do not advance past the first
+			if count > chr {
+				break
+			}
+		}
+		byteOffset += w
+
+	}
+
+	return byteOffset
 }
 
 func (self Position) EndOfLineIn(content string) Position {